Document number:  PL22.16/10-0212 = WG21 N3222
Date:  2010-11-29
Project:  Programming Language C++
Reference:  ISO/IEC IS 14882:2003
Reply to:  William M. Miller
 Edison Design Group, Inc.
 wmm@edg.com


C++ Standard Core Language Defect Reports, Revision 74


This document contains the C++ core language issues that have been categorized as Defect Reports by the Committee (J16 + WG21), that is, issues with status "DR," "WP," "CD1," "CD2," and "TC1," along with their proposed resolutions. ONLY RESOLUTIONS FOR ISSUES WITH TC1 STATUS ARE PART OF THE INTERNATIONAL STANDARD FOR C++. The other issues are provided for informational purposes only, as an indication of the intent of the Committee. They should not be considered definitive until or unless they appear in an approved Technical Corrigendum or revised International Standard for C++.

This document is part of a group of related documents that together describe the issues that have been raised regarding the C++ Standard. The other documents in the group are:

For more information, including a description of the meaning of the issue status codes and instructions on reporting new issues, please see the Active Issues List.

Section references in this document reflect the section numbering of document PL22.16/10-0116 = WG21 N3126.


Issues with "DR" Status


248. Identifier characters

Section: _N2691_.E  [extendid]     Status: DR     Submitter: John Spicer     Date: 6 Oct 2000

[Voted into the WP at the November, 2010 meeting as paper N3146.]

The list of identifier characters specified in the C++ standard annex _N2691_.E [extendid] and the C99 standard annex D are different. The C99 standard includes more characters.

The C++ standard says that the characters are from "ISO/IEC PDTR 10176" while the C99 standard says "ISO/IEC TR 10176". I'm guessing that the PDTR is an earlier draft of the TR.

Should the list in the C++ standard be updated?

Tom Plum: In my opinion, the "identifier character" issue has not been resolved with certainty within SC22.

One critical difference in C99 was the decision to allow a compiler to accept more characters than are given in the annex. This allows for future expansion.

The broader issue concerns the venue in which the "identifier character" issue will receive ongoing resolution.

Notes from 10/00 meeting:

The core language working group expressed a strong preference (13/0/5 in favor/opposed/abstaining) that the list of identifier characters should be extensible, as is the case in C99. However, the fact that this topic is under active discussion by other bodies was deemed sufficient reason to defer any changes to the C++ specification until the situation is more stable.

Notes from October, 2005 meeting:

The working group expressed interest in the kind of approach taken by XML 1.1, in which the definition of an identifier character is done by excluding large ranges of the Unicode character set and accepting any character outside those ranges, rather than by affirmatively designating each identifier character in each language. As noted above, consideration of this issue was previously deferred pending other related standardization efforts. Clark Nelson will investigate whether these have reached a point at which progress on this issue in C++ is now possible.

Additional note (May, 2008):

Issue 663 also deals with this appendix, and the proposed resolution there is to update the table to reflect the newest available technical report, ISO/IEC TR 10176:2003. That resolution might be seen as sufficient for this issue, as well. However, that approach does not address several of the concerns mentioned in the discussion above: coordination with WG14, the extensibility of the list of identifiers, the alternative approach used in the XML specification, etc.




1102. Better example of undefined behavior

Section: 1.9  [intro.execution]     Status: DR     Submitter: GB     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment GB 6

There are core issues surrounding the undefined behavior of dereferencing a null pointer. It appears the intent is that dereferencing is well defined, but using the result of the dereference will yield undefined behavior. This topic is too confused to be the reference example of undefined behavior, or should be stated more precisely if it is to be retained.

(See also issue 232.)

Proposed resolution (September, 2010):

Change 1.9 [intro.execution] paragraph 4 as follows:

Certain other operations are described in this International Standard as undefined (for example, the effect of dereferencing the null pointer attempting to modify a const object). [Note:...



1173. Unclear specification of effects of signal handling

Section: 1.9  [intro.execution]     Status: DR     Submitter: GB     Date: 2010-08-06

[Voted into the WP at the November, 2010 meeting.]

N3092 comment GB 7

The current wording of 1.9 [intro.execution] paragraph 6 could be read as saying that any signal would leave the program in an unspecified state after completing.

Proposed resolution (August, 2010):

Change 1.9 [intro.execution] paragraph 6 as follows:

When the processing of the abstract machine is interrupted by receipt of a signal, the values of objects which are neither

are unspecified during the execution of the signal handler, and the value of any object not in either of these two categories that is modified by the handler becomes undefined.




1103. Reversion of phase 1 and 2 transformations in raw string literals

Section: 2.2  [lex.phases]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 13
N3092 comment US 14

“Raw” strings are still only Pittsburgh-rare strings: the reversion in phase 3 only applies to an r-char-sequence. It should apply to the entire raw string literal.

Proposed resolution (August, 2010):

  1. Change 2.2 [lex.phases] paragraph 1 phase 1 as follows:

  2. ...(An implementation may use any internal encoding, so long as an actual extended character encountered in the source file, and the same extended character expressed in the source file as a universal-character-name (i.e., using the \uXXXX notation), are handled equivalently except where this replacement is reverted in a raw string literal.).)
  3. Change 2.2 [lex.phases] paragraph 1 phase 3 as follows:

  4. ...[Example: see the handling of < within a #include preprocessing directive. —end example] Within the r-char-sequence of a raw string literal, any transformations performed in phases 1 and 2 (trigraphs, universal-character-names, and line splicing) are reverted.
  5. Change 2.2 [lex.phases] paragraph 1 phase 5 as follows:

  6. Each source character set member and universal-character-name in a character literal or a string literal, as well as each escape sequence and universal-character-name in a character literal or a non-raw string literal, is converted to the corresponding member of the execution character set (2.14.3 [lex.ccon], 2.14.5 [lex.string]); if there is no corresponding member, it is converted to an implementation-defined member other than the null (wide) character.
  7. Change 2.3 [lex.charset] paragraph 2 as follows:

  8. ...Additionally, if the hexadecimal value for a universal-character-name outside the c-char-sequence, s-char-sequence, or r-char-sequence of a character or string literal corresponds to a control character (in either of the ranges 0x000x1F or 0x7F0x9F, both inclusive) or to a character in the basic source character set, the program is ill-formed. [Footnote: A sequence of characters resembling a universal-character-name in an r-char-sequence (2.14.5 [lex.string]) does not form a universal-character-name. —end footnote]
  9. Change 2.5 [lex.pptoken] paragraph 3 as follows:

  10. If the input stream has been parsed into preprocessing tokens up to a given character:
  11. Delete footnote 24 in 2.14.5 [lex.string] paragraph 2:

  12. Use of characters with trigraph equivalents in a d-char-sequence may produce unintended results.
  13. Insert the following examples after 2.14.5 [lex.string] paragraph 4:

  14. [Example: The raw string

      R"a(
      )\
      a"
      )a"
    

    is equivalent to "\n)\\\na\"\n". The raw string

      R"(??)"
    

    is equivalent to "\?\?". The raw string

      R"#(
      )??="
      )#"
    

    is equivalent to "\n)\?\?=\"\n". —end example]




1104. Global-scope template arguments vs the <: digraph

Section: 2.6  [lex.digraph]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 15

Passing a name qualified by the global scope operator :: as a template argument can inadvertently trigger recognition of the <: digraph, causing a syntax error. This should be handled by a lexical rule similar to the special treament given to >> so that <:: would be recognized as an open angle-bracket followed by the scope-resolution operator.

Proposed resolution (August, 2010):

Insert a bullet into the list in 2.5 [lex.pptoken] paragraph 3 as follows:

If the input stream has been parsed into preprocessing tokens up to a given character:



1105. Issues relating to TR 10176:2003

Section: 2.11  [lex.name]     Status: DR     Submitter: CA     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting as paper N3146.]

N3092 comment CA 24
  1. “Combining characters should not appear as the first character of an identifier.” [Reference: ISO/IEC TR 10176:2003 (Annex A)] This is not reflected in FCD.

  2. Restrictions on the first character of an identifier are not observed as recommended in TR 10176:2003. The inclusion of digits (outside of those in the basic character set) under identifer-nondigit is implied by FCD.

  3. It is implied that only the “main listing” from Annex A is included for C++. That is, the list ends with the Special Characters section. This is not made explicit in FCD. Existing practice in C++03 as well as WG 14 (C, as of N1425) and WG 4 (COBOL, as of N4315) is to include a list in a normative Annex.

  4. Specify width sensitivity as implied by C++03: \uFF21 is not the same as A. Case sensitivity is already stated in 2.11 [lex.name].

Notes from the August, 2010 meeting:

CWG expressed interest in an approach by which all characters except for those in specifically-excluded categories would be acceptable identifier characters. This suggestion will be brought up with WG14 as a liaison matter in hopes of agreeing on a uniform approach between C and C++.




1106. Need more detail in nullptr keyword description

Section: 2.14.7  [lex.nullptr]     Status: DR     Submitter: DE     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment DE 3

It is not sufficiently clear that std::nullptr_t is a distinct type and neither a pointer type nor a pointer-to-member type. Add a note in 2.14.7 [lex.nullptr] stating that, preferably with cross-references to the normative statements in 3.9 [basic.types].

Proposed resolution (September, 2010):

Change 2.14.7 [lex.nullptr] paragraph 1 as follows:

The pointer literal is the keyword nullptr. It is a prvalue of type std::nullptr_t. [Note: std::nullptr_t is a distinct type that is neither a pointer type nor a pointer to member type; rather, a prvalue of this type is a null pointer constant and can be converted to a null pointer value or null member pointer value. See 4.10 [conv.ptr] and 4.11 [conv.mem]. —end note]



1107. Overload resolution for user-defined integer literals

Section: 2.14.8  [lex.ext]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 17

In general, the parameter type of a literal operator must be the same as the argument passed to it. That is not the case for a user-defined-character-literal, where the argument could inadvertently match a literal operator intended for use with user-defined-integer-literals:

    typedef unsigned long long ULL;
    int operator "" X(ULL);
    int i = 'c'X; // operator"" X(ULL('c'))

Suggested resolution: Add the following phrase to the description in paragraph 6:

S shall contain a literal operator whose parameter type is the same as the type of ch.

Proposed resolution (August, 2010):

Change 2.14.8 [lex.ext] paragraph 6 as follows:

If L is a user-defined-character-literal, let ch be the literal without its ud-suffix. The S shall contain a literal operator (13.5.8 [over.literal]) whose only parameter has the type of ch and the literal L is treated as a call of the form
    operator "" X (ch )



1112. constexpr variables should have internal linkage like const

Section: 3.5  [basic.link]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 24

One of the critieria for giving a name internal linkage is “a variable that is explicitly declared const and neither explicitly declared extern nor previously declared to have external linkage.” This should presumably apply to variables declared constexpr as well.

Proposed resolution (August, 2010):

Change 3.5 [basic.link] paragraph 3 bullet 2 as follows:




1113. Linkage of namespace member of unnamed namespace

Section: 3.5  [basic.link]     Status: DR     Submitter: DE     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment DE 4

It is odd that, in the following example, N has no linkage but g has external linkage:

    namespace {
      namespace N // has no linkage
      {
        void g(); // has external linkage
      }
    }

Proposed resolution (August, 2010):

Change 3.5 [basic.link] paragraph 4 as follows:

An unnamed namespace or a namespace declared directly or indirectly within an unnamed namespace has internal linkage. All other namespaces have external linkage. A name having namespace scope that has not been given internal linkage above has external linkage the same linkage as the enclosing namespace if it is the name of



1114. Incorrect use of placement new in example

Section: 3.8  [basic.life]     Status: DR     Submitter: GB     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment GB 18

The example in 3.8 [basic.life] paragraph 9 reads,

    struct B {
      B();
      ~B();
    };

    const B b;

    void h() {
      b.~B();
      new (&b) const B;  // undefined behavior
    }

Assuming that the placement new is intended to use the operator defined in the Standard library, the new-expression is ill-formed, because there is no implicit conversion from “pointer to const B” to void*.

Proposed resolution (August, 2010):

Change the example in 3.8 [basic.life] paragraph 9 as follows:

    ...
    new (const_cast<B *>(&b)) const B;  // undefined behavior
    ...



964. Incorrect description of when the lvalue-to-rvalue conversion applies

Section: 3.10  [basic.lval]     Status: DR     Submitter: Doug Gregor     Date: 14 September, 2009

[Voted into the WP at the November, 2010 meeting.]

3.10 [basic.lval] paragraph 7 says,

Whenever an lvalue appears in a context where an rvalue is expected, the lvalue is converted to an rvalue

That is not correct in the context of an attempt to bind an rvalue reference to an lvalue (8.5.3 [dcl.init.ref]).

Proposed resolution (October, 2009):

Change 3.10 [basic.lval] paragraph 7 as follows:

Whenever an lvalue appears in a context where an rvalue is expected and an lvalue is not explicitly prohibited (as, for example, in 8.5.3 [dcl.init.ref]), the lvalue it is converted to an rvalue; see 4.1 [conv.lval], 4.2 [conv.array], and 4.3 [conv.func].

Notes from the March, 2010 meeting:

This resolution needs to be reconsidered in light of the new expression taxonomy.

Proposed resolution (September, 2010):

Change 3.10 [basic.lval] paragraph 2 as follows:

Whenever a glvalue appears in a context where a prvalue is expected, the glvalue is converted to a prvalue; see 4.1 [conv.lval], 4.2 [conv.array], and 4.3 [conv.func]. [Note: An attempt to bind an rvalue reference to an lvalue is not such a context; see 8.5.3 [dcl.init.ref]. —end note]



1117. Incorrect note about xvalue member access expressions

Section: 5  [expr]     Status: DR     Submitter: JP     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment JP 1

One of the bullets in the note in 5 [expr] paragraph 6 says that an expression is an xvalue if it is:

This is incorrect if the type of the non-static data member is a reference type (cf 5.2.5 [expr.ref] paragraph 4.)

Proposed resolution (September, 2010):

Change 5 [expr] paragraph 6 bullet 3 as follows:




1034. Attributes for return statements in lambdas

Section: 5.1.2  [expr.prim.lambda]     Status: DR     Submitter: Daveed Vandevoorde     Date: 2010-02-18

[Voted into the WP at the November, 2010 meeting.]

5.1.2 [expr.prim.lambda] paragraph 4 bullet 1 says,

if the compound-statement if [sic] of the form

the type of the returned expression...

The problem (besides the typo “if”) is that the attribute-specifier for a return statement precedes, rather than following, the keyword (6 [stmt.stmt] paragraph 1).

Proposed resolution (September, 2010):

Change 5.1.2 [expr.prim.lambda] paragraph 4 bullet 1 as follows:




1062. Syntax of attribute-specifiers in lambdas

Section: 5.1.2  [expr.prim.lambda]     Status: DR     Submitter: Peter Sommerlad     Date: 2010-03-23

[Voted into the WP at the November, 2010 meeting.]

N3092 comment CH 4

The adoption of paper N3067 at the March, 2010 meeting moved the position of the optional attribute-specifier in a function declarator from immediately following the parameter-declaration-clause to after the exception-specification. However, the grammar in 5.1.2 [expr.prim.lambda] paragraph 1 and the verbal description in paragraph 5 still have the attribute-specifier in a lambda-declarator at its old position. These should be updated to reflect the new function declarator syntax.

Proposed resolution (August, 2010):

  1. Change the grammar in 5.1.2 [expr.prim.lambda] paragraph 1 as follows:

  2. Change 5.1.2 [expr.prim.lambda] paragraph 5 as follows:

  3. ...Any attribute-specifiers appearing immediately after the lambda-expression's parameter-declaration-clause appertain An attribute-specifier in a lambda-declarator appertains to the type of the corresponding function call operator...



1083. Passing an object to ellipsis with non-trivial move constructor

Section: 5.2.2  [expr.call]     Status: DR     Submitter: Jason Merrill     Date: 2010-06-22

[Voted into the WP at the November, 2010 meeting.]

5.2.2 [expr.call] paragraph 7 should mention a non-trivial move constructor as well, for consistency with “trivially copyable.”

Proposed resolution (September, 2010):

Change 5.2.2 [expr.call] paragraph 7 as follows:

...Passing a potentially-evaluated argument of class type (Clause 9 [class]) with having a non-trivial copy constructor, a non-trivial move constructor, or a non-trivial destructor, with no corresponding parameter is conditionally-supported, with implementation-defined semantics. If the argument...



1119. Missing case in description of member access ambiguity

Section: 5.2.5  [expr.ref]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 32

According to 5.2.5 [expr.ref] paragraph 5,

If E2 is a non-static data member or a non-static member function, the program is ill-formed if the class of which E2 is directly a member is an ambiguous base (10.2 [class.member.lookup]) of the naming class (11.2 [class.access.base]) of E2.

This does not cover the following case:

    struct A { int i; };
    struct B: A { };
    struct C: A, B { };
    void f(C* p) {
      p->A::i; // Should be ambiguous
    }

Notes (August, 2010):

The example in the FCD National Body comment is incorrect: it is missing the A:: in the next-to-last line.

The ambiguity actually is covered in the Standard but in an unexpected location: 11.2 [class.access.base] paragraph 6:

If a class member access operator, including an implicit “this->,” is used to access a non-static data member or non-static member function, the reference is ill-formed if the left operand (considered as a pointer in the “.” operator case) cannot be implicitly converted to a pointer to the naming class of the right operand.

An explanatory note, including a cross-reference to 11.2 [class.access.base], should be added to 5.2.5 [expr.ref] paragraph 6.

Proposed resolution (September, 2010):

Change 5.2.5 [expr.ref] paragraph 5 as follows:

If E2 is a non-static data member or a non-static member function, the program is ill-formed if the class of which E2 is directly a member is an ambiguous base (10.2 [class.member.lookup]) of the naming class (11.2 [class.access.base]) of E2. [Note: The program is also ill-formed if the naming class is an ambiguous base of the class type of the object expression; see 11.2 [class.access.base]. —end note]



1011. Standard conversions that cannot be inverted

Section: 5.2.9  [expr.static.cast]     Status: DR     Submitter: John Spicer     Date: 2009-12-03

[Voted into the WP at the November, 2010 meeting.]

According to 5.2.9 [expr.static.cast] paragraph 7, static_cast can be used to perform the inverse of any standard conversion sequence except the lvalue-to-rvalue, array-to-pointer, function-to-pointer, and boolean conversions. The null pointer and null pointer-to-member conversions should also be listed — it should not be permitted to cast a pointer or pointer-to-member to either integral type or std::nullptr_t.

Proposed resolution (October, 2010):

  1. Change 4.10 [conv.ptr] paragraph 1 as follows:

  2. ...A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is distinguishable from every other value of pointer to object or pointer to function type. Such a conversion is called a null pointer conversion. Two null pointer values...
  3. Change 4.11 [conv.mem] paragraph 1 as follows:

  4. A null pointer constant (4.10 [conv.ptr]) can be converted to a pointer to member type; the result is the null member pointer value of that type and is distinguishable from any pointer to member not created from a null pointer constant. Such a conversion is called a null member pointer conversion. Two null member pointer values...
  5. Change 5.2.9 [expr.static.cast] paragraph 7 as follows:

  6. The inverse of any standard conversion sequence (Clause 4 [conv]), other than the not containing an lvalue-to-rvalue (4.1 [conv.lval]), array-to-pointer (4.2 [conv.array]), function-to-pointer (4.3 [conv.func]), and null pointer (4.10 [conv.ptr]), null member pointer (4.11 [conv.mem]), or boolean (4.12 [conv.bool]) conversions, can be performed explicitly using static_cast. A program is ill-formed...



1086. const_cast to rvalue reference to function type

Section: 5.2.11  [expr.const.cast]     Status: DR     Submitter: Steve Adamczyk     Date: 2010-06-25

[Voted into the WP at the November, 2010 meeting.]

5.2.11 [expr.const.cast] paragraph 1 refers to casting to an rvalue reference to function type. This was an accidental edit in the application of the value categories and should be removed.

Proposed resolution (October, 2010):

Change 5.2.11 [expr.const.cast] paragraph 1 as follows:

If T is an lvalue reference to object type or an rvalue reference to function type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue (4.1 [conv.lval]), array-to-pointer (4.2 [conv.array]), and function-to-pointer (4.3 [conv.func]) standard conversions are performed on the expression v...
[Drafting note: with this change to the wording, an attempt to cast a function lvalue to either an lvalue or rvalue reference will be ill-formed because the function-to-pointer conversion will be applied to the operand and the resulting operand base type will be incompatible with the target base type.]


1121. Unnecessary ambiguity error in formation of pointer to member

Section: 5.3.1  [expr.unary.op]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 33

The resolution of issue 983 restored an error, inadvertently removed by the resolution of issue 39, for the formation of a member of an ambiguous base class. For example:

    struct B { int i; };
    struct I1: B { };
    struct I2: B { };
    struct D: I1, I2 { };
    int B::* pm = &D::i;    // Originally and again ambiguous

This error is not necessary, because the result of taking the address of a member of an ambiguous base class is a pointer to a member of that class; an actual ambiguity would occur only if that pointer to a base class member is converted to a pointer to a member of the derived class. (See issue 203, which suggests changing the result of taking the address of a member to reflect the naming class of the member instead of the class of which it is directly a member; if that change were made, the ambiguity error would be needed.)

The resolution of issue 983 should be reverted.

Proposed resolution (August, 2010):

  1. Change 5.3.1 [expr.unary.op] paragraph 3 as follows:

  2. ...If the operand is a qualified-id naming a non-static member m of some class C with type T, the result has type “pointer to member of class C of type T and is a prvalue designating C::m; the program is ill formed if C is an ambiguous base (10.2) of the class designated by the nested-name-specifier of the qualified-id. Otherwise...
  3. Change 10.2 [class.member.lookup] paragraph 13 as follows:

  4. [Note: Even if the result of name lookup is unambiguous, use of a name found in multiple subobjects might still be ambiguous (4.11 [conv.mem], 5.2.5 [expr.ref], 5.3.1 [expr.unary.op], 11.2 [class.access.base]). —end note]...



1122. Circular definition of std::size_t

Section: 5.3.3  [expr.sizeof]     Status: DR     Submitter: GB     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment GB 24

The return type of the sizeof operator is defined as being of type std::size_t, defined in library clause 18.2 [support.types]. This, in turn, says that size_t is defined in the C standard, which in turn says that size_t is defined as the type of the result of the sizeof operator!

The C definition of sizeof returns an implementation-defined unsigned integer type, recommended not to have “an integer conversion rank greater than signed long int, unless the implementation supports objects large enough to make this necessary.”

Proposed resolution (September, 2010):

Add the following three paragraphs after 18.2 [support.types] paragraph 4:

The type ptrdiff_t is an implementation-defined signed integer type that can hold the difference of two subscripts in an array object, as described in 5.7 [expr.add].

The type size_t is an implementation-defined unsigned integer type that is large enough to contain the size in bytes of any object.

[Note: It is recommended that implementations choose types for ptrdiff_t and size_t whose integer conversion ranks (4.13 [conv.rank]) are no greater than that of signed long int unless a larger size is necessary to contain all the possible values. —end note]




1061. Negative array bounds in a new-expression

Section: 5.3.4  [expr.new]     Status: DR     Submitter: Sean Hunt     Date: 2010-03-23

[Voted into the WP at the November, 2010 meeting.]

Recent changes have added the requirement (5.3.4 [expr.new] paragraph 7),

If the value of that expression is such that the size of the allocated object would exceed the implementation-defined limit, no storage is obtained and the new-expression terminates by throwing an exception of a type that would match a handler (15.3 [except.handle]) of type std::bad_array_new_length (18.6.2.2 [new.badlength]).

Given this checking, is there any current reason for the statement in the preceding paragraph,

If the value of the expression is negative, the behavior is undefined.

Presumably for most negative expressions on most platforms, a negative value would result in a too-large request anyway, and even if not the check could easily be expanded to look explicitly for a negative value in addition to a too-large request.

Proposed resolution (September, 2010):

  1. Change 5.3.4 [expr.new] paragraphs 6 and 7 as follows:

  2. ...If the value of the expression is negative, the behavior is undefined. [Example: given the definition int n = 42, new float[n][5] is well-formed (because n is the expression of a noptr-new-declarator), but new float[5][n] is ill-formed (because n is not a constant expression). If n is negative, the effect of new float[n][5] is undefined.end example]

    When the value of the expression in a noptr-new-declarator is zero, the allocation function is called to allocate an array with no elements. If the value of that expression is less than zero or such that the size of the allocated object would exceed the implementation-defined limit, no storage is obtained and the new-expression terminates by throwing an exception of a type that would match a handler (15.3 [except.handle]) of type std::bad_array_new_length (18.6.2.2 [new.badlength]).

  3. Change 18.6.2.2 [new.badlength] paragraph 1 as follows:

  4. The class bad_array_new_length defines the type of objects thrown as exceptions by the implementation to report an attempt to allocate an array of size less than zero or greater than an implementation-defined limit (5.3.4 [expr.new]).



1037. Requirements for operands of delete-expressions and deallocation functions

Section: 5.3.5  [expr.delete]     Status: DR     Submitter: Daniel Krügler     Date: 2010-03-01

[Voted into the WP at the November, 2010 meeting.]

According to 5.3.5 [expr.delete] paragraph 2,

...in the first alternative (delete object), the value of the operand of delete shall be a pointer to a non-array object or a pointer to a subobject (1.8 [intro.object]) representing a base class of such an object (Clause 10 [class.derived]). If not, the behavior is undefined. In the second alternative (delete array), the value of the operand of delete shall be the pointer value which resulted from a previous array new-expression.79 If not, the behavior is undefined.

The second part of this specification makes it clear that an array object being deleted must have been allocated via new. However, the first part, for the non-array object, completely omits this vital requirement, requiring only that it not be an array.

The corresponding requirement for an argument to a deallocation function is found in 3.7.4.2 [basic.stc.dynamic.deallocation] paragraph 3:

...the value supplied to operator delete(void*) in the standard library shall be one of the values returned by a previous invocation of either operator new(std::size_t) or operator new(std::size_t, const std::nothrow_t&) in the standard library, and the value supplied to operator delete[](void*) in the standard library shall be one of the values returned by a previous invocation of either operator new[](std::size_t) or operator new[](std::size_t, const std::nothrow_t&) in the standard library.

This correctly states the required provenance of the pointer, but it does so using “shall,” which is inappropriate for a runtime requirement. This wording should be recast in terms of undefined behavior if the requirement is not met.

Proposed resolution (October, 2010):

  1. Change 5.3.5 [expr.delete] paragraph 2 as follows:

  2. If the operand has a class type, the operand is converted to a pointer type by calling the above-mentioned conversion function, and the converted operand is used in place of the original operand for the remainder of this section. In either alternative, the value of the operand of delete may be a null pointer value. If it is not a null pointer value, in In the first alternative (delete object), the value of the operand of delete shall may be a null pointer value, a pointer to a non-array object created by a previous new-expression, or a pointer to a subobject (1.8 [intro.object]) representing a base class of such an object (Clause 10 [class.derived]). If not, the behavior is undefined. In the second alternative (delete array), the value of the operand of delete shall may be the a null pointer value or a pointer value which that resulted from a previous array new-expression.79 If not, the behavior is undefined. [Note: this means that the syntax of the delete-expression must match the type of the object allocated by new new, not the syntax of the new-expression. —end note]...
  3. Change 3.7.4.2 [basic.stc.dynamic.deallocation] paragraph 3 as follows:

  4. ...Otherwise, the behavior is undefined if the value supplied to operator delete(void*) in the standard library shall be is not one of the values returned by a previous invocation of either operator new(std::size_t) or operator new(std::size_t, const std::nothrow_t&) in the standard library, and the behavior is undefined if the value supplied to operator delete[](void*) in the standard library shall be is not one of the values returned by a previous invocation of either operator new[](std::size_t) or operator new[](std::size_t, const std::nothrow_t&) in the standard library.



1123. Destructors should be noexcept by default

Section: 5.3.7  [expr.unary.noexcept]     Status: DR     Submitter: FI     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting as paper N3204.]

N3092 comment FI 17

Destructors should by default be noexcept. Such a rule should be obeyed even for cases where a destructor is defaulted. Then a throwing destructor would need to be declared noexcept(false), and the resulting code breakage is acceptable.

Notes from the August, 2010 meeting:

CWG agreed with the suggested direction.

(Duplicate of issue 1147.)




1125. Unclear definition of “potential constant expression”

Section: 5.19  [expr.const]     Status: DR     Submitter: DE     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting as paper N3218.]

N3092 comment DE 8

In the definition of “potential constant expression” in 5.19 [expr.const] paragraph 6, it is unclear how “arbitrary” the substitution of the function parameters is. Does it mean “there exists a value for which the result is a constant expression” or does it mean “for all possible values, the result needs to be a constant expression?” Example:

    constexpr int f(int x){ return x + 1; }

is a constant expression under the first interpretation, but not under the second (because overflow occurs for x == INT_MAX, cf 5.19 [expr.const] paragraph 2 bullet 5). The answer also affects expressions such as:

    constexpr int f2(bool v) { return v ? throw 0 : 0; }
    constexpr int f3(bool v) { return v && (throw 0, 0); }

See also issue 1129.




1126. constexpr functions in const initializers

Section: 5.19  [expr.const]     Status: DR     Submitter: GB     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment GB 25

It does not appear to be clearly enough stated that the example

    constexpr int f() { return 42 + 84; }
    const int sz = f();
    int a[sz];

is equivalent to

    const int sz = 42 + 84;
    int a[sz];

Proposed resolution (August, 2010):

Change 5.19 [expr.const] paragraph 1 as follows:

Certain contexts require expressions that satisfy additional requirements as detailed in this sub-clause; other contexts have different semantics depending on whether or not an expression satisfies these requirements. Such expressions Expressions that satisfy these requirements are called constant expressions. [Note: Those Constant expressions can be evaluated during translation. —end note]



1128. attribute-specifiers in decl-specifier-seqs

Section: 7.1  [dcl.spec]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 39

The current wording of 7.1 [dcl.spec] paragraph 1 says,

The optional attribute-specifier in a decl-specifier-seq appertains to the type determined by the decl-specifier-seq (8.3 [dcl.meaning]).

However, decl-specifier-seq is a recursive production. The intent is that the attribute-specifier appertains to the type determined by the top-level decl-specifier-seq, but this makes it sound as if it appertains to the one that directly contains the attribute-specifier.

Proposed resolution (August, 2010):

Change 7.1 [dcl.spec] paragraph 1 as follows:

The optional attribute-specifier in a decl-specifier-seq appertains to the type determined by the decl-specifier-seq preceding decl-specifiers (8.3 [dcl.meaning]).



1129. Default nothrow for constexpr functions

Section: 7.1.5  [dcl.constexpr]     Status: DR     Submitter: GB     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment GB 29

A constexpr function is not permitted to return via an exception. This should be recognised, and a function declared constexpr without an explicit exception specification should be treated as if declared noexcept(true) rather than the usual noexcept(false). For a function template declared constexpr without an explicit exception specification, it should be considered noexcept(true) if and only if the constexpr keyword is respected on a given instantiation.

See also issue 1125.

Notes from the August, 2010 meeting:

The premise is not correct: an exception is forbidden only when a constexpr function is invoked in a context that requires a constant expression. Used as an ordinary function, it can throw.

Proposed resolution (August, 2010):

Change 5.3.7 [expr.unary.noexcept] paragraph 3 bullet 1 as follows:




1075. Grammar does not allow template alias in type-name

Section: 7.1.6.2  [dcl.type.simple]     Status: DR     Submitter: Jason Merrill     Date: 2010-06-06

[Voted into the WP at the November, 2010 meeting.]

According to 14.2 [temp.names] paragraph 7,

A template-id that names a template alias specialization is a type-name.

However, the grammar for type-name in 7.1.6.2 [dcl.type.simple] does not include a production for simple-template-id.

Proposed resolution (September, 2010):

Change the definition of type-name in 7.1.6.2 [dcl.type.simple] paragraph 1 as follows:




1130. Function parameter type adjustments and decltype

Section: 7.1.6.2  [dcl.type.simple]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 40

The description of decltype does not specify whether the type of a parameter is the declared type or the type as adjusted in 8.3.5 [dcl.fct] paragraph 5:

    auto f(int a[])->decltype(a);     // ill-formed or int*?
    auto g(const int i)->decltype(i); // int or const int?

Suggested resolution: Clarify the wording to indicate that the type of a parameter is after the array- and function-to-pointer decay but before the removal of cv-qualification.

Proposed resolution (August, 2010):

Change 8.3.5 [dcl.fct] paragraph 5 as follows:

...After producing the list of parameter types, several transformations take place upon these types to determine the function type. Any any top-level cv-qualifiers modifying a parameter type is are deleted when forming the function type. [Example: the type void(*)(const int) becomes void(*)(int)end example] Such cv-qualifiers affect only the definition of the parameter within the body of the function; they do not affect the function type. If a storage-class-specifier modifies a parameter type, the specifier is deleted. [Example: register char* becomes char*end example] Such storage-class-specifiers affect only the definition of the parameter within the body of the function; they do not affect the function type. The resulting list of transformed parameter types and the presence or absence of the ellipsis or a function parameter pack is the function's parameter-type-list. [Note: This transformation does not affect the types of the parameters. For example, int(*)(const int p, decltype(p)*) and int(*)(int, const int*) are identical types. —end note]



1131. Template aliases in elaborated-type-specifiers

Section: 7.1.6.3  [dcl.type.elab]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 41

The current wording disallows use of typedef-names in elaborated-type-specifiers. This prohibition should also apply to template aliases:

    struct A { };
    template<typename T> using X = A;
    struct X<int>* p2; // ill-formed

Proposed resolution (August, 2010):

Change 7.1.6.3 [dcl.type.elab] paragraph 2 as follows:

...If the identifier resolves to a typedef-name or the simple-template-id resolves to an alias template specialization, the elaborated-type-specifier is ill-formed. [Note:...

[Note: this wording assumes the change from “template alias” to “alias template” requested by comment FI 11 on FCD N3092.]




1012. Undeprecating static

Section: 7.3.1.1  [namespace.unnamed]     Status: DR     Submitter: Ville Voutilainen     Date: 2009-12-09

[Voted into the WP at the November, 2010 meeting.]

N3092 comment FI 6

Although 7.3.1.1 [namespace.unnamed] states that the use of the static keyword for declaring variables in namespace scope is deprecated because the unnamed namespace provides a superior alternative, it is unlikely that the feature will be removed at any point in the foreseeable future, especially in light of C compatibility concerns. The Committee should consider removing the deprecation.

Proposed resolution (August, 2010):

  1. Delete 7.3.1.1 [namespace.unnamed] paragraph 2:

  2. The use of the static keyword is deprecated when declaring variables in a namespace scope (see annex D [depr]); the unnamed-namespace provides a superior alternative.
  3. Delete D.2 [depr.static]:

  4. D.2 static keyword[depr.static]

    The use of the static keyword is deprecated when declaring objects in namespace scope (see 3.3.6 [basic.scope.namespace]).




341. extern "C" namespace member function versus global variable

Section: 7.5  [dcl.link]     Status: DR     Submitter: Steve Adamczyk     Date: 1 Mar 2002

[Voted into the WP at the November, 2010 meeting.]

Here's an interesting case:

  int f;
  namespace N {
    extern "C" void f () {}
  }
As far as I can tell, this is not precluded by the ODR section (3.2 [basic.def.odr]) or the extern "C" section (7.5 [dcl.link]). However, I believe many compilers do not do name mangling on variables and (more-or-less by definition) on extern "C" functions. That means the variable and the function in the above end up having the same name at link time. EDG's front end, g++, and the Sun compiler all get essentially the same error, which is a compile-time assembler-level error because of the duplicate symbols (in other words, they fail to check for this, and the assembler complains). MSVC++ 7 links the program without error, though I'm not sure how it is interpreted.

Do we intend for this case to be valid? If not, is it a compile time error (required), or some sort of ODR violation (no diagnostic required)? If we do intend for it to be valid, are we forcing many implementations to break binary compatibility by requiring them to mangle variable names?

Personally, I favor a compile-time error, and an ODR prohibition on such things in separate translation units.

Notes from the 4/02 meeting:

The working group agreed with the proposal. We feel a diagnostic should be required for declarations within one translation unit. We also noted that if the variable in global scope in the above example were declared static we would still expect an error.

Relevant sections in the standard are 7.5 [dcl.link] paragraph 6 and 3.5 [basic.link] paragraph 9. We feel that the definition should be written such that the entities in conflict are not "the same entity" but merely not allowed together.

Additional note (September, 2004)

This problem need not involve a conflict between a function and a variable; it can also arise with two variable declarations:

    int x;
    namespace N {
        extern "C" int x;
    }

Proposed resolution (March, 2008):

Change 7.5 [dcl.link] paragraph 6 as follows:

At most one function with a particular name can have C language linkage. Two declarations for a function with C language linkage with the same function name (ignoring the namespace names that qualify it) that appear in different namespace scopes refer to the same function. Two declarations for an object with C language linkage with the same name (ignoring the namespace names that qualify it) that appear in different namespace scopes refer to the same object. A function or object with C linkage shall not be declared with the same name (clause 3 [basic]) as an object or reference declared in global scope, unless both declarations denote the same object; no diagnostic is required if the declarations appear in different translation units. [Note: because of the one definition rule (3.2 [basic.def.odr]), only Only one definition for a function or object with C linkage may appear in the program (see 3.2 [basic.def.odr]); that is, implies that such a function or object must not be defined in more than one namespace scope. For example,

    int x;
    namespace A {
      extern "C" int f();
      extern "C" int g() { return 1; }
      extern "C" int h();
      extern "C" int x();               // ill-formed: same name as global-scope object x
    }

    namespace B {
      extern "C" int f();               // A::f and B::f refer
                                        // to the same function
      extern "C" int g() { return 1; }  // ill-formed, the function g
                                        // with C language linkage
                                        // has two definitions
    }

    int A::f() { return 98; }           // definition for the function f
                                        // with C language linkage
    extern "C" int h() { return 97; }
                                        // definition for the function h
                                        // with C language linkage
                                        // A::h and ::h refer to the same function

end note]

Notes from the September, 2008 meeting:

It should also be possible to declare references with C name linkage (although the meaning the first sentence of 7.5 [dcl.link] paragraph 1 with respect to the meaning of such a declaration is not clear), which would mean that the changed wording should refer to declaring “the same entity” instead of “the same object.” The formulation here would probably benefit from the approach currently envisioned for issues 570 and 633, in which “variable” is defined as being either an object or a reference.

Proposed resolution (February, 2010):

Change 7.5 [dcl.link] paragraph 6 as follows:

At most one function with a particular name can have C language linkage. Two declarations for a function with C language linkage with the same function name (ignoring the namespace names that qualify it) that appear in different namespace scopes refer to the same function. Two declarations for an object or reference with C language linkage with the same name (ignoring the namespace names that qualify it) that appear in different namespace scopes refer to the same object or reference. An entity with C language linkage shall not be declared with the same name as an entity in global scope, unless both declarations denote the same object or reference; no diagnostic is required if the declarations appear in different translation units. [Note: because of the one definition rule (3.2 [basic.def.odr]), only Only one definition for a function or object an entity with C linkage may appear in the program (see 3.2 [basic.def.odr]); that is, implies that such a function or object an entity must not be defined in more than one namespace scope. end note] For example, [Example:

  int x;
  namespace A {
    extern "C" int f();
    extern "C" int g() { return 1; }
    extern "C" int h();
    extern "C" int x();               // ill-formed: same name as global-scope object x
  }

  namespace B {
   extern "C" int f();                // A::f and B::f refer
                                      // to the same function
   extern "C" int g() { return 1; }   // ill-formed, the function g
                                      // with C language linkage
                                      // has two definitions
  }

  int A::f() { return 98; }           // definition for the function f
                                      // with C language linkage
  extern "C" int h() { return 97; }
                                      // definition for the function h
                                      // with C language linkage
                                      // A::h and ::h refer to the same function

end note example]

Additional note (February, 2010):

The proposed wording above does not cover the case where two different entities with C linkage are declared in different namespaces, only the case where one of the entities is in global scope.

Proposed resolution (August, 2010):

Change 7.5 [dcl.link] paragraph 6 as follows:

At most one function with a particular name can have C language linkage. Two declarations for a function with C language linkage with the same function name (ignoring the namespace names that qualify it) that appear in different namespace scopes refer to the same function. Two declarations for an object a variable with C language linkage with the same name (ignoring the namespace names that qualify it) that appear in different namespace scopes refer to the same object variable. An entity with C language linkage shall not be declared with the same name as an entity in global scope, unless both declarations denote the same entity; no diagnostic is required if the declarations appear in different translation units. A variable with C language linkage shall not be declared with the same name as a function with C language linkage (ignoring the namespace names that qualify the respective names); no diagnostic is required if the declarations appear in different translation units. [Note: because of the one definition rule (3.2 [basic.def.odr]), only Only one definition for a function or object an entity with a given name with C language linkage may appear in the program (see 3.2 [basic.def.odr]); that is, implies such a function or object an entity must not be defined in more than one namespace scope. end note] For example, [Example:

  int x;
  namespace A {
    extern "C" int f();
    extern "C" int g() { return 1; }
    extern "C" int h();
    extern "C" int x();               // ill-formed: same name as global-scope object x
  }

  namespace B {
    extern "C" int f();               // A::f and B::f refer
                                      // to the same function
    extern "C" int g() { return 1; }  // ill-formed, the function g
                                      // with C language linkage
                                      // has two definitions
  }

  int A::f() { return 98; }           // definition for the function f
                                      // with C language linkage
  extern "C" int h() { return 97; }
                                      // definition for the function h
                                      // with C language linkage
                                      // A::h and ::h refer to the same function

end note example]

[Note to editor: please consider reformatting the comments in the example.]




972. Allowing multiple attribute-specifiers

Section: 7.6.1  [dcl.attr.grammar]     Status: DR     Submitter: Daveed Vandevoorde     Date: 5 October, 2009

[Voted into the WP at the November, 2010 meeting as part of paper N3190.

The current syntax requires that multiple attributes that appertain to the same entity be grouped into a single attribute-specifier. The migration from existing vendor-specific attributes would be easier if the syntax allowed multiple attribute-specifiers at each location where an attribute-specifier currently appears.

Proposed resolution (October, 2010):

  1. Replace every occurrence of attribute-specifier with attribute-specifier-seq except in 7.6.1 [dcl.attr.grammar] paragraphs 1, 3, and 6 (but including paragraph 4).

  2. Insert the following production at the beginning of the grammar of 7.6.1 [dcl.attr.grammar] paragraph 1:




1036. Alignment attribute in an exception-declaration

Section: 7.6.2  [dcl.align]     Status: DR     Submitter: Daveed Vandevoorde     Date: 2010-02-26

[Voted into the WP at the November, 2010 meeting.]

The Standard explicitly bans alignment attributes for function parameters (7.6.2 [dcl.align] paragraph 1), but it is silent regarding the “parameter” of an exception handler. This should be clarified one way or the other.

Proposed resolution (October, 2010):

Change 7.6.2 [dcl.align] paragraph 1 as follows:

...The attribute may be followed by an ellipsis. The attribute may be applied to a variable that is neither a function parameter nor declared with the register storage class specifier and to a class data member that is not a bit-field or to a class data member, but it shall not be applied to a bit-field, a function parameter, the formal parameter of a catch clause (15.3 [except.handle]), or a variable declared with the register storage class specifier. The attribute may also be applied to the declaration of a class or enumeration type.



1065. [[hiding]] with [[override]]

Section: 7.6.5  [dcl.attr.override]     Status: DR     Submitter: Daveed Vandevoorde     Date: 2010-03-24

[Voted into the WP at the November, 2010 meeting as part of paper N3206.]

The meaning of the [[base_check]] and [[hiding]] attributes is defined in terms of hiding as described in 3.3.10 [basic.scope.hiding]. In that section, however, hiding is orthogonal to overriding: practically by definition, a function that overrides a base class virtual function also hides it. According to the current specification, the [[override]] and [[hiding]] attributes would always need to be specified together on every overriding function in a [[base_check]] class. This is presumably unintended, so the current wording should be amended so that [[override]] implies [[hiding]] or some such.




994. braced-init-list as a default argument

Section: 8.3.5  [dcl.fct]     Status: DR     Submitter: Daniel Krügler     Date: 25 Oct, 2009

[Voted into the WP at the November, 2010 meeting.]

It seems strange that it is possible to call a function with an explict argument of {} but that it is not possible to specify that same argument as a default in a function declaration.

Rationale (August, 2010):

This was previously considered and rejected by EWG.

Note (October, 2010):

Additional discussion has indicated a potential willingness to revisit this question.

Proposed resolution (November, 2010):

See paper N3217.




1069. Incorrect function type with trailing-return-type

Section: 8.3.5  [dcl.fct]     Status: DR     Submitter: James Widman     Date: 2010-04-30

[Voted into the WP at the November, 2010 meeting.]

In 8.3.5 [dcl.fct] paragraph 2, the type of a function declarator with a trailing-return-type is said to be

“function of (parameter-declaration-clause) cv-qualifier-seqopt ref-qualifieropt returning type-id”.

This formulation incorrectly omits the derived-declarator-type-list modifier for the type, and it should refer to “the trailing-type-specifier-seq of the trailing-return-type” as the return type instead of type-id (which is left over from before the introduction of trailing-return-type).

Proposed resolution (September, 2010):

Change 8.3.5 [dcl.fct] paragraph 2 as follows:

...T shall be the single type-specifier auto. The type of the declarator-id in D is “derived-declarator-type-list function of (parameter-declaration-clause) cv-qualifier-seqopt ref-qualifieropt returning type-id trailing-return-type”. The optional...



1134. When is an explicitly-defaulted function defined?

Section: 8.4.2  [dcl.fct.def.default]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 47

8.4.2 [dcl.fct.def.default] paragraph 4 says,

A special member function is user-provided if it is user-declared and not explicitly defaulted on its first declaration. A user-provided explicitly-defaulted function is defined at the point where it is explicitly defaulted...

The second sentence should say “user-declared” instead of “user-provided.”

Proposed resolution (September, 2010):

Change 8.4.2 [dcl.fct.def.default] paragraph 4 as follows:

...A special member function is user-provided if it is user-declared and not explicitly defaulted on its first declaration. A user-provided explicitly-defaulted function (i.e., explicitly defaulted after its first declaration) is defined at the point where it is explicitly defaulted; if such a function is implicitly defined as deleted, the program is ill-formed. [Note:...
[Drafting note: the suggestion in the NB comment is incorrect. The proposed resolution clarifies the intent.]


694. Zero- and value-initialization of union objects

Section: 8.5  [dcl.init]     Status: DR     Submitter: Clark Nelson     Date: 14 May, 2008

[Voted into the WP at the November, 2010 meeting.]

The C committee is considering changing the definition of zero-initialization of unions to guarantee that the bytes of the entire union are set to zero before assigning 0, converted to the appropriate type, to the first member. The argument (summarized here) is for backward compatibility. The C++ Committee may want to consider the same change.

Proposed resolution (August, 2008):

Change bullet 3 of 8.5 [dcl.init] paragraph 5 (in the first list, dealing with zero-initialization) as follows:

[Drafting notes: Ask a C liaison about the progress of WG14 paper N1311, which deals with this issue. Since the adoption of WG21 paper N2544, unions may have static data members, hence the change to refer to the first non-static data member and the deletion of the footnote.]

Notes from the September, 2008 meeting:

It was observed that padding bytes in structs are zero-initialized in C, so if we are changing the treatment of unions in this way we should consider adding the C behavior for padding bytes at the same time. In particular, using memcmp to compare structs only works reliably if the padding bytes are zero-initialized.

Additional notes (February, 2010):

The C Committee has changed its approach to this question and is no longer using a two-phase process; in C, zero-initialization is specific to program start-up and thus is not appropriate for use with thread-local storage. See C paper N1387.

Proposed resolution (October, 2010):

Change 8.5 [dcl.init] paragraph 5 as follows:

To zero-initialize an object or reference of type T means:




1070. Missing initializer clauses in aggregate initialization

Section: 8.5.1  [dcl.init.aggr]     Status: DR     Submitter: Jason Merrill     Date: 2010-06-01

[Voted into the WP at the November, 2010 meeting.]

Issue 990 added the following text to 8.5.4 [dcl.init.list] paragraph 3:

A better way to handle this would be to delete that bullet and change 8.5.1 [dcl.init.aggr] paragraph 7 as follows:

If there are fewer initializer-clauses in the list than there are members in the aggregate, then each member not explicitly initialized shall be value-initialized (8.5 [dcl.init]) initialized from an empty initializer list (8.5.4 [dcl.init.list]).

This makes { } less of a special case and makes the following example work:

    struct A {
       A(std::initializer_list<int>);
    };
    struct B {
       int i;
       A a;
    };
    B b = { 1 };

Proposed resolution (August, 2010):

  1. Delete 8.5.4 [dcl.init.list] paragraph 3 bullet 2, includeing the example:

  2. Change 8.5.1 [dcl.init.aggr] paragraph 7 as follows:

  3. If there are fewer initializer-clauses in the list than there are members in the aggregate, then each member not explicitly initialized shall be value-initialized (8.5 [dcl.init]) initialized from an empty initializer list (8.5.4 [dcl.init.list]).



1138. Rvalue-ness check for rvalue reference binding is wrong

Section: 8.5.3  [dcl.init.ref]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 48
N3092 comment GB 37
N3092 comment DE 10

The requirement that an rvalue reference must be bound to an rvalue is found in 8.5.3 [dcl.init.ref] paragraph 5 bullet 2:

This is not quite correct, as it is phrased in terms of the value category of the initializer expression itself rather than that of the result of any conversions applied to the initializer. It should be permitted to bind an rvalue reference to a temporary created from an lvalue, for instance, or to the rvalue result of a conversion function for an lvalue object of class type. Also, it should not be permitted to bind an rvalue reference to the lvalue result of a conversion function for a class rvalue.

Proposed resolution (August, 2010):

Change 8.5.3 [dcl.init.ref] paragraph 5 as follows:

A reference to type “cv1 T1” is initialized by an expression of type “cv2 T2” as follows:

In all cases except the last (i.e., creating and initializing a temporary from the initializer expression), the reference is said to bind directly to the initializer expression.

This resolution also resolves issue 1139.




1139. Rvalue reference binding to scalar xvalues

Section: 8.5.3  [dcl.init.ref]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 49

The current wording of 8.5.3 [dcl.init.ref] paragraph 5 does not specify direct binding of an rvalue reference to a scalar xvalue; instead, it requires creation of a temporary and binding the rvalue reference to the temporary.

Proposed resolution (August, 2010):

This issue is resolved by the resolution of 1138.




1072. Scoped enumerator with the same name as its containing class

Section: 9.2  [class.mem]     Status: DR     Submitter: Pavel Minaev     Date: 2010-06-02

[Voted into the WP at the November, 2010 meeting.]

9.2 [class.mem] paragraph 13 requires that all enumerators of a member enumeration type have names different from that of the containing class. This is only necessary for an unscoped enumeration; scoped enumerators are not in the scope of the containing class.

Proposed resolution (September, 2010):

Change 9.2 [class.mem] paragraph 14 bullet 4 as follows:




1142. friend declaration of member function of containing class

Section: 9.3  [class.mfct]     Status: DR     Submitter: US     Date: 2010-08-02

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 52

The current wording of 9.3 [class.mfct] paragraph 7 allows friend declarations to name member functions “after their class has been defined.” This appears to prohibit a friend declaration in a nested class defined inside its containing class that names a member function of the containing class, because the containing class is still considered to be incomplete at that point.

Proposed resolution (September, 2010):

Change 9.3 [class.mfct] paragraph 7 as follows:

Member Previously-declared member functions may be mentioned in friend declarations after their class has been defined.



1144. Remove access declarations

Section: 11.3  [class.access.dcl]     Status: DR     Submitter: US     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 56

Access declarations were deprecated in the 1998 standard and have no benefits over using-declarations. They should be removed in C++0x.

Proposed resolution (August, 2010):

  1. Delete 11.3 [class.access.dcl].

  2. Delete D.3 [depr.access.dcl].

  3. Delete the following production from the grammar in 9.2 [class.mem] paragraph 1:

  4. Change 9.2 [class.mem] paragraph 1 as follows:
  5. ...Except when used to declare friends (11.4) or to introduce the name of a member of a base class into a derived class (7.3.3, 11.3), member-declarations declare members of the class...
  6. Delete 7.3.3 [namespace.udecl] paragraph 19:

  7. [Note: use of access-declarations (11.3 [class.access.dcl]) is deprecated; member using-declarations provide a better alternative. —end note]



1029. Type of a destructor call

Section: 12.4  [class.dtor]     Status: DR     Submitter: Johannes Schaub     Date: 2010-02-08

[Voted into the WP at the November, 2010 meeting.]

The Standard does not define the type of a destructor call. Although that is not of any practical importance, it should do so as a matter of completeness. (5.2.4 [expr.pseudo] paragraph 1 defines the type of a pseudo-destructor call as void.)

Proposed resolution (September, 2010):

Change 5.2.2 [expr.call] paragraph 3 as follows:

The If the postfix-expression designates a destructor (12.4 [class.dtor]), the type of the function call expression is void; otherwise, the type of the function call expression is the return type of the statically chosen function (i.e., ignoring the virtual keyword), even if the type of the function actually called is different. This type shall be a complete object type, a reference type or the type void.



1146. exception-specifications of defaulted functions

Section: 12.4  [class.dtor]     Status: DR     Submitter: GB     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment GB 39
N3092 comment GB 41

The note in 12.4 [class.dtor] paragraph 4 says,

An explictly defaulted definition has no implicit exception-specification.

There are similar notes in 12.8 [class.copy] paragraphs 15 and 29.

However, 8.4.2 [dcl.fct.def.default] paragraph 2 bullet 4 says that a special member function that is explicitly defaulted on its first declaration

is implicitly considered to have the same exception-specification as if it had been implicitly declared (15.4 [except.spec])

The notes are incorrect.

Proposed resolution (August, 2010):

  1. Change 12.4 [class.dtor] paragraph 4 as follows:

  2. [Note: an implicitly- declared destructor has an exception-specification (15.4). An explictly defaulted definition has no implicit exception-specification.end note]
  3. Change 12.8 [class.copy] paragraph 15 as follows:

  4. [Note: an implicitly-declared copy/move constructor has an exception-specification (15.4). An explicitly-defaulted definition (8.4.2) has no implicit exception-specification.end note]
  5. Change 12.8 [class.copy] paragraph 29 as follows:

  6. [Note: An implicitly-declared copy/move assignment operator has an exception- specification (15.4). An explicitly-defaulted definition has no implicit exception-specification.end note]



1147. Destructors should be default nothrow

Section: 12.4  [class.dtor]     Status: DR     Submitter: GB     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting as paper N3204.]

N3092 comment GB 40
N3092 comment CH 9

A user-declared destructor that does not supply an exception specification should be considered as if declared noexcept(true) rather than noexcept(false).

(Duplicate of issue 1123.)




1020. Implicitly-defined copy constructors and explicit base class constructors

Section: 12.8  [class.copy]     Status: DR     Submitter: Niels Dekker     Date: 2010-01-12

[Voted into the WP at the November, 2010 meeting.]

N3092 comment FI 19

It is not clear whether the current specification allows a defaulted copy constructor to call an explicit constructor to copy a base or member subobject, and if so, whether that is desirable or not. See also issues 535 and 1118.

Proposed resolution (August, 2010):

This issue is resolved by the resolution of issue 1051.




1051. Reference members and generated copy constructors

Section: 12.8  [class.copy]     Status: DR     Submitter: Steve Adamczyk     Date: 2010-03-11

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 62

The new wording describing generated copy constructors (12.8 [class.copy] paragraph 16) does not describe the initialization of members with reference type.

See also issue 992.

Proposed resolution (October, 2010):

  1. Change 12.8 [class.copy] paragraph 12 as follows:

  2. An implicitly-declared copy/move constructor is an inline public member of its class. A defaulted copy/move constructor for a class X is defined as deleted (8.4.3 [dcl.fct.def.delete]) if X has:

  3. Change 12.8 [class.copy] paragraph 16 as follows:

  4. The implicitly-defined copy/move constructor for a non-union class X performs a memberwise copy/move of its subobjects bases and members. [Note: brace-or-equal-initializers of non-static data members are ignored. See also the example in 12.6.2 [class.base.init]. —end note] The order of copying initialization is the same as the order of initialization of bases and members in a user-defined constructor (see 12.6.2 [class.base.init]). Let x be either the parameter of the constructor or, for the move constructor, an xvalue referring to the parameter. Each subobject base or non-static data member is copied/moved in the manner appropriate to its type:

    Virtual base class subobjects shall be copied initialized only once by the implicitly-defined copy/move constructor (see 12.6.2 [class.base.init]).

  5. Delete 12.8 [class.copy] paragraph 17:

  6. The implicitly-defined move constructor for a non-union class X performs a memberwise move of its subobjects. [Note: brace-or-equal-initializers of non-static data members are ignored. See also the example in 12.6.2 [class.base.init]. —end note] The order of moving is the same as the order of initialization of bases and members in a user-defined constructor (see 12.6.2 [class.base.init]). Given a parameter named x, each base or non-static data member is moved in the manner appropriate to its type:

    Virtual base class subobjects shall be moved only once by the implicitly-defined move constructor (see 12.6.2 [class.base.init]).

  7. Change 12.8 [class.copy] paragraph 18 as follows:

  8. The implicitly-defined copy/move constructor for a union X copies the object representation (3.9 [basic.types]) of X.
  9. Change 12.8 [class.copy] paragraph 28 as follows:

  10. A copy/move assignment operator that is defaulted and not defined as deleted is implicitly defined when is assigned a value of its class type or a value of a class type derived from its class type it is used (3.2 [basic.def.odr]) (e.g., when it is selected by overload resolution to assign to an object of its class type) or when it is explicitly defaulted after its first declaration.
  11. Change 12.8 [class.copy] paragraph 30 as follows:

  12. The implicitly-defined copy/move assignment operator for a non-union class X performs memberwise copy/move assignment of its subobjects. The direct base classes of X are assigned first, in the order of their declaration in the base-specifier-list, and then the immediate non-static data members of X are assigned, in the order in which they were declared in the class definition. Let x be either the parameter of the function or, for the move assignment operator, an xvalue referring to the parameter. Each subobject is assigned in the manner appropriate to its type:

    It is unspecified whether subobjects representing virtual base classes are assigned more than once by the implicitly-defined copy assignment operator. [Example:

      struct V { };
      struct A : virtual V { };
      struct B : virtual V { };
      struct C : B, A { };
    

    It is unspecified whether the virtual base class subobject V is assigned twice by the implicitly-defined copy assignment operator for C. —end example] [Note: This does not apply to move assignment, as a defaulted move assignment operator is deleted if the class has virtual bases. —end note]

  13. Delete 12.8 [class.copy] paragraph 31:

  14. The implicitly-defined move assignment operator for a non-union class X performs memberwise assignment of its subobjects. The direct base classes of X are assigned first, in the order of their declaration in the base-specifier-list, and then the immediate non-static data members of X are assigned, in the order in which they were declared in the class definition. Given a parameter named x, each subobject is assigned in the manner appropriate to its type:

This resolution also resolves issues 1020, 1064 and 1066.




1064. Defaulted move constructor for a union

Section: 12.8  [class.copy]     Status: DR     Submitter: Mike Miller     Date: 2010-03-23

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 63

The new wording in 12.8 [class.copy] specifies the behavior of an implicitly-defined copy constructor for a non-union class (paragraph 16), an implicitly-defined move constructor for a non-union class (paragraph 17), and an implicitly-defined copy constructor for a union (paragraph 18), but not an implicitly-defined move constructor for a union.

Proposed resolution (August, 2010):

This issue is resolved by the resolution of issue 1051.




1066. When is a copy/move assignment operator implicitly defined?

Section: 12.8  [class.copy]     Status: DR     Submitter: Mike Miller     Date: 2010-03-24

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 64

According to 12.8 [class.copy] paragraph 28,

A copy/move assignment operator that is defaulted and not defined as deleted is implicitly defined when an object of its class type is assigned a value of its class type or a value of a class type derived from its class type or when it is explicitly defaulted after its first declaration.

This sounds as if any assignment to a class object, regardless of whether it is a copy or a move assignment, defines both the copy and move operators. Presumably an assignment should only define the assignment operator chosen by overload resolution for the operation. (Compare the corresponding wording in paragraph 14 for the copy/move constructors: “...implicitly defined if it is used to initialize an object of its class type...”)

Proposed resolution (August, 2010):

This issue is resolved by the resolution of issue 1051.




1148. Copy elision and move construction of function parameters

Section: 12.8  [class.copy]     Status: DR     Submitter: DE     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment DE 11

It is unclear whether copy elision is permitted when returning a parameter of class type. If not, it should still be possible to move, rather than copy, the return value.

Suggested resolution: Amend paragraph 34 to explicitly exclude function parameters from copy elision. Amend paragraph 35 to include function parameters as eligible for move-construction.

Proposed resolution (September, 2010):

  1. Change 12.8 [class.copy] paragraph 34 bullets 1 and 2 as follows:

  2. Change 12.8 [class.copy] paragraph 35 as follows:

    When the criteria for elision of a copy operation are met, or would be met save for the fact that the source object is a function parameter, and the object to be copied is designated by an lvalue, overload resolution...
[Drafting note: the change to paragraph 35 intentionally omits catch-clause parameters out of concern that a rethrow during the move would throw a stripped exception object. This should not be problematic in most cases, since exception objects are typically small.]


1016. Overloadable declarations, function templates, and references

Section: 13  [over]     Status: DR     Submitter: Johannes Schaub     Date: 2009-12-27

[Voted into the WP at the November, 2010 meeting.]

According to 13 [over] paragraph 1,

Only function declarations can be overloaded; object and type declarations cannot be overloaded.

There are two problems with this statement. First, it does not allow for overloading function templates. (There may be other places in the Standard that refer to “functions” but should include function templates, as well.)

Second, the restriction on “object” declarations should presumably be on “variable” declarations instead, since one can also not overload reference declarations.

Proposed resolution (September, 2010):

Change 13 [over] paragraph 1 as follows:

...Only function and function template declarations can be overloaded; object variable and type declarations cannot be overloaded.



1087. Additional applications of issue 899

Section: 13.3.1.4  [over.match.copy]     Status: DR     Submitter: Jason Merrill     Date: 2010-06-25

[Voted into the WP at the November, 2010 meeting.]

The resolution of issue 899 needs to be extended to cover move constructors and template constructors as well.

Proposed resolution (August, 2010):

For example

    struct C {
       template <class T = int> C(C&, T = 0);
    };

    struct A {
       explicit operator C&() const;
    };

    int main() {
       A a;
       C c (a); // should use template constructor
    }



1152. Rules for determining existence of implicit conversion sequence

Section: 13.3.2  [over.match.viable]     Status: DR     Submitter: US     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 67

To determine whether there is an implicit conversion sequence that converts the argument to the corresponding parameter, 13.3.2 [over.match.viable] paragraph 3 uses 13.3.3.1 [over.best.ics] instead of just saying “there is an ICS if-and-only-if a copy initialization would be well-formed.” Apparently this is intended, but to a casual reader or an implementor reading these rules for the first time for a new implementation, it's not clear why that's desirable. A note should be added to explain the rationale.

Proposed resolution (August, 2010):

Change 13.3.3.1.4 [over.ics.ref] paragraph 3 as follows:

Except for an implicit object parameter, for which see 13.3.1 [over.match.funcs], a standard conversion sequence cannot be formed if it requires binding an lvalue reference to non-const other than a reference to a non-volatile const type to an rvalue or binding an rvalue reference to an lvalue. [Note:...



1153. Type matching in address of overloaded function

Section: 13.4  [over.over]     Status: DR     Submitter: US     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 68

Overload resolution within the operand of a unary & operator is done by selecting the function “whose type matches the target type required in the context.” The criterion for determining whether the types match, however, is not defined. At least three possibilities suggest themselves:

  1. The types are identical.

  2. The source type can be implicitly converted to the target type.

  3. The expression would be well-formed if the function under consideration were not overloaded.

This question arises for pointer-to-member types, where there is an implicit conversion from a pointer-to-base-member to a pointer-to-derived-member, as well as when the context is an explicit type conversion (which allows, for static_cast, a conversion from pointer-to-derived-member to a pointer-to-base-member and, in the reinterpret_cast interpretation of functional and old-style casts, essentially any conversion).

Notes from the August, 2010 meeting:

CWG observed that the only case in which the types might not match exactly was for pointers to member functions. In this case, the approach should be to ignore the class of which the functions are members and just match (exactly) on the function type.

Proposed resolution (September, 2010):

  1. Change 13.4 [over.over] paragraph 1 as follows:

  2. ...The function selected is the one whose type matches is identical to the function type of the target type required in the context. [Note: That is, the class of which the function is a member is ignored when matching a pointer-to-member-function type. —end note] The target can be...
  3. Change 13.4 [over.over] paragraph 3 as follows:
  4. Non-member functions and static member functions match targets of type “pointer-to-function” or “reference- to-function.” Nonstatic member functions match targets of type “pointer-to-member-function;.the function type of the pointer to member is used to select the member function from the set of overloaded member functions. If a non-static member function is selected, the reference to the overloaded function name is required to have the form of a pointer to member as described in 5.3.1 [expr.unary.op].



1009. Missing cases in the declarator-id of a function template declaration

Section: 14  [temp]     Status: DR     Submitter: Johannes Schaub     Date: 2009-11-28

[Voted into the WP at the November, 2010 meeting.]

According to 14 [temp] paragraph 2,

In a function template declaration, the last component of the declarator-id shall be a template-name or operator-function-id (i.e., not a template-id).

This is too restrictive; it should also allow conversion-function-ids and literal-operator-ids.

Proposed resolution (September, 2010):

Change 14 [temp] paragraph 2 as follows:

A template-declaration can appear only as a namespace scope or class scope declaration. In a function template declaration, the last component of the declarator-id shall not be a template-id template-name or operator-function-id (i.e., not a template-id). [Note: in That last component may be an identifier, an operator-function-id, a conversion-function-id, or a literal-operator-id. In a class template declaration, if the class name is a simple-template-id, the declaration declares a class template partial specialization (14.5.5 [temp.class.spec]). —end note]



1006. std::nullptr_t as a non-type template parameter

Section: 14.1  [temp.param]     Status: DR     Submitter: Mike Miller     Date: 2009-11-20

[Voted into the WP at the November, 2010 meeting.]

std::nullptr_t is not currently allowed by 14.1 [temp.param] paragraph 4 to be used as the type of a non-type template parameter. However, this could arise for a template with a non-type template parameter with a dependent type in a template intended for use with pointers, e.g.,

    template<typename T, T t> void f();
    ...
    f<std::nullptr_t, nullptr>();

or in a case of delegation.

Proposed resolution (September, 2010):

Change 14.1 [temp.param] paragraph 4 as follows:

A non-type template-parameter shall have one of the following (optionally cv-qualified) types:




1025. Use of a reference as a non-type template argument

Section: 14.3.2  [temp.arg.nontype]     Status: DR     Submitter: Mike Miller     Date: 2010-01-31

[Voted into the WP at the November, 2010 meeting.]

The current wording of 14.3.2 [temp.arg.nontype] paragraph 1 does not prevent the use a reference as a non-type template argument. It simply requires

the address of an object or function with external linkage... expressed as & id-expression...

This would presumably (but unintentionally?) allow an example like the following:

    struct S { };
    template<S*> struct X { };
    S s;
    S& ref = s;
    X<&ref> xr;   // well-formed?

The expression &ref is not a constant expression, but the current wording of 14.3.2 [temp.arg.nontype] does not require a constant expression.

Proposed resolution (September, 2010):

Change 14.3.2 [temp.arg.nontype] paragraph 1 bullet 3 as follows:




1154. Address of thread_local variable as non-type template argument

Section: 14.3.2  [temp.arg.nontype]     Status: DR     Submitter: US     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 69

The standard permits the address of a thread_local object as a non-type template argument. The addresses of these objects are not constant, however. Such template arguments should require objects of static storage duration.

Proposed resolution (August, 2010):

This issue is resolved by the resolution of issue 1155.




1155. Internal-linkage non-type template arguments

Section: 14.3.2  [temp.arg.nontype]     Status: DR     Submitter: US     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment DE 12

Now that local classes can be used as template arguments, it seems odd that there are “external linkage” restrictions on non-type template parameters. The addresses of objects and functions with internal linkage should be permitted as well.

Proposed resolution (August, 2010):

Change 14.3.2 [temp.arg.nontype] paragraph 1 bullet 3 as follows:

This resolution also resolves issue 1154.




1156. Partial ordering in a non-call context

Section: 14.5.6.2  [temp.func.order]     Status: DR     Submitter: US     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 70

14.8.2.4 [temp.deduct.partial] paragraph 3 specifies that the deduction used in partial ordering in a non-call context is based on the complete function type of the function templates. The wording in 14.5.6.2 [temp.func.order] paragraph 2 (and echoed in paragraph 4) reflects an earlier specification, however, saying that the deduction uses only “the function parameter types, or in the case of a conversion function the return type.” This is a contradiction. The wording in 14.5.6.2 [temp.func.order] should be changed.

Proposed resolution (September, 2010):

  1. Change 14.5.6.2 [temp.func.order] paragraph 2 as follows:

  2. Partial ordering selects which of two function templates is more specialized than the other by transforming each template in turn (see next paragraph) and performing template argument deduction using the function parameter types, or in the case of a conversion function the return type. The deduction process determines whether one of the templates is more specialized than the other. If so, the more specialized template is the one chosen by the partial ordering process.
  3. Change 14.5.6.2 [temp.func.order] paragraph 4 as follows:

  4. Using the transformed function template's function parameter list, or in the case of a conversion function its transformed return type, perform type deduction against the function parameter list (or return type) type of the other function template. The mechanism for performing these deductions is given in 14.8.2.4 [temp.deduct.partial].



1158. Recursive instantiation via alias template

Section: 14.5.7  [temp.alias]     Status: DR     Submitter: US     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 73

The current wording of 7.1.3 [dcl.typedef] paragraph 2 requires that the identifier in an alias-declaration “...shall not appear in the type-id.” With template aliases, however, the name of the alias can be used indirectly:

    template<typename T> struct A;
    template<typename T> using B=typename A<T>::U;
    template<typename T> struct A {
      typedef B<T> U;
    };
    B<short> b;

Here the instantiation of B<short> causes the instantiation of A<short>, and the typedef in A<short> ends up attempting to use B<short>, which is still in the process of being instantiated.

There should be an explicit prohibition of such uses.

Proposed resolution (August, 2010):

Add the following as a new paragraph at the end of 14.5.7 [temp.alias]:

The type-id in an alias template declaration shall not refer to the alias template being declared. The type produced by an alias template specialization shall not directly or indirectly make use of that specialization. [Example:

  template <class T> struct A;
  template <class T> using B = typename A<T>::U;
  template <class T> struct A {
    typedef B<T> U;
  };
  B<short> b; // Error: instantiation of B<short> uses own type via A<short>::U.

end example]

[Note: this wording assumes the change from “template alias” to “alias template” requested by comment FI 11 on FCD N3092.]


1159. Class and enumeration definitions in template aliases

Section: 14.5.7  [temp.alias]     Status: DR     Submitter: US     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 74

An alias-declaration allows a class or enumeration type to be defined in its type-id (7.1.6 [dcl.type] paragraph 3). However, it's not clear that this is desirable when the alias-declaration is part of a template alias:

    template<typename T> using A =
      struct { void f(T) { } };

Proposed resolution (August, 2010):

Change 7.1.6 [dcl.type] paragraph 3 as follows:

...A type-specifier-seq shall not define a class or enumeration unless it appears in the type-id of an alias-declaration (7.1.3 [dcl.typedef]) that is not the declaration of a template-declaration.



1161. Dependent nested-name-specifier in a pointer-to-member declarator

Section: 14.6  [temp.res]     Status: DR     Submitter: GB     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment GB 44

It is not clear from the current wording of 14.6 [temp.res] whether the following is well-formed or not:

    template<typename T> struct id { typedef T type; };
    template<typename T> void f() { int id<T>::type::*p = 0; }
    struct A { };
    int main() { f<A>(); }

If typename is required before the use of id<T>::type, it is not permitted by the current syntax.

Proposed resolution (August, 2010):

Change 14.6 [temp.res] paragraph 5 as follows:

A qualified name used as the name in a mem-initializer-id, a base-specifier, or an elaborated-type-specifier is implicitly assumed to name a type, without the use of the typename keyword. In a nested-name-specifier that immediately contains a nested-name-specifier that depends on a template parameter, the identifier or simple-template-id is implicitly assumed to name a type, without the use of the typename keyword. [Note: the typename keyword is not permitted by the syntax of these constructs. —end note]



1160. Definitions of template members and the current instantiation

Section: 14.6.2.1  [temp.dep.type]     Status: DR     Submitter: GB     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment GB 43

The current rules for determining whether a name refers to the current instantiation, given in 14.6.2.1 [temp.dep.type] paragraph 1, do not cover the case when a template-id matching a primary template or partial specialization appears in the definition of a member of the template.

Proposed resolution (August, 2010):

Change 14.6.2.1 [temp.dep.type] paragraph 1 as follows:

In the definition of a class template, a nested class of a class template, a member of a class template, or a member of a nested class of a class template, a A name refers to the current instantiation if it is



1047. When is typeid value-dependent?

Section: 14.6.2.3  [temp.dep.constexpr]     Status: DR     Submitter: Steve Adamczyk     Date: 2010-03-08

[Voted into the WP at the November, 2010 meeting.]

The Standard should, but does not currently, say that typeid is value-dependent if its expression or type is type-dependent.

Proposed resolution (September, 2010):

Change 14.6.2.3 [temp.dep.constexpr] paragraph 2 as follows:

...Expressions of the following form are value-dependent if the unary-expression or expression is type-dependent or the type-id is dependent:




1164. Partial ordering of f(T&) and f(T&&)

Section: 14.8.2.1  [temp.deduct.call]     Status: DR     Submitter: US     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 77

The following example is ambiguous:

    template<typename T> int f(T&);
    template<typename T> int f(T&&);
    int i;
    int j = f(i);

Because of the special deduction rule for lvalues passed to rvalue-reference parameters, deduction produces f(int&) for both templates, and they are indistinguishable.

Because f(T&) accepts a strict subset of the things that f(T&&) does, it should be considered more specialized by the partial ordering rules.

Proposed resolution (August, 2010):

Change 14.8.2.4 [temp.deduct.partial] paragraph 9 as follows:

If, for a given type, deduction succeeds in both directions (i.e., the types are identical after the transformations above) and both P and A were reference types (before being replaced with the type referred to above):

[Editing note: this change transforms the running text at the end of the paragraph into a bulleted list.]




1165. Exceptions when destroying array elements

Section: 15.2  [except.ctor]     Status: DR     Submitter: US     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment US 78

According to 15.2 [except.ctor] paragraph 2,

An object that is partially constructed or partially destroyed will have destructors executed for all of its fully constructed base classes and non-variant members, that is, for subobjects for which the principal constructor (12.6.2 [class.base.init]) has completed execution and the destructor has not yet begun execution.

This wording leaves unclear whether the remaining elements of an array will be destroyed if the destructor for one of the elements exits via an exception: an array element is a subobject (1.8 [intro.object] paragraph 2), but it is not a base class or non-variant member.

Proposed resolution (September, 2010):

Change 15.2 [except.ctor] paragraph 2 as follows:

An object that is partially constructed or partially destroyed of any storage duration whose initialization or destruction is terminated by an exception will have destructors executed for all of its fully constructed base classes and non-variant members subobjects (excluding the variant members of a union-like class), that is, for subobjects for which the principal constructor (12.6.2 [class.base.init]) has completed execution and the destructor has not yet begun execution. Similarly, if the non-delegating constructor...



1168. Additional reasons to call std::terminate

Section: 15.5.1  [except.terminate]     Status: DR     Submitter: GB     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment GB 47

The list of reasons for which std::terminate is called needs to be extended to cover several additional cases in C++0x:

Proposed resolution (August, 2010):

  1. Change 15.5.1 [except.terminate] paragraph 1 as follows:

  2. In the following some situations exception handling must be abandoned for less subtle error handling techniques:. [Note: These situations are:

    end note]

  3. Insert the following as a new paragraph following 15.1 [except.throw] paragraph 6:

  4. An exception is considered caught...

    If the exception handling mechanism, after completing evaluation of the expression to be thrown but before the exception is caught, calls a function that exits via an exception, std::terminate is called (15.5.1 [except.terminate]). [Example:

        struct C {
          C() { }
          C(const C&) { throw 0; }
        };
    
        int main() {
          try {
            throw C();   // calls std::terminate()
          } catch(C) { }
        }
    

    end example]

  5. Change 15.2 [except.ctor] paragraph 3 as follows:

  6. The process of calling destructors for automatic objects constructed on the path from a try block to a throw-expression is called “stack unwinding.” [Note: If a destructor called during stack unwinding exits with an exception, std::terminate is called (15.5.1 [except.terminate]). [Note: So destructors should generally catch exceptions and not let them propagate out of the destructor. —end note]
  7. Change 3.6.2 [basic.start.init] paragraph 6 as follows:

  8. [Note: If the initialization of a non-local variable with static or thread storage duration terminates by throwing exits via an exception, std::terminate is called (see 15.5.1 [except.terminate]). end note]
  9. Change 3.6.3 [basic.start.term] paragraph 1 as follows:

  10. ...[Note: If the destruction of a non-local an object with static or thread storage duration terminates by throwing exits via an exception, std::terminate is called (see 15.5.1 [except.terminate]). end note]
  11. Change 18.5 [support.start.term] paragraph 8 bullet 1 as follows:




1171. Partial stack unwinding with noexcept violation

Section: 15.5.1  [except.terminate]     Status: DR     Submitter: Merrill     Date: 2010-08-04

[Voted into the WP at the November, 2010 meeting.]

The current wording of 15.5.1 [except.terminate] paragraph 2 makes it sound as if stack unwinding in the case of a noexcept violation is an all-or-nothing proposition. It would be useful to be able to partially unwind the stack, in particular, not to call destructors for the function containing the noexcept-specification.

Proposed resolution (August, 2010):

Change 15.5.1 [except.terminate] paragraph 2 as follows:

...In the situation where the search for a handler (15.3 [except.handle]) encounters the outermost block of a function with a noexcept-specification that does not allow the exception (15.4 [except.spec]), it is implementation-defined whether the stack is unwound, unwound partially, or not unwound at all before std::terminate() is called. In all other situations, the stack shall not be unwound...



1169. Missing feature macro for strict pointer safety

Section: 16.8  [cpp.predefined]     Status: DR     Submitter: GB     Date: 2010-08-03

[Voted into the WP at the November, 2010 meeting.]

N3092 comment DE 13

The recommendations of document N2693 included a feature macro to enable a program to determine whether the implementation enforces strict pointer safety or not, but this macro is not specified in 16.8 [cpp.predefined].

Proposed resolution (August, 2010):

Add the following to the end of 16.8 [cpp.predefined] paragraph 2:






Issues with "WP" Status


676. static_assert-declarations and general requirements for declarations

Section: 3.1  [basic.def]     Status: WP     Submitter: Alisdair Meredith     Date: 12 February, 2008

[Voted into WP at August, 21010 meeting.]

3.1 [basic.def] makes statements about declarations that do not appear to apply to static_assert-declarations. For example, paragraph 1 says,

A declaration (clause 7 [dcl.dcl]) introduces names into a translation unit or redeclares names introduced by previous declarations. A declaration specifies the interpretation and attributes of these names.

What name is being declared or described by a static_assert-declaration?

Also, paragraph 2 lists the kinds of declarations that are not definitions, and a static_assert-declaration is not among them. Is it intentional that static_assert-declarations are definitions?

Proposed resolution (March, 2010):

Change 3.1 [basic.def] paragraphs 1-2 as follows:

A declaration (Clause 7 [dcl.dcl]) may introduces one or more names into a translation unit or redeclares names introduced by previous declarations. A If so, the declaration specifies the interpretation and attributes of these names. A declaration may also have effects including

A declaration is a definition unless it declares a function without specifying the function's body (8.4 [dcl.fct.def]), it contains the extern specifier (7.1.1 [dcl.stc]) or a linkage-specification25 (7.5 [dcl.link]) and neither an initializer nor a function-body, it declares a static data member in a class definition (9.4 [class.static]), it is a class name declaration (9.1 [class.name]), it is an opaque-enum-declaration (7.2 [dcl.enum]), or it is a typedef declaration (7.1.3 [dcl.typedef]), a using-declaration (7.3.3 [namespace.udecl]), a static_assert-declaration (Clause 7 [dcl.dcl]), an attribute-declaration (Clause 7 [dcl.dcl]), an empty-declaration (Clause 7 [dcl.dcl]), or a using-directive (7.3.4 [namespace.udir]). [Example:...




678. Language linkage of member function parameter types and the ODR

Section: 3.2  [basic.def.odr]     Status: WP     Submitter: James Widman     Date: 15 February, 2008

[Voted into WP at August, 21010 meeting.]

I thought this case would result in undefined behavior according to 3.2 [basic.def.odr]:

    // t.h:
    struct A { void (*p)(); };

    // t1.cpp:
    #include "t.h" // A::p is a pointer to C++ func

    // t2.cpp:
    extern "C" {
    #include "t.h" // A::p is a pointer to C func
    }

...but I don't see how any of the bullets in the list in paragraph 5 apply.

Proposed resolution (March, 2010):

Add a new bullet following 3.2 [basic.def.odr] paragraph 5, second bullet:

...Given such an entity named D defined in more than one translation unit, then




373. Lookup on namespace qualified name in using-directive

Section: 3.4.6  [basic.lookup.udir]     Status: WP     Submitter: Clark Nelson     Date: 15 August 2002

[Voted into WP at August, 2010 meeting.]

Is this case valid? G++ compiles it.

namespace X {
  namespace Y {
    struct X {
      void f()
      {
        using namespace X::Y;
        namespace Z = X::Y;
      }
    };
  }
}

The relevant citation from the standard is 3.4.6 [basic.lookup.udir]: "When looking up a namespace-name in a using-directive or namespace-alias-definition, only namespace names are considered." This statement could reasonably be interpreted to apply only to the last element of a qualified name, and that's the way EDG and Microsoft seem to interpret it.

However, since a class can't contain a namespace, it seems to me that this interpretation is, shall we say, sub optimal. If the X qualifiers in the above example are interpreted as referring to the struct X, an error of some sort is inevitable, since there can be no namespace for the qualified name to refer to. G++ apparently interprets 3.4.6 [basic.lookup.udir] as applying to nested-name-specifiers in those contexts as well, which makes a valid interpretation of the test possible.

I'm thinking it might be worth tweaking the words in 3.4.6 [basic.lookup.udir] to basically mandate the more useful interpretation. Of course a person could argue that the difference would matter only to a perverse program. On the other hand, namespaces were invented specifically to enable the building of programs that would otherwise be considered perverse. Where name clashes are concerned, one man's perverse is another man's real world.

Proposed Resolution (November, 2006):

Change 3.4.6 [basic.lookup.udir] paragraph 1 as follows:

When looking up a namespace-name in a using-directive or namespace-alias-definition, In a using-directive or namespace-alias-definition, during the lookup for a namespace-name or for a name in a nested-name-specifier, only namespace names are considered.



619. Completeness of array types

Section: 3.9  [basic.types]     Status: WP     Submitter: Steve Clamage     Date: 16 February 2007

[Voted into WP at August, 21010 meeting.]

Is the following example well-formed?

    struct S {
        static char a[5];
    };
    char S::a[];    // Unspecified bound in definition

3.5 [basic.link] paragraph 10 certainly makes allowance for declarations to differ in the presence or absence of a major array bound. However, 3.1 [basic.def] paragraph 5 says that

A program is ill-formed if the definition of any object gives the object an incomplete type (3.9 [basic.types]).

3.9 [basic.types] paragraph 7 says,

The declared type of an array object might be an array of unknown size and therefore be incomplete at one point in a translation unit and complete later on; the array types at those two points (“array of unknown bound of T” and “array of N T”) are different types.

This wording appears to make no allowance for the C concept of “composite type;” instead, each declaration is said to have its own type. By this interpretation, the example is ill-formed, because the type declared by the definition of S::a is incomplete.

If the example is intended to be well-formed, the Standard needs explicit wording stating that an omitted array bound in a declaration is implicitly taken from that of a visible declaration of that object, if any.

Notes from the April, 2007 meeting:

The CWG agreed that this usage should be permitted.

Proposed resolution (June, 2008):

  1. Change 8.3.4 [dcl.array] paragraph 1 as follows:

  2. ...If Except as noted below, if the constant expression is omitted, the type of the identifier of D is “derived-declarator-type-list array of unknown bound of T,” an incomplete object type...
  3. Change 8.3.4 [dcl.array] paragraph 3 as follows:

  4. When several “array of” specifications are adjacent, a multidimensional array is created; only the first of the constant expressions that specify the bounds of the arrays can may be omitted only for the first member of the sequence. [Note: this elision is useful for function parameters of array types, and when the array is external and the definition, which allocates storage, is given elsewhere. —end note] In addition to declarations in which an incomplete object type is allowed, an array bound may be omitted in the declaration of a function parameter (8.3.5 [dcl.fct]). The first constant-expression can An array bound may also be omitted when the declarator is followed by an initializer (8.5 [dcl.init]). In this case the bound is calculated from the number of initial elements (say, N) supplied (8.5.1 [dcl.init.aggr]), and the type of the identifier of D is “array of N T.” Furthermore, if there is a visible declaration of the name declared by the declarator-id (if any) in which the bound was specified, an omitted array bound is taken to be the same as in that earlier declaration.

Notes from the September, 2008 meeting:

The proposed resolution does not capture the result favored by the CWG: array bound information should be accumulated across declarations within the same scope, but a block extern declaration in a nested scope should not inherit array bound information from the outer declaration. (This is consistent with the treatment of default arguments in function declarations.) For example:

    int a[5];
    void f() {
        extern int a[];
        sizeof(a);
    }

Although there was some confusion about the C99 wording dealing with this case, it is probably well-formed in C99. However, it should be ill-formed in C++, because we want to avoid the concept of “compatible types” as it exists in C.

Proposed resolution (March, 2010):

  1. Change 8.3.4 [dcl.array] paragraph 1 as follows:

  2. ...If Except as noted below, if the constant expression is omitted, the type of the identifier of D is “derived-declarator-type-list array of unknown bound of T,” an incomplete object type...
  3. Change 8.3.4 [dcl.array] paragraphs 3-4 as follows:

  4. When several “array of” specifications are adjacent, a multidimensional array is created; only the first of the constant expressions that specify the bounds of the arrays can may be omitted only for the first member of the sequence. [Note: this elision is useful for function parameters of array types, and when the array is external and the definition, which allocates storage, is given elsewhere. —end note] In addition to declarations in which an incomplete object type is allowed, an array bound may be omitted in some cases in the declaration of a function parameter (8.3.5 [dcl.fct]). The first constant-expression can An array bound may also be omitted when the declarator is followed by an initializer (8.5 [dcl.init]). In this case the bound is calculated from the number of initial elements (say, N) supplied (8.5.1 [dcl.init.aggr]), and the type of the identifier of D is “array of N T.” Furthermore, if there is a preceding declaration of the entity in the same scope in which the bound was specified, an omitted array bound is taken to be the same as in that earlier declaration, and similarly for the definition of a static data member of a class.

    [Example:...

    ...can reasonably appear in an expression. Finally,

      extern int x[10];
      struct S {
        static int y[10];
      };
    
      int x[];                //OK: bound is 10
      int S::y[];             //OK: bound is 10
    
      void f() {
        extern int x[];
        int i = sizeof(x);    //error: incomplete object type
      }
    

    end example]




948. constexpr in conditions

Section: 6.4  [stmt.select]     Status: WP     Submitter: Gabriel dos Reis     Date: 2 August, 2009

[Voted into WP at August, 21010 meeting.]

The grammar for condition in 6.4 [stmt.select] paragraph 1 does not allow for the constexpr specifier. This was not intended by the original proposal.

Proposed resolution (March, 2010):

  1. Change the definition of condition in 6.4 [stmt.select] paragraph 1 as follows:

  2. Insert the following text as a new paragraph at the end of 6.4 [stmt.select]:

  3. In the decl-specifier-seq of a condition, each decl-specifier shall be either a type-specifier or constexpr.



864. braced-init-list in the range-based for statement

Section: 6.5.4  [stmt.ranged]     Status: WP     Submitter: James Widman     Date: 7 April, 2009

[Voted into WP at August, 21010 meeting.]

The intent is that the range-based for statement should be able to be used with a braced-init-list as the range over which to iterate. However, this does not work grammatically: a braced-init-list is not an expression, as required by the syntax in 6.5.4 [stmt.ranged] paragraph 1:

Even if this were resolved, the “equivalent to” code is not correct. It contains the declaration,

This has a similar problem, in that 7.1.6.4 [dcl.spec.auto] paragraph 3 requires that the initializer have one of the forms

which does not allow for a braced-initializer-list. In addition, although not allowed by the grammar, 7.1.6.4 [dcl.spec.auto] paragraph 6 treats the braced-init-list specially, in order for the type deduction to work correctly:

Obtain P from T by replacing the occurrences of auto with either a new invented type template parameter U or, if the initializer is a braced-init-list (8.5.4 [dcl.init.list]), with std::initializer_list<U>.

The problem here is that a parenthesized initializer, as in the code expansion of the range-based for statement, is not a braced-init-list.

Proposed resolution (June, 2010):

  1. Change 6.5 [stmt.iter] paragraph 1 as follows:

  2. Iteration statements specify looping.




    [Note: a for-init-statement ends with a semicolon. —end note]

  3. Change 6.5.4 [stmt.ranged] paragraph 1 as follows:

  4. The For a range-based for statement of the form

    let range-init be equivalent to the expression surrounded by parentheses:

    [Footnote: this ensures that a top-level comma operator cannot be reinterpreted as a delimiter between init-declarators in the declaration of __range. —end footnote] and for a range-based for statement of the form

    let range-init be equivalent to the braced-init-list. In each case, a range-based for statement is equivalent to

      {
        auto && __range = ( expression ) range-init;
        for ( auto __begin = begin-expr,
        ...
    

    Note to editor:

    The formatting in the preceding change for range-init follows that of the existing text for begin-expr and end-expr. However, CWG is concerned that this style makes all of these elements look too much like grammar nonterminals and asks that the editor consider some other formatting convention.




700. Constexpr member functions of class templates

Section: 7.1.5  [dcl.constexpr]     Status: WP     Submitter: Jens Maurer     Date: 27 June, 2008

[Voted into WP at August, 21010 meeting.]

7.1.5 [dcl.constexpr] paragraph 5 applies only to “the instantiated template specialization of a constexpr function template;” it should presumably apply to non-template member functions of a class template, as well.

Notes from the September, 2008 meeting:

This question is more involved than it might appear. For example, a constexpr member function is implicitly const; if the constexpr specifier is ignored, does that make the member function non-const? Also, should this provision apply only to dependent expressions in the function? Should it be an error if no constexpr function can be instantiated from the template, along the lines of the permission given in 14.6 [temp.res] paragraph 8 for an implementation to diagnose a template definition from which no valid specialization can be instantiated?

Notes from the July, 2009 meeting:

The consensus of the CWG was that an “ignored” constexpr specifier in this case simply means that the specialization is not constexpr, not that it is not const. The CWG also decided not to address the question of non-dependent expressions that render a function template specialization non-constexpr, leaving it to quality of implementation whether a (warning) diagnostic is issued in such cases.

Proposed resolution (February, 2010):

Change 7.1.5 [dcl.constexpr] paragraph 5 as follows:

If the instantiated template specialization of a constexpr function template or member function of a class template would fail to satisfy the requirements for a constexpr function or constexpr constructor, the constexpr specifier is ignored that specialization is not a constexpr function or constexpr constructor. [Note: if the function is a member function it will still be const as described below. Implementations are encouraged to issue a warning if a function was rendered not constexpr by a non-dependent construct. —end note]



860. Explicit qualification of constexpr member functions

Section: 7.1.5  [dcl.constexpr]     Status: WP     Submitter: Daniel Krügler     Date: 6 April, 2009

[Voted into WP at August, 21010 meeting.]

7.1.5 [dcl.constexpr] paragraph 6 says,

A constexpr specifier for a non-static member function that is not a constructor declares that member function to be const (9.3.1 [class.mfct.non-static]).

Is a const qualifier on such a member function redundant or ill-formed?

Notes from the July, 2009 meeting:

The CWG agreed that a const qualifier on a constexpr member function is simply redundant and not an error.

Proposed resolution (February, 2010):

Change 7.1.5 [dcl.constexpr] paragraph 6 as follows:

A constexpr specifier for a non-static member function that is not a constructor declares that member function to be const (9.3.1 [class.mfct.non-static]). [Note: the constexpr specifier has no other effect on the function type. —end note] The keyword const is ignored if it appears in the cv-qualifier-seq of the function declarator of the declaration of such a member function. The class of which that function is a member shall be a literal type (3.9 [basic.types]). [Example:...



892. Missing requirements for constexpr constructors

Section: 7.1.5  [dcl.constexpr]     Status: WP     Submitter: Alisdair Meredith     Date: 8 May, 2009

[Voted into WP at August, 21010 meeting.]

The rules for constexpr constructors are missing some necessary requirements. In particular, there is no requirement that a brace-or-equal-initializer for a non-static data member be a constant expression, and the requirement for constexpr constructors for initializing non-static data members applies only to members named in a mem-initializer, allowing a non-constexpr default constructor to be invoked.

Proposed resolution (February, 2010):

Change 7.1.5 [dcl.constexpr] paragraph 4 as follows:

The definition of a constexpr constructor shall satisfy the following constraints:




508. Non-constructed value-initialized objects

Section: 8.5  [dcl.init]     Status: WP     Submitter: Alisdair Meredith     Date: 18 Mar 2005

[Voted into WP at August, 2010 meeting.]

According to the definition of value initialization (8.5 [dcl.init] paragraph 5), non-union class types without user-declared constructors are value-initialized by value-initializing each of their members rather than by executing the (generated) default constructor. However, a number of other items in the Standard are described in relationship to the execution of the constructor:

Proposed resolution (October, 2005):

Add the indicated words to 8.5 [dcl.init] paragraph 6:

A program that calls for default-initialization or value-initialization of an entity of reference type is ill-formed. If T is a cv-qualified type, the cv-unqualified version of T is used for these definitions of zero-initialization, default-initialization, and value-initialization. Even when value-initialization of an object does not call that object's constructor, the object is deemed to have been fully constructed once its initialization is complete and thus subject to provisions of this International Standard applying to “constructed” objects, objects “for which the constructor has completed execution,” etc.

Notes from April, 2006 meeting:

There was some concern about whether this wording covered (or needed to cover) cases where an object is “partially constructed.” Another approach might be simply to define value initialization to be “construction.” Returned to “drafting” status for further investigation.

Proposed resolution (February, 2010):

Change 8.5 [dcl.init] paragraph 7 as follows:

To value-initialize an object of type T means:

An object that is value-initialized is deemed to be constructed and thus subject to provisions of this International Standard applying to “constructed” objects, objects “for which the constructor has completed,” etc., even if no constructor is invoked for the object's initialization.




615. Incorrect description of variables that can be initialized

Section: 8.5  [dcl.init]     Status: WP     Submitter: comp.std.c++     Date: 30 January 2007

[Voted into WP at August, 21010 meeting.]

8.5 [dcl.init] paragraph 2 reads,

Automatic, register, static, and external variables of namespace scope can be initialized by arbitrary expressions involving literals and previously declared variables and functions.

Both “automatic” and “static” are used to describe storage durations, “register” is a storage class specifier which indicates the object has automatic storage duration, “external” describes linkage, and “namespace scope” is a kind of scope. Automatic, register, static and external, together with namespace scope, are used to restrict the “variables.”

Register objects are only a sub-set of automatic objects and thus the word “register” is redundant and should be elided. If register objects are to be emphasized, they should be mentioned like “Automatic (including register)...”

Variables having namespace scope can never be automatic; they can only be static, with either external or internal linkage. Therefore, there are in fact no “automatic variables of namespace scope,” and the “static” in “static variables of namespace scope” is useless.

In fact, automatic and static variables already compose all variables with either external linkage or not, and thus the “external” becomes redundant, too, and the quoted sentence seems to mean that all variables of namespace scope can be initialized by arbitrary expressions. But this is not true because not all internal variables of namespace scope can. Therefore, the restrictive “external” is really necessary, not redundant.

As a result, the erroneous restrictive “automatic, register, static” should be removed and the quoted sentence may be changed to:

External variables of namespace scope can be initialized by arbitrary expressions involving literals and previously declared variables and functions.

Notes from the April, 2007 meeting:

This sentence is poorly worded, but the analysis given in the issue description is incorrect. The intent is simply that the storage class of a variable places no restrictions on the kind of expression that can be used to initialize it (in contrast to C, where variables of static storage duration can only be initialized by constant expressions).

Proposed resolution (June, 2008):

Change 8.5 [dcl.init] paragraph 2 as follows:

Automatic, register, static, and external variables of namespace scope Variables of automatic, thread, and static storage duration can be initialized by arbitrary expressions involving literals and previously declared variables and functions...

Notes from the September, 2008 meeting:

The existing wording is intended to exclude block-scope extern declarations but to allow initializers in all other forms of variable declarations. The best way to phrase that is probably to say that all variable definitions (except for function parameters, where the initializer syntax is used for default arguments) can have arbitrary expressions as initializers, regardless of storage duration.

Proposed resolution (February, 2010):

Change 8.5 [dcl.init] paragraph 2 as follows:

Automatic, register, thread_local, static, and namespace-scoped external variables can be initialized by Except for objects declared with the constexpr specifier, for which see 7.1.5 [dcl.constexpr], an initializer in the definition of a variable can consist of arbitrary expressions involving literals and previously declared variables and functions, regardless of the variable's storage duration. [Example:...



924. alias-declaration as a class member

Section: 9.2  [class.mem]     Status: WP     Submitter: Alisdair Meredith     Date: 23 June, 2009

N3092 comment FI 10

[Voted into WP at August, 21010 meeting.]

The grammar for member-declaration in 9.2 [class.mem] does not include a production for the alias-declaration form of typedef declarations, meaning that something like

    struct S {
      using UINT = unsigned int;
    };

is ill-formed. This seems like an oversight.

Proposed resolution (February, 2010):

In the grammar in 9.2 [class.mem], add the indicated production to the definition of member-declaration:




741. “plain” long long bit-fields

Section: 9.6  [class.bit]     Status: WP     Submitter: Mike Miller     Date: 7 November, 2008

[Voted into WP at August, 21010 meeting.]

The type long long is missing from the list of bit-field types in 9.6 [class.bit] paragraph 3 for which the implementation can choose the signedness. This was presumably an oversight. (If that is the case, we may want to reconsider the handling of 4.5 [conv.prom] paragraph 3: a long long bit-field that the implementation treats as unsigned will — pending the outcome of issue 739 — still promote to signed long long, which can lead to unexpected results for bit-fields with the same number of bits as long long.)

Proposed resolution (February, 2010):

Change 9.6 [class.bit] paragraph 3 as follows:

...It is implementation-defined whether a plain (neither explicitly signed nor unsigned) char, short, int or, long, or long long bit-field is signed or unsigned...



738. constexpr not permitted by the syntax of constructor declarations

Section: 12.1  [class.ctor]     Status: WP     Submitter: James Widman     Date: 27 October, 2008

[Voted into WP at August, 21010 meeting.]

According to 12.1 [class.ctor] paragraph 1, only function-specifiers are permitted in the declaration of a constructor, and constexpr is not a function-specifier. (See also issue 263, in which the resolution of a similar concern regarding the friend specifier did not change 12.1 [class.ctor] paragraph 1 but perhaps should have done so.)

Proposed resolution (February, 2010):

Change 12.1 [class.ctor] paragraph 1 as follows:

Constructors do not have names. A special declarator syntax using an optional sequence of function-specifiers (7.1.2 [dcl.fct.spec]) followed by the constructor's class name followed by a parameter list is used to declare or define the constructor. The syntax uses

in that order. In such a declaration, optional parentheses around the constructor class name are ignored. [Example:...




655. Initialization not specified for forwarding constructors

Section: 12.6.2  [class.base.init]     Status: WP     Submitter: Alisdair Meredith     Date: 17 October 2007

[Voted into WP at August, 21010 meeting.]

The changes for delegating constructors overlooked the need to change 12.6.2 [class.base.init] paragraph 3:

The expression-list in a mem-initializer is used to initialize the base class or non-static data member subobject denoted by the mem-initializer-id. The semantics of a mem-initializer are as follows:

The initialization of each base and member constitutes a full-expression. Any expression in a mem-initializer is evaluated as part of the full-expression that performs the initialization.

This paragraph deals only with subobjects; it needs to be made more general to apply to the complete object as well when the mem-initializer-id designates the constructor's class.

Proposed resolution (June, 2008):

Change 12.6.2 [class.base.init] paragraph 3 as follows:

The expression-list in a mem-initializer is used to initialize the base class or non-static data member subobject denoted by the mem-initializer-id. The semantics of a mem-initializer are A mem-initializer in which the mem-initializer-id names the constructor's class initializes the object by invoking the selected target constructor with the mem-initializer's expression-list. A mem-initializer in which the mem-initializer-id names a base class or non-static data member initializes the designated subobject as follows:

...

The initialization of each base and member performed by each mem-initializer constitutes a full-expression. Any expression...

Notes from the September, 2008 meeting:

This text was significantly modified by N2756 (nonstatic data member initializers) and needs to be reworked in light of those changes.

Proposed resolution (February, 2010):

  1. Change 12.6.2 [class.base.init] paragraph 7 as follows:

  2. The expression-list or braced-init-list in a mem-initializer is used to initialize the base class or non-static data member subobject denoted by the mem-initializer-id designated subobject (or, in the case of a delegating constructor, the complete class object) according to the initialization rules of 8.5 [dcl.init] for direct-initialization.

    [Example: ...

    end example] The initialization of each base and member performed by each mem-initializer constitutes a full-expression...

  3. Change 12.6.2 [class.base.init] paragraph 8 as follows:

  4. If In a non-delegating constructor, if a given non-static data member or base class is not named by a mem-initializer-id...
  5. Change 12.6.2 [class.base.init] paragraph 10 as follows:

  6. Initialization In a non-delegating constructor, initialization proceeds in the following order:
  7. Change 12.6.2 [class.base.init] paragraph 12 as follows (this is an unrelated change correcting an error noticed while preparing the resolution of this issue):

  8. Names in the expression-list or braced-init-list of a mem-initializer are evaluated in the scope of the constructor...
  9. Change the next-to-last bullet of the note in 8.5.4 [dcl.init.list] paragraph 1 as follows:




691. Template parameter packs in class template partial specializations

Section: 14.1  [temp.param]     Status: WP     Submitter: Doug Gregor     Date: 9 April, 2008

[Voted into WP at August, 21010 meeting.]

14.1 [temp.param] paragraph 11 currently says,

If a template-parameter of a class template is a template parameter pack, it shall be the last template-parameter. [Note: These are not requirements for function templates because template arguments might be deduced (14.8.2 [temp.deduct])...

This restriction was only meant to apply to primary class templates, not partial specializations.

Suggested resolution:

If a template-parameter of a primary class template is a template parameter pack, it shall be the last template-parameter. [Note: These are not requirements for function templates or class template partial specializations because template arguments might be deduced (14.8.2 [temp.deduct])...

Proposed resolution (February, 2010):

Change 14.1 [temp.param] paragraph 11 as follows:

If a template-parameter of a class template has a default template-argument, each subsequent template-parameter shall either have a default template-argument supplied or be a template parameter pack. If a template-parameter of a primary class template is a template parameter pack, it shall be the last template-parameter. [Note: These are not requirements for function templates or class template partial specializations because template arguments might can be deduced (14.8.2 [temp.deduct]). [Example:...



96. Syntactic disambiguation using the template keyword

Section: 14.2  [temp.names]     Status: WP     Submitter: John Spicer     Date: 16 Feb 1999

[Voted into WP at August, 2010 meeting.]

The following is the wording from 14.2 [temp.names] paragraphs 4 and 5 that discusses the use of the "template" keyword following . or -> and in qualified names.

The whole point of this feature is to say that the "template" keyword is needed to indicate that a "<" begins a template parameter list in certain contexts. The constraints in paragraph 5 leave open to debate certain cases.

First, I think it should be made more clear that the template name must be followed by a template argument list when the "template" keyword is used in these contexts. If we don't make this clear, we would have to add several semantic clarifications instead. For example, if you say "p->template f()", and "f" is an overload set containing both templates and nontemplates: a) is this valid? b) are the nontemplates in the overload set ignored? If the user is forced to write "p->template f<>()" it is clear that this is valid, and it is equally clear that nontemplates in the overload set are ignored. As this feature was added purely to provide syntactic guidance, I think it is important that it otherwise have no semantic implications.

I propose that paragraph 5 be modified to:

(See also issue 30 and document J16/00-0008 = WG21 N1231.)

Notes from 04/00 meeting:

The discussion of this issue revived interest in issues 11 and 109.

Notes from the October 2003 meeting:

We reviewed John Spicer's paper N1528 and agreed with his recommendations therein.

Proposed resolution (February, 2010):

Change 14.2 [temp.names] paragraph 5 as follows:

If a A name prefixed by the keyword template is not the name of a template, shall be a template-id or the name shall refer to a class template the program is ill-formed. [Note: the keyword template may not be applied to non-template members of class templates. —end note] [Note: as is the case with the typename prefix, the template prefix is allowed in cases where it is not strictly necessary; i.e., when the nested-name-specifier or the expression on the left of the -> or . is not dependent on a template-parameter, or the use does not appear in the scope of a template. —end note] [Example:

  template <class T> struct A {
    void f(int);
    template <class U> void f(U); };

  template <class T> void f(T t) {
    A<T> a;
    a.template f<>(t); // OK: calls template
    a.template f(t);   // error: not a template-id
  }

  template <class T> struct B {template <class T2> struct C {}; };
  // OK: T::template C names a class template:
  template <class T, template <class X> class TT = T::template C> struct D {};
  D<B<int>> db;

end example]




431. Defect in wording in 14.2

Section: 14.2  [temp.names]     Status: WP     Submitter: Mat Marcus     Date: 10 August 2003

[Voted into WP at August, 2010 meeting.]

Consider this example:

   class Foo {
   public:
       template< typename T > T *get();
   };

   template< typename U >
   U *testFoo( Foo &foo ) {
       return foo.get< U >(); //#1
   }

I am under the impression that this should compile without requiring the insertion of the template keyword before get in the expression at //#1. This notion is supported by this note excerpted from 14.2 [temp.names]/5:

[Note: just as is the case with the typename prefix, the template prefix is allowed in cases where it is not strictly necessary; i.e., when the expression on the left of the -> or ., or the nested-name-specifier is not dependent on a template parameter.]

But 14.2 [temp.names]/4 contains this text:

When the name of a member template specialization appears after . or -> in a postfix-expression, or after nested-name-specifier in a qualified-id, and the postfix-expression or qualified-id explicitly depends on a template-parameter (14.6.2), the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template.

The only way that I can read this to support my assumption above is if I assume that the phrase postfix-expression is used twice above with different meaning. That is I read the first use as referring to the full expression while the second use refers to the subexpression preceding the operator. Is this the correct determination of intent? I find this text confusing. Would it be an improvement if the second occurrence of "postfix-expression" should be replaced by "the subexpression preceding the operator". Of course that begs the question "where is subexpression actually defined in the standard?"

John Spicer: I agree that the code should work, and that we should tweak the wording.

Proposed resolution (March, 2010):

Change 14.2 [temp.names] paragraph 4 as follows:

When the name of a member template specialization appears after . or -> in a postfix-expression, or after a nested-name-specifier in a qualified-id, and the object or pointer expression of the postfix-expression or the nested-name-specifier in the qualified-id explicitly depends on a template-parameter template parameter (14.6.2 [temp.dep]) but does not refer to a member of the current instantiation (14.6.2.1 [temp.dep.type]), the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template. [Example:...



773. Parentheses in address non-type template arguments

Section: 14.3.2  [temp.arg.nontype]     Status: WP     Submitter: Doug Gregor     Date: 11 February, 2009

[Voted into WP at August, 21010 meeting.]

According to 14.3.2 [temp.arg.nontype] paragraph 1, bullet 3, one of the acceptable forms of a non-type, non-template template argument is:

the address of an object or function... expressed as & id-expression where the & is optional if the name refers to a function or array, or if the corresponding template-parameter is a reference

It is not clear from this whether a template argument like (&i) satisfies the requirement or not.

Notes from the March, 2009 meeting:

The consensus of the CWG was that the parentheses should be allowed.

Proposed resolution (February, 2010):

Change 14.3.2 [temp.arg.nontype] paragraph 1 bullet 3 as follows:

[Drafting note: The change requiring the omission of the & in the reference case fixes an existing problem that is not related to this issue.]




674. “matching specialization” for a friend declaration

Section: 14.5.4  [temp.friend]     Status: WP     Submitter: James Widman     Date: 7 February, 2008

[Voted into WP at August, 21010 meeting.]

14.5.4 [temp.friend] paragraph 1 bullet 3 says:

I'm not sure this says what it's supposed to say. For example:

    namespace N {
        template<class T> int f(T);
    }

    class A {
        friend int N::f(int);
        int m;
        A();
    };

    namespace N {
        template< class T > int f(T) {
            A a;            // ok for T=int?
            return a.m;     // ok for T=int?
        }
    }

    int m = N::f(42);       // ok?
    char c = N::f('a');     // Clearly ill-formed.

The key is that the wording talks about a “matching specialization,” which to me means that N::f<int> is befriended only if that specialization existed in N before the friend declaration. So it's ill-formed as written, but if we move the call to N::f<int> up to a point before the definition of A, it's well-formed.

That seems surprising, especially given that the first bullet does not require a pre-existing specialization. So I suggest replacing bullet 3 with something like:

Proposed resolution (June, 2010):

Change 14.5.4 [temp.friend] paragraph 1 bullet 3 as follows:

...For a friend function declaration that is not a template declaration:

(This resolution depends on that of issue 873; in particular, the cross-reference to 14.8.2.6 [temp.deduct.decl] refers to a new section added by the resolution of that issue.)




532. Member/nonmember operator template partial ordering

Section: 14.5.6.2  [temp.func.order]     Status: WP     Submitter: Nathan Sidwell     Date: 16 September 2005

[Voted into WP at August, 2010 meeting.]

The Standard does not specify how member and nonmember function templates are to be ordered. This question arises with an example like the following:

    struct A {
        template<class T> void operator<<(T&);
    };

    template<class T> struct B { };
    template<class T> void operator<<(A&, B<T>&);

    int main() {
        A a;
        B<A> b;
        a << b;
    }

The two candidates for “a << b” are:

  1. A::operator<< <B<A> >(B<A>&)
  2. ::operator<< <A>(A&, B<A>&)

How should we treat the implicit this parameter of #1 and the explicit first parameter of #2?

The difference between option 1 and option 2 can be seen in the following example:

    struct A { };

    template<class T> struct B {
        template<typename R> int operator*(R&);   // #1
    };

    template <typename T> int operator*(T&, A&);  // #2

    int main() {
        A a;
        B<A> b;
        b * a;
    }

Should this select #1, select #2, or be ambiguous? Option 1 will select #2, because “A&” is more specialized than “T&”. Option 2 will make this example ambiguous, because “B<A>&” is more specialized than “T&”.

If one were considering two non-member templates,

    template <typename T> int operator*(T&, A&);                 // #2
    template <typename T, typename R> int operator*(B<A>&, R&);  // #3

the current rules would make these unordered. Option 2 thus seems more consistent with this existing behavior.

Notes from the April, 2006 meeting:

The group favored option 2.

Proposed resolution (February, 2010):

Change 14.5.6.2 [temp.func.order] paragraph 3 as follows:

...and substitute it for each occurrence of that parameter in the function type of the template. If only one of the function templates is a non-static member, that function template is considered to have a new first parameter inserted in its function parameter list. The new parameter is of type “reference to cv A,” where cv are the cv-qualifiers of the function template (if any) and A is the class of which the function template is a member. [Note: This allows a non-static member to be ordered with respect to a nonmember function and for the results to be equivalent to the ordering of two equivalent nonmembers. —end note] [Example:

  struct A { };
  template<class T> struct B {
    template<typename R> int operator*(R&); // #1
  };

  template<typename T, typename R> int operator*(T&, R&); // #2

  // The declaration of B::operator* is transformed into the equivalent of
  // template<typename R> int operator*(B<A>&, R&);  // #1a

  int main() {
    A a;
    B<A> b;
    b * a;   // calls #1a
  }

end example]




448. Set of template functions in call with dependent explicit argument

Section: 14.6.1  [temp.local]     Status: WP     Submitter: Mark Mitchell     Date: 4 Jan 2004

[Voted into WP at August, 2010 meeting.]

Is this program valid?

  template <typename T> int g(int);
  class h{};
  template <typename T> int l(){h j; return g<T>(j);}
  template <typename T> int g(const h&);
  class j{};
  int jj(){return l<j>();}

The key issue is when "g" is looked up, i.e., whether both overloaded template "g" functions are available at the call site or only the first. Clearly, the entire postfix-expression "g<T>(j)" is dependent, but when is the set of available template functions determined?

For consistency with the rules about when the set of available overloads is determined when calling a function given by an unqualified-id, I would think that we should postpone determining the set of template functions if (and only if) any of the explicit template arguments are dependent.

John Spicer: I agree that there should be a core issue for this. The definition of "dependent name" (14.6.2 [temp.dep] paragraph 1) should probably be modified to cover this case. It currently only handles cases where the function name is a simple identifier.

Notes from the March 2004 meeting:

A related issue is a call with a qualified name and dependent arguments, e.g., x::y(depa, depb).

Proposed resolution (June, 2010):

  1. Change 14.6.2 [temp.dep] paragraph 1 as follows:

  2. ...In an expression of the form:

    where the postfix-expression is an unqualified-id id-expression, the unqualified-id id-expression denotes a dependent name if and only if any of the expressions in the expression-list is a type-dependent expression (14.6.2.2 [temp.dep.expr]) or if the unqualified-id of the id-expression is a template-id in which any of the template arguments depends on a template parameter. If an operand of an operator is a type-dependent expression, the operator also denotes a dependent name. Such names are unbound and are looked up at the point of the template instantiation (14.6.4.1 [temp.point]) in both the context of the template definition and the context of the point of instantiation.

  3. Change 14.6.4.2 [temp.dep.candidate] paragraph 1 as follows:

  4. For a function call that depends on a template parameter, if the function name is an unqualified-id or if the function is called using operator notation, the candidate functions are found using the usual lookup rules (3.4.1 [basic.lookup.unqual], 3.4.2 [basic.lookup.argdep], 3.4.3 [basic.lookup.qual]) except that:

    If the function name is an unqualified-id and the call would be ill-formed or would find a better match had the lookup within the associated namespaces considered all the function declarations with external linkage introduced in those namespaces in all translation units, not just considering those declarations found in the template definition and template instantiation contexts, then the program has undefined behavior.




458. Hiding of member template parameters by other members

Section: 14.6.1  [temp.local]     Status: WP     Submitter: Gabriel Dos Reis     Date: 2 Feb 2004

[Voted into WP at August, 2010 meeting.]

The list of cases in 14.6.1 [temp.local] about when a template parameter is hidden seems to be incomplete.

Consider

      // example-1
    struct S {
       int C;
       template<class> void f();
    };

    template<class C>
      void S::f()
      {
         C c;           // #1
      }

Someone asked whether line #1 is well-formed and I responded "no" based on my understanding of the rules in 14.6.1. After a second looking, I've realized that the above case is currently missing from the list.

The list in 14.6.1 covers cases like

     // example-2
   template<class T>
     struct S {
        int C;
        void f();
     };

   template<class C>
     void S<C>::f()
     {
       C c;     // ERROR: 'C' is 'S::C' not the template parameter
     }
or
     // example-3
   struct A { int C; }

   template<class C>
      struct S : A {
        C c;    // ERROR: 'C' is 'A::C', not the template parameter
      };
But the case of a 'member template' is missing. I believe it should follow the same rule as above. The reason is this.

In the case listed in 14.6.1 (having to do with members of classes), the "algorithm" seems to be this:

  1. put the "template parameter scope"[1] on the top of active scope stack. That will make the template parameter declarations the innermost bindings.
  2. Enter the class scope. That will push more scopes on the stack. In particular, any bindings from non-dependent base classes or from the class definition will hide any previous bindings, especially the template parameter declarations.
The above formulation uniformly covers paragraphs 5 and 7 of section 14.6.1 and gives a general view of how name lookup is supposed to happen.

I believe that any rule, coherent with 14.6.1/5 and 14.6.1/7, for covering the cases of member templates (example-1) will be described by the above "algorithm".

Am I missing something?

[1] of course, the standard text does not formally speak of "template parameter scope", but we all know that the template parameters "live" somewhere. I'm using that terminology to designate the declarative region of the template parameters.

Mike Miller: I have a somewhat different perspective on this question. I think your example-1 is fundamentally different from your example-2 and example-3. Looking, for instance, at your example-2, I see four nested scopes:

     namespace scope
       template scope (where the parameter is)
         class S scope
           S::f() block scope

Naturally, S::C hides the template parameter C. The same is true of your example-3, with three scopes:

     namespace scope
       template scope
         class S scope (includes 10.2 base class lookup)

Again, it's clear that the C inherited from A hides the template parameter in the containing scope.

The scopes I see in your example-1, however, are different:

     namespace scope
       struct S scope
         template scope (where the parameter is)
           S::f() block scope

Here it seems clear to me that the template parameter hides the class member.

It might help to look at the case where the function template is defined inline in the class:

     struct S {
        int C;
        template<class C> int f() {
            C c;   // #1
        }
     };

It would be pretty strange, I think, if the #1 C were the member and not the template parameter. It would also be odd if the name lookup were different between an inline definition and an out-of-line definition.

See also issue 459.

Notes from the March 2004 meeting:

Basically, the standard is okay. We think Gaby's desired cases like #1 should be ill-formed.

There is a wording problem in 14.6.1 [temp.local] paragraph 7. It says:

In the definition of a member of a class template that appears outside of the class template definition, the name of a member of this template hides the name of a template-parameter.

It should say "hides the name of a template-parameter of the class template (but not a template-parameter of the member, if the member is itself a template)" or words to that effect.

Proposed resolution (February, 2010):

Change 14.6.1 [temp.local] paragraph 8 as follows:

In the definition of a member of a class template that appears outside of the class template definition, the name of a member of this the class template hides the name of a template-parameter of any enclosing class templates (but not a template-parameter of the member, if the member is a class or function template). [Example:

  template<class T> struct A {
    struct B { /* ... */ };
    typedef void C;
    void f();
    template<class U> void g(U);
  };

  template<class B> void A<B>::f() {
    B b;   // A's B, not the template parameter
  }

  template<class B> template<class C> void A<B>::g(C) {
    B b;   // A's B, not the template parameter
    C c;   // the template parameter C, not A's C
  }



502. Dependency of nested enumerations and enumerators

Section: 14.6.2.1  [temp.dep.type]     Status: WP     Submitter: Mark Mitchell     Date: 05 Feb 2005

[Voted into WP at August, 2010 meeting.]

The Standard is currently silent on the dependency status of enumerations and enumerators that are members of class templates. There are three questions that must be answered in this regard:

  1. Are enumeration members of class templates dependent types?

    It seems clear that nested enumerations must be dependent. For example:

        void f(int);
    
        template<typename T> struct S {
            enum E { e0 };
            void g() {
                f(e0);
            }
        };
    
        void f(S<int>::E);
    
        void x() {
            S<int> si;
            si->g();       // Should call f(S<int>::E)
        }
    
  2. Is sizeof applied to a nested enumeration a value-dependent expression (14.6.2.3 [temp.dep.constexpr])?

    There are three distinct cases that might have different answers to this question:

    14.6.2.3 [temp.dep.constexpr] paragraph 2 says that a sizeof expression is value-dependent if the type of the operand is type-dependent. Unless enumerations are given special treatment, all three of these examples will have value-dependent sizes. This could be surprising for the first case, at least, if not the second as well.

  3. Are nested enumerators value-dependent expressions?

    Again the question of dependent initializers comes into play. As an example, consider:

        template<short I> struct S {
            enum E { e0, e1 = I, e2 };
        };
    

    There seem to be three possible approaches as to whether the enumerators of E are value-dependent:

    1. The enumerators of a nested enumeration are all value-dependent, regardless of whether they have a value-dependent initializer or not. This is the current position of 14.6.2.3 [temp.dep.constexpr] paragraph 2, which says that an identifier is value-dependent if it is a name declared with a dependent type.

    2. The enumerators of a nested enumeration are all value-dependent if any of the enumeration's enumerators has a value-dependent initializer. In this approach, e0 would be value-dependent, even though it is clear that it has the value 0.

    3. An enumerator of a nested enumeration is value-dependent only if it has a value-dependent initializer (explict or implicit). This approach would make e1 and e2 value-dependent, but not e0.

    An example that bears on the third approach is the following:

        template<typename T> struct S {
            enum E { N = UINT_MAX, O = T::O };
            int a[N + 2];
        };
    

    With the normal treatment of enumerations, the type of a might be either int[UINT_MAX+2] or int[1], depending on whether the value of T::O was such that the underlying type of E is unsigned int or long.

    One possibility for addressing this problem under the third approach would be to treat a given enumerator as having the type of its initializer in such cases, rather than the enumeration type. This would be similar to the way enumerators are treated within the enumerator list, before the enumeration declaration is complete (7.2 [dcl.enum] paragraph 5). The argument against this is that it makes arithmetic using enumerators behave differently when the enumeration is a member of a class template and when it is not.

Notes from the April, 2005 meeting:

The CWG agreed on the following positions:

  1. Nested enumerations are dependent types.

  2. The result of the sizeof operator applied to a nested enumeration is value-dependent unless there are no dependent initializers in its definition; the first case above is not dependent, while the second and third are dependent.

  3. The approach described in 3.C above is correct. This is similar to the treatment of static const integral data members, which are dependent only if their initializer is dependent.

Notes from the October, 2005 meeting:

There was no consensus among the CWG regarding question #3 (which enumerators should be considered value-dependent). The argument in favor of 3.C is principally that the values of enumerators with non-dependent initializers are known at definition time, so there is no need to treat them as dependent.

One objection to 3.C is that, according to the consensus of the CWG, the enumeration type is dependent and thus even the known values of the enumeration would have a dependent type, which could affect the results when such enumerations are used in expressions. A possible response to this concern would be to treat non-dependent initializers as having the type of the initializer rather than the enumeration type, similar to the treatment of enumerators within the enumerator-list (7.2 [dcl.enum] paragraph 5). However, this approach would be inconsistent with the treatment of other enumeration types. It would also interfere with overload resolution (e.g., the call in the example under question #1 above would resolve to f(int) with this approach rather than f(S<int>::E)).

Those in favor of option 3.A also suggested that it would be simpler and require less drafting: if all the enumerators have the (dependent) type of the enumeration, 14.6.2.3 [temp.dep.constexpr] paragraph 2 already says that a name with a dependent type is value-dependent, so nothing further would need to be said. Option 3.C would require additional caveats to exempt some enumerators.

The proponents of 3.A also pointed out that there are many other cases where a known value with a dependent type is treated as dependent:

    static const T t = 0;
    ... A<t> ...

or

    template <int I> void f() {
        g(I-I);
    }

With regard to current practice, g++ and MSVC++ implement 3.A, while EDG implements 3.C.

Notes from the July, 2009 meeting:

The consensus of the CWG was that all the types and values are dependent.

Proposed resolution (June, 2010):

Change 14.6.2.1 [temp.dep.type] paragraph 6 as follows:

A type is dependent if it is




546. Explicit instantiation of class template members

Section: 14.7.2  [temp.explicit]     Status: WP     Submitter: Martin Sebor     Date: 29 October 2005

[Voted into WP at August, 2010 meeting.]

Issue 470 specified the explicit instantiation of members of explicitly-instantiated class templates. In restricting the affected members to those “whose definition is visible at the point of instantiation,” however, this resolution introduced an incompatibility between explicitly instantiating a member function or static data member and explicitly instantiating the class template of which it is a member (14.7.2 [temp.explicit] paragraph 3 requires only that the class template definition, not that of the member function or static data member, be visible at the point of the explicit instantiation). It would be better to treat the member instantiations the same, regardless of whether they are directly or indirectly explicitly instantiated.

Notes from the April, 2006 meeting:

In forwarding document J16/06-0057 = WG21 N1987 to be approved by the full Committee, the CWG reaffirmed its position that explicitly instantiating a class template only explicitly instantiates those of its members that have been defined before the point of the explicit instantiation. The effect of the position advocated above would be to require all non-exported member functions to be defined in the translation unit in which the class template is explicitly instantiated (cf paragraph 4), and we did not want to require that. We did agree that the “visible” terminology should be replaced by wording along the lines of “has been defined.”

Proposed resolution (February, 2010):

Change 14.7.2 [temp.explicit] paragraph 8 as follows:

An explicit instantiation definition that names a class template specialization explicitly instantiates the class template specialization and is only an explicit instantiation definition of members whose definition is visible that have been defined at the point of instantiation.



605. Linkage of explicit specializations

Section: 14.7.3  [temp.expl.spec]     Status: WP     Submitter: Steve Clamage     Date: 30 November 2006

[Voted into WP at August, 2010 meeting.]

Given

    template <class T> static T f(T   t) { ... }
    template <>             int f(int t) { ... }

what is the linkage of f(int)?

Section 14 [temp] paragraph 4 says,

Entities generated from a template with internal linkage are distinct from all entities generated in other translation units.

But is the explicit specialization “generated from” the primary template? Does it inherit the local linkage? If so, where do I find a reference saying so explicitly?

James Widman: Data points: EDG 3.8 inherits, GCC 4.0 does not.

Mike Miller: There's a pretty strong presumption that the linkage of an explicit specialization cannot be different from that of its primary template, given that storage class specifiers cannot appear in an explicit specialization (7.1.1 [dcl.stc] paragraph 1).

Notes from the April, 2007 meeting:

The CWG agreed that the linkage of an explicit specialization must be that of the template. Gabriel dos Reis will investigate the reason for the different behavior of g++.

Proposed resolution (March, 2010):

Change 14 [temp] paragraph 4 as follows:

...Entities generated from Specializations (explicit or implicit) of a template with that has internal linkage are distinct from all entities generated specializations in other translation units...



621. Template argument deduction from function return types

Section: 14.7.3  [temp.expl.spec]     Status: WP     Submitter: Richard Corden     Date: 16 February 2007

[Voted into WP at August, 21010 meeting.]

It does not appear that the following example is well-formed, although most compilers accept it:

    template <typename T> T foo();
    template <> int foo();

The reason is that 14.7.3 [temp.expl.spec] paragraph 11 only allows trailing template-arguments to be omitted if they “can be deduced from the function argument type,” and there are no function arguments in this example.

14.7.3 [temp.expl.spec] should probably say “function type” instead of “function argument type.” Also, a subsection should probably be added to 14.8.2 [temp.deduct] to cover “Deducing template arguments from declarative contexts” or some such. It would be essentially the same as 14.8.2.2 [temp.deduct.funcaddr] except that the function type from the declaration would be used as the type of P.

Proposed resolution (March, 2008):

  1. Insert the following as a new subsection after 14.8.2.5 [temp.deduct.type]:

  2. 14.9.2.6 Deducing template arguments in a declaration that names a specialization of a function template [temp.deduct.funcdecl]

    Template arguments can be deduced from the function type specified when declaring a specialization of a function template. [Note: this can occur in the context of an explicit specialization, an explicit instantiation, or a friend declaration. —end note] The function template's function type and the declared type are used as the types of P and A, and the deduction is done as described in 14.8.2.5 [temp.deduct.type].

  3. Change 14.7.3 [temp.expl.spec] paragraph 11 as follows:

  4. A trailing template-argument can be left unspecified in the template-id naming an explicit function template specialization provided it can be deduced from the function argument type (14.9.2.6 [temp.deduct.funcdecl])...

Notes from the September, 2008 meeting:

The proposed resolution is probably more than is needed. Instead of a complete new section, the material could become a paragraph in 14.5.6 [temp.fct].

Proposed resolution (February, 2010):

Add the following paragraph at the end of 14.5.6 [temp.fct]:

In a declaration that names a specialization of a function template, template arguments can be deduced from the function type. [Note: this can occur in the context of an explicit specialization, an explicit instantiation, or a friend declaration. —end note] The function template's function type and the declared type are used as the types of P and A and the deduction is done as described in 14.8.2.5 [temp.deduct.type].

Proposed resolution (March, 2010):

This issue is resolved by the resolution of issue 873.




941. Explicit specialization of deleted function template

Section: 14.7.3  [temp.expl.spec]     Status: WP     Submitter: Spicer     Date: 14 July, 2009

[Voted into WP at August, 21010 meeting.]

According to 14.7.3 [temp.expl.spec] paragraph 1, only non-deleted function templates may be explicitly specialized. There doesn't appear to be a compelling need for this restriction, however, and it could be useful to forbid use of implicitly-instantiated specializations while still allowing use of explicitly-specialized versions.

Proposed resolution (February, 2010):

Change 14.7.3 [temp.expl.spec] paragraph 1 as follows:

An explicit specialization of any of the following:

can be declared...




575. Criteria for deduction failure

Section: 14.8.2  [temp.deduct]     Status: WP     Submitter: James Widman     Date: 19 April 2006

[Voted into WP at August, 2010 meeting.]

The last two sentences of 14.8.2 [temp.deduct] paragraph 5 read:

When all template arguments have been deduced or obtained from default template arguments, all uses of template parameters in non-deduced contexts are replaced with the corresponding deduced or default argument values. If the substitution results in an invalid type, as described above, type deduction fails.

Shouldn't the substitution occur for all uses of the parameters, so that any of them could result in deduction failure?

Proposed resolution (October, 2006):

Change 14.8.2 [temp.deduct] paragraph 5 as follows:

...When all template arguments have been deduced or obtained from default template arguments, all uses of template parameters in non-deduced contexts the function type are replaced with the corresponding deduced or default argument values. If the substitution results in an invalid type, as described above, type deduction fails.

Notes from the September, 2008 meeting:

This issue was returned to "drafting" status in order to coordinate the wording with the concepts proposal.

Proposed resolution (March, 2010):

Change 14.8.2 [temp.deduct] paragraph 5 as follows:

When all template arguments have been deduced or obtained from default template arguments, all uses of template parameters in non-deduced contexts the template parameter list of the template and the function type are replaced with the corresponding deduced or default argument values. If the substitution results in an invalid type, as described above, type deduction fails.



709. Enumeration names as nested-name-specifiers in deduction failure

Section: 14.8.2  [temp.deduct]     Status: WP     Submitter: Doug Gregor     Date: 23 Aug, 2008

[Voted into WP at August, 21010 meeting.]

The current rules in 14.8.2 [temp.deduct] say that type deduction fails as a result of attempting to use a type that is not a class type in a qualified name. However, it is now possible to use enumeration names as nested-name-specifiers, so this rule needs to be updated accordingly.

Proposed resolution (February, 2010):

Change the third bullet of the note in 14.8.2 [temp.deduct] paragraph 8 as follows:

[Note: Type deduction may fail for the following reasons:




873. Deducing rvalue references in declarative contexts

Section: 14.8.2.5  [temp.deduct.type]     Status: WP     Submitter: John Spicer     Date: 16 April, 2009

[Voted into WP at August, 21010 meeting.]

14.8.2.1 [temp.deduct.call] paragraph 3 gives the deduction of rvalue references special treatment in the context of a function call:

If P is of the form T&&, where T is a template parameter, and the argument is an lvalue, the type A& is used in place of A for type deduction.

A similar provision is needed, but is not present, in declarative contexts. For example:

    template<typename T> void f(T&&);
    template<> void f(int&) { }    // #1
    template<> void f(int&&) { }   // #2
    void g(int i) {
        f(i);    // calls f<int&>(int&), i.e., #1
        f(0);    // calls f<int>(int&&), i.e., #2
    }

There need to be rules that deduce the template arguments for the specializations in the same way that the arguments are deduced in the calls.

Proposed resolution (February, 2010):

  1. Change 14.8.2.5 [temp.deduct.type] paragraph 10 as follows:

  2. Similarly, if P has a form that contains (T), then each parameter type Pi of the respective parameter-type-list of P is compared with the corresponding parameter type Ai of the corresponding parameter-type-list of A. If P and A are function types that originated from deduction when taking the address of a function template (14.8.2.2 [temp.deduct.funcaddr]) or when deducing template arguments from a function declaration ([temp.deduct.decl]) and Pi and Ai are parameters of the top-level parameter-type-list of P and A, respectively, Pi is adjusted if it is an rvalue reference to a cv-unqualified template parameter and Ai is an lvalue reference, in which case the type of Pi is changed to be the template parameter type (i.e., T&& is changed to simply T). [Note: As a result, when Pi is T&& and Ai is X&, the adjusted Pi will be T, causing T to be deduced as X&. —end note] [Example:

      template<typename T> void f(T&&);
      template<> void f(int&) { }      // #1
      template<> void f(int&&) { }     // #2
      void g(int i) {
        f(i);   // calls f<int&>(int&), i.e., #1
        f(0);   // calls f<int>(int&&), i.e., #2
      }
    

    end example]

    If the parameter-declaration corresponding to Pi is a function parameter pack...

  3. Add a new section under 14.8.2 [temp.deduct], either before or after 14.8.2.5 [temp.deduct.type], as follows:

  4. 14.8.2.x Deducing template arguments from a function declaration [temp.deduct.decl]

    In a declaration whose declarator-id refers to a specialization of a function template, template argument deduction is performed to identify the specialization to which the declaration refers. Specifically, this is done for explicit instantiations (14.7.2 [temp.explicit]), explicit specializations (14.7.3 [temp.expl.spec]), and certain friend declarations (14.5.4 [temp.friend]). This is also done to determine whether a function template specialization matches a placement operator new (3.7.4.2 [basic.stc.dynamic.deallocation], 5.3.4 [expr.new]). In all these cases, P is the type of the function template being considered as a potential match and A is the function type from the declaration. The deduction is done as described in 14.8.2.5 [temp.deduct.type].

    If, for the set of function templates so considered, there is either no match or more than one match after partial ordering has been considered (14.5.6.2 [temp.func.order]), deduction fails and the declaration is ill-formed.

(Note that the resolution of issue 674 depends on this resolution.)




475. When is std::uncaught_exception() true? (take 2)

Section: 15.5.3  [except.uncaught]     Status: WP     Submitter: Martin Sebor     Date: 27 Sep 2004

[Voted into WP at August, 2010 meeting.]

See also issue 37.

Given this piece of code and S having a user-defined ctor, at precisely which point must std::uncaught_exception() return true and where false?

    try { S s0; throw s0; } catch (S s2) { }

My understanding of the semantics of the code is as follows:

  1. The throw expression creates a temporary for a copy of s0, say s1, using the copy ctor of S. In this invocation of the copy ctor uncaught_exception() must return true.
  2. s0 is destroyed during stack unwinding. In the invocation of S dtor uncaught_exception() must still return true.
  3. The variable s2 is initialized from s1 by invoking the copy ctor of S. In this invocation uncaught_exception() must also return true.
  4. s2 and s1 are destroyed. In the invocations of S dtor uncaught_exception() must return false.

Is my understanding correct?

15.1 [except.throw] paragraph 3 talks about “the exception object” when describing the semantics of the throw-expression:

a throw-expression initializes a temporary object, called the exception object...

However, 15.5.1 [except.terminate] paragraph 1 talks about “the expression to be thrown” when enumerating the conditions under which terminate() is called:

when the exception handling mechanism, after completing evaluation of the expression to be thrown but before the exception is caught (15.1 [except.throw]), calls a user function that exits via an uncaught exception...

And, 15.5.3 [except.uncaught] paragraph 1 refers to “the object to be thrown” in the description of uncaught_exception():

The function std::uncaught_exception() returns true after completing evaluation of the object to be thrown...

Are all these objects one and the same? I believe the answer is important in case the construction of the temporary exception object throws another exception.

Suppose they are the same. Then uncaught_exception() invoked from the copy ctor for s1 (from the example [above]) must return false and a new exception (e.g., bad_alloc) may be thrown and caught by a matching handler (i.e., without calling terminate()).

But if they are not the same, then uncaught_exception() invoked from the copy ctor for s1 must return true and throwing another exception would end up calling terminate(). This would, IMO, have pretty severe consequences on writing exception safe exception classes.

As in the first case, different compilers behave differently, with most compilers not calling terminate() when the ctor for the temporary exception object throws. Unfortunately, the two compilers that I trust the most do call terminate().

FWIW, my feeling is that it should be possible for the copy ctor invoked to initialize the temporary exception object to safely exit by throwing another exception, and that the new exception should be allowed to be caught without calling terminate.

Mike Miller: The way I see this, a throw-expression has an assignment-expression as an operand. This expression is “the expression to be thrown.” Evaluation of this expression yields an object; this object is “the object to be thrown.” This object is then copied to the exception object.

Martin Sebor: Here's a survey of the return value from uncaught_exception() in the various stages of exception handling, implemented by current compilers:

expr temp unwind handlr 2nd ex
HP aCC 6 0 0 1 0 OK
Compaq C++ 6.5 0 0 1 1 ABRT
EDG eccp 3.4 0 1 1 1 ABRT
g++ 3.4.2 0 0 1 0 OK
Intel C++ 7.0 0 0 1 0 OK
MIPSpro 7.4.1 0 0 1 1 ABRT
MSVC 7.0 0 0 1 0 OK
SunPro 5.5 1 1 1 0 OK
VisualAge 6.0 0 1 1 1 OK

In the table above:

Proposed resolution (October, 2004):

  1. Change 15.1 [except.throw] paragraph 3 as follows:

    A throw-expression initializes a temporary object, called the exception object, the by copying the thrown object (i.e., the result of evaluating its assignment-expression operand) to it. The type of which the exception object is determined by removing any top-level cv-qualifiers from the static type of the operand of throw and adjusting the type from “array of T” or “function returning T” to “pointer to T” or “pointer to function returning T,” respectively. [Note: the temporary object created for by a throw-expression that whose operand is a string literal is never of type char* or wchar_t*; that is, the special conversions for string literals from the types “array of const char” and “array of const wchar_t” to the types “pointer to char” and “pointer to wchar_t,” respectively (4.2 [conv.array]), are never applied to the operand of a throw-expression. —end note] The temporary is an lvalue and is used to initialize the variable named in the matching handler (15.3 [except.handle]). The type of the operand of a throw-expression shall not be an incomplete type, or a pointer to an incomplete type other than (possibly cv-qualified) void. [...]
  2. Change the note in 15.3 [except.handle] paragraph 3 as follows:

    [Note: a throw-expression operand that which is an integral constant expression of integer type that evaluates to zero does not match a handler of pointer type; that is, the null pointer constant conversions (4.10 [conv.ptr], 4.11 [conv.mem]) do not apply. —end note]
  3. Change 15.5.1 [except.terminate] paragraph 1 bullet 1 as follows:

    when the exception handling mechanism, after completing evaluation of the expression to be thrown operand of throw but before the exception is caught (15.1 [except.throw]), calls a user function that exits via an uncaught exception,
  4. Change 15.5.3 [except.uncaught] paragraph 1 as follows:

    The function std::uncaught_exception() returns true after completing evaluation of the object to be thrown operand of throw until completing the initialization of the exception-declaration in the matching handler (18.8.5 [uncaught]).
  5. Change 18.8.5 [uncaught] paragraph 1 by adding the indicated words:

    Returns: true after completing evaluation of the operand of a throw-expression until either completing initialization of the exception-declaration in the matching handler or entering unexpected() due to the throw; or after entering terminate() for any reason other than an explicit call to terminate(). [Note: This includes stack unwinding (15.2 [except.ctor]). —end note]

Notes from the April, 2005 meeting:

The CWG discussed this resolution both within the group and with other interested parties. Among the points that were made:

The CWG felt that more input from a wider audience was necessary before a decision could be made on the appropriate resolution.

Notes from the April, 2006 meeting:

The CWG agreed with the position that std::uncaught_exception() should return false during the copy to the exception object and that std::terminate() should not be called if that constructor exits with an exception. The issue was returned to “drafting” status for rewording to reflect this position.

Additional notes (September, 2007):

Although this issue deals primarily with when std::uncaught_exception() begins to return true, the specification of when it begins to return false is also problematic. There are two parallel sections that define the meaning of std::uncaught_exception() and each has a different problem. 15.5.3 [except.uncaught] reads,

The function std::uncaught_exception() returns true after completing evaluation of the object to be thrown until completing the initialization of the exception-declaration in the matching handler (18.8.5 [uncaught]).

The problem here is that whether an exception is considered caught (the underlying condition tested by the function) is here presented in terms of having initialized the exception-declaration, while in other places it is specified by having an active handler for the exception, e.g., 15.1 [except.throw] paragraph 6:

An exception is considered caught when a handler for that exception becomes active (15.3 [except.handle]).

This distinction is important because of 15.3 [except.handle] paragraph 3:

A handler is considered active when initialization is complete for the formal parameter (if any) of the catch clause. [Note: the stack will have been unwound at that point. —end note] Also, an implicit handler is considered active when std::terminate() or std::unexpected() is entered due to a throw.

Note that there is no exception-declaration to be initialized for the std::terminate() and std::unexpected() cases; nevertheless, according to 18.8.5 [uncaught], std::uncaught_exception() is supposed to return false when one of those two functions is entered.

The specification in 18.8.5 [uncaught] is not well phrased, however, and is open to misinterpretation. It reads,

Returns: true after completing evaluation of a throw-expression until either completing initialization of the exception-declaration in the matching handler or entering unexpected() due to the throw; or after entering terminate() for any reason other than an explicit call to terminate().

The problem here is lack of parallelism: does “after entering terminate” refer to the condition for returning true or false? This would be better phrased along the lines of

Returns: true after completing evaluation of a throw-expression until a handler for the exception becomes active (15.3 [except.handle]).

Proposed resolution (March, 2010):

  1. Change 15.5.1 [except.terminate] paragraph 1 bullet 1 as follows:

  2. In the following situations exception handling must be abandoned for less subtle error handling techniques:

  3. Change 15.5.3 [except.uncaught] paragraph 1 as follows:

  4. The function std::uncaught_exception() returns true after completing evaluation of the object to be thrown the initialization of the exception object (15.1 [except.throw]) until completing the initialization of the exception-declaration in the matching handler activation of a handler for the exception (15.3 [except.handle], 18.8.5 [uncaught])...
  5. Change 18.8.5 [uncaught] paragraph 1 as follows:

  6. Returns: true after completing evaluation of a throw-expression initializing an exception object 15.1 [except.throw] until either completing initialization of the exception-declaration in the matching handler or entering unexpected() due to the throw; or after entering terminate() for any reason other than an explicit call to terminate() a handler for the exception (including unexpected() or terminate()) is activated (15.3 [except.handle]). [Note: This includes stack unwinding (15.2 [except.ctor]). —end note]





Issues with "CD1" Status


663. Valid Cyrillic identifier characters

Section: _N2691_.E  [extendid]     Status: CD1     Submitter: Steve Clamage     Date: 30 November 2007

[Voted into the WP at the June, 2008 meeting.]

The C99 and C++ Standards disagree about the validity of two Cyrillic characters for use in identifiers. C++ (_N2691_.E [extendid]) says that 040d is valid in an identifier but that 040e is not; C99 (Annex D) says exactly the opposite. In fact, both characters should be accepted in identifiers; see the Unicode chart.

Proposed resolution (February, 2008):

The reference in paragraph 2 should be changed to ISO/IEC TR 10176:2003 and the table should be changed to conform to the one in that document (beginning on page 34).




357. Definition of signature should include name

Section: 1.3  [intro.defs]     Status: CD1     Submitter: Steve Clamage     Date: 26 May 2002

[Voted into WP at April, 2007 meeting.]

Section 1.3 [intro.defs], definition of "signature" omits the function name as part of the signature. Since the name participates in overload resolution, shouldn't it be included in the definition? I didn't find a definition of signature in the ARM, but I might have missed it.

Fergus Henderson: I think so. In particular, 17.6.3.3.2 [global.names] reserves certain "function signatures" for use by the implementation, which would be wrong unless the signature includes the name.

-2- Each global function signature declared with external linkage in a header is reserved to the implementation to designate that function signature with external linkage.

-5- Each function signature from the Standard C library declared with external linkage is reserved to the implementation for use as a function signature with both extern "C" and extern "C++" linkage, or as a name of namespace scope in the global namespace.

Other uses of the term "function signature" in the description of the standard library also seem to assume that it includes the name.

James Widman:

Names don't participate in overload resolution; name lookup is separate from overload resolution. However, the word “signature” is not used in clause 13 [over]. It is used in linkage and declaration matching (e.g., 14.5.6.1 [temp.over.link]). This suggests that the name and scope of the function should be part of its signature.

Proposed resolution (October, 2006):

  1. Replace 1.3 [intro.defs] “signature” with the following:

  2. the name and the parameter-type-list (8.3.5 [dcl.fct]) of a function, as well as the class or namespace of which it is a member. If a function or function template is a class member its signature additionally includes the cv-qualifiers (if any) on the function or function template itself. The signature of a function template additionally includes its return type and its template parameter list. The signature of a function template specialization includes the signature of the template of which it is a specialization and its template arguments (whether explicitly specified or deduced). [Note: Signatures are used as a basis for name-mangling and linking. —end note]
  3. Delete paragraph 3 and replace the first sentence of 14.5.6.1 [temp.over.link] as follows:

  4. The signature of a function template specialization consists of the signature of the function template and of the actual template arguments (whether explicitly specified or deduced).

    The signature of a function template consists of its function signature, its return type and its template parameter list is defined in 1.3 [intro.defs]. The names of the template parameters are significant...

(See also issue 537.)




537. Definition of “signature”

Section: 1.3  [intro.defs]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 12 October 2005

[Voted into WP at April, 2007 meeting.]

The standard defines “signature” in two places: 1.3 [intro.defs] and 14.5.6.1 [temp.over.link] paragraphs 3-4. The former seems to be meant as a formal definition (I think it's the only place covering the nontemplate case), yet it lacks some bits mentioned in the latter (specifically, the notion of a “signature of a function template,” which is part of every signature of the associated function template specializations).

Also, I think the 1.3 [intro.defs] words “the information about a function that participates in overload resolution” isn't quite right either. Perhaps, “the information about a function that distinguishes it in a set of overloaded functions?”

Eric Gufford:

In 1.3 [intro.defs] the definition states that “Function signatures do not include return type, because that does not participate in overload resolution,” while 14.5.6.1 [temp.over.link] paragraph 4 states “The signature of a function template consists of its function signature, its return type and its template parameter list.” This seems inconsistent and potentially confusing. It also seems to imply that two identical function templates with different return types are distinct signatures, which is in direct violation of 13.3 [over.match]. 14.5.6.1 [temp.over.link] paragraph 4 should be amended to include verbiage relating to overload resolution.

Either return types are included in function signatures, or they're not, across the board. IMHO, they should be included as they are an integral part of the function declaration/definition irrespective of overloads. Then verbiage should be added about overload resolution to distinguish between signatures and overload rules. This would help clarify things, as it is commonly understood that overload resolution is based on function signature.

In short, the term “function signature” should be made consistent, and removed from its (implicit, explicit or otherwise) linkage to overload resolution as it is commonly understood.

James Widman:

The problem is that (a) if you say the return type is part of the signature of a non-template function, then you have overloading but not overload resolution on return types (i.e., what we have now with function templates). I don't think anyone wants to make the language uglier in that way. And (b) if you say that the return type is not part of the signature of a function template, you will break code. Given those alternatives, it's probably best to maintain the status quo (which the implementors appear to have rendered faithfully).

Proposed resolution (September, 2006):

This issue is resolved by the resolution of issue 357.




513. Non-class “most-derived” objects

Section: 1.8  [intro.object]     Status: CD1     Submitter: Marc Schoolderman     Date: 20 Mar 2005

[Voted into WP at April, 2006 meeting.]

The standard uses “most derived object” in some places (for example, 1.3 [intro.defs] “dynamic type,” 5.3.5 [expr.delete]) to refer to objects of both class and non-class type. However, 1.8 [intro.object] only formally defines it for objects of class type.

Possible fix: Change the wording in 1.8 [intro.object] paragraph 4 from

an object of a most derived class type is called a most derived object

to

an object of a most derived class type, or of non-class type, is called a most derived object

Proposed resolution (October, 2005):

Add the indicated words to 1.8 [intro.object] paragraph 4:

If a complete object, a data member (9.2 [class.mem]), or an array element is of class type, its type is considered the most derived class, to distinguish it from the class type of any base class subobject; an object of a most derived class type, or of a non-class type, is called a most derived object.



637. Sequencing rules and example disagree

Section: 1.9  [intro.execution]     Status: CD1     Submitter: Ofer Porat     Date: 2 June 2007

[Voted into the WP at the September, 2008 meeting.]

In 1.9 [intro.execution] paragraph 16, the following expression is still listed as an example of undefined behavior:

    i = ++i + 1;

However, it appears that the new sequencing rules make this expression well-defined:

  1. The assignment side-effect is required to be sequenced after the value computations of both its LHS and RHS (5.17 [expr.ass] paragraph 1).

  2. The LHS (i) is an lvalue, so its value computation involves computing the address of i.

  3. In order to value-compute the RHS (++i + 1), it is necessary to first value-compute the lvalue expression ++i and then do an lvalue-to-rvalue conversion on the result. This guarantees that the incrementation side-effect is sequenced before the computation of the addition operation, which in turn is sequenced before the assignment side effect. In other words, it yields a well-defined order and final value for this expression.

It should be noted that a similar expression

    i = i++ + 1;

is still not well-defined, since the incrementation side-effect remains unsequenced with respect to the assignment side-effect.

It's unclear whether making the expression in the example well-defined was intentional or just a coincidental byproduct of the new sequencing rules. In either case either the example should be fixed, or the rules should be changed.

Clark Nelson: In my opinion, the poster's argument is perfectly correct. The rules adopted reflect the CWG's desired outcome for issue 222. At the Portland meeting, I presented (and still sympathize with) Tom Plum's case that these rules go a little too far in nailing down required behavior; this is a consequence of that.

One way or another, a change needs to be made, and I think we should seriously consider weakening the resolution of issue 222 to keep this example as having undefined behavior. This could be done fairly simply by having the sequencing requirements for an assignment expression depend on whether it appears in an lvalue context.

James Widman: How's this for a possible re-wording?

In all cases, the side effect of the assignment expression is sequenced after the value computations of the right and left operands. Furthermore, if the assignment expression appears in a context where an lvalue is required, the side effect of the assignment expression is sequenced before its value computation.

Notes from the February, 2008 meeting:

There was no real support in the CWG for weakening the resolution of issue 222 and returning the example to having undefined behavior. No one knew of an implementation that doesn't already do the (newly) right thing for such an example, so there was little motivation to go out of our way to increase the domain of undefined behavior. So the proposed resolution is to change the example to one that definitely does have undependable behavior in existing practice, and undefined behavior under the new rules.

Also, the new formulation of the sequencing rules approved in Oxford contained the wording that by and large resolved issue 222, so with the resolution of this issue, we can also close issue 222.

Proposed resolution (March, 2008):

Change the example in 1.9 [intro.execution] paragraph 16 as follows:

    i = v[i++];             // the behavior is undefined
    i = 7, i++, i++;        // i becomes 9
    i = ++i i++ + 1;        // the behavior is undefined
    i = i + 1;              // the value of i is incremented



639. What makes side effects “different” from one another?

Section: 1.9  [intro.execution]     Status: CD1     Submitter: James Widman     Date: 26 July 2007

[Voted into the WP at the September, 2008 meeting.]

Is the behavior undefined in the following example?

    void f() {
         int n = 0;
         n = --n;
    }

1.9 [intro.execution] paragraph 16 says,

If a side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined.

It's not clear to me whether the two side-effects in n=--n are “different.” As far as I can tell, it seems that both side-effects involve the assignment of -1 to n, which in a sense makes them non-“different.” But I don't know if that's the intent. Would it be better to say “another” instead of “a different?”

On a related note, can we include this example to illustrate?

    void f( int, int );
    void g( int a ) { f( a = -1, a = -1 ); } // Undefined?

Proposed resolution (March, 2008):

Change 1.9 [intro.execution] paragraph 16 as follows:

...If a side effect on a scalar object is unsequenced relative to either a different another side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined. [Example:

    void f(int, int);
    void g(int i, int* v) {
        i = v[i++];         // the behavior is undefined
        i = 7, i++, i++;    // i becomes 9

        i = ++i + 1;        // the behavior is undefined
        i = i + 1;          // the value of i is incremented

        f(i = -1, i = -1);  // the behavior is undefined
    }

end example] When calling...




362. Order of initialization in instantiation units

Section: 2.2  [lex.phases]     Status: CD1     Submitter: Mark Mitchell     Date: 2 July 2002

[Voted into WP at March 2004 meeting.]

Should this program do what its author obviously expects? As far as I can tell, the standard says that the point of instantiation for Fib<n-1>::Value is the same as the point of instantiation as the enclosing specialization, i.e., Fib<n>::Value. What in the standard actually says that these things get initialized in the right order?

  template<int n>
  struct Fib { static int Value; };

  template <>
  int Fib<0>::Value = 0;

  template <>
  int Fib<1>::Value = 1;

  template<int n>
  int Fib<n>::Value = Fib<n-1>::Value + Fib<n-2>::Value;

  int f ()
  {
    return Fib<40>::Value;
  }

John Spicer: My opinion is that the standard does not specify the behavior of this program. I thought there was a core issue related to this, but I could not find it. The issue that I recall proposed tightening up the static initialization rules to make more cases well defined.

Your comment about point of instantiation is correct, but I don't think that really matters. What matters is the order of execution of the initialization code at execution time. Instantiations don't really live in "translation units" according to the standard. They live in "instantiation units", and the handling of instantiation units in initialization is unspecified (which should probably be another core issue). See 2.2 [lex.phases] paragraph 8.

Notes from October 2002 meeting:

We discussed this and agreed that we really do mean the the order is unspecified. John Spicer will propose wording on handling of instantiation units in initialization.

Proposed resolution (April 2003):

TC1 contains the following text in 3.6.2 [basic.start.init] paragraph 1:

Objects with static storage duration defined in namespace scope in the same translation unit and dynamically initialized shall be initialized in the order in which their definition appears in the translation unit.

This was revised by issue 270 to read:

Dynamic initialization of an object is either ordered or unordered. Explicit specializations and definitions of class template static data members have ordered initialization. Other class template static data member instances have unordered initialization. Other objects defined in namespace scope have ordered initialization. Objects defined within a single translation unit and with ordered initialization shall be initialized in the order of their definitions in the translation unit. The order of initialization is unspecified for objects with unordered initialization and for objects defined in different translation units.

This addresses this issue but while reviewing this issue some additional changes were suggested for the above wording:

Dynamic initialization of an object is either ordered or unordered. Definitions of explicitly specialized Explicit specializations and definitions of class template static data members have ordered initialization. Other class template static data members (i.e., implicitly or explicitly instantiated specializations) instances have unordered initialization. Other objects defined in namespace scope have ordered initialization. Objects defined within a single translation unit and with ordered initialization shall be initialized in the order of their definitions in the translation unit. The order of initialization is unspecified for objects with unordered initialization and for objects defined in different translation units.



558. Excluded characters in universal character names

Section: 2.3  [lex.charset]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 8 February 2006

[Moved to DR at October 2007 meeting.]

C99 and C++ differ in their approach to universal character names (UCNs).

Issue 248 already covers the differences in UCNs allowed for identifiers, but a more fundamental issue is that of UCNs that correspond to codes reserved by ISO 10676 for surrogate pair forms.

Specifically, C99 does not allow UCNs whose short names are in the range 0xD800 to 0xDFFF. I think C++ should have the same constraint. If someone really wants to place such a code in a character or string literal, they should use a hexadecimal escape sequence instead, for example:

    wchar_t  w1 = L'\xD900'; // Okay.
    wchar_t  w2 = L'\uD900'; // Error, not a valid character.

(Compare 6.4.3 paragraph 2 in ISO/IEC 9899/1999 with 2.3 [lex.charset] paragraph 2 in the C++ standard.)

Proposed resolution (October, 2007):

This issue is resolved by the adoption of paper J16/07-0030 = WG21 N2170.




505. Conditionally-supported behavior for unknown character escapes

Section: 2.14.3  [lex.ccon]     Status: CD1     Submitter: Mike Miller     Date: 14 Apr 2005

[Voted into WP at the October, 2006 meeting.]

The current wording of 2.14.3 [lex.ccon] paragraph 3 states,

If the character following a backslash is not one of those specified, the behavior is undefined.

Paper J16/04-0167=WG21 N1727 suggests that such character escapes be ill-formed. In discussions at the Lillehammer meeting, however, the CWG felt that the newly-approved category of conditionally-supported behavior would be more appropriate.

Proposed resolution (April, 2006):

Change the next-to-last sentence of 2.14.3 [lex.ccon] paragraph 3 from:

If the character following a backslash is not one of those specified, the behavior is undefined.

to:

Escape sequences in which the character following the backslash is not listed in Table 6 are conditionally-supported, with implementation-defined semantics.



309. Linkage of entities whose names are not simply identifiers, in introduction

Section: 3  [basic]     Status: CD1     Submitter: Mike Miller     Date: 17 Sep 2001

[Voted into the WP at the June, 2008 meeting.]

3 [basic] paragraph 8, while not incorrect, does not allow for linkage of operators and conversion functions. It says:

An identifier used in more than one translation unit can potentially refer to the same entity in these translation units depending on the linkage (3.5 [basic.link]) of the identifier specified in each translation unit.

Proposed Resolution (November, 2006):

This issue is resolved by the proposed resolution of issue 485.




485. What is a “name”?

Section: 3  [basic]     Status: CD1     Submitter: Gabriel Dos Reis     Date: 9 Nov 2004

[Voted into the WP at the June, 2008 meeting.]

Clause 3 [basic] paragraph 4 says:

A name is a use of an identifier (2.11 [lex.name]) that denotes an entity or label (6.6.4 [stmt.goto], 6.1 [stmt.label]).

Just three paragraphs later, it says

Two names are the same if

The last two bullets contradict the definition of name in paragraph 4 because they are not identifiers.

This definition affects other parts of the Standard, as well. For example, in 3.4.2 [basic.lookup.argdep] paragraph 1,

When an unqualified name is used as the postfix-expression in a function call (5.2.2 [expr.call]), other namespaces not considered during the usual unqualified lookup (3.4.1 [basic.lookup.unqual]) may be searched, and in those namespaces, namespace-scope friend function declarations (11.4 [class.friend]) not otherwise visible may be found.

With the current definition of name, argument-dependent lookup apparently does not apply to function-notation calls to overloaded operators.

Another related question is whether a template-id is a name or not and thus would trigger an argument-dependent lookup. Personally, I have always viewed a template-id as a name, just like operator+.

Proposed Resolution (November, 2006):

  1. Change clause 3 [basic] paragraphs 3-8 as follows:

    1. An entity is a value, object, subobject, base class subobject, array element, variable, reference, function, instance of a function, enumerator, type, class member, template, template specialization, namespace, or parameter pack.

    2. A name is a use of an identifier identifier (2.11 [lex.name]), operator-function-id (13.5 [over.oper]), conversion-function-id (12.3.2 [class.conv.fct]), or template-id (14.2 [temp.names]) that denotes an entity or label (6.6.4 [stmt.goto], 6.1 [stmt.label]). A variable is introduced by the declaration of an object. The variable’s name denotes the object.

    3. Every name that denotes an entity is introduced by a declaration. Every name that denotes a label is introduced either by a goto statement (6.6.4 [stmt.goto]) or a labeled-statement (6.1 [stmt.label]).

    4. A variable is introduced by the declaration of an object. The variable's name denotes the object.

    5. Some names denote types, classes, enumerations, or templates. In general, it is necessary to determine whether or not a name denotes one of these entities before parsing the program that contains it. The process that determines this is called name lookup (3.4 [basic.lookup]).

    6. Two names are the same if

      • they are identifiers identifiers composed of the same character sequence; or

      • they are the names of overloaded operator functions operator-function-ids formed with the same operator; or

      • they are the names of user-defined conversion functions conversion-function-ids formed with the same type., or

      • they are template-ids that refer to the same class or function (14.4 [temp.type]).

    7. An identifier A name used in more than one translation unit can potentially refer to the same entity in these translation units depending on the linkage (3.5 [basic.link]) of the identifier name specified in each translation unit.

  2. Change 3.3.7 [basic.scope.class] paragraph 1 item 5 as follows:

    The potential scope of a declaration that extends to or past the end of a class definition also extends to the regions defined by its member definitions, even if the members are defined lexically outside the class (this includes static data member definitions, nested class definitions, member function definitions (including the member function body and any portion of the declarator part of such definitions which follows the identifier declarator-id, including a parameter-declaration-clause and any default arguments (8.3.6 [dcl.fct.default]).

    [Drafting note: This last change is not really mandated by the issue, but it's another case of “identifier” confusion.]

(This proposed resolution also resolves issue 309.)




261. When is a deallocation function "used?"

Section: 3.2  [basic.def.odr]     Status: CD1     Submitter: Mike Miller     Date: 7 Nov 2000

[Moved to DR at October 2002 meeting.]

3.2 [basic.def.odr] paragraph 2 says that a deallocation function is "used" by a new-expression or delete-expression appearing in a potentially-evaluated expression. 3.2 [basic.def.odr] paragraph 3 requires only that "used" functions be defined.

This wording runs afoul of the typical implementation technique for polymorphic delete-expressions in which the deallocation function is invoked from the virtual destructor of the most-derived class. The problem is that the destructor must be defined, because it's virtual, and if it contains an implicit reference to the deallocation function, the deallocation function must also be defined, even if there are no relevant new-expressions or delete-expressions in the program.

For example:

        struct B { virtual ~B() { } };

        struct D: B {
            void operator delete(void*);
            ~D() { }
        };

Is it required that D::operator delete(void*) be defined, even if no B or D objects are ever created or deleted?

Suggested resolution: Add the words "or if it is found by the lookup at the point of definition of a virtual destructor (12.4 [class.dtor])" to the specification in 3.2 [basic.def.odr] paragraph 2.

Notes from 04/01 meeting:

The consensus was in favor of requiring that any declared non-placement operator delete member function be defined if the destructor for the class is defined (whether virtual or not), and similarly for a non-placement operator new if a constructor is defined.

Proposed resolution (10/01):

In 3.2 [basic.def.odr] paragraph 2, add the indicated text:

An allocation or deallocation function for a class is used by a new expression appearing in a potentially-evaluated expression as specified in 5.3.4 [expr.new] and 12.5 [class.free]. A deallocation function for a class is used by a delete expression appearing in a potentially-evaluated expression as specified in 5.3.5 [expr.delete] and 12.5 [class.free]. A non-placement allocation or deallocation function for a class is used by the definition of a constructor of that class. A non-placement deallocation function for a class is used by the definition of the destructor of that class, or by being selected by the lookup at the point of definition of a virtual destructor (12.4 [class.dtor]). [Footnote: An implementation is not required to call allocation and deallocation functions from constructors or destructors; however, this is a permissible implementation technique.]




289. Incomplete list of contexts requiring a complete type

Section: 3.2  [basic.def.odr]     Status: CD1     Submitter: Mike Miller     Date: 25 May 2001

[Moved to DR at October 2002 meeting.]

3.2 [basic.def.odr] paragraph 4 has a note listing the contexts that require a class type to be complete. It does not list use as a base class as being one of those contexts.

Proposed resolution (10/01):

In 3.2 [basic.def.odr] paragraph 4 add a new bullet at the end of the note as the next-to-last bullet:




433. Do elaborated type specifiers in templates inject into enclosing namespace scope?

Section: 3.3.2  [basic.scope.pdecl]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 2 September 2003

[Voted into WP at March 2004 meeting.]

Consider the following translation unit:

  template<class T> struct S {
    void f(union U*);  // (1)
  };
  template<class T> void S<T>::f(union U*) {}  // (2)
  U *p;  // (3)

Does (1) introduce U as a visible name in the surrounding namespace scope?

If not, then (2) could presumably be an error since the "union U" in that definition does not find the same type as the declaration (1).

If yes, then (3) is OK too. However, we have gone through much trouble to allow template implementations that do not pre-parse the template definitions, but requiring (1) to be visible would change that.

A slightly different case is the following:

  template<typename> void f() { union U *p; }
  U *q;  // Should this be valid?

Notes from October 2003 meeting:

There was consensus that example 1 should be allowed. (Compilers already parse declarations in templates; even MSVC++ 6.0 accepts this case.) The vote was 7-2.

Example 2, on the other hand, is wrong; the union name goes into a block scope anyway.

Proposed resolution:

In 3.3.2 [basic.scope.pdecl] change the second bullet of paragraph 5 as follows:

for an elaborated-type-specifier of the form
   class-key identifier
if the elaborated-type-specifier is used in the decl-specifier-seq or parameter-declaration-clause of a function defined in namespace scope, the identifier is declared as a class-name in the namespace that contains the declaration; otherwise, except as a friend declaration, the identifier is declared in the smallest non-class, non-function-prototype scope that contains the declaration. [Note: These rules also apply within templates.] [Note: ...]



432. Is injected class name visible in base class specifier list?

Section: 3.3.7  [basic.scope.class]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 29 August 2003

[Voted into WP at March 2004 meeting.]

Consider the following example (inspired by a question from comp.lang.c++.moderated):

  template<typename> struct B {};
  template<typename T> struct D: B<D> {};

Most (all?) compilers reject this code because D is handled as a template name rather than as the injected class name.

9 [class]/2 says that the injected class name is "inserted into the scope of the class."

3.3.7 [basic.scope.class]/1 seems to be the text intended to describe what "scope of a class" means, but it assumes that every name in that scope was introduced using a "declarator". For an implicit declaration such as the injected-class name it is not clear what that means.

So my questions:

  1. Should the injected class name be available in the base class specifiers?
    John Spicer: I do not believe the injected class name should be available in the base specifier. I think the semantics of injected class names should be as if a magic declaration were inserted after the opening "{" of the class definition. The injected class name is a member of the class and members don't exist at the point where the base specifiers are scanned.
  2. Do you agree the wording should be clarified whatever the answer to the first question?
    John Spicer: I believe the 3.3.7 [basic.scope.class] wording should be updated to reflect the fact that not all names come from declarators.

Notes from October 2003 meeting:

We agree with John Spicer's suggested answers above.

Proposed Resolution (October 2003):

The answer to question 1 above is No and no change is required.

For question 1, change 3.3.7 [basic.scope.class] paragraph 1 rule 1 to:

1) The potential scope of a name declared in a class consists not only of the declarative region following the name's point of declaration declarator, but also of all function bodies, default arguments, and constructor ctor-initializers in that class (including such things in nested classes). The point of declaration of an injected-class-name (clause 9 [class]) is immediately following the opening brace of the class definition.

(Note that this change overlaps a change in issue 417.)

Also change 3.3.2 [basic.scope.pdecl] by adding a new paragraph 8 for the injected-class-name case:

The point of declaration for an injected-class-name (clause 9 [class]) is immediately following the opening brace of the class definition.

Alternatively this paragraph could be added after paragraph 5 and before the two note paragraphs (i.e. it would become paragraph 5a).




139. Error in friend lookup example

Section: 3.4.1  [basic.lookup.unqual]     Status: CD1     Submitter: Mike Miller     Date: 14 Jul 1999

[Moved to DR at 10/01 meeting.]

The example in 3.4.1 [basic.lookup.unqual] paragraph 3 is incorrect:

    typedef int f;
    struct A {
        friend void f(A &);
        operator int();
        void g(A a) {
            f(a);
        }
    };
Regardless of the resolution of other issues concerning the lookup of names in friend declarations, this example is ill-formed (the function and the typedef cannot exist in the same scope).

One possible repair of the example would be to make f a class with a constructor taking either A or int as its parameter.

(See also issues 95, 136, 138, 143, 165, and 166.)

Proposed resolution (04/01):

  1. Change the example in 3.4.1 [basic.lookup.unqual] paragraph 3 to read:

        typedef int f;
        namespace N {
            struct A {
                friend int f(A &);
                operator int();
                void g(A a) {
                    int i = f(a);
                          // f is the typedef, not the friend function:
                          // equivalent to int(a)
                }
            };
        }
    
  2. Delete the sentence immediately following the example:

    The expression f(a) is a cast-expression equivalent to int(a).



514. Is the initializer for a namespace member in the scope of the namespace?

Section: 3.4.1  [basic.lookup.unqual]     Status: CD1     Submitter: Mike Miller     Date: 24 Mar 2005

[Voted into WP at the October, 2006 meeting.]

Is the following code well-formed?

    namespace N {
      int i;
      extern int j;
    }
    int N::j = i;

The question here is whether the lookup for i in the initializer of N::j finds the declaration in namespace N or not. Implementations differ on this question.

If N::j were a static data member of a class, the answer would be clear: both 3.4.1 [basic.lookup.unqual] paragraph 12 and 8.5 [dcl.init] paragraph 11 say that the initializer “is in the scope of the member's class.” There is no such provision for namespace members defined outside the namespace, however.

The reasoning given in 3.4.1 [basic.lookup.unqual] may be instructive:

A name used in the definition of a static data member of class X (9.4.2 [class.static.data]) (after the qualified-id of the static member) is looked up as if the name was used in a member function of X.

It is certainly the case that a name used in a function that is a member of a namespace is looked up in that namespace (3.4.1 [basic.lookup.unqual] paragraph 6), regardless of whether the definition is inside or outside that namespace. Initializers for namespace members should probably be looked up the same way.

Proposed resolution (April, 2006):

Add a new paragraph following 3.4.1 [basic.lookup.unqual] paragraph 12:

If a variable member of a namespace is defined outside of the scope of its namespace then any name used in the definition of the variable member (after the declarator-id) is looked up as if the definition of the variable member occurred in its namespace. [Example:

    namespace N {
      int i = 4;
      extern int j;
    }

    int i = 2;

    int N::j = i;	// N::j == 4

end example]




143. Friends and Koenig lookup

Section: 3.4.2  [basic.lookup.argdep]     Status: CD1     Submitter: Mike Miller     Date: 21 Jul 1999

[Moved to DR at 4/02 meeting.]

Paragraphs 1 and 2 of 3.4.2 [basic.lookup.argdep] say, in part,

When an unqualified name is used as the postfix-expression in a function call (5.2.2 [expr.call] )... namespace-scope friend function declarations (11.4 [class.friend] ) not otherwise visible may be found... the set of declarations found by the lookup of the function name [includes] the set of declarations found in the... classes associated with the argument types.
The most straightforward reading of this wording is that if a function of namespace scope (as opposed to a class member function) is declared as a friend in a class, and that class is an associated class in a function call, the friend function will be part of the overload set, even if it is not visible to normal lookup.

Consider the following example:

    namespace A {
	class S;
    };
    namespace B {
	void f(A::S);
    };
    namespace A {
	class S {
	    int i;
	    friend void B::f(S);
	};
    }
    void g() {
	A::S s;
	f(s); // should find B::f(A::S)
    }
This example would seem to satisfy the criteria from 3.4.2 [basic.lookup.argdep] : A::S is an associated class of the argument, and A::S has a friend declaration of the namespace-scope function B::f(A::S), so Koenig lookup should include B::f(A::S) as part of the overload set in the call.

Another interpretation is that, instead of finding the friend declarations in associated classes, one only looks for namespace-scope functions, visible or invisible, in the namespaces of which the the associated classes are members; the only use of the friend declarations in the associated classes is to validate whether an invisible function declaration came from an associated class or not and thus whether it should be included in the overload set or not. By this interpretation, the call f(s) in the example will fail, because B::f(A::S) is not a member of namespace A and thus is not found by the lookup.

Notes from 10/99 meeting: The second interpretation is correct. The wording should be revised to make clear that Koenig lookup works by finding "invisible" declarations in namespace scope and not by finding friend declarations in associated classes.

Proposed resolution (04/01): The "associated classes" are handled adequately under this interpretation by 3.4.2 [basic.lookup.argdep] paragraph 3, which describes the lookup in the associated namespaces as including the friend declarations from the associated classes. Other mentions of the associated classes should be removed or qualified to avoid the impression that there is a lookup in those classes:

  1. In 3.4.2 [basic.lookup.argdep], change

    When an unqualified name is used as the postfix-expression in a function call (5.2.2 [expr.call]), other namespaces not considered during the usual unqualified lookup (3.4.1 [basic.lookup.unqual]) may be searched, and namespace-scope friend function declarations (11.4 [class.friend]) not otherwise visible may be found.

    to

    When an unqualified name is used as the postfix-expression in a function call (5.2.2 [expr.call]), other namespaces not considered during the usual unqualified lookup (3.4.1 [basic.lookup.unqual]) may be searched, and in those namespaces, namespace-scope friend function declarations (11.4 [class.friend]) not otherwise visible may be found.
  2. In 3.4.2 [basic.lookup.argdep] paragraph 2, delete the words and classes in the following two sentences:

    If the ordinary unqualified lookup of the name finds the declaration of a class member function, the associated namespaces and classes are not considered. Otherwise the set of declarations found by the lookup of the function name is the union of the set of declarations found using ordinary unqualified lookup and the set of declarations found in the namespaces and classes associated with the argument types.

(See also issues 95, 136, 138, 139, 165, 166, and 218.)




218. Specification of Koenig lookup

Section: 3.4.2  [basic.lookup.argdep]     Status: CD1     Submitter: Hyman Rosen     Date: 28 Mar 2000

[Voted into WP at April, 2007 meeting.]

The original intent of the Committee when Koenig lookup was added to the language was apparently something like the following:

  1. The name in the function call expression is looked up like any other unqualified name.
  2. If the ordinary unqualified lookup finds nothing or finds the declaration of a (non-member) function, function template, or overload set, argument-dependent lookup is done and any functions found in associated namespaces are added to the result of the ordinary lookup.

This approach is not reflected in the current wording of the Standard. Instead, the following appears to be the status quo:

  1. Lookup of an unqualified name used as the postfix-expression in the function call syntax always performs Koenig lookup (3.4.1 [basic.lookup.unqual] paragraph 3).
  2. Unless ordinary lookup finds a class member function, the result of Koenig lookup always includes the declarations found in associated namespaces (3.4.2 [basic.lookup.argdep] paragraph 2), regardless of whether ordinary lookup finds a declaration and, if so, what kind of entity is found.
  3. The declarations from associated namespaces are not limited to functions and template functions by anything in 3.4.2 [basic.lookup.argdep]. However, if Koenig lookup results in more than one declaration and at least one of the declarations is a non-function, the program is ill-formed (7.3.4 [namespace.udir], paragraph 4; although this restriction is in the description of the using-directive, the wording applies to any lookup that spans namespaces).

John Spicer: Argument-dependent lookup was created to solve the problem of looking up function names within templates where you don't know which namespace to use because it may depend on the template argument types (and was then expanded to permit use in nontemplates). The original intent only concerned functions. The safest and simplest change is to simply clarify the existing wording to that effect.

Bill Gibbons: I see no reason why non-function declarations should not be found. It would take a special rule to exclude "function objects", as well as pointers to functions, from consideration. There is no such rule in the standard and I see no need for one.

There is also a problem with the wording in 3.4.2 [basic.lookup.argdep] paragraph 2:

If the ordinary unqualified lookup of the name finds the declaration of a class member function, the associated namespaces and classes are not considered.

This implies that if the ordinary lookup of the name finds the declaration of a data member which is a pointer to function or function object, argument-dependent lookup is still done.

My guess is that this is a mistake based on the incorrect assumption that finding any member other than a member function would be an error. I would just change "class member function" to "class member" in the quoted sentence.

Mike Miller: In light of the issue of "short-circuiting" Koenig lookup when normal lookup finds a non-function, perhaps it should be written as "...finds the declaration of a class member, an object, or a reference, the associated namespaces..."?

Andy Koenig: I think I have to weigh in on the side of extending argument-dependent lookup to include function objects and pointers to functions. I am particularly concerned about [function objects], because I think that programmers should be able to replace functions by function objects without changing the behavior of their programs in fundamental ways.

Bjarne Stroustrup: I don't think we could seriously argue from first principles that [argument-dependent lookup should find only function declarations]. In general, C++ name lookup is designed to be independent of type: First we find the name(s), then, we consider its(their) meaning. 3.4 [basic.lookup] states "The name lookup rules apply uniformly to all names ..." That is an important principle.

Thus, I consider text that speaks of "function call" instead of plain "call" or "application of ()" in the context of koenig lookup an accident of history. I find it hard to understand how 5.2.2 [expr.call] doesn't either disallow all occurrences of x(y) where x is a class object (that's clearly not intended) or requires koenig lookup for x independently of its type (by reference from 3.4 [basic.lookup]). I suspect that a clarification of 5.2.2 [expr.call] to mention function objects is in order. If the left-hand operand of () is a name, it should be looked up using koenig lookup.

John Spicer: This approach causes otherwise well-formed programs to be ill-formed, and it does so by making names visible that might be completely unknown to the author of the program. Using-directives already do this, but argument-dependent lookup is different. You only get names from using-directives if you actually use using-directives. You get names from argument-dependent lookup whether you want them or not.

This basically breaks an important reason for having namespaces. You are not supposed to need any knowledge of the names used by a namespace.

But this example breaks if argument-dependent lookup finds non-functions and if the translation unit includes the <list> header somewhere.

    namespace my_ns {
        struct A {};
        void list(std::ostream&, A&);

        void f() {
            my_ns::A a;
            list(cout, a);
        }
    }

This really makes namespaces of questionable value if you still need to avoid using the same name as an entity in another namespace to avoid problems like this.

Erwin Unruh: Before we really decide on this topic, we should have more analysis on the impact on programs. I would also like to see a paper on the possibility to overload functions with function surrogates (no, I won't write one). Since such an extension is bound to wait until the next official update, we should not preclude any outcome of the discussion.

I would like to have a change right now, which leaves open several outcomes later. I would like to say that:

Koenig lookup will find non-functions as well. If it finds a variable, the program is ill-formed. If the primary lookup finds a variable, Koenig lookup is done. If the result contains both functions and variables, the program is ill-formed. [Note: A future standard will assign semantics to such a program.]

I myself are not comfortable with this as a long-time result, but it prepares the ground for any of the following long term solutions:

The note is there to prevent compiler vendors to put their own extensions in here.

(See also issues 113 and 143.)

Notes from 04/00 meeting:

Although many agreed that there were valid concerns motivating a desire for Koenig lookup to find non-function declarations, there was also concern that supporting this capability would be more dangerous than helpful in the absence of overload resolution for mixed function and non-function declarations.

A straw poll of the group revealed 8 in favor of Koenig lookup finding functions and function templates only, while 3 supported the broader result.

Notes from the 10/01 meeting:

There was unanimous agreement on one less controversial point: if the normal lookup of the identifier finds a non-function, argument-dependent lookup should not be done.

On the larger issue, the primary point of consensus is that making this change is an extension, and therefore it should wait until the point at which we are considering extensions (which could be very soon). There was also consensus on the fact that the standard as it stands is not clear: some introductory text suggests that argument-dependent lookup finds only functions, but the more detailed text that describes the lookup does not have any such restriction.

It was also noted that some existing implementations (e.g., g++) do find some non-functions in some cases.

The issue at this point is whether we should (1) make a small change to make the standard clear (presumably in the direction of not finding the non-functions in the lookup), and revisit the issue later as an extension, or (2) leave the standard alone for now and make any changes only as part of considering the extension. A straw vote favored option (1) by a strong majority.

Additional Notes (September, 2006):

Recent discussion of this issue has emphasized the following points:

  1. The concept of finding function pointers and function objects as part of argument-dependent lookup is not currently under active discussion in the Evolution Working Group.

  2. The major area of concern with argument-dependent lookup is finding functions in unintended namespaces. There are current proposals to deal with this concern either by changing the definition of “associated namespace” so that fewer namespaces are considered or to provide a mechanism for enabling or disabling ADL altogether. Although this concern is conceptually distinct from the question of whether ADL finds function pointers and function objects, it is related in the sense that the current rules are perceived as finding too many functions (because of searching too many namespaces), and allowing function pointers and function objects would also increase the number of entities found by ADL.

  3. Any expansion of ADL to include function pointers and function objects must necessarily update the overloading rules to specify how they interact with functions and function templates in the overload set. Current implementation experience (g++) is not helpful in making this decision because, although it performs a uniform lookup and finds non-function entities, it diagnoses an error in overload resolution if non-function entities are in the overload set.

  4. There is a possible problem if types are found by ADL: it is not clear that overloading between callable entities (functions, function templates, function pointers, and function objects) and types (where the postfix syntax means a cast or construction of a temporary) is reasonable or useful.

James Widman:

There is a larger debate here about whether ADL should find object names; the proposed wording below is only intended to answer the request for wording to clarify the status quo (option 1 above) and not to suggest the outcome of the larger debate.

Proposed Resolution (October, 2006):

  1. Replace the normative text in 3.4.2 [basic.lookup.argdep] paragraph 3 with the following (leaving the text of the note and example unchanged):

    Let X be the lookup set produced by unqualified lookup (3.4.1 [basic.lookup.unqual]) and let Y be the lookup set produced by argument dependent lookup (defined as follows). If X contains

    • a declaration of a class member, or
    • a block-scope function declaration that is not a using-declaration, or
    • a declaration that is neither a function nor a function template

    then Y is empty. Otherwise Y is the set of declarations found in the namespaces associated with the argument types as described below. The set of declarations found by the lookup of the name is the union of X and Y.

  2. Change 3.4.1 [basic.lookup.unqual] paragraph 4 as indicated:

    When considering an associated namespace, the lookup is the same as the lookup performed when the associated namespace is used as a qualifier (3.4.3.2 [namespace.qual]) except that:

    • Any using-directives in the associated namespace are ignored.
    • Any namespace-scope friend functions or friend function templates declared in associated classes are visible within their respective namespaces even if they are not visible during an ordinary lookup (11.4 [class.friend]).
    • All names except those of (possibly overloaded) functions and function templates are ignored.




403. Reference to a type as a template-id

Section: 3.4.2  [basic.lookup.argdep]     Status: CD1     Submitter: John Spicer     Date: 18 Sep 2003

[Voted into WP at March 2004 meeting.]

Spun off from issue 384.

3.4.2 [basic.lookup.argdep] says:

If T is a template-id, its associated namespaces and classes are the namespace in which the template is defined; for member templates, the member template's class; the namespaces and classes associated with the types of the template arguments provided for template type parameters (excluding template template parameters); the namespaces in which any template template arguments are defined; and the classes in which any member templates used as template template arguments are defined. [Note: non-type template arguments do not contribute to the set of associated namespaces. ]
There is a problem with the term "is a template-id". template-id is a syntactic construct and you can't really talk about a type being a template-id. Presumably, this is intended to mean "If T is the type of a class template specialization ...".

Proposed Resolution (October 2003):

In 3.4.2 [basic.lookup.argdep], paragraph 2, bullet 8, replace

If T is a template-id ...
with
If T is a class template specialization ...




557. Does argument-dependent lookup cause template instantiation?

Section: 3.4.2  [basic.lookup.argdep]     Status: CD1     Submitter: Mike Miller     Date: 8 February 2006

[Voted into WP at the October, 2006 meeting.]

One might assume from 14.7.1 [temp.inst] paragraph 1 that argument-dependent lookup would require instantiation of any class template specializations used in argument types:

Unless a class template specialization has been explicitly instantiated (14.7.2 [temp.explicit]) or explicitly specialized (14.7.3 [temp.expl.spec]), the class template specialization is implicitly instantiated when the specialization is referenced in a context that requires a completely-defined object type or when the completeness of the class type affects the semantics of the program.

A complete class type is required to determine the associated classes and namespaces for the argument type (to determine the class's bases) and to determine the friend functions declared by the class, so the completeness of the class type certainly “affects the semantics of the program.”

This conclusion is reinforced by the second bullet of 3.4.2 [basic.lookup.argdep] paragraph 2:

A class template specialization is a class type, so the second bullet would appear to apply, requiring the specialization to be instantiated in order to determine its base classes.

However, bullet 8 of that paragraph deals explicitly with class template specializations:

Note that the class template specialization itself is not listed as an associated class, unlike other class types, and there is no mention of base classes. If bullet 8 were intended as a supplement to the treatment of class types in bullet 2, one would expect phrasing along the lines of, “In addition to the associated namespaces and classes for all class types...” or some such; instead, bullet 8 reads like a self-contained and complete specification.

If argument-dependent lookup does not cause implicit instantiation, however, examples like the following fail:

    template <typename T> class C {
        friend void f(C<T>*) { }
    };
    void g(C<int>* p) {
        f(p);    // found by ADL??
    }

Implementations differ in whether this example works or not.

Proposed resolution (April, 2006):

  1. Change bullet 2 of 3.4.2 [basic.lookup.argdep] paragraph 2 as indicated:

  2. Delete bullet 8 of 3.4.2 [basic.lookup.argdep] paragraph 2:




298. T::x when T is cv-qualified

Section: 3.4.3.1  [class.qual]     Status: CD1     Submitter: Steve Adamczyk     Date: 7 Jul 2001

[Voted into WP at April 2003 meeting.]

Can a typedef T to a cv-qualified class type be used in a qualified name T::x?

    struct A { static int i; };
    typedef const A CA;
    int main () {
      CA::i = 0;  // Okay?
    }

Suggested answer: Yes. All the compilers I tried accept the test case.

Proposed resolution (10/01):

In 3.4.3.1 [class.qual] paragraph 1 add the indicated text:

If the nested-name-specifier of a qualified-id nominates a class, the name specified after the nested-name-specifier is looked up in the scope of the class (10.2 [class.member.lookup]), except for the cases listed below. The name shall represent one or more members of that class or of one of its base classes (clause 10 [class.derived]). If the class-or-namespace-name of the nested-name-specifier names a cv-qualified class type, it nominates the underlying class (the cv-qualifiers are ignored).

Notes from 4/02 meeting:

There is a problem in that class-or-namespace-name does not include typedef names for cv-qualified class types. See 7.1.3 [dcl.typedef] paragraph 4:

Argument and text removed from proposed resolution (October 2002):

7.1.3 [dcl.typedef] paragraph 5:

Here's a good question: in this example, should X be used as a name-for-linkage-purposes (FLP name)?

  typedef class { } const X;

Because a type-qualifier is parsed as a decl-specifier, it isn't possible to declare cv-qualified and cv-unqualified typedefs for a type in a single declaration. Also, of course, there's no way to declare a typedef for the cv-unqualified version of a type for which only a cv-qualified version has a name. So, in the above example, if X isn't used as the FLP name, then there can be no FLP name. Also note that a FLP name usually represents a parameter type, where top-level cv-qualifiers are usually irrelevant anyway.

Data points: for the above example, Microsoft uses X as the FLP name; GNU and EDG do not.

My recommendation: for consistency with the direction we're going on this issue, for simplicity of description (e.g., "the first class-name declared by the declaration"), and for (very slightly) increased utility, I think Microsoft has this right.

If the typedef declaration defines an unnamed class type (or enum type), the first typedef-name declared by the declaration to be have that class type (or enum type) or a cv-qualified version thereof is used to denote the class type (or enum type) for linkage purposes only (3.5 [basic.link]). [Example: ...

Proposed resolution (October 2002):

3.4.4 [basic.lookup.elab] paragraphs 2 and 3:

This sentence is deleted twice:

... If this name lookup finds a typedef-name, the elaborated-type-specifier is ill-formed. ...

Note that the above changes are included in N1376 as part of the resolution of issue 245.

5.1.1 [expr.prim.general] paragraph 7:

This is only a note, and it is at least incomplete (and quite possibly inaccurate), despite (or because of) its complexity. I propose to delete it.

... [Note: a typedef-name that names a class is a class-name (9.1 [class.name]). Except as the identifier in the declarator for a constructor or destructor definition outside of a class member-specification (12.1 [class.ctor], 12.4 [class.dtor]), a typedef-name that names a class may be used in a qualified-id to refer to a constructor or destructor. ]

7.1.3 [dcl.typedef] paragraph 4:

My first choice would have been to make this the primary statement about the equivalence of typedef-name and class-name, since the equivalence comes about as a result of a typedef declaration. Unfortunately, references to class-name point to 9.1 [class.name], so it would seem that the primary statement should be there instead. To avoid the possiblity of conflicts in the future, I propose to make this a note.

[Note: A typedef-name that names a class type, or a cv-qualified version thereof, is also a class-name (9.1 [class.name]). If a typedef-name is used following the class-key in an elaborated-type-specifier (7.1.6.3 [dcl.type.elab]), or in the class-head of a class declaration (9 [class]), or is used as the identifier in the declarator for a constructor or destructor declaration (12.1 [class.ctor], 12.4 [class.dtor]), to identify the subject of an elaborated-type-specifier (7.1.6.3 [dcl.type.elab]), class declaration (clause 9 [class]), constructor declaration (12.1 [class.ctor]), or destructor declaration (12.4 [class.dtor]), the program is ill-formed. ] [Example: ...

7.1.6.3 [dcl.type.elab] paragraph 2:

This is the only remaining (normative) statement that a typedef-name can't be used in an elaborated-type-specifier. The reference to template type-parameter is deleted by the resolution of issue 283.

... If the identifier resolves to a typedef-name or a template type-parameter, the elaborated-type-specifier is ill-formed. [Note: ...

8 [dcl.decl] grammar rule declarator-id:

When I looked carefully into the statement of the rule prohibiting a typedef-name in a constructor declaration, it appeared to me that this grammar rule (inadvertently?) allows something that's always forbidden semantically.

9.1 [class.name] paragraph 5:

Unlike the prohibitions against appearing in an elaborated-type-specifier or constructor or destructor declarator, each of which was expressed more than once, the prohibition against a typedef-name appearing in a class-head was previously stated only in 7.1.3 [dcl.typedef]. It seems to me that that prohibition belongs here instead. Also, it seems to me important to clarify that a typedef-name that is a class-name is still a typedef-name. Otherwise, the various prohibitions can be argued around easily, if perversely ("But that isn't a typedef-name, it's a class-name; it says so right there in 9.1 [class.name].")

A typedef-name (7.1.3 [dcl.typedef]) that names a class type or a cv-qualified version thereof is also a class-name, but shall not be used in an elaborated-type-specifier; see also 7.1.3 [dcl.typedef]. as the identifier in a class-head.

12.1 [class.ctor] paragraph 3:

The new nonterminal references are needed to really nail down what we're talking about here. Otherwise, I'm just eliminating redundancy. (A typedef-name that doesn't name a class type is no more valid here than one that does.)

A typedef-name that names a class is a class-name (7.1.3 [dcl.typedef]); however, a A typedef-name that names a class shall not be used as the identifier class-name in the declarator declarator-id for a constructor declaration.

12.4 [class.dtor] paragraph 1:

The same comments apply here as to 12.1 [class.ctor].

... A typedef-name that names a class is a class-name (7.1.3); however, a A typedef-name that names a class shall not be used as the identifier class-name following the ~ in the declarator for a destructor declaration.



318. struct A::A should not name the constructor of A

Section: 3.4.3.1  [class.qual]     Status: CD1     Submitter: John Spicer     Date: 18 Oct 2001

[Voted into WP at April 2003 meeting.]

A use of an injected-class-name in an elaborated-type-specifier should not name the constructor of the class, but rather the class itself, because in that context we know that we're looking for a type. See issue 147.

Proposed Resolution (revised October 2002):

This clarifies the changes made in the TC for issue 147.

In 3.4.3.1 [class.qual] paragraph 1a replace:

If the nested-name-specifier nominates a class C, and the name specified after the nested-name-specifier, when looked up in C, is the injected class name of C (clause 9 [class]), the name is instead considered to name the constructor of class C.

with

In a lookup in which the constructor is an acceptable lookup result, if the nested-name-specifier nominates a class C and the name specified after the nested-name-specifier, when looked up in C, is the injected class name of C (clause 9 [class]), the name is instead considered to name the constructor of class C. [Note: For example, the constructor is not an acceptable lookup result in an elaborated type specifier so the constructor would not be used in place of the injected class name.]

Note that issue 263 updates a part of the same paragraph.

Append to the example:

  struct A::A a2;  // object of type A



400. Using-declarations and the "struct hack"

Section: 3.4.3.2  [namespace.qual]     Status: CD1     Submitter: Mark Mitchell     Date: 22 Jan 2003

[Voted into WP at March 2004 meeting.]

Consider this code:

  struct A { int i; struct i {}; };
  struct B { int i; struct i {}; };
  struct D : public A, public B { using A::i; void f (); };
  void D::f () { struct i x; }

I can't find anything in the standard that says definitively what this means. 7.3.3 [namespace.udecl] says that a using-declaration shall name "a member of a base class" -- but here we have two members, the data member A::i and the class A::i.

Personally, I'd find it more attractive if this code did not work. I'd like "using A::i" to mean "lookup A::i in the usual way and bind B::i to that", which would mean that while "i = 3" would be valid in D::f, "struct i x" would not be. However, if there were no A::i data member, then "A::i" would find the struct and the code in D::f would be valid.

John Spicer: I agree with you, but unfortunately the standard committee did not.

I remembered that this was discussed by the committee and that a resolution was adopted that was different than what I hoped for, but I had a hard time finding definitive wording in the standard.

I went back though my records and found the paper that proposed a resolution and the associated committee motion that adopted the proposed resolution The paper is N0905, and "option 1" from that paper was adopted at the Stockholm meeting in July of 1996. The resolution is that "using A::i" brings in everything named i from A.

3.4.3.2 [namespace.qual] paragraph 2 was modified to implement this resolution, but interestingly that only covers the namespace case and not the class case. I think the class case was overlooked when the wording was drafted. A core issue should be opened to make sure the class case is handled properly.

Notes from April 2003 meeting:

This is related to issue 11. 7.3.3 [namespace.udecl] paragraph 10 has an example for namespaces.

Proposed resolution (October 2003):

Add a bullet to the end of 3.4.3.1 [class.qual] paragraph 1:

Change the beginning of 7.3.3 [namespace.udecl] paragraph 4 from

A using-declaration used as a member-declaration shall refer to a member of a base class of the class being defined, shall refer to a member of an anonymous union that is a member of a base class of the class being defined, or shall refer to an enumerator for an enumeration type that is a member of a base class of the class being defined.

to

In a using-declaration used as a member-declaration, the nested-name-specifier shall name a base class of the class being defined. Such a using-declaration introduces the set of declarations found by member name lookup (10.2 [class.member.lookup], 3.4.3.1 [class.qual]).



245. Name lookup in elaborated-type-specifiers

Section: 3.4.4  [basic.lookup.elab]     Status: CD1     Submitter: Jack Rouse     Date: 14 Sep 2000

[Voted into WP at April 2003 meeting.]

I have some concerns with the description of name lookup for elaborated type specifiers in 3.4.4 [basic.lookup.elab]:

  1. Paragraph 2 has some parodoxical statements concerning looking up names that are simple identifers:

    If the elaborated-type-specifier refers to an enum-name and this lookup does not find a previously declared enum-name, the elaborated-type-specifier is ill-formed. If the elaborated-type-specifier refers to an [sic] class-name and this lookup does not find a previously declared class-name... the elaborated-type-specifier is a declaration that introduces the class-name as described in 3.3.2 [basic.scope.pdecl]."

    It is not clear how an elaborated-type-specifier can refer to an enum-name or class-name given that the lookup does not find such a name and that class-name and enum-name are not part of the syntax of an elaborated-type-specifier.

  2. The second sentence quoted above seems to suggest that the name found will not be used if it is not a class name. typedef-name names are ill-formed due to the sentence preceding the quote. If lookup finds, for instance, an enum-name then a new declaration will be created. This differs from C, and from the enum case, and can have surprising effects:

        struct S {
           enum E {
               one = 1
           };
           class E* p;     // declares a global class E?
        };
    

    Was this really the intent? If this is the case then some more work is needed on 3.4.4 [basic.lookup.elab]. Note that the section does not make finding a type template formal ill-formed, as is done in 7.1.6.3 [dcl.type.elab]. I don't see anything that makes a type template formal name a class-name. So the example in 7.1.6.3 [dcl.type.elab] of friend class T; where T is a template type formal would no longer be ill-formed with this interpretation because it would declare a new class T.

(See also issue 254.)

Notes from the 4/02 meeting:

This will be consolidated with the changes for issue 254. See also issue 298.

Proposed resolution (October 2002):

As given in N1376=02-0034. Note that the inserts and strikeouts in that document do not display correctly in all browsers; <del> --> <strike> and <ins> --> <b>, and the similar changes for the closing delimiters, seem to do the trick.




254. Definitional problems with elaborated-type-specifiers

Section: 3.4.4  [basic.lookup.elab]     Status: CD1     Submitter: Clark Nelson     Date: 26 Oct 2000

[Voted into WP at April 2003 meeting.]

  1. The text in 3.4.4 [basic.lookup.elab] paragraph 2 twice refers to the possibility that an elaborated-type-specifier might have the form

            class-key identifier ;
    

    However, the grammar for elaborated-type-specifier does not include a semicolon.

  2. In both 3.4.4 [basic.lookup.elab] and 7.1.6.3 [dcl.type.elab], the text asserts that an elaborated-type-specifier that refers to a typedef-name is ill-formed. However, it is permissible for the form of elaborated-type-specifier that begins with typename to refer to a typedef-name.

    This problem is the result of adding the typename form to the elaborated-type-name grammar without changing the verbiage correspondingly. It could be fixed either by updating the verbiage or by moving the typename syntax into its own production and referring to both nonterminals when needed.

(See also issue 180. If this issue is resolved in favor of a separate nonterminal in the grammar for the typename forms, the wording in that issue's resolution must be changed accordingly.)

Notes from 04/01 meeting:

The consensus was in favor of moving the typename forms out of the elaborated-type-specifier grammar.

Notes from the 4/02 meeting:

This will be consolidated with the changes for issue 245.

Proposed resolution (October 2002):

As given in N1376=02-0034.




141. Non-member function templates in member access expressions

Section: 3.4.5  [basic.lookup.classref]     Status: CD1     Submitter: fvali     Date: 31 July 1999

[Voted into the WP at the June, 2008 meeting.]

3.4.5 [basic.lookup.classref] paragraph 1 says,

In a class member access expression (5.2.5 [expr.ref] ), if the . or -> token is immediately followed by an identifier followed by a <, the identifier must be looked up to determine whether the < is the beginning of a template argument list (14.2 [temp.names] ) or a less-than operator. The identifier is first looked up in the class of the object expression. If the identifier is not found, it is then looked up in the context of the entire postfix-expression and shall name a class or function template.

There do not seem to be any circumstances in which use of a non-member template function would be well-formed as the id-expression of a class member access expression.

Proposed Resolution (November, 2006):

Change 3.4.5 [basic.lookup.classref] paragraph 1 as follows:

In a class member access expression (5.2.5 [expr.ref]), if the . or -> token is immediately followed by an identifier followed by a <, the identifier must be looked up to determine whether the < is the beginning of a template argument list (14.2 [temp.names]) or a less-than operator. The identifier is first looked up in the class of the object expression. If the identifier is not found, it is then looked up in the context of the entire postfix-expression and shall name a class or function template...



305. Name lookup in destructor call

Section: 3.4.5  [basic.lookup.classref]     Status: CD1     Submitter: Mark Mitchell     Date: 19 May 2001

[Voted into WP at the October, 2006 meeting.]

I believe this program is invalid:

    struct A {
    };

    struct C {
      struct A {};
      void f ();
    };

    void C::f () {
      ::A *a;
      a->~A ();
    }
The problem is that 3.4.5 [basic.lookup.classref] says that you have to look up A in both the context of the pointed-to-type (i.e., ::A), and in the context of the postfix-expression (i.e., the body of C::f), and that if the name is found in both places it must name the same type in both places.

The EDG front end does not issue an error about this program, though.

Am I reading the standardese incorrectly?

John Spicer: I think you are reading it correctly. I think I've been hoping that this would get changed. Unlike other dual lookup contexts, this is one in which the compiler already knows the right answer (the type must match that of the left hand of the -> operator). So I think that if either of the types found matches the one required, it should be sufficient. You can't say a->~::A(), which means you are forced to say a->::A::~A(), which disables the virtual mechanism. So you would have to do something like create a local typedef for the desired type.

See also issues 244, 399, and 466.

Proposed resolution (April, 2006):

  1. Remove the indicated text from 3.4.5 [basic.lookup.classref] paragraph 2:

    If the id-expression in a class member access (5.2.5 [expr.ref]) is an unqualified-id, and the type of the object expression is of a class type C (or of pointer to a class type C), the unqualified-id is looked up in the scope of class C...
  2. Change 3.4.5 [basic.lookup.classref] paragraph 3 as indicated:

    If the unqualified-id is ~type-name, the type-name is looked up in the context of the entire postfix-expression. and If the type T of the object expression is of a class type C (or of pointer to a class type C), the type-name is also looked up in the context of the entire postfix-expression and in the scope of class C. The type-name shall refer to a class-name. If type-name is found in both contexts, the name shall refer to the same class type. If the type of the object expression is of scalar type, the type-name is looked up in the scope of the complete postfix-expression. At least one of the lookups shall find a name that refers to (possibly cv-qualified) T. [Example:
    
        struct A { };
    
        struct B {
          struct A { };
          void f(::A* a);
        };
    
        void B::f(::A* a) {
          a->~A();  // OK, lookup in *a finds the injected-class-name
        }
    
    end example]

[Note: this change also resolves issue 414.]




381. Incorrect example of base class member lookup

Section: 3.4.5  [basic.lookup.classref]     Status: CD1     Submitter: Steve Adamczyk     Date: 8 Nov 2002

[Voted into WP at October 2004 meeting.]

The example in 3.4.5 [basic.lookup.classref] paragraph 4 is wrong (see 11.2 [class.access.base] paragraph 5; the cast to the naming class can't be done) and needs to be corrected. This was noted when the final version of the algorithm for issue 39 was checked against it.

Proposed Resolution (October 2003):

Remove the entire note at the end of 3.4.5 [basic.lookup.classref] paragraph 4, including the entire example.




414. Multiple types found on destructor lookup

Section: 3.4.5  [basic.lookup.classref]     Status: CD1     Submitter: John Spicer     Date: 1 May 2003

[Voted into WP at the October, 2006 meeting.]

By 3.4.5 [basic.lookup.classref] paragraph 3, the following is ill-formed because the two lookups of the destructor name (in the scope of the class of the object and in the surrounding context) find different Xs:

  struct X {};
  int main() {
    X x;
    struct X {};
    x.~X();  // Error?
  }

This is silly, because the compiler knows what the type has to be, and one of the things found matches that. The lookup should require only that one of the lookups finds the required class type.

Proposed resolution (April, 2005):

This issue is resolved by the resolution of issue 305.




216. Linkage of nameless class-scope enumeration types

Section: 3.5  [basic.link]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 13 Mar 2000

[Moved to DR at 10/01 meeting.]

3.5 [basic.link] paragraph 4 says (among other things):
A name having namespace scope has external linkage if it is the name of
That prohibits for example:
    typedef enum { e1 } *PE;
    void f(PE) {}  // Cannot declare a function (with linkage) using a 
		   // type with no linkage.

However, the same prohibition was not made for class scope types. Indeed, 3.5 [basic.link] paragraph 5 says:

In addition, a member function, static data member, class or enumeration of class scope has external linkage if the name of the class has external linkage.

That allows for:

    struct S {
       typedef enum { e1 } *MPE;
       void mf(MPE) {}
    };

My guess is that this is an unintentional consequence of 3.5 [basic.link] paragraph 5, but I would like confirmation on that.

Proposed resolution:

Change text in 3.5 [basic.link] paragraph 5 from:

In addition, a member function, static data member, class or enumeration of class scope has external linkage if the name of the class has external linkage.
to:
In addition, a member function, a static data member, a named class or enumeration of class scope, or an unnamed class or enumeration defined in a class-scope typedef declaration such that the class or enumeration has the typedef name for linkage purposes (7.1.3 [dcl.typedef]), has external linkage if the name of the class has external linkage.



319. Use of names without linkage in declaring entities with linkage

Section: 3.5  [basic.link]     Status: CD1     Submitter: Clark Nelson     Date: 29 Oct 2001

[Voted into WP at October 2004 meeting.]

According to 3.5 [basic.link] paragraph 8, "A name with no linkage ... shall not be used to declare an entity with linkage." This would appear to rule out code such as:

  typedef struct {
    int i;
  } *PT;
  extern "C" void f(PT);
[likewise]
  static enum { a } e;
which seems rather harmless to me.

See issue 132, which dealt with a closely related issue.

Andrei Iltchenko submitted the same issue via comp.std.c++ on 17 Dec 2001:

Paragraph 8 of Section 3.5 [basic.link] contains the following sentences: "A name with no linkage shall not be used to declare an entity with linkage. If a declaration uses a typedef name, it is the linkage of the type name to which the typedef refers that is considered."

The problem with this wording is that it doesn't cover cases where the type to which a typedef-name refers has no name. As a result it's not clear whether, for example, the following program is well-formed:

#include <vector>

int  main()
{
   enum  {   sz = 6u   };
   typedef int  (* aptr_type)[sz];
   typedef struct  data  {
      int   i,  j;
   }  * elem_type;
   std::vector<aptr_type>   vec1;
   std::vector<elem_type>   vec2;
}

Suggested resolution:

My feeling is that the rules for whether or not a typedef-name used in a declaration shall be treated as having or not having linkage ought to be modelled after those for dependent types, which are explained in 14.6.2.1 [temp.dep.type].

Add the following text at the end of Paragraph 8 of Section 3.5 [basic.link] and replace the following example:

In case of the type referred to by a typedef declaration not having a name, the newly declared typedef-name has linkage if and only if its referred type comprises no names of no linkage excluding local names that are eligible for appearance in an integral constant-expression (5.19 [expr.const]). [Note: if the referred type contains a typedef-name that does not denote an unnamed class, the linkage of that name is established by the recursive application of this rule for the purposes of using typedef names in declarations.] [Example:
  void f()
  {
     struct A { int x; };        // no linkage
     extern A a;                 // ill-formed
     typedef A Bl
     extern B b;                 // ill-formed

     enum  {   sz = 6u   };
     typedef int  (* C)[sz];     // C has linkage because sz can
                                 // appear in a constant expression
  }
--end example.]

Additional issue (13 Jan 2002, from Andrei Iltchenko):

Paragraph 2 of Section 14.3.1 [temp.arg.type] is inaccurate and unnecessarily prohibits a few important cases; it says "A local type, a type with no linkage, an unnamed type or a type compounded from any of these types shall not be used as a template-argument for a template-parameter." The inaccuracy stems from the fact that it is not a type but its name that can have a linkage.

For example based on the current wording of 14.3.1 [temp.arg.type], the following example is ill-formed.

  #include <vector>
  struct  data  {
    int   i,  j;
  };
  int  main()
  {
    enum  {   sz = 6u   };
    std::vector<int(*)[sz]>   vec1; // The types 'int(*)[sz]' and 'data*'
    std::vector<data*>        vec2; // have no names and are thus illegal
                                    // as template type arguments.
  }

Suggested resolution:

Replace the whole second paragraph of Section 14.3.1 [temp.arg.type] with the following wording:

A type whose name does not have a linkage or a type compounded from any such type shall not be used as a template-argument for a template-parameter. In case of a type T used as a template type argument not having a name, T constitutes a valid template type argument if and only if the name of an invented typedef declaration referring to T would have linkage; see 3.5. [Example:
  template <class T> class X { /* ... */ };
  void f()
  {
    struct S { /* ... */ };
    enum  {   sz = 6u   };

    X<S> x3;                     // error: a type name with no linkage
                                 // used as template-argument
    X<S*> x4;                    // error: pointer to a type name with
                                 // no linkage used as template-argument
    X<int(*)[sz]> x5;            // OK: since the name of typedef int
                                 // (*pname)[sz] would have linkage
  }
--end example] [Note: a template type argument may be an incomplete type (3.9 [basic.types]).]

Proposed resolution:

This is resolved by the changes for issue 389. The present issue was moved back to Review status in February 2004 because 389 was moved back to Review.




389. Unnamed types in entities with linkage

Section: 3.5  [basic.link]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 31 Oct 2002

[Voted into WP at October 2004 meeting.]

3.5 [basic.link] paragraph 8 says (among other things):

A name with no linkage (notably, the name of a class or enumeration declared in a local scope (3.3.3 [basic.scope.local])) shall not be used to declare an entity with linkage. If a declaration uses a typedef name, it is the linkage of the type name to which the typedef refers that is considered.

I would expect this to catch situations such as the following:

  // File 1:
  typedef struct {} *UP;
  void f(UP) {}

  // File 2:
  typedef struct {} *UP; // Or: typedef struct {} U, *UP;
  void f(UP);

The problem here is that most implementations must generate the same mangled name for "f" in two translation units. The quote from the standard above isn't quite clear, unfortunately: There is no type name to which the typedef refers.

A related situation is the following:

  enum { no, yes } answer;
The variable "answer" is declared as having external linkage, but it is declared with an unnamed type. Section 3.5 [basic.link] talks about the linkage of names, however, and does therefore not prohibit this. There is no implementation issue for most compilers because they do not ordinarily mangle variable names, but I believe the intent was to allow that implementation technique.

Finally, these problems are much less relevant when declaring names with internal linkage. For example, I would expect there to be few problems with:

  typedef struct {} *UP;
  static void g(UP);

I recently tried to interpret 3.5 [basic.link] paragraph 8 with the assumption that types with no names have no linkage. Surprisingly, this resulted in many diagnostics on variable declarations (mostly like "answer" above).

I'm pretty sure the standard needs clarifying words in this matter, but which way should it go?

See also issue 319.

Notes from April 2003 meeting:

There was agreement that this check is not needed for variables and functions with extern "C" linkage, and a change there is desirable to allow use of legacy C headers. The check is also not needed for entities with internal linkage, but there was no strong sentiment for changing that case.

We also considered relaxing this requirement for extern "C++" variables but decided that we did not want to change that case.

We noted that if extern "C" functions are allowed an additional check is needed when such functions are used as arguments in calls of function templates. Deduction will put the type of the extern "C" function into the type of the template instance, i.e., there would be a need to mangle the name of an unnamed type. To plug that hole we need an additional requirement on the template created in such a case.

Proposed resolution (April 2003, revised slightly October 2003 and March 2004):

In 3.5 [basic.link] paragraph 8, change

A name with no linkage (notably, the name of a class or enumeration declared in a local scope (3.3.3 [basic.scope.local])) shall not be used to declare an entity with linkage. If a declaration uses a typedef name, it is the linkage of the type name to which the typedef refers that is considered.

to

A type is said to have linkage if and only if A type without linkage shall not be used as the type of a variable or function with linkage, unless the variable or function has extern "C" linkage (7.5 [dcl.link]). [Note: in other words, a type without linkage contains a class or enumeration that cannot be named outside of its translation unit. An entity with external linkage declared using such a type could not correspond to any other entity in another translation unit of the program and is thus not permitted. Also note that classes with linkage may contain members whose types do not have linkage, and that typedef names are ignored in the determination of whether a type has linkage.]

Change 14.3.1 [temp.arg.type] paragraph 2 from (note: this is the wording as updated by issue 62)

The following types shall not be used as a template-argument for a template type-parameter:

to

A type without linkage (3.5 [basic.link]) shall not be used as a template-argument for a template type-parameter.

Once this issue is ready, issue 319 should be moved back to ready as well.




474. Block-scope extern declarations in namespace members

Section: 3.5  [basic.link]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 23 Jul 2004

[Voted into WP at October 2005 meeting.]

Consider the following bit of code:

    namespace N {
      struct S {
        void f();
      };
    }
    using namespace N;
    void S::f() {
      extern void g();  // ::g or N::g?
    }

In 3.5 [basic.link] paragraph 7 the Standard says (among other things),

When a block scope declaration of an entity with linkage is not found to refer to some other declaration, then that entity is a member of the innermost enclosing namespace.

The question then is whether N is an “enclosing namespace” for the local declaration of g()?

Proposed resolution (October 2004):

Add the following text as a new paragraph at the end of 7.3.1 [namespace.def]:

The enclosing namespaces of a declaration are those namespaces in which the declaration lexically appears, except for a redeclaration of a namespace member outside its original namespace (e.g., a definition as specified in 7.3.1.2 [namespace.memdef]). Such a redeclaration has the same enclosing namespaces as the original declaration. [Example:
  namespace Q {
    namespace V {
      void f(); // enclosing namespaces are the global namespace, Q, and Q::V
      class C { void m(); };
    }
    void V::f() { // enclosing namespaces are the global namespace, Q, and Q::V
      extern void h(); // ... so this declares Q::V::h
    }
    void V::C::m() { // enclosing namespaces are the global namespace, Q, and Q::V
    }
  }

end example]




270. Order of initialization of static data members of class templates

Section: 3.6.2  [basic.start.init]     Status: CD1     Submitter: Jonathan H. Lundquist     Date: 9 Feb 2001

[Moved to DR at 4/02 meeting.]

The Standard does not appear to address how the rules for order of initialization apply to static data members of class templates.

Suggested resolution: Add the following verbiage to either 3.6.2 [basic.start.init] or 9.4.2 [class.static.data]:

Initialization of static data members of class templates shall be performed during the initialization of static data members for the first translation unit to have static initialization performed for which the template member has been instantiated. This requirement shall apply to both the static and dynamic phases of initialization.

Notes from 04/01 meeting:

Enforcing an order of initialization on static data members of class templates will result in substantial overhead on access to such variables. The problem is that the initialization be required as the result of instantiation in a function used in the initialization of a variable in another translation unit. In current systems, the order of initialization of static data data members of class templates is not predictable. The proposed resolution is to state that the order of initialization is undefined.

Proposed resolution (04/01, updated slightly 10/01):

Replace the following sentence in 3.6.2 [basic.start.init] paragraph 1:

Objects with static storage duration defined in namespace scope in the same translation unit and dynamically initialized shall be initialized in the order in which their definition appears in the translation unit.

with

Dynamic initialization of an object is either ordered or unordered. Explicit specializations and definitions of class template static data members have ordered initialization. Other class template static data member instances have unordered initialization. Other objects defined in namespace scope have ordered initialization. Objects defined within a single translation unit and with ordered initialization shall be initialized in the order of their definitions in the translation unit. The order of initialization is unspecified for objects with unordered initialization and for objects defined in different translation units.

Note that this wording is further updated by issue 362.

Note (07/01):

Brian McNamara argues against the proposed resolution. The following excerpt captures the central point of a long message on comp.std.c++:

I have a class for representing linked lists which looks something like
    template <class T>
    class List {
       ...  static List<T>* sentinel; ...
    };
 
    template <class T>
    List<T>* List<T>::sentinel( new List<T> ); // static member definition

The sentinel list node is used to represent "nil" (the null pointer cannot be used with my implementation, for reasons which are immaterial to this discussion). All of the List's non-static member functions and constructors depend upon the value of the sentinel. Under the proposed resolution for issue #270, Lists cannot be safely instantiated before main() begins, as the sentinel's initialization is "unordered".

(Some readers may propose that I should use the "singleton pattern" in the List class. This is undesirable, for reasons I shall describe at the end of this post at the location marked "[*]". For the moment, indulge me by assuming that "singleton" is not an adequate solution.)

Though this is a particular example from my own experience, I believe it is representative of a general class of examples. It is common to use static data members of a class to represent the "distinguished values" which are important to instances of that class. It is imperative that these values be initialized before any instances of the class are created, as the instances depend on the values.

In a comp.std.c++ posting on 28 Jul 2001, Brian McNamara proposes the following alternative resolution:

Replace the following sentence in 3.6.2 [basic.start.init] paragraph 1:

Objects with static storage duration defined in namespace scope in the same translation unit and dynamically initialized shall be initialized in the order in which their definition appears in the translation unit.
with
Objects with static storage duration defined in namespace scope shall be initialized in the order described below.
and then after paragraph 1, add this text:
Dynamic initialization is either ordered or quasi-ordered. Explicit specializations of class template static data members have ordered initialization. Other class template static data member instances have quasi-ordered initialization. All other objects defined in namespace scope have ordered initialization. The order of initialization is specified as follows:
along with a non-normative note along the lines of
[ Note: The intention is that translation units can each be compiled separately with no knowledge of what objects may be re-defined in other translation units. Each translation unit can contain a method which initializes all objects (both quasi-ordered and ordered) as though they were ordered. When these translation units are linked together to create an executable program, all of these objects can be initialized by simply calling the initialization methods (one from each translation unit) in any order. Quasi-ordered objects require some kind of guard to ensure that they are not initialized more than once (the first attempt to initialize such an object should succeed; any subsequent attempts should simply be ignored). ]

Erwin Unruh replies: There is a point which is not mentioned with this posting. It is the cost for implementing the scheme. It requires that each static template variable is instantiated in ALL translation units where it is used. There has to be a flag for each of these variables and this flag has to be checked in each TU where the instantiation took place.

I would reject this idea and stand with the proposed resolution of issue 270.

There just is no portable way to ensure the "right" ordering of construction.

Notes from 10/01 meeting:

The Core Working Group reaffirmed its previous decision.




441. Ordering of static reference initialization

Section: 3.6.2  [basic.start.init]     Status: CD1     Submitter: Mike Miller     Date: 1 Dec 2003

[Voted into WP at April 2005 meeting.]

I have a couple of questions about 3.6.2 [basic.start.init], "Initialization of non-local objects." I believe I recall some discussion of related topics, but I can't find anything relevant in the issues list.

The first question arose when I discovered that different implementations treat reference initialization differently. Consider, for example, the following (namespace-scope) code:

  int i;
  int& ir = i;
  int* ip = &i;
Both initializers, "i" and "&i", are constant expressions, per 5.19 [expr.const] paragraph 4-5 (a reference constant expression and an address constant expression, respectively). Thus, both initializations are categorized as static initialization, according to 3.6.2 [basic.start.init] paragraph 1:
Zero-initialization and initialization with a constant expression are collectively called static initialization; all other initialization is dynamic initialization.

However, that does not mean that both ir and ip must be initialized at the same time:

Objects of POD types (3.9) with static storage duration initialized with constant expressions (5.19) shall be initialized before any dynamic initialization takes place.

Because "int&" is not a POD type, there is no requirement that it be initialized before dynamic initialization is performed, and implementations differ in this regard. Using a function called during dynamic initialization to print the values of "ip" and "&ir", I found that g++, Sun, HP, and Intel compilers initialize ir before dynamic initialization and the Microsoft compiler does not. All initialize ip before dynamic initialization. I believe this is conforming (albeit inconvenient :-) behavior.

So, my first question is whether it is intentional that a reference of static duration, initialized with a reference constant expression, need not be initialized before dynamic initialization takes place, and if so, why?

The second question is somewhat broader. As 3.6.2 [basic.start.init] is currently worded, it appears that there are no requirements on when ir is initialized. In fact, there is a whole category of objects -- non-POD objects initialized with a constant expression -- for which no ordering is specified. Because they are categorized as part of "static initialization," they are not subject to the requirement that they "shall be initialized in the order in which their definition appears in the translation unit." Because they are not POD types, they are not required to be initialized before dynamic initialization occurs. Am I reading this right?

My preference would be to change 3.6.2 [basic.start.init] paragraph 1 so that 1) references are treated like POD objects with respect to initialization, and 2) "static initialization" applies only to POD objects and references. Here's some sample wording to illustrate:

Suggested resolution:

Objects with static storage duration (3.7.1) shall be zero-initialized (8.5) before any other initialization takes place. Initializing a reference, or an object of POD type, of static storage duration with a constant expression (5.19) is called constant initialization. Together, zero-initialization and constant initialization are called static initialization; all other initialization is dynamic initialization. Static initialization shall be performed before any dynamic initialization takes place. [Remainder unchanged.]

Proposed Resolution:

Change 3.6.2 [basic.start.init] paragraph 1 as follows:

Objects with static storage duration (3.7.1) shall be zero-initialized (8.5) before any other initialization takes place. Initializing a reference, or an object of POD type, of static storage duration with a constant expression (5.19) is called constant initialization. Together, zero-initialization and constant initialization are Zero-initialization and initialization with a constant expression are collectively called static initialization; all other initialization is dynamic initialization. Static initialization shall be performed Objects of POD types (3.9) with static storage duration initialized with constant expressions (5.19) shall be initialized before any dynamic initialization takes place.



688. Constexpr constructors and static initialization

Section: 3.6.2  [basic.start.init]     Status: CD1     Submitter: Peter Dimov     Date: 26 March, 2008

[Voted into the WP at the September, 2008 meeting (resolution in paper N2757).]

Given this literal type,

    struct X {
        constexpr X() { }
    };

and this definition,

    static X x;

the current specification does not require that x be statically initialized because it is not “initialized with a constant expression” (3.6.1 [basic.start.main] paragraph 1).

Lawrence Crowl:

This guarantee is essential for atomics.

Jens Maurer:

Suggestion:

A reference with static storage duration or an object of literal type with static storage duration can be initialized with a constant expression (5.19 [expr.const]) or with a constexpr constructor; this is called constant initialization.

(Not spelling out “default constructor” makes it easier to handle multiple-parameter constexpr constructors, where there isn't “a” constant expression but several.)

Peter Dimov:

In addition, there is a need to enforce static initialization for non-literal types: std::shared_ptr, std::once_flag, and std::atomic_* all have nontrivial copy constructors, making them non-literal types. However, we need a way to ensure that a constexpr constructor called with constant expressions will guarantee static initialization, regardless of the nontriviality of the copy constructor.

Proposed resolution (April, 2008):

  1. Change 3.6.2 [basic.start.init] paragraph 1 as follows:

  2. ...A reference with static storage duration and an object of trivial or literal type with static storage duration can be initialized with a constant expression (5.19 [expr.const]); this If a reference with static storage duration is initialized with a constant expression (5.19 [expr.const]) or if the initialization of an object with static storage duration satisfies the requirements for the object being declared with constexpr (7.1.5 [dcl.constexpr]), that initialization is called constant initialization...
  3. Change 6.7 [stmt.dcl] paragraph 4 as follows:

  4. ...A local object of trivial or literal type (3.9 [basic.types]) with static storage duration initialized with constant-expressions is initialized Constant initialization (3.6.2 [basic.start.init]) of a local entity with static storage duration is performed before its block is first entered...
  5. Change 7.1.5 [dcl.constexpr] paragraph 7 as follows:

  6. A constexpr specifier used in an object declaration declares the object as const. Such an object shall be initialized, and every expression that appears in its initializer (8.5 [dcl.init]) shall be a constant expression. Every implicit conversion used in converting the initializer expressions and every constructor call used for the initialization shall be one of those allowed in a constant expression (5.19 [expr.const])...
  7. Replace 8.5.1 [dcl.init.aggr] paragraph 14 as follows:

  8. When an aggregate with static storage duration is initialized with a brace-enclosed initializer-list, if all the member initializer expressions are constant expressions, and the aggregate is a trivial type, the initialization shall be done during the static phase of initialization (3.6.2 [basic.start.init]); otherwise, it is unspecified whether the initialization of members with constant expressions takes place during the static phase or during the dynamic phase of initialization. [Note: The order of initialization for aggregates with static storage duration is specified in 3.6.2 [basic.start.init] and 6.7 [stmt.dcl]. —end note]

(Note: the change to 3.6.2 [basic.start.init] paragraph 1 needs to be reconciled with the conflicting change in issue 684.)




28. 'exit', 'signal' and static object destruction

Section: 3.6.3  [basic.start.term]     Status: CD1     Submitter: Martin J. O'Riordan     Date: 19 Oct 1997

[Voted into the WP at the June, 2008 meeting.]

The C++ standard has inherited the definition of the 'exit' function more or less unchanged from ISO C.

However, when the 'exit' function is called, objects of static extent which have been initialised, will be destructed if their types posses a destructor.

In addition, the C++ standard has inherited the definition of the 'signal' function and its handlers from ISO C, also pretty much unchanged.

The C standard says that the only standard library functions that may be called while a signal handler is executing, are the functions 'abort', 'signal' and 'exit'.

This introduces a bit of a nasty turn, as it is not at all unusual for the destruction of static objects to have fairly complex destruction semantics, often associated with resource release. These quite commonly involve apparently simple actions such as calling 'fclose' for a FILE handle.

Having observed some very strange behaviour in a program recently which in handling a SIGTERM signal, called the 'exit' function as indicated by the C standard.

But unknown to the programmer, a library static object performed some complicated resource deallocation activities, and the program crashed.

The C++ standard says nothing about the interaction between signals, exit and static objects. My observations, was that in effect, because the destructor called a standard library function other than 'abort', 'exit' or 'signal', while transitively in the execution context of the signal handler, it was in fact non-compliant, and the behaviour was undefined anyway.

This is I believe a plausible judgement, but given the prevalence of this common programming technique, it seems to me that we need to say something a lot more positive about this interaction.

Curiously enough, the C standard fails to say anything about the analogous interaction with functions registered with 'atexit' ;-)

Proposed Resolution (10/98):

The current Committee Draft of the next version of the ISO C standard specifies that the only standard library function that may be called while a signal handler is executing is 'abort'. This would solve the above problem.

[This issue should remain open until it has been decided that the next version of the C++ standard will use the next version of the C standard as the basis for the behavior of 'signal'.]

Notes (November, 2006):

C89 is slightly contradictory here: It allows any signal handler to terminate by calling abort, exit, longjmp, but (for asynchronous signals, i.e. not those produced by abort or raise) then makes calling any library function other than signal with the current signal undefined behavior (C89 7.7.1.1). For synchronous signals, C99 forbids calls to raise, but imposes no other restrictions. For asynchronous signals, C99 allows only calls to abort, _Exit, and signal with the current signal (C99 7.14.1.1). The current C++ WP refers to “plain old functions” and “conforming C programs” (18.10 [support.runtime] paragraph 6).

Proposed Resolution (November, 2006):

Change the footnote in 18.10 [support.runtime] paragraph 6 as follows:

In particular, a signal handler using exception handling is very likely to have problems. Also, invoking std::exit may cause destruction of objects, including those of the standard library implementation, which, in general, yields undefined behavior in a signal handler (see 1.9 [intro.execution]).



521. Requirements for exceptions thrown by allocation functions

Section: 3.7.4.1  [basic.stc.dynamic.allocation]     Status: CD1     Submitter: Alisdair Meredith     Date: 22 May 2005

[Voted into WP at the October, 2006 meeting.]

According to 3.7.4.1 [basic.stc.dynamic.allocation] paragraph 3,

Any other allocation function that fails to allocate storage shall only indicate failure by throwing an exception of class std::bad_alloc (18.6.2.1 [bad.alloc]) or a class derived from std::bad_alloc.

Shouldn't this statement have the usual requirements for an unambiguous and accessible base class?

Proposed resolution (April, 2006):

Change the last sentence of 3.7.4.1 [basic.stc.dynamic.allocation] paragraph 3 as indicated:

Any other allocation function that fails to allocate storage shall only indicate failure only by throwing an exception of class std::bad_alloc (18.6.2.1 [bad.alloc]) or a class derived from std::bad_alloc a type that would match a handler (15.3 [except.handle]) of type std::bad_alloc (18.6.2.1 [bad.alloc]).



220. All deallocation functions should be required not to throw

Section: 3.7.4.2  [basic.stc.dynamic.deallocation]     Status: CD1     Submitter: Herb Sutter     Date: 31 Mar 2000

[Voted into the WP at the September, 2008 meeting (resolution in paper N2757).]

[Picked up by evolution group at October 2002 meeting.]

The default global operators delete are specified to not throw, but there is no requirement that replacement global, or class-specific, operators delete must not throw. That ought to be required.

In particular:

We already require that all versions of an allocator's deallocate() must not throw, so that part is okay.

Rationale (04/00):

  1. Replacement deallocation functions are already required not to throw an exception (cf 17.6.3.8 [res.on.functions] paragraph 2, as applied to 18.6.1.1 [new.delete.single] paragraph 12 and 18.6.1.2 [new.delete.array] paragraph 11).
  2. Section 17.6.3.6 [replacement.functions] is describing the signatures of the functions to be replaced; exception specfications are not part of the signature.
  3. There does not appear to be any pressing need to require that class member deallocation functions not throw.

Note (March, 2008):

The Evolution Working Group has accepted the intent of this issue and referred it to CWG for action for C++0x (see paper J16/07-0033 = WG21 N2173).

Proposed resolution (March, 2008):

Change 3.7.4.2 [basic.stc.dynamic.deallocation] paragraph 3 as follows:

A deallocation function shall not terminate by throwing an exception. The value of the first argument supplied to a deallocation function...



348. delete and user-written deallocation functions

Section: 3.7.4.2  [basic.stc.dynamic.deallocation]     Status: CD1     Submitter: Ruslan Abdikeev     Date: 1 April 2002

[Voted into WP at October 2005 meeting.]

Standard is clear on behaviour of default allocation/deallocation functions. However, it is surpisingly vague on requirements to the behaviour of user-defined deallocation function and an interaction between delete-expression and deallocation function. This caused a heated argument on fido7.su.c-cpp newsgroup.

Resume:

It is not clear if user-supplied deallocation function is called from delete-expr when the operand of delete-expr is the null pointer (5.3.5 [expr.delete]). If it is, standard does not specify what user-supplied deallocation function shall do with the null pointer operand (18.6.1 [new.delete]). Instead, Standard uses the term "has no effect", which meaning is too vague in context given (5.3.5 [expr.delete]).

Description:

Consider statements

   char* p= 0; //result of failed non-throwing ::new char[]
   ::delete[] p;
Argument passed to delete-expression is valid - it is the result of a call to the non-throwing version of ::new, which has been failed. 5.3.5 [expr.delete] paragraph 1 explicitly prohibit us to pass 0 without having the ::new failure.

Standard does NOT specify whether user-defined deallocation function should be called in this case, or not.

Specifically, standard says in 5.3.5 [expr.delete] paragraph 2:

...if the value of the operand of delete is the null pointer the operation has no effect.
Standard doesn't specify term "has no effect". It is not clear from this context, whether the called deallocation function is required to have no effect, or delete-expression shall not call the deallocation function.

Furthermore, in para 4 standard says on default deallocation function:

If the delete-expression calls the implementation deallocation function (3.7.4.2 [basic.stc.dynamic.deallocation]), if the operand of the delete expression is not the null pointer constant, ...
Why it is so specific on interaction of default deallocation function and delete-expr?

If "has no effect" is a requirement to the deallocation function, then it should be stated in 3.7.4.2 [basic.stc.dynamic.deallocation], or in 18.6.1.1 [new.delete.single] and 18.6.1.2 [new.delete.array], and it should be stated explicitly.

Furthermore, standard does NOT specify what actions shall be performed by user-supplied deallocation function if NULL is given (18.6.1.1 [new.delete.single] paragraph 12):

Required behaviour: accept a value of ptr that is null or that was returned by an earlier call to the default operator new(std::size_t) or operator new(std::size_t, const std::nothrow_t&).

The same corresponds to ::delete[] case.

Expected solution:

  1. Make it clear that delete-expr will not call deallocation function if null pointer is given (in 5.3.5 [expr.delete]).
  2. Specify what user deallocation function shall do when null is given (either in 3.7.4.2 [basic.stc.dynamic.deallocation], or in 18.6.1.1 [new.delete.single], and 18.6.1.2 [new.delete.array]).

Notes from October 2002 meeting:

We believe that study of 18.6.1.1 [new.delete.single] paragraphs 12 and 13, 18.6.1.2 [new.delete.array] paragraphs 11 and 12, and 3.7.4.2 [basic.stc.dynamic.deallocation] paragraph 3 shows that the system-provided operator delete functions must accept a null pointer and ignore it. Those sections also show that a user-written replacement for the system-provided operator delete functions must accept a null pointer. There is no requirement that such functions ignore a null pointer, which is okay -- perhaps the reason for replacing the system-provided functions is to do something special with null pointer values (e.g., log such calls and return).

We believe that the standard should not require an implementation to call a delete function with a null pointer, but it must allow that. For the system-provided delete functions or replacements thereof, the standard already makes it clear that the delete function must accept a null pointer. For class-specific delete functions, we believe the standard should require that such functions accept a null pointer, though it should not mandate what they do with null pointers.

5.3.5 [expr.delete] needs to be updated to say that it is unspecified whether or not the operator delete function is called with a null pointer, and 3.7.4.2 [basic.stc.dynamic.deallocation] needs to be updated to say that any deallocation function must accept a null pointer.

Proposed resolution (October, 2004):

  1. Change 5.3.5 [expr.delete] paragraph 2 as indicated:

    If the operand has a class type, the operand is converted to a pointer type by calling the above-mentioned conversion function, and the converted operand is used in place of the original operand for the remainder of this section. In either alternative, if the value of the operand of delete is the null pointer the operation has no effect may be a null pointer value. If it is not a null pointer value, in In the first alternative (delete object), the value of the operand of delete shall be a pointer to a non-array object or a pointer to a sub-object (1.8 [intro.object]) representing a base class of such an object (clause 10 [class.derived])...
  2. Change 5.3.5 [expr.delete] paragraph 4 as follows (note that the old wording reflects the changes proposed by issue 442:

    The cast-expression in a delete-expression shall be evaluated exactly once. If the delete-expression calls the implementation deallocation function (3.7.4.2 [basic.stc.dynamic.deallocation]), and if the value of the operand of the delete expression is not a null pointer, the deallocation function will deallocate the storage referenced by the pointer thus rendering the pointer invalid. [Note: the value of a pointer that refers to deallocated storage is indeterminate. —end note]

  3. Change 5.3.5 [expr.delete] paragraphs 6-7 as follows:

    The If the value of the operand of the delete-expression is not a null pointer value, the delete-expression will invoke the destructor (if any) for the object or the elements of the array being deleted. In the case of an array, the elements will be destroyed in order of decreasing address (that is, in reverse order of the completion of their constructor; see 12.6.2 [class.base.init]).

    The If the value of the operand of the delete-expression is not a null pointer value, the delete-expression will call a deallocation function (3.7.4.2 [basic.stc.dynamic.deallocation]). Otherwise, it is unspecified whether the deallocation function will be called. [Note: The deallocation function is called regardless of whether the destructor for the object or some element of the array throws an exception. —end note]

  4. Change 3.7.4.2 [basic.stc.dynamic.deallocation] paragraph 3 as indicated:

    The value of the first argument supplied to one of the a deallocation functions provided in the standard library may be a null pointer value; if so, and if the deallocation function is one supplied in the standard library, the call to the deallocation function has no effect. Otherwise, the value supplied to operator delete(void*) in the standard library shall be one of the values returned by a previous invocation of either operator new(std::size_t) or operator new(std::size_t, const std::nothrow_t&) in the standard library, and the value supplied to operator delete[](void*) in the standard library shall be one of the values returned by a previous invocation of either operator new[](std::size_t) or operator new[](std::size_t, const std::nothrow_t&) in the standard library.

[Note: this resolution also resolves issue 442.]




119. Object lifetime and aggregate initialization

Section: 3.8  [basic.life]     Status: CD1     Submitter: Jack Rouse     Date: 20 May 1999

[Moved to DR at 4/02 meeting.]

Jack Rouse: 3.8 [basic.life] paragraph 1 includes:

The lifetime of an object is a runtime property of the object. The lifetime of an object of type T begins when:
Consider the code:
    struct B {
        B( int = 0 );
        ~B();
    };

    struct S {
        B b1;
    };

    int main()
    {
        S s = { 1 };
        return 0;
    }
In the code above, class S does have a non-trivial constructor, the default constructor generated by the compiler. According the text above, the lifetime of the auto s would never begin because a constructor for S is never called. I think the second case in the text needs to include aggregate initialization.

Mike Miller: I see a couple of ways of fixing the problem. One way would be to change "the constructor call has completed" to "the object's initialization is complete."

Another would be to add following "a class type with a non-trivial constructor" the phrase "that is not initialized with the brace notation (8.5.1 [dcl.init.aggr] )."

The first formulation treats aggregate initialization like a constructor call; even POD-type members of an aggregate could not be accessed before the aggregate initialization completed. The second is less restrictive; the POD-type members of the aggregate would be usable before the initialization, and the members with non-trivial constructors (the only way an aggregate can acquire a non-trivial constructor) would be protected by recursive application of the lifetime rule.

Proposed resolution (04/01):

In 3.8 [basic.life] paragraph 1, change

If T is a class type with a non-trivial constructor (12.1 [class.ctor]), the constructor call has completed.

to

If T is a class type with a non-trivial constructor (12.1 [class.ctor]), the initialization is complete. [Note: the initialization can be performed by a constructor call or, in the case of an aggregate with an implicitly-declared non-trivial default constructor, an aggregate initialization (8.5.1 [dcl.init.aggr]).]



274. Cv-qualification and char-alias access to out-of-lifetime objects

Section: 3.8  [basic.life]     Status: CD1     Submitter: Mike Miller     Date: 14 Mar 2001

[Voted into WP at April 2003 meeting.]

The wording in 3.8 [basic.life] paragraph 6 allows an lvalue designating an out-of-lifetime object to be used as the operand of a static_cast only if the conversion is ultimately to "char&" or "unsigned char&". This description excludes the possibility of using a cv-qualified version of these types for no apparent reason.

Notes on 04/01 meeting:

The wording should be changed to allow cv-qualified char types.

Proposed resolution (04/01):

In 3.8 [basic.life] paragraph 6 change the third bullet:

to read:




404. Unclear reference to construction with non-trivial constructor

Section: 3.8  [basic.life]     Status: CD1     Submitter: Mike Miller     Date: 8 Apr 2003

[Voted into WP at March 2004 meeting.]

3.8 [basic.life] paragraph 1 second bullet says:

if T is a class type with a non-trivial constructor (12.1), the constructor call has completed.

This is confusing; what was intended is probably something like

if T is a class type and the constructor invoked to create the object is non-trivial (12.1), the constructor call has completed.

Proposed Resolution (October 2003):

As given above.




594. Coordinating issues 119 and 404 with delegating constructors

Section: 3.8  [basic.life]     Status: CD1     Submitter: Tom Plum     Date: 30 August 2006

[Voted into the WP at the September, 2008 meeting.]

In ISO/IEC 14882:2003, the second bullet of 3.8 [basic.life] paragraph 1 reads,

if T is a class type with a non-trivial constructor (12.1 [class.ctor]), the constructor call has completed.

Issue 119 pointed out that aggregate initialization can be used with some classes with a non-trivial implicitly-declared default constructor, and that in such cases there is no call to the object's constructor. The resolution for that issue was to change the previously-cited wording to read,

If T is a class type with a non-trivial constructor (12.1 [class.ctor], the initialization is complete.

Later (but before the WP was revised with the wording from the resolution of issue 119), issue 404 changed the 2003 wording to read,

If T is a class type and the constructor invoked to create the object is non-trivial (12.1 [class.ctor]), the constructor call has completed.

thus reversing the effect of issue 119, whose whole purpose was to cover objects with non-trivial constructors that are not invoked.

Through an editorial error, the post-Redmond draft (N1905) still contained the original 2003 wording that should have been replaced by the resolution of issue 119, in addition to the new wording from the resolution:

if T is a class type and the constructor invoked to create the object is non-trivial (12.1 [class.ctor]), the constructor call has completed. the initialization is complete.

Finally, during the application of the edits for delegating constructors (N1986), this editing error was “fixed” by retaining the original 2003 wording (which was needed for the application of the change specified in N1986), so that the current draft (N2009) reads,

if T is a class type and the constructor invoked to create the object is non-trivial (12.1 [class.ctor]), the principal constructor call 12.6.2 [class.base.init]) has completed.

Because the completion of the call to the principal constructor corresponds to the point at which the object is “fully constructed” (15.2 [except.ctor] paragraph 2), i.e., its initialization is complete, I believe that the exact wording of the issue 119 resolution would be correct and should be restored verbatim.

Proposed resolution (June, 2008):

Change 3.8 [basic.life] paragraph 1 as follows:

The lifetime of an object is a runtime property of the object. An object is said to have non-trivial initialization if it is of a class or aggregate type and it or one of its members is initialized by a constructor other than a trivial default constructor. [Note: Initialization by a trivial copy constructor is non-trivial initialization. —end note] The lifetime of an object of type T begins when:

The lifetime of an object of type T ends when...




644. Should a trivial class type be a literal type?

Section: 3.9  [basic.types]     Status: CD1     Submitter: Alisdair Meredith     Date: 8 Aug 2007

[Voted into the WP at the June, 2008 meeting.]

The original proposed wording for 3.9 [basic.types] paragraph 11 required a constexpr constructor for a literal class only “if the class has at least one user-declared constructor.” This wording was dropped during the review by CWG out of a desire to ensure that literal types not have any uninitialized members. Thus, a class like

    struct pixel {
        int x, y;
    };

is not a literal type. However, if an object of that type is aggregate-initialized or value-initialized, there can be no uninitialized members; the missing wording should be restored in order to permit use of expressions like pixel().x as constant expressions.

Proposed resolution (February, 2008):

Change 3.9 [basic.types] paragraph 10 as follows:

A type is a literal type if it is:



158. Aliasing and qualification conversions

Section: 3.10  [basic.lval]     Status: CD1     Submitter: Mike Stump     Date: 20 Aug 1999

[Moved to DR at 4/02 meeting.]

3.10 [basic.lval] paragraph 15 lists the types via which an lvalue can be used to access the stored value of an object; using an lvalue type that is not listed results in undefined behavior. It is permitted to add cv-qualification to the actual type of the object in this access, but only at the top level of the type ("a cv-qualified version of the dynamic type of the object").

However, 4.4 [conv.qual] paragraph 4 permits a "conversion [to] add cv-qualifiers at levels other than the first in multi-level pointers." The combination of these two rules allows creation of pointers that cannot be dereferenced without causing undefined behavior. For instance:

    int* jp;
    const int * const * p1 = &jp;
    *p1;    // undefined behavior!

The reason that *p1 results in undefined behavior is that the type of the lvalue is const int * const", which is not "a cv-qualified version of" int*.

Since the conversion is permitted, we must give it defined semantics, hence we need to fix the wording in 3.10 [basic.lval] to include all possible conversions of the type via 4.4 [conv.qual].

Proposed resolution (04/01):

Add a new bullet to 3.10 [basic.lval] paragraph 15, following "a cv-qualified version of the dynamic type of the object:"




649. Optionally ill-formed extended alignment requests

Section: 3.11  [basic.align]     Status: CD1     Submitter: Mike Miller     Date: 12 Aug 2007

[Voted into the WP at the September, 2008 meeting.]

The requirements on an implementation when presented with an alignment-specifier not supported by that implementation in that context are contradictory: 3.11 [basic.align] paragraph 9 says,

If a request for a specific extended alignment in a specific context is not supported by an implementation, the implementation may reject the request as ill-formed. The implementation may also silently ignore the requested alignment.

In contrast, 7.6.2 [dcl.align] paragraph 2, bullet 4 says simply,

with no provision to “silently ignore” the requested alignment. These two passages need to be reconciled.

If the outcome of the reconciliation is to grant implementations the license to accept and ignore extended alignment requests, the specification should be framed in terms of mechanisms that already exist in the Standard, such as undefined behavior and/or conditionally-supported constructs; “ill-formed” is a category that is defined by the Standard, not something that an implementation can decide.

Notes from the February, 2008 meeting:

The consensus was that such requests should be ill-formed and require a diagnostic. However, it was also observed that an implementation need not reject an ill-formed program; the only requirement is that it issue a diagnostic. It would thus be permissible for an implementation to “noisily ignore” (as opposed to “silently ignoring”) an unsupported alignment request.

Proposed resolution (June, 2008):

Change 3.11 [basic.align] paragraph 9 as follows:

If a request for a specific extended alignment in a specific context is not supported by an implementation, the implementation may reject the request as program is ill-formed. The implementation may also silently ignore the requested alignment. [Note: aAdditionally, a request for runtime allocation of dynamic memory storage for which the requested alignment cannot be honored may shall be treated as an allocation failure. end note]



519. Null pointer preservation in void* conversions

Section: 4.10  [conv.ptr]     Status: CD1     Submitter: comp.std.c++     Date: 19 May 2005

[Voted into WP at April, 2006 meeting.]

The C standard says in 6.3.2.3, paragraph 4:

Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal.

C++ appears to be incompatible with the first sentence in only two areas:

    A *a = 0;
    void *v = a;

C++ (4.10 [conv.ptr] paragraph 2) says nothing about the value of v.

    void *v = 0;
    A *b = (A*)v; // aka static_cast<A*>(v)

C++ (5.2.9 [expr.static.cast] paragraph 10) says nothing about the value of b.

Suggested changes:

  1. Add the following sentence to 4.10 [conv.ptr] paragraph 2:

  2. The null pointer value is converted to the null pointer value of the destination type.
  3. Add the following sentence to 5.2.9 [expr.static.cast] paragraph 10:

  4. The null pointer value (4.10 [conv.ptr]) is converted to the null pointer value of the destination type.

Proposed resolution (October, 2005):

  1. Add the indicated words to 4.10 [conv.ptr] paragraph 2:

  2. An rvalue of type “pointer to cv T,” where T is an object type, can be converted to an rvalue of type “pointer to cv void”. The result of converting a “pointer to cv T” to a “pointer to cv void” points to the start of the storage location where the object of type T resides, as if the object is a most derived object (1.8 [intro.object]) of type T (that is, not a base class subobject). The null pointer value is converted to the null pointer value of the destination type.
  3. Add the indicated words to 5.2.9 [expr.static.cast] paragraph 11:

  4. An rvalue of type “pointer to cv1 void” can be converted to an rvalue of type “pointer to cv2 T,” where T is an object type and cv2 is the same cv-qualification as, or greater cv-qualification than, cv1. The null pointer value is converted to the null pointer value of the destination type. A value of type pointer to object converted to “pointer to cv void” and back, possibly with different cv-qualification, shall have its original value...



654. Conversions to and from nullptr_t

Section: 4.10  [conv.ptr]     Status: CD1     Submitter: Jason Merrill     Date: 7 October 2007

[Voted into the WP at the June, 2008 meeting as paper N2656.]

In the interest of promoting use of nullptr instead of the integer literal 0 as the null pointer constant, the proposal accepted by the Committee does not provide for converting a zero-valued integral constant to type std::nullptr_t. However, this omission reduces the utility of the feature for use in the library for smart pointers. In particular, the addition of that conversion (along with a converting constructor accepting a std::nullptr_t) would allow smart pointers to be used just like ordinary pointers in expressions like:

    if (p == 0) { }
    if (0 == p) { }
    if (p != 0) { }
    if (0 != p) { }
    p = 0;

The existing use of the “unspecified bool type” idiom supports this usage, but being able to use std::nullptr_t instead would be simpler and more elegant.

Jason Merrill: I have another reason to support the conversion as well: it seems to me very odd for nullptr_t to be more restrictive than void*. Anything we can do with an arbitrary pointer, we ought to be able to do with nullptr_t as well. Specifically, since there is a standard conversion from literal 0 to void*, and there is a standard conversion from void* to bool, nullptr_t should support the same conversions.

This changes two of the example lines in the proposal as adopted:

    if (nullptr) ;      // error, no conversion to bool
    if (nullptr == 0) ; // error

become

    if (nullptr) ;      // evaluates to false
    if( nullptr == 0 ); // evaluates to true

And later,

    char* ch3 = expr ? nullptr : nullptr; // ch3 is the null pointer value
    char* ch4 = expr ? 0 : nullptr;       // ch4 is the null pointer value
    int n3 = expr ? nullptr : nullptr;    // error, nullptr_t can’t be converted to int
    int n4 = expr ? 0 : nullptr;          // error, nullptr_t can’t be converted to int

I would also allow reinterpret_cast from nullptr_t to integral type, with the same semantics as a reinterpret_cast from the null pointer value to integral type.

Basically, I would like nullptr_t to act like a void* which is constrained to always be (void*)0.




480. Is a base of a virtual base also virtual?

Section: 4.11  [conv.mem]     Status: CD1     Submitter: Mark Mitchell     Date: 18 Oct 2004

[Voted into WP at the October, 2006 meeting.]

When the Standard refers to a virtual base class, it should be understood to include base classes of virtual bases. However, the Standard doesn't actually say this anywhere, so when 4.11 [conv.mem] (for example) forbids casting to a derived class member pointer from a virtual base class member pointer, it could be read as meaning:

  struct B {};
  struct D : public B {};
  struct D2 : virtual public D {};

  int B::*p;
  int D::*q;

  void f() {
    static_cast<int D2::*>(p);  // permitted
    static_cast<int D2::*>(q);  // forbidden
  }

Proposed resolution (October, 2005):

  1. Change 4.11 [conv.mem] paragraph 2 as indicated:

  2. ...If B is an inaccessible (clause 11 [class.access]), ambiguous (10.2 [class.member.lookup]) or virtual (10.1 [class.mi]) base class of D, or a base class of a virtual base class of D, a program that necessitates this conversion is ill-formed...
  3. Change 5.2.9 [expr.static.cast] paragraph 2 as indicated:

  4. ...and B is not neither a virtual base class of D nor a base class of a virtual base class of D...
  5. Change 5.2.9 [expr.static.cast] paragraph 9 as indicated:

  6. ...and B is not neither a virtual base class of D nor a base class of a virtual base class of D...



222. Sequence points and lvalue-returning operators

Section: 5  [expr]     Status: CD1     Submitter: Andrew Koenig     Date: 20 Dec 1999

[Voted into the WP at the September, 2008 meeting.]

I believe that the committee has neglected to take into account one of the differences between C and C++ when defining sequence points. As an example, consider

    (a += b) += c;

where a, b, and c all have type int. I believe that this expression has undefined behavior, even though it is well-formed. It is not well-formed in C, because += returns an rvalue there. The reason for the undefined behavior is that it modifies the value of `a' twice between sequence points.

Expressions such as this one are sometimes genuinely useful. Of course, we could write this particular example as

    a += b; a += c;

but what about

    void scale(double* p, int n, double x, double y) {
        for (int i = 0; i < n; ++i) {
            (p[i] *= x) += y;
        }
    }

All of the potential rewrites involve multiply-evaluating p[i] or unobvious circumlocations like creating references to the array element.

One way to deal with this issue would be to include built-in operators in the rule that puts a sequence point between evaluating a function's arguments and evaluating the function itself. However, that might be overkill: I see no reason to require that in

    x[i++] = y;

the contents of `i' must be incremented before the assignment.

A less stringent alternative might be to say that when a built-in operator yields an lvalue, the implementation shall not subsequently change the value of that object as a consequence of that operator.

I find it hard to imagine an implementation that does not do this already. Am I wrong? Is there any implementation out there that does not `do the right thing' already for (a += b) += c?

5.17 [expr.ass] paragraph 1 says,

The result of the assignment operation is the value stored in the left operand after the assignment has taken place; the result is an lvalue.

What is the normative effect of the words "after the assignment has taken place"? I think that phrase ought to mean that in addition to whatever constraints the rules about sequence points might impose on the implementation, assignment operators on built-in types have the additional constraint that they must store the left-hand side's new value before returning a reference to that object as their result.

One could argue that as the C++ standard currently stands, the effect of x = y = 0; is undefined. The reason is that it both fetches and stores the value of y, and does not fetch the value of y in order to compute its new value.

I'm suggesting that the phrase "after the assignment has taken place" should be read as constraining the implementation to set y to 0 before yielding the value of y as the result of the subexpression y = 0.

Note that this suggestion is different from asking that there be a sequence point after evaluation of an assignment. In particular, I am not suggesting that an order constraint be imposed on any side effects other than the assignment itself.

Francis Glassborow:

My understanding is that for a single variable:

  1. Multiple read accesses without a write are OK
  2. A single read access followed by a single write (of a value dependant on the read, so that the read MUST happen first) is OK
  3. A write followed by an actual read is undefined behaviour
  4. Multiple writes have undefined behaviour

It is the 3) that is often ignored because in practice the compiler hardly ever codes for the read because it already has that value but in complicated evaluations with a shortage of registers, that is not always the case. Without getting too close to the hardware, I think we both know that a read too close to a write can be problematical on some hardware.

So, in x = y = 0;, the implementation must NOT fetch a value from y, instead it has to "know" what that value will be (easy because it has just computed that in order to know what it must, at some time, store in y). From this I deduce that computing the lvalue (to know where to store) and the rvalue to know what is stored are two entirely independent actions that can occur in any order commensurate with the overall requirements that both operands for an operator be evaluated before the operator is.

Erwin Unruh:

C distinguishes between the resulting value of an assignment and putting the value in store. So in C a compiler might implement the statement x=y=0; either as x=0;y=0; or as y=0;x=0; In C the statement (x += 5) += 7; is not allowed because the first += yields an rvalue which is not allowed as left operand to +=. So in C an assignment is not a sequence of write/read because the result is not really "read".

In C++ we decided to make the result of assignment an lvalue. In this case we do not have the option to specify the "value" of the result. That is just the variable itself (or its address in a different view). So in C++, strictly speaking, the statement x=y=0; must be implemented as y=0;x=y; which makes a big difference if y is declared volatile.

Furthermore, I think undefined behaviour should not be the result of a single mentioning of a variable within an expression. So the statement (x +=5) += 7; should NOT have undefined behaviour.

In my view the semantics could be:

  1. if the result of an assignment is used as an rvalue, its value is that of the variable after assignment. The actual store takes place before the next sequence point, but may be before the value is used. This is consistent with C usage.
  2. if the result of an assignment is used as an lvalue to store another value, then the new value will be stored in the variable before the next sequence point. It is unspecified whether the first assigned value is stored intermediately.
  3. if the result of an assignment is used as an lvalue to take an address, that address is given (it doesn't change). The actual store of the new value takes place before the next sequence point.

Jerry Schwarz:

My recollection is different from Erwin's. I am confident that the intention when we decided to make assignments lvalues was not to change the semantics of evaluation of assignments. The semantics was supposed to remain the same as C's.

Ervin seems to assume that because assignments are lvalues, an assignment's value must be determined by a read of the location. But that was definitely not our intention. As he notes this has a significant impact on the semantics of assignment to a volatile variable. If Erwin's interpretation were correct we would have no way to write a volatile variable without also reading it.

Lawrence Crowl:

For x=y=0, lvalue semantics implies an lvalue to rvalue conversion on the result of y=0, which in turn implies a read. If y is volatile, lvalue semantics implies both a read and a write on y.

The standard apparently doesn't state whether there is a value dependence of the lvalue result on the completion of the assignment. Such a statement in the standard would solve the non-volatile C compatibility issue, and would be consistent with a user-implemented operator=.

Another possible approach is to state that primitive assignment operators have two results, an lvalue and a corresponding "after-store" rvalue. The rvalue result would be used when an rvalue is required, while the lvalue result would be used when an lvalue is required. However, this semantics is unsupportable for user-defined assignment operators, or at least inconsistent with all implementations that I know of. I would not enjoy trying to write such two-faced semantics.

Erwin Unruh:

The intent was for assignments to behave the same as in C. Unfortunately the change of the result to lvalue did not keep that. An "lvalue of type int" has no "int" value! So there is a difference between intent and the standard's wording.

So we have one of several choices:

I think the last one has the least impact on existing programs, but it is an ugly solution.

Andrew Koenig:

Whatever we may have intended, I do not think that there is any clean way of making

    volatile int v;
    int i;

    i = v = 42;
have the same semantics in C++ as it does in C. Like it or not, the subexpression v = 42 has the type ``reference to volatile int,'' so if this statement has any meaning at all, the meaning must be to store 42 in v and then fetch the value of v to assign it to i.

Indeed, if v is volatile, I cannot imagine a conscientious programmer writing a statement such as this one. Instead, I would expect to see

    v = 42;
    i = v;
if the intent is to store 42 in v and then fetch the (possibly changed) value of v, or
    v = 42;
    i = 42;
if the intent is to store 42 in both v and i.

What I do want is to ensure that expressions such as ``i = v = 42'' have well-defined semantics, as well as expressions such as (i = v) = 42 or, more realistically, (i += v) += 42 .

I wonder if the following resolution is sufficient:

Append to 5.17 [expr.ass] paragraph 1:

There is a sequence point between assigning the new value to the left operand and yielding the result of the assignment expression.

I believe that this proposal achieves my desired effect of not constraining when j is incremented in x[j++] = y, because I don't think there is a constraint on the relative order of incrementing j and executing the assignment. However, I do think it allows expressions such as (i += v) += 42, although with different semantics from C if v is volatile.

Notes on 10/01 meeting:

There was agreement that adding a sequence point is probably the right solution.

Notes from the 4/02 meeting:

The working group reaffirmed the sequence-point solution, but we will look for any counter-examples where efficiency would be harmed.

For drafting, we note that ++x is defined in 5.3.2 [expr.pre.incr] as equivalent to x+=1 and is therefore affected by this change. x++ is not affected. Also, we should update any list of all sequence points.

Notes from October 2004 meeting:

Discussion centered around whether a sequence point “between assigning the new value to the left operand and yielding the result of the expression” would require completion of all side effects of the operand expressions before the value of the assignment expression was used in another expression. The consensus opinion was that it would, that this is the definition of a sequence point. Jason Merrill pointed out that adding a sequence point after the assignment is essentially the same as rewriting

    b += a

as

    b += a, b

Clark Nelson expressed a desire for something like a “weak” sequence point that would force the assignment to occur but that would leave the side effects of the operands unconstrained. In support of this position, he cited the following expression:

    j = (i = j++)

With the proposed addition of a full sequence point after the assignment to i, the net effect is no change to j. However, both g++ and MSVC++ behave differently: if the previous value of j is 5, the value of the expression is 5 but j gets the value 6.

Clark Nelson will investigate alternative approaches and report back to the working group.

Proposed resolution (March, 2008):

See issue 637.




351. Sequence point error: unspecified or undefined?

Section: 5  [expr]     Status: CD1     Submitter: Andrew Koenig     Date: 23 April 2002

[Voted into WP at March 2004 meeting.]

I have found what looks like a bug in clause 5 [expr], paragraph 4:

Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored. The requirements of this paragraph shall be met for each allowable ordering of the subexpressions of a full expression; otherwise the behavior is undefined. Example:
        i = v[i++];                     // the behavior is unspecified
        i = 7, i++, i++;                // i becomes 9

        i = ++i + 1;                    // the behavior is unspecified
        i = i + 1;                      // the value of i is incremented
--end example]

So which is it, unspecified or undefined?

Notes from October 2002 meeting:

We should find out what C99 says and do the same thing.

Proposed resolution (April 2003):

Change the example in clause 5 [expr], paragraph 4 from

[Example:
i = v[i++];                     //  the behavior is unspecified
i = 7, i++, i++;                //   i  becomes  9

i = ++i + 1;                    //  the behavior is unspecified
i = i + 1;                      //  the value of  i  is incremented
--- end example]

to (changing "unspecified" to "undefined" twice)

[Example:
i = v[i++];                     //  the behavior is undefined
i = 7, i++, i++;                //   i  becomes  9

i = ++i + 1;                    //  the behavior is undefined
i = i + 1;                      //  the value of  i  is incremented
--- end example]



451. Expressions with invalid results and ill-formedness

Section: 5  [expr]     Status: CD1     Submitter: Gennaro Prota     Date: 19 Jan 2004

[Voted into WP at October 2005 meeting.]

Clause 5 [expr] par. 5 of the standard says:

If during the evaluation of an expression, the result is not mathematically defined or not in the range of representable values for its type, the behavior is undefined, unless such an expression is a constant expression (5.19), in which case the program is ill-formed.

Well, we do know that except in some contexts (e.g. controlling expression of a #if, array bounds), a compiler is not required to evaluate constant-expressions in compile time, right?

Now, let us consider, the following simple snippet:

  if (a && 1/0)
      ...
with a, to fix our attention, being *not* a constant expression. The quote above seems to say that since 1/0 is a constant (sub-)expression, the program is ill-formed. So, is it the intent that such ill-formedness is diagnosable at run-time? Or is it the intent that the above gives undefined behavior (if 1/0 is evaluated) and is not ill-formed?

I think the intent is actually the latter, so I propose the following rewording of the quoted section:

If an expression is evaluated but its result is not mathematically defined or not in the range of representable values for its type the behavior is undefined, unless such an expression is a constant expression (5.19) that shall be evaluated during program translation, in which case the program is ill-formed.

Rationale (March, 2004):

We feel the standard is clear enough. The quoted sentence does begin "If during the evaluation of an expression, ..." so the rest of the sentence does not apply to an expression that is not evaluated.

Note (September, 2004):

Gennaro Prota feels that the CWG missed the point of his original comment: unless a constant expression appears in a context that requires a constant expression, an implementation is permitted to defer its evaluation to runtime. An evaluation that fails at runtime cannot affect the well-formedness of the program; only expressions that are evaluated at compile time can make a program ill-formed.

The status has been reset to “open” to allow further discussion.

Proposed resolution (October, 2004):

Change paragraph 5 of 5 [expr] as indicated:

If during the evaluation of an expression, the result is not mathematically defined or not in the range of representable values for its type, the behavior is undefined, unless such an expression is a constant expression appears where an integral constant expression is required (5.19 [expr.const]), in which case the program is ill-formed.



122. template-ids as unqualified-ids

Section: 5.1.1  [expr.prim.general]     Status: CD1     Submitter: Mike Miller     Date: 3 June 1999

[Moved to DR at 10/01 meeting.]

5.1.1 [expr.prim.general] paragraph 11 reads,

A template-id shall be used as an unqualified-id only as specified in 14.7.2 [temp.explicit] , 14.7 [temp.spec] , and 14.5.5 [temp.class.spec] .

What uses of template-ids as unqualified-ids is this supposed to prevent? And is the list of referenced sections correct/complete? For instance, what about 14.8.1 [temp.arg.explicit], "Explicit template argument specification?" Does its absence from the list in 5.1.1 [expr.prim.general] paragraph 11 mean that "f<int>()" is ill-formed?

This is even more confusing when you recall that unqualified-ids are contained in qualified-ids:

qualified-id: ::opt nested-name-specifier templateopt unqualified-id

Is the wording intending to say "used as an unqualified-id that is not part of a qualified-id?" Or something else?

Proposed resolution (10/00):

Remove the referenced sentence altogether.




125. Ambiguity in friend declaration syntax

Section: 5.1.1  [expr.prim.general]     Status: CD1     Submitter: Martin von Loewis     Date: 7 June 1999

[Voted into WP at March 2004 meeting.]

The example below is ambiguous.

    struct A{
      struct B{};
    };

    A::B C();

    namespace B{
      A C();
    }

    struct Test {
      friend A::B ::C();
    };
Here, it is not clear whether the friend declaration denotes A B::C() or A::B C(), yet the standard does not resolve this ambiguity.

The ambiguity arises since both the simple-type-specifier (7.1.6.2 [dcl.type.simple] paragra 1) and an init-declararator (8 [dcl.decl] paragraph 1) contain an optional :: and an optional nested-name-specifier (5.1.1 [expr.prim.general] paragraph 1). Therefore, two different ways to analyse this code are possible:

simple-type-specifier = A::B
init-declarator = ::C()
simple-declaration = friend A::B ::C();
or
simple-type-specifier = A
init-declarator = ::B::C()
simple-declaration = friend A ::B::C();
Since it is a friend declaration, the init-declarator may be qualified, and start with a global scope.

Suggested Resolution: In the definition of nested-name-specifier, add a sentence saying that a :: token immediately following a nested-name-specifier is always considered as part of the nested-name-specifier. Under this interpretation, the example is ill-formed, and should be corrected as either

    friend A (::B::C)();   //or
    friend A::B (::C)();

An alternate suggestion — changing 7.1 [dcl.spec] to say that

The longest sequence of tokens that could possibly be a type name is taken as the decl-specifier-seq of a declaration.

— is undesirable because it would make the example well-formed rather than requiring the user to disambiguate the declaration explicitly.

Proposed resolution (04/01):

(See below for problem with this, from 10/01 meeting.)

In 5.1.1 [expr.prim.general] paragraph 7,

  1. Before the grammar for qualified-id, start a new paragraph 7a with the text

    A qualified-id is an id-expression that contains the scope resolution operator ::.
  2. Following the grammar fragment, insert the following:

    The longest sequence of tokens that could form a qualified-id constitutes a single qualified-id. [Example:

        // classes C, D; functions F, G, namespace N; non-class type T
        friend C ::D::F();   // ill-formed, means friend (C::D::F)();
        friend C (::D::F)(); // well-formed
        friend N::T ::G();   // ill-formed, means friend (N::T::G)();
        friend N::T (::G)(); // well-formed
    

    end example]

  3. Start a new paragraph 7b following the example.

(This resolution depends on that of issue 215.)

Notes from 10/01 meeting:

It was pointed out that the proposed resolution does not deal with cases like X::Y where X is a type but not a class type. The working group reaffirmed its decision that the disambiguation should be syntactic only, i.e., it should depend only on whether or not the name is a type.

Jason Merrill :

At the Seattle meeting, I suggested that a solution might be to change the class-or-namespace-name in the nested-name-specifier rule to just be "identifier"; there was some resistance to this idea. FWIW, I've tried this in g++. I had to revise the idea so that only the second and subsequent names were open to being any identifier, but that seems to work just fine.

So, instead of

it would be

Or some equivalent but right-associative formulation, if people feel that's important, but it seems irrelevant to me.

Clark Nelson :

Personally, I prefer the left-associative rule. I think it makes it easier to understand. I was thinking about this production a lot at the meeting, considering also some issues related to 301. My formulation was getting kind of ugly, but with a left-associative rule, it gets a lot nicer.

Your proposal isn't complete, however, as it doesn't allow template arguments without an explicit template keyword. You probably want to add an alternative for:

There is admittedly overlap between this alternative and

but I think they're both necessary.

Notes from the 4/02 meeting:

The changes look good. Clark Nelson will merge the two proposals to produce a single proposed resolution.

Proposed resolution (April 2003):

nested-name-specifier is currently defined in 5.1.1 [expr.prim.general] paragraph 7 as:

The proposed definition is instead:

Issue 215 is addressed by using type-name instead of class-name in the first alternative. Issue 125 (this issue) is addressed by using identifier instead of anything more specific in the third alternative. Using left association instead of right association helps eliminate the need for class-or-namespace-name (or type-or-namespace-name, as suggested for issue 215).

It should be noted that this formulation also rules out the possibility of A::template B::, i.e. using the template keyword without any template arguments. I think this is according to the purpose of the template keyword, and that the former rule allowed such a construct only because of the difficulty of formulation of a right-associative rule that would disallow it. But I wanted to be sure to point out this implication.

Notes from April 2003 meeting:

See also issue 96.

The proposed change resolves only part of issue 215.




113. Visibility of called function

Section: 5.2.2  [expr.call]     Status: CD1     Submitter: Christophe de Dinechin     Date: 5 May 1999

[Moved to DR at 10/01 meeting.]

Christophe de Dinechin: In 5.2.2 [expr.call] , paragraph 2 reads:

If no declaration of the called function is visible from the scope of the call the program is ill-formed.
I think nothing there or in the previous paragraph indicates that this does not apply to calls through pointer or virtual calls.

Mike Miller: "The called function" is unfortunate phraseology; it makes it sound as if it's referring to the function actually called, as opposed to the identifier in the postfix expression. It's wrong with respect to Koenig lookup, too (the declaration need not be visible if it can be found in a class or namespace associated with one or more of the arguments).

In fact, this paragraph should be a note. There's a general rule that says you have to find an unambiguous declaration of any name that is used (3.4 [basic.lookup] paragraph 1); the only reason this paragraph is here is to contrast with C's implicit declaration of called functions.

Proposed resolution:

Change section 5.2.2 [expr.call] paragraph 2 from:
If no declaration of the called function is visible from the scope of the call the program is ill-formed.
to:
[Note: if a function or member function name is used, and name lookup (3.4 [basic.lookup]) does not find a declaration of that name, the program is ill-formed. No function is implicitly declared by such a call. ]

(See also issue 218.)




118. Calls via pointers to virtual member functions

Section: 5.2.2  [expr.call]     Status: CD1     Submitter: Martin O'Riordan     Date: 17 May 1999

[Voted into the WP at the June, 2008 meeting.]

Martin O'Riordan: Having gone through all the relevant references in the IS, it is not conclusive that a call via a pointer to a virtual member function is polymorphic at all, and could legitimately be interpreted as being static.

Consider 5.2.2 [expr.call] paragraph 1:

The function called in a member function call is normally selected according to the static type of the object expression (clause 10 [class.derived] ), but if that function is virtual and is not specified using a qualified-id then the function actually called will be the final overrider (10.3 [class.virtual] ) of the selected function in the dynamic type of the object expression.
Here it is quite specific that you get the polymorphic call only if you use the unqualified syntax. But, the address of a member function is "always" taken using the qualified syntax, which by inference would indicate that call with a PMF is static and not polymorphic! Not what was intended.

Yet other references such as 5.5 [expr.mptr.oper] paragraph 4:

If the dynamic type of the object does not contain the member to which the pointer refers, the behavior is undefined.
indicate that the opposite may have been intended, by stating that it is the dynamic type and not the static type that matters. Also, 5.5 [expr.mptr.oper] paragraph 6:
If the result of .* or ->* is a function, then that result can be used only as the operand for the function call operator (). [Example:
        (ptr_to_obj->*ptr_to_mfct)(10);
calls the member function denoted by ptr_to_mfct for the object pointed to by ptr_to_obj. ]
which also implies that it is the object pointed to that determines both the validity of the expression (the static type of 'ptr_to_obj' may not have a compatible function) and the implicit (polymorphic) meaning. Note too, that this is stated in the non-normative example text.

Andy Sawyer: Assuming the resolution is what I've assumed it is for the last umpteen years (i.e. it does the polymorphic thing), then the follow on to that is "Should there also be a way of selecting the non-polymorphic behaviour"?

Mike Miller: It might be argued that the current wording of 5.2.2 [expr.call] paragraph 1 does give polymorphic behavior to simple calls via pointers to members. (There is no qualified-id in obj.*pmf, and the IS says that if the function is not specified using a qualified-id, the final overrider will be called.) However, it clearly says the wrong thing when the pointer-to-member itself is specified using a qualified-id (obj.*X::pmf).

Bill Gibbons: The phrase qualified-id in 5.2.2 [expr.call] paragraph 1 refers to the id-expression and not to the "pointer-to-member expression" earlier in the paragraph:

For a member function call, the postfix expression shall be an implicit (9.3.1 [class.mfct.non-static] , 9.4 [class.static] ) or explicit class member access (5.2.5 [expr.ref] ) whose id-expression is a function member name, or a pointer-to-member expression (5.5 [expr.mptr.oper] ) selecting a function member.

Mike Miller: To be clear, here's an example:

    struct S {
	virtual void f();
    };
    void (S::*pmf)();
    void g(S* sp) {
	sp->f();         // 1: polymorphic
	sp->S::f();      // 2: non-polymorphic
	(sp->S::f)();    // 3: non-polymorphic
	(sp->*pmf)();    // 4: polymorphic
	(sp->*&S::f)();  // 5: polymorphic
    }

Notes from October 2002 meeting:

This was moved back to open for lack of a champion. Martin O'Riordan is not expected to be attending meetings.

Proposed resolution (February, 2008):

  1. Change 5.2.2 [expr.call] paragraph 1 as follows:

    ... For a member function call, the postfix expression shall be an implicit (9.3.1 [class.mfct.non-static], 9.4 [class.static]) or explicit class member access (5.2.5 [expr.ref]) whose id-expression is a function member name, or a pointer-to-member expression (5.5 [expr.mptr.oper]) selecting a function member. The first expression in the postfix expression is then called the object expression, and; the call is as a member of the object pointed to or referred to by the object expression (5.2.5 [expr.ref], 5.5 [expr.mptr.oper]). In the case of an implicit class member access, the implied object is the one pointed to by this. [Note: a member function call of the form f() is interpreted as (*this).f() (see 9.3.1 [class.mfct.non-static]). —end note] If a function or member function name is used, the name can be overloaded (clause 13 [over]), in which case the appropriate function shall be selected according to the rules in 13.3 [over.match]. The function called in a member function call is normally selected according to the static type of the object expression (clause 10 [class.derived]), but if that function is virtual and is not specified using a qualified-id then the function actually called will be the final overrider (10.3 [class.virtual]) of the selected function in the dynamic type of the object expression If the selected function is non-virtual, or if the id-expression in the class member access expression is a qualified-id, that function is called. Otherwise, its final overrider (10.3 [class.virtual]) in the dynamic type of the object expression is called. ...
  2. Change 5.5 [expr.mptr.oper] paragraph 4 as follows:

    The first operand is called the object expression. If the dynamic type of the object expression does not contain the member to which the pointer refers, the behavior is undefined.



506. Conditionally-supported behavior for non-POD objects passed to ellipsis

Section: 5.2.2  [expr.call]     Status: CD1     Submitter: Mike Miller     Date: 14 Apr 2005

[Voted into WP at the October, 2006 meeting.]

The current wording of 5.2.2 [expr.call] paragraph 7 states:

When there is no parameter for a given argument, the argument is passed in such a way that the receiving function can obtain the value of the argument by invoking va_arg (18.10 [support.runtime]). The lvalue-to-rvalue (4.1 [conv.lval]), array-to-pointer (4.2 [conv.array]), and function-to-pointer (4.3 [conv.func]) standard conversions are performed on the argument expression. After these conversions, if the argument does not have arithmetic, enumeration, pointer, pointer to member, or class type, the program is ill-formed. If the argument has a non-POD class type (clause 9 [class]), the behavior is undefined.

Paper J16/04-0167=WG21 N1727 suggests that passing a non-POD object to ellipsis be ill-formed. In discussions at the Lillehammer meeting, however, the CWG felt that the newly-approved category of conditionally-supported behavior would be more appropriate.

Proposed resolution (October, 2005):

Change 5.2.2 [expr.call] paragraph 7 as indicated:

...After these conversions, if the argument does not have arithmetic, enumeration, pointer, pointer to member, or class type, the program is ill-formed. If the argument has a non-POD class type (clause 9), the behavior is undefined. Passing an argument of non-POD class type (clause 9) with no corresponding parameter is conditionally-supported, with implementation-defined semantics.



634. Conditionally-supported behavior for non-POD objects passed to ellipsis redux

Section: 5.2.2  [expr.call]     Status: CD1     Submitter: Howard Hinnant     Date: 6 Jun 2007

[Voted into the WP at the September, 2008 meeting.]

Issue 506 changed passing a non-POD class type to an ellipsis from undefined behavior to conditionally-supported behavior. As a result, an implementation could conceivably reject code like the following:

    struct two {char _[2];};

    template <class From, class To>
    struct is_convertible
    {
    private:
            static From f;

            template <class U> static char test(const U&);
            template <class U> static two test(...);
    public:
            static const bool value = sizeof(test<To>(f)) == 1;
    };

    struct A
    {
         A();
    };

    int main()
    {
         const bool b = is_convertible<A,int>::value;  // b == false
    }

This technique has become popular in template metaprogramming, and no non-POD object is actually passed at runtime. Concepts will eliminate much (perhaps not all) of the need for this kind of programming, but legacy code will persist.

Perhaps this technique should be officially supported by allowing implementations to reject passing a non-POD type to ellipsis only if it appears in a potentially-evaluated expression?

Notes from the July, 2007 meeting:

The CWG agreed with the suggestion to allow such calls in unevaluated contexts.

Proposed resolution (September, 2007):

Change 5.2.2 [expr.call] paragraph 7 as follows:

...Passing an a potentially-evaluated argument of non-trivial class type (clause 9 [class]) with no corresponding parameter is conditionally-supported, with implementation-defined semantics...



466. cv-qualifiers on pseudo-destructor type

Section: 5.2.4  [expr.pseudo]     Status: CD1     Submitter: Mark Mitchell     Date: 18 Mar 2004

[Voted into WP at April, 2006 meeting.]

5.2.4 [expr.pseudo] paragraph 2 says both:

The type designated by the pseudo-destructor-name shall be the same as the object type.
and also:
The cv-unqualified versions of the object type and of the type designated by the pseudo-destructor-name shall be the same type.
Which is it? "The same" or "the same up to cv-qualifiers"? The second sentence is more generous than the first. Most compilers seem to implement the less restrictive form, so I guess that's what I think we should do.

See also issues 305 and 399.

Proposed resolution (October, 2005):

Change 5.2.4 [expr.pseudo] paragraph 2 as follows:

The left-hand side of the dot operator shall be of scalar type. The left-hand side of the arrow operator shall be of pointer to scalar type. This scalar type is the object type. The type designated by the pseudo-destructor-name shall be the same as the object type. The cv-unqualified versions of the object type and of the type designated by the pseudo-destructor-name shall be the same type. Furthermore, the two type-names in a pseudo-destructor-name of the form shall designate the same scalar type. The cv-unqualified versions of the object type and of the type designated by the pseudo-destructor-name shall be the same type.



421. Is rvalue.field an rvalue?

Section: 5.2.5  [expr.ref]     Status: CD1     Submitter: Gabriel Dos Reis     Date: 15 June 2003

[Voted into WP at March 2004 meeting.]

Consider

  typedef
    struct {
      int a;
    } A;

  A f(void)
  {
    A a;
    return a;
  }

  int main(void)
  {
    int* p = &f().a;   // #1
  }

Should #1 be rejected? The standard is currently silent.

Mike Miller: I don't believe the Standard is silent on this. I will agree that the wording of 5.2.5 [expr.ref] paragraph 4 bullet 2 is unfortunate, as it is subject to misinterpretation. It reads,

If E1 is an lvalue, then E1.E2 is an lvalue.
The intent is, "and not otherwise."

Notes from October 2003 meeting:

We agree the reference should be an rvalue, and a change along the lines of that recommended by Mike Miller is reasonable.

Proposed Resolution (October 2003):

Change the second bullet of 5.2.5 [expr.ref] paragraph 4 to read:

If E1 is an lvalue, then E1.E2 is an lvalue; otherwise, it is an rvalue.



492. typeid constness inconsistent with example

Section: 5.2.8  [expr.typeid]     Status: CD1     Submitter: Ron Natalie     Date: 15 Dec 2004

[Voted into WP at April, 2006 meeting.]

There is an inconsistency between the normative text in section 5.2.8 [expr.typeid] and the example that follows.

Here is the relevant passage (starting with paragraph 4):

When typeid is applied to a type-id, the result refers to a std::type_info object representing the type of the type-id. If the type of the type-id is a reference type, the result of the typeid expression refers to a std::type_info object representing the referenced type.

The top-level cv-qualifiers of the lvalue expression or the type-id that is the operand of typeid are always ignored.

and the example:

    typeid(D) == typeid(const D&); // yields true

The second paragraph above says the “type-id that is the operand”. This would be const D&. In this case, the const is not at the top-level (i.e., applied to the operand itself).

By a strict reading, the above should yield false.

My proposal is that the strict reading of the normative test is correct. The example is wrong. Different compilers here give different answers.

Proposed resolution (April, 2005):

Change the second sentence of 5.2.8 [expr.typeid] paragraph 4 as follows:

If the type of the type-id is a reference to a possibly cv-qualified type, the result of the typeid expression refers to a std::type_info object representing the cv-unqualified referenced type.



54. Static_cast from private base to derived class

Section: 5.2.9  [expr.static.cast]     Status: CD1     Submitter: Steve Adamczyk     Date: 13 Oct 1998

[Voted into WP at October 2004 meeting.]

Is it okay to use a static_cast to cast from a private base class to a derived class? That depends on what the words "valid standard conversion" in paragraph 8 mean — do they mean the conversion exists, or that it would not get an error if it were done? I think the former was intended — and therefore a static_cast from a private base to a derived class would be allowed.

Rationale (04/99): A static_cast from a private base to a derived class is not allowed outside a member from the derived class, because 4.10 [conv.ptr] paragraph 3 implies that the conversion is not valid. (Classic style casts work.)

Reopened September 2003:

Steve Adamczyk: It makes some sense to disallow the inverse conversion that is pointer-to-member of derived to pointer-to-member of private base. There's less justification for the pointer-to-private-base to pointer-to-derived case. EDG, g++ 3.2, and MSVC++ 7.1 allow the pointer case and disallow the pointer-to-member case. Sun disallows the pointer case as well.

  struct B {};
  struct D : private B {};
  int main() {
    B *p = 0;
    static_cast<D *>(p);  // Pointer case: should be allowed
    int D::*pm = 0;
    static_cast<int B::*>(pm);  // Pointer-to-member case: should get error
  }

There's a tricky case with old-style casts: because the static_cast interpretation is tried first, you want a case like the above to be considered a static_cast, but then issue an error, not be rejected as not a static cast; if you did the latter, you would then try the cast as a reinterpret_cast.

Ambiguity and casting to a virtual base should likewise be errors after the static_cast interpretation is selected.

Notes from the October 2003 meeting:

There was lots of sentiment for making things symmetrical: the pointer case should be the same as the pointer-to-member case. g++ 3.3 now issues errors on both cases.

We decided an error should be issued on both cases. The access part of the check should be done later; by some definition of the word the static_cast is valid, and then later an access error is issued. This is similar to the way standard conversions work.

Proposed Resolution (October 2003):

Replace paragraph 5.2.9 [expr.static.cast]/6:

The inverse of any standard conversion sequence (clause 4 [conv]), other than the lvalue-to-rvalue (4.1 [conv.lval]), array-to-pointer (4.2 [conv.array]), function-to-pointer (4.3 [conv.func]), and boolean (4.12 [conv.bool]) conversions, can be performed explicitly using static_cast. The lvalue-to-rvalue (4.1 [conv.lval]), array-to-pointer (4.2 [conv.array]), and function-to-pointer (4.3 [conv.func]) conversions are applied to the operand. Such a static_cast is subject to the restriction that the explicit conversion does not cast away constness (5.2.11 [expr.const.cast]), and the following additional rules for specific cases:

with two paragraphs:

The inverse of any standard conversion sequence (clause 4 [conv]), other than the lvalue-to-rvalue (4.1 [conv.lval]), array-to-pointer (4.2 [conv.array]), function-to-pointer (4.3 [conv.func]), and boolean (4.12 [conv.bool]) conversions, can be performed explicitly using static_cast. A program is ill-formed if it uses static_cast to perform the inverse of an ill-formed standard conversion sequence.[Example:

struct B {};
struct D : private B {};
void f() {
  static_cast<D*>((B*)0); // Error: B is a private base of D.
  static_cast<int B::*>((int D::*)0); // Error: B is a private base of D.
}
--- end example]

The lvalue-to-rvalue (4.1 [conv.lval]), array-to-pointer (4.2 [conv.array]), and function-to-pointer (4.3 [conv.func]) conversions are applied to the operand. Such a static_cast is subject to the restriction that the explicit conversion does not cast away constness (5.2.11 [expr.const.cast]), and the following additional rules for specific cases:

In addition, modify the second sentence of 5.4 [expr.cast]/5. The first two sentences of 5.4 [expr.cast]/5 presently read:

The conversions performed by can be performed using the cast notation of explicit type conversion. The same semantic restrictions and behaviors apply.

Change the second sentence to read:

The same semantic restrictions and behaviors apply, with the exception that in performing a static_cast in the following situations the conversion is valid even if the base class is inaccessible:

Remove paragraph 5.4 [expr.cast]/7, which presently reads:

In addition to those conversions, the following static_cast and reinterpret_cast operations (optionally followed by a const_cast operation) may be performed using the cast notation of explicit type conversion, even if the base class type is not accessible:



427. static_cast ambiguity: conversion versus cast to derived

Section: 5.2.9  [expr.static.cast]     Status: CD1     Submitter: Mark Mitchell     Date: 5 July 2003

[Voted into WP at October 2004 meeting.]

Consider this code:

  struct B {};

  struct D : public B { 
    D(const B&);
  };

  extern B& b;

  void f() {
    static_cast<const D&>(b);
  }

The rules for static_cast permit the conversion to "const D&" in two ways:

  1. D is derived from B, and b is an lvalue, so a cast to D& is OK.
  2. const D& t = b is valid, using the constructor for D. [Ed. note: actually, this should be parenthesized initialization.]

The first alternative is 5.2.9 [expr.static.cast]/5; the second is 5.2.9 [expr.static.cast]/2.

Presumably the first alternative is better -- it's the "simpler" conversion. The standard does not seem to make that clear.

Steve Adamczyk: I take the "Otherwise" at the beginning of 5.2.9 [expr.static.cast]/3 as meaning that the paragraph 2 interpretation is used if available, which means in your example above interpretation 2 would be used. However, that's not what EDG's compiler does, and I agree that it's not the "simpler" conversion.

Proposed Resolution (October 2003):

Move paragraph 5.2.9/5:

An lvalue of type ``cv1 B'', where B is a class type, can be cast to type ``reference to cv2 D'', where D is a class derived (clause 10 [class.derived]) from B, if a valid standard conversion from ``pointer to D'' to ``pointer to B'' exists (4.10 [conv.ptr]), cv2 is the same cv-qualification as, or greater cv-qualification than, cv1, and B is not a virtual base class of D. The result is an lvalue of type ``cv2 D.'' If the lvalue of type ``cv1 B'' is actually a sub-object of an object of type D, the lvalue refers to the enclosing object of type D. Otherwise, the result of the cast is undefined. [Example:

  struct B {};
  struct D : public B {};
  D d;
  B &br = d;

  static_cast<D&>(br);            //  produces lvalue to the original  d  object
--- end example]

before paragraph 5.2.9 [expr.static.cast]/2.

Insert Otherwise, before the text of paragraph 5.2.9 [expr.static.cast]/2 (which will become 5.2.9 [expr.static.cast]/3 after the above insertion), so that it reads:

Otherwise, an expression e can be explicitly converted to a type T using a static_cast of the form static_cast<T>(e) if the declaration "T t(e);" is well-formed, for some invented temporary variable t (8.5 [dcl.init]). The effect of such an explicit conversion is the same as performing the declaration and initialization and then using the temporary variable as the result of the conversion. The result is an lvalue if T is a reference type (8.3.2 [dcl.ref]), and an rvalue otherwise. The expression e is used as an lvalue if and only if the initialization uses it as an lvalue.




439. Guarantees on casting pointer back to cv-qualified version of original type

Section: 5.2.9  [expr.static.cast]     Status: CD1     Submitter: Mark Mitchell     Date: 30 Oct 2003

[Voted into WP at April 2005 meeting.]

Paragraph 5.2.9 [expr.static.cast] paragraph 10 says that:

A value of type pointer to object converted to "pointer to cv void" and back to the original pointer type will have its original value.

That guarantee should be stronger. In particular, given:

  T* p1 = new T;
  const T* p2 = static_cast<const T*>(static_cast<void *>(p1));
  if (p1 != p2)
    abort ();
there should be no call to "abort". The last sentence of Paragraph 5.2.9 [expr.static.cast] paragraph 10 should be changed to read:

A value of type pointer to object converted to "pointer to cv void" and back to the original pointer type (or a variant of the original pointer type that differs only in the cv-qualifiers applied to the object type) will have its original value. [Example:
T* p1 = new T;
const T* p2 = static_cast<const T*>(static_cast<void *>(p1));
bool b = p1 == p2; // b will have the value true.
---end example.]

Proposed resolution:

Change 5.2.9 [expr.static.cast] paragraph 10 as indicated:

A value of type pointer to object converted to "pointer to cv void" and back to the original pointer type, possibly with different cv-qualification, will have its original value. [Example:

  T* p1 = new T;
  const T* p2 = static_cast<const T*>(static_cast<void *>(p1));
  bool b = p1 == p2; // b will have the value true.

---end example]

Rationale: The wording "possibly with different cv-qualification" was chosen over the suggested wording to allow for changes in cv-qualification at different levels in a multi-level pointer, rather than only at the object type level.




671. Explicit conversion from a scoped enumeration type to integral type

Section: 5.2.9  [expr.static.cast]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 22 December 2007

[Voted into the WP at the September, 2008 meeting.]

There appears to be no provision in the Standard for explicit conversion of a value of a scoped enumeration type to an integral type, even though the inverse conversion is permitted. That is,

    enum class E { e };
    static_cast<E>(0);       // #1: OK
    static_cast<int>(E::e);  // #2: error

This is because values of scope enumeration types (intentionally) cannot be implicitly converted to integral types (4.5 [conv.prom] and 4.7 [conv.integral]) and 5.2.9 [expr.static.cast] was not updated to permit #2, although #1 is covered by paragraph 8.

Proposed resolution (June, 2008):

Add the following as a new paragraph following 5.2.9 [expr.static.cast] paragraph 8:

A value of a scoped enumeration type (7.2 [dcl.enum]) can be explicitly converted to an integral type. The value is unchanged if the original value can be represented by the specified type. Otherwise, the resulting value is unspecified.



195. Converting between function and object pointers

Section: 5.2.10  [expr.reinterpret.cast]     Status: CD1     Submitter: Steve Clamage     Date: 12 Jan 2000

[Voted into WP at April 2005 meeting.]

It is currently not permitted to cast directly between a pointer to function type and a pointer to object type. This conversion is not listed in 5.2.9 [expr.static.cast] and 5.2.10 [expr.reinterpret.cast] and thus requires a diagnostic to be issued. However, if a sufficiently long integral type exists (as is the case in many implementations), it is permitted to cast between pointer to function types and pointer to object types using that integral type as an intermediary.

In C the cast results in undefined behavior and thus does not require a diagnostic, and Unix C compilers generally do not issue one. This fact is used in the definition of the standard Unix function dlsym, which is declared to return void* but in fact may return either a pointer to a function or a pointer to an object. The fact that C++ compilers are required to issue a diagnostic is viewed as a "competitive disadvantage" for the language.

Suggested resolution: Add wording to 5.2.10 [expr.reinterpret.cast] allowing conversions between pointer to function and pointer to object types, if the implementation has an integral data type that can be used as an intermediary.

Several points were raised in opposition to this suggestion:


  1. Early C++ supported this conversion and it was deliberately removed during the standardization process.
  2. The existence of an appropriate integral type is irrelevant to whether the conversion is "safe." The condition should be on whether information is lost in the conversion or not.
  3. There are numerous ways to address the problem at an implementation level rather than changing the language. For example, the compiler could recognize the specific case of dlsym and omit the diagnostic, or the C++ binding to dlsym could be changed (using templates, for instance) to circumvent the violation.
  4. The conversion is, in fact, not supported by C; the dlsym function is simply relying on non-mandated characteristics of C implementations, and we would be going beyond the requirements of C compatibility in requiring (some) implementations to support the conversion.
  5. This issue is in fact not a defect (omitted or self-contradictory requirements) in the current Standard; the proposed change would actually be an extension and should not be considered until the full review of the IS.
  6. dlsym appears not to be used very widely, and the declaration in the header file is not problematic, only calls to it. Since C code generally requires some porting to be valid C++ anyway, this should be considered one of those items that requires porting.

Martin O'Riordan suggested an alternative approach:


The advantage of this approach is that it would permit writing portable, well-defined programs involving such conversions. However, it breaks the current degree of compatibility between old and new casts, and it adds functionality to dynamic_cast which is not obviously related to its current meaning.

Notes from 04/00 meeting:

Andrew Koenig suggested yet another approach: specify that "no diagnostic is required" if the implementation supports the conversion.

Later note:

It was observed that conversion between function and data pointers is listed as a "common extension" in C99.

Notes on the 10/01 meeting:

It was decided that we want the conversion defined in such a way that it always exists but is always undefined (as opposed to existing only when the size relationship is appropriate, and being implementation-defined in that case). This would allow an implementation to issue an error at compile time if the conversion does not make sense.

Bill Gibbons notes that the definitions of the other similar casts are inconsistent in this regard. Perhaps they should be updated as well.

Proposed resolution (April 2003):

After 5.2.10 [expr.reinterpret.cast] paragraph 6, insert:

A pointer to a function can be explicitly converted to a pointer to a function of a different type. The effect of calling a function through a pointer to a function type (8.3.5 [dcl.fct]) that is not the same as the type used in the definition of the function is undefined. Except that converting an rvalue of type ``pointer to T1'' to the type ``pointer to T2'' (where T1 and T2 are function types) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified. [Note: see also 4.10 [conv.ptr] for more details of pointer conversions. ] It is implementation defined whether a conversion from pointer to object to pointer to function and/or a conversion from pointer to function to pointer to object exist.
and in paragraph 10:
An lvalue expression of type T1 can be cast to the type ``reference to T2'' if T1 and T2 are object types and an expression of type ``pointer to T1'' can be explicitly converted to the type ``pointer to T2'' using a reinterpret_cast. That is, a reference cast reinterpret_cast< T& >(x) has the same effect as the conversion *reinterpret_cast< T* >(&x) with the built-in & and * operators. The result is an lvalue that refers to the same object as the source lvalue, but with a different type. No temporary is created, no copy is made, and constructors (12.1 [class.ctor]) or conversion functions (12.3 [class.conv]) are not called.

Drafting Note:

If either conversion exists, the implementation already has to define the behavior (paragraph 3).

Notes from April 2003 meeting:

The new consensus is that if the implementation allows this cast, pointer-to-function converted to pointer-to-object converted back to the original pointer-to-function should work; anything else is undefined behavior. If the implementation does not allow the cast, it should be ill-formed.

Tom Plum is investigating a new concept, that of a "conditionally-defined" feature, which may be applicable here.

Proposed Resolution (October, 2004):

(See paper J16/04-0067 = WG21 N1627 for background material and rationale for this resolution. The resolution proposed here differs only editorially from the one in the paper.)

  1. Insert the following into 1.3 [intro.defs] and renumber all following definitions accordingly:

    1.3.2  conditionally-supported behavior

    behavior evoked by a program construct that is not a mandatory requirement of this International Standard. If a given implementation supports the construct, the behavior shall be as described herein; otherwise, the implementation shall document that the construct is not supported and shall treat a program containing an occurrence of the construct as ill-formed (1.3 [intro.defs]).

  2. Add the indicated words to 1.4 [intro.compliance] paragraph 2, bullet 2:

  3. Insert the following as a new paragraph following 5.2.10 [expr.reinterpret.cast] paragraph 7:

    Converting a pointer to a function to a pointer to an object type or vice versa evokes conditionally-supported behavior. In any such conversion supported by an implementation, converting from an rvalue of one type to the other and back (possibly with different cv-qualification) shall yield the original pointer value; mappings between pointers to functions and pointers to objects are otherwise implementation-defined.
  4. Change 7.4 [dcl.asm] paragraph 1 as indicated:

    The meaning of an An asm declaration evokes conditionally-supported behavior. If supported, its meaning is implementation-defined.
  5. Change 7.5 [dcl.link] paragraph 2 as indicated:

    The string-literal indicates the required language linkage. The meaning of the string-literal is implementation-defined. A linkage-specification with a string that is unknown to the implementation is ill-formed. This International Standard specifies the semantics of C and C++ language linkage. Other values of the string-literal evoke conditionally-supported behavior, with implementation-defined semantics. [Note: Therefore, a linkage-specification with a string-literal that is unknown to the implementation requires a diagnostic. When the string-literal in a linkage-specification names a programming language, the spelling of the programming language's name is implementation-defined. [Note: It is recommended that the spelling be taken from the document defining that language, for example Ada (not ADA) and Fortran or FORTRAN (depending on the vintage). The semantics of a language linkage other than C++ or C are implementation-defined. ]
  6. Change 14 [temp] paragraph 4 as indicated:

    A template, a template explicit specialization (14.7.3 [temp.expl.spec]), or a class template partial specialization shall not have C linkage. If the linkage of one of these is something other than C or C++, the behavior is implementation-defined result is conditionally-supported behavior, with implementation-defined semantics.



463. reinterpret_cast<T*>(0)

Section: 5.2.10  [expr.reinterpret.cast]     Status: CD1     Submitter: Gennaro Prota     Date: 14 Feb 2004

[Voted into WP at April, 2006 meeting.]

Is reinterpret_cast<T*>(null_pointer_constant) guaranteed to yield the null pointer value of type T*?

I think a committee clarification is needed. Here's why: 5.2.10 [expr.reinterpret.cast] par. 8 talks of "null pointer value", not "null pointer constant", so it would seem that

  reinterpret_cast<T*>(0)
is a normal int->T* conversion, with an implementation-defined result.

However a little note to 5.2.10 [expr.reinterpret.cast] par. 5 says:

Converting an integral constant expression (5.19) with value zero always yields a null pointer (4.10), but converting other expressions that happen to have value zero need not yield a null pointer.
Where is this supported in normative text? It seems that either the footnote or paragraph 8 doesn't reflect the intent.

SUGGESTED RESOLUTION: I think it would be better to drop the footnote #64 (and thus the special case for ICEs), for two reasons:

a) it's not normative anyway; so I doubt anyone is relying on the guarantee it hints at, unless that guarantee is given elsewhere in a normative part

b) users expect reinterpret_casts to be almost always implementation dependent, so this special case is a surprise. After all, if one wants a null pointer there's static_cast. And if one wants reinterpret_cast semantics the special case requires doing some explicit cheat, such as using a non-const variable as intermediary:

   int v = 0;
   reinterpret_cast<T*>(v); // implementation defined

   reinterpret_cast<T*>(0); // null pointer value of type T*
   const int w = 0;
   reinterpret_cast<T*>(w); // null pointer value of type T*

It seems that not only that's providing a duplicate functionality, but also at the cost to hide what seems the more natural one.

Notes from October 2004 meeting:

This footnote was added in 1996, after the invention of reinterpret_cast, so the presumption must be that it was intentional. At this time, however, the CWG feels that there is no reason to require that reinterpret_cast<T*>(0) produce a null pointer value as its result.

Proposed resolution (April, 2005):

  1. Delete the footnote in 5.2.10 [expr.reinterpret.cast] paragraph 5 reading,

    Converting an integral constant expression (5.19 [expr.const]) with value zero always yields a null pointer (4.10 [conv.ptr]), but converting other expressions that happen to have value zero need not yield a null pointer.
  2. Add the indicated note to 5.2.10 [expr.reinterpret.cast] paragraph 8:

    The null pointer value (4.10 [conv.ptr]) is converted to the null pointer value of the destination type. [Note: A null pointer constant, which has integral type, is not necessarily converted to a null pointer value. —end note]



324. Can "&" be applied to assignment to bit-field?

Section: 5.3.1  [expr.unary.op]     Status: CD1     Submitter: Alasdair Grant     Date: 27 Nov 2001

[Voted into WP at October 2003 meeting.]

An assignment returns an lvalue for its left operand. If that operand refers to a bit field, can the "&" operator be applied to the assignment? Can a reference be bound to it?

  struct S { int a:3; int b:3; int c:3; };

  void f()
  {
    struct S s;
    const int *p = &(s.b = 0);     // (a)
    const int &r = (s.b = 0);      // (b)
          int &r2 = (s.b = 0);     // (c)
  }

Notes from the 4/02 meeting:

The working group agreed that this should be an error.

Proposed resolution (October 2002):

In 5.3.2 [expr.pre.incr] paragraph 1 (prefix "++" and "--" operators), change

The value is the new value of the operand; it is an lvalue.
to
The result is the updated operand; it is an lvalue, and it is a bit-field if the operand is a bit-field.

In 5.16 [expr.cond] paragraph 4 ("?" operator), add the indicated text:

If the second and third operands are lvalues and have the same type, the result is of that type and is an lvalue and it is a bit-field if the second or the third operand is a bit-field, or if both are bit-fields.

In 5.17 [expr.ass] paragraph 1 (assignment operators) add the indicated text (the original text is as updated by issue 221, which is DR but not in TC1):

The assignment operator (=) and the compound assignment operators all group right-to-left. All require a modifiable lvalue as their left operand and return an lvalue with the type and value of the left operand after the assignment has taken place. The result in all cases is a bit-field if the left operand is a bit-field.

Note that issue 222 adds (non-conflicting) text at the end of this same paragraph (5.17 [expr.ass] paragraph 1).

In 5.18 [expr.comma] paragraph 1 (comma operator), change:

The type and value of the result are the type and value of the right operand; the result is an lvalue if its right operand is.
to
The type and value of the result are the type and value of the right operand; the result is an lvalue if the right operand is an lvalue, and is a bit-field if the right operand is an lvalue and a bit-field.

Relevant related text (no changes required):

5.3.1 [expr.unary.op] paragraph 4:

The operand of & shall not be a bit-field.

8.5.3 [dcl.init.ref] paragraph 5, bullet 1, sub-bullet 1 (regarding binding a reference to an lvalue):

... is an lvalue (but is not a bit-field) ...




256. Overflow in size calculations

Section: 5.3.4  [expr.new]     Status: CD1     Submitter: James Kanze     Date: 15 Oct 2000

[Voted into the WP at the September, 2008 meeting.]

[Picked up by evolution group at October 2002 meeting.]

(See also issue 476.)

The size requested by an array allocation is computed by multiplying the number of elements requested by the size of each element and adding an implementation-specific amount for overhead. It is possible for this calculation to overflow. Is an implementation required to detect this situation and, for instance, throw std::bad_alloc?

On one hand, the maximum allocation size is one of the implementation limits specifically mentioned in Annex B [implimits], and, according to 1.4 [intro.compliance] paragraph 2, an implementation is only required to "accept and correctly execute" programs that do not violate its resource limits.

On the other hand, it is difficult or impossible for user code to detect such overflows in a portable fashion, especially given that the array allocation overhead is not fixed, and it would be a service to the user to handle this situation gracefully.

Rationale (04/01):

Each implementation is required to document the maximum size of an object (Annex B [implimits]). It is not difficult for a program to check array allocations to ensure that they are smaller than this quantity. Implementations can provide a mechanism in which users concerned with this problem can request extra checking before array allocations, just as some implementations provide checking for array index and pointer validity. However, it would not be appropriate to require this overhead for every array allocation in every program.

(See issue 624 for a request to reconsider this resolution.)

Note (March, 2008):

The Evolution Working Group has accepted the intent of this issue and referred it to CWG for action for C++0x (see paper J16/07-0033 = WG21 N2173).

Proposed resolution (September, 2008):

This issue is resolved by the resolution of issue 624, given in paper N2757.




299. Conversion on array bound expression in new

Section: 5.3.4  [expr.new]     Status: CD1     Submitter: Mark Mitchell     Date: 19 Jul 2001

[Voted into WP at October 2005 meeting.]

In 5.3.4 [expr.new], the standard says that the expression in an array-new has to have integral type. There's already a DR (issue 74) that says it should also be allowed to have enumeration type. But, it should probably also say that it can have a class type with a single conversion to integral type; in other words the same thing as in 6.4.2 [stmt.switch] paragraph 2.

Suggested resolution:

In 5.3.4 [expr.new] paragraph 6, replace "integral or enumeration type (3.9.1 [basic.fundamental])" with "integral or enumeration type (3.9.1 [basic.fundamental]), or a class type for which a single conversion function to integral or enumeration type exists".

Proposed resolution (October, 2004):

Change 5.3.4 [expr.new] paragraph 6 as follows:

Every constant-expression in a direct-new-declarator shall be an integral constant expression (5.19 [expr.const]) and evaluate to a strictly positive value. The expression in a direct-new-declarator shall have be of integral type, or enumeration type (3.9.1), or a class type for which a single conversion function to integral or enumeration type exists (12.3 [class.conv]). If the expression is of class type, the expression is converted by calling the conversion function, and the result of the conversion is used in place of the original expression. The value of the expression shall bewith a non-negative value. [Example: ...

Proposed resolution (April, 2005):

Change 5.3.4 [expr.new] paragraph 6 as follows:

Every constant-expression in a direct-new-declarator shall be an integral constant expression (5.19 [expr.const]) and evaluate to a strictly positive value. The expression in a direct-new-declarator shall have integral or enumeration type (3.9.1 [basic.fundamental]) with a non-negative value be of integral type, enumeration type, or a class type for which a single conversion function to integral or enumeration type exists (12.3 [class.conv]). If the expression is of class type, the expression is converted by calling that conversion function, and the result of the conversion is used in place of the original expression. If the value of the expression is negative, the behavior is undefined. [Example: ...



429. Matching deallocation function chosen based on syntax or signature?

Section: 5.3.4  [expr.new]     Status: CD1     Submitter: John Wilkinson     Date: 18 July 2003

[Voted into WP at October 2004 meeting.]

What does this example do?

  #include <stdio.h>
  #include <stdlib.h>

  struct A {
        void* operator new(size_t alloc_size, size_t dummy=0) {
                printf("A::operator new()\n");
                return malloc(alloc_size);
        };

        void operator delete(void* p, size_t s) {
                printf("A::delete %d\n", s);
        };


        A()  {printf("A constructing\n"); throw 2;};

  };

  int
  main() {
    try {
        A* ap = new A;
        delete ap;
    }
    catch(int) {printf("caught\n"); return 1;}
  }  

The fundamental issue here is whether the deletion-on-throw is driven by the syntax of the new (placement or non-placement) or by signature matching. If the former, the operator delete would be called with the second argument equal to the size of the class. If the latter, it would be called with the second argument 0.

Core issue 127 (in TC1) dealt with this topic. It removed some wording in 15.2 [except.ctor] paragraph 2 that implied a syntax-based interpretation, leaving wording in 5.3.4 [expr.new] paragraph 19 that is signature-based. But there is no accompanying rationale to confirm an explicit choice of the signature-based approach.

EDG and g++ get 0 for the second argument, matching the presumed core issue 127 resolution. But maybe this should be revisited.

Notes from October 2003 meeting:

There was widespread agreement that the compiler shouldn't just silently call the delete with either of the possible values. In the end, we decided it's smarter to issue an error on this case and force the programmer to say what he means.

Mike Miller's analysis of the status quo: 3.7.4.2 [basic.stc.dynamic.deallocation] paragraph 2 says that "operator delete(void*, std::size_t)" is a "usual (non-placement) deallocation function" if the class does not declare "operator delete(void*)." 3.7.4.1 [basic.stc.dynamic.allocation] does not use the same terminology for allocation functions, but the most reasonable way to understand the uses of the term "placement allocation function" in the Standard is as an allocation function that has more than one parameter and thus can (but need not) be called using the "new-placement" syntax described in 5.3.4 [expr.new]. (In considering issue 127, the core group discussed and endorsed the position that, "If a placement allocation function has default arguments for all its parameters except the first, it can be called using non-placement syntax.")

5.3.4 [expr.new] paragraph 19 says that any non-placement deallocation function matches a non-placement allocation function, and that a placement deallocation function matches a placement allocation function with the same parameter types after the first -- i.e., a non-placement deallocation function cannot match a placement allocation function. This makes sense, because non-placement ("usual") deallocation functions expect to free memory obtained from the system heap, which might not be the case for storage resulting from calling a placement allocation function.

According to this analysis, the example shows a placement allocation function and a non-placement deallocation function, so the deallocation function should not be invoked at all, and the memory will just leak.

Proposed Resolution (October 2003):

Add the following text at the end of 5.3.4 [expr.new] paragraph 19:

If the lookup finds the two-parameter form of a usual deallocation function (3.7.4.2 [basic.stc.dynamic.deallocation]), and that function, considered as a placement deallocation function, would have been selected as a match for the allocation function, the program is ill-formed. [Example:
struct S { 
  // Placement allocation function:
  static void* operator new(std::size_t, std::size_t); 

  // Usual (non-placement) deallocation function:
  static void operator delete(void*, std::size_t); 
}; 

S* p = new (0) S; // ill-formed: non-placement deallocation function matches 
                  // placement allocation function 
--- end example]



624. Overflow in calculating size of allocation

Section: 5.3.4  [expr.new]     Status: CD1     Submitter: Jens Maurer     Date: 8 March 2007

[Voted into the WP at the September, 2008 meeting (resolution in paper N2757).]

Issue 256 was closed without action, principally on the the grounds that an implementation could provide a means (command-line option, #pragma, etc.) for requesting that the allocation size be checked for validity, but that “it would not be appropriate to require this overhead for every array allocation in every program.”

This rationale may be giving too much weight to the overhead such a check would add, especially when compared to the likely cost of actually doing the storage allocation. In particular, the test essentially amounts to something like

    if (max_allocation_size / sizeof(T) < num_elements)
        throw std::bad_alloc();

(noting that max_allocation_size/sizeof(T) is a compile-time constant). It might make more sense to turn the rationale around and require the check, assuming that implementations could provide a mechanism for suppressing it if needed.

Suggested resolution:

In 5.3.4 [expr.new] paragraph 7, add the following words before the example:

If the value of the expression is such that the size of the allocated object would exceed the implementation-defined limit, an exception of type std::bad_alloc is thrown and no storage is obtained.

Note (March, 2008):

The Evolution Working Group has accepted the intent of issue 256 and referred it to CWG for action for C++0x (see paper J16/07-0033 = WG21 N2173).

Proposed resolution (March, 2008):

As suggested.

Notes from the June, 2008 meeting:

The CWG felt that this situation should not be treated like an out-of-memory situation and thus an exception of type std::bad_alloc (or, alternatively, returning a null pointer for a throw() allocator) would not be appropriate.

Proposed resolution (June, 2008):

Change 5.3.4 [expr.new] paragraph 8 as follows:

If the value of the expression in a direct-new-declarator is such that the size of the allocated object would exceed the implementation-defined limit, no storage is obtained and the new-expression terminates by throwing an exception of a type that would match a handler (15.3 [except.handle]) of type std::length_error (19.2.4 [length.error]). Otherwise, if When the value of the that expression in a direct-new-declarator is zero, the allocation function is called to allocate an array with no elements.

[Drafting note: std::length_error is thrown by std::string and std::vector and thus appears to be the right choice for the exception to be thrown here.]




288. Misuse of "static type" in describing pointers

Section: 5.3.5  [expr.delete]     Status: CD1     Submitter: James Kuyper     Date: 19 May 2001

[Voted into the WP at the June, 2008 meeting.]

For delete expressions, 5.3.5 [expr.delete] paragraph 1 says

The operand shall have a pointer type, or a class type having a single conversion function to a pointer type.

However, paragraph 3 of that same section says:

if the static type of the operand is different from its dynamic type, the static type shall be a base class of the operand's dynamic type and the static type shall have a virtual destructor or the behavior is undefined.

Since the operand must be of pointer type, its static type is necessarily the same as its dynamic type. That clause is clearly referring to the object being pointed at, and not to the pointer operand itself.

Correcting the wording gets a little complicated, because dynamic and static types are attributes of expressions, not objects, and there's no sub-expression of a delete-expression which has the relevant types.

Suggested resolution:

then there is a static type and a dynamic type that the hypothetical expression (* const-expression) would have. If that static type is different from that dynamic type, then that static type shall be a base class of that dynamic type, and that static type shall have a virtual destructor, or the behavior is undefined.

There's precedent for such use of hypothetical constructs: see 5.10 [expr.eq] paragraph 2, and 8.1 [dcl.name] paragraph 1.

10.3 [class.virtual] paragraph 3 has a similar problem. It refers to

the type of the pointer or reference denoting the object (the static type).

The type of the pointer is different from the type of the reference, both of which are different from the static type of '*pointer', which is what I think was actually intended. Paragraph 6 contains the exact same wording, in need of the same correction. In this case, perhaps replacing "pointer or reference" with "expression" would be the best fix. In order for this fix to be sufficient, pointer->member must be considered equivalent to (*pointer).member, in which case the "expression" referred to would be (*pointer).

12.5 [class.free] paragraph 4 says that
if a delete-expression is used to deallocate a class object whose static type has...

This should be changed to

if a delete-expression is used to deallocate a class object through a pointer expression whose dereferenced static type would have...

The same problem occurs later, when it says that the

static and dynamic types of the object shall be identical

In this case you could replace "object" with "dereferenced pointer expression".

Footnote 104 says that

5.3.5 [expr.delete] requires that ... the static type of the delete-expression's operand be the same as its dynamic type.

This would need to be changed to

the delete-expression's dereferenced operand

Proposed resolution (December, 2006):

  1. Change 5.3.5 [expr.delete] paragraph 3 as follows:

  2. In the first alternative (delete object), if the static type of the operand object to be deleted is different from its dynamic type, the static type shall be a base class of the operand’s dynamic type of the object to be deleted and the static type shall have a virtual destructor or the behavior is undefined. In the second alternative (delete array) if the dynamic type of the object to be deleted differs from its static type, the behavior is undefined.
  3. Change the footnote in 12.5 [class.free] paragraph 4 as follows:

  4. A similar provision is not needed for the array version of operator delete because 5.3.5 [expr.delete] requires that in this situation, the static type of the delete-expression’s operand object to be deleted be the same as its dynamic type.
  5. Change the footnote in 12.5 [class.free] paragraph 5 as follows:

  6. If the static type in the delete-expression of the object to be deleted is different from the dynamic type and the destructor is not virtual the size might be incorrect, but that case is already undefined; see 5.3.5 [expr.delete].

[Drafting notes: No change is required for 10.3 [class.virtual] paragraph 7 because “the type of the pointer” includes the pointed-to type. No change is required for 12.5 [class.free] paragraph 4 because “...used to deallocate a class object whose static type...” already refers to the object (and not the operand expression).]




353. Is deallocation routine called if destructor throws exception in delete?

Section: 5.3.5  [expr.delete]     Status: CD1     Submitter: Duane Smith     Date: 30 April 2002

[Voted into WP at April 2003 meeting.]

In a couple of comp.std.c++ threads, people have asked whether the Standard guarantees that the deallocation function will be called in a delete-expression if the destructor throws an exception. Most/all people have expressed the opinion that the deallocation function must be called in this case, although no one has been able to cite wording in the Standard supporting that view.

#include <new.h>
#include <stdio.h>
#include <stdlib.h>

static int flag = 0;

inline 
void operator delete(void* p) throw() 
{
   if (flag)
        printf("in deallocation function\n");
   free(p);
}

struct S {
    ~S() { throw 0; }
};

void f() {
    try {
        delete new S;
    }
    catch(...) { }
}

int main() {
       flag=1;
       f();
       flag=0;
       return 0;
}

Proposed resolution (October 2002):

Add to 5.3.5 [expr.delete] paragraph 7 the highlighted text:

The delete-expression will call a deallocation function (3.7.4.2 [basic.stc.dynamic.deallocation]) [Note: The deallocation function is called regardless of whether the destructor for the object or some element of the array throws an exception. ]



442. Incorrect use of null pointer constant in description of delete operator

Section: 5.3.5  [expr.delete]     Status: CD1     Submitter: Matthias Hofmann     Date: 2 Dec 2003

[Voted into WP at October 2005 meeting.]

After some discussion in comp.lang.c++.moderated we came to the conclusion that there seems to be a defect in 5.3.5 [expr.delete]/4, which says:

The cast-expression in a delete-expression shall be evaluated exactly once. If the delete-expression calls the implementation deallocation function (3.7.3.2), and if the operand of the delete expression is not the null pointer constant, the deallocation function will deallocate the storage referenced by the pointer thus rendering the pointer invalid. [Note: the value of a pointer that refers to deallocated storage is indeterminate. ]

In the second sentence, the term "null pointer constant" should be changed to "null pointer". In its present form, the passage claims that the deallocation function will deallocate the storage refered to by a null pointer that did not come from a null pointer constant in the delete expression. Besides, how can the null pointer constant be the operand of a delete expression, as "delete 0" is an error because delete requires a pointer type or a class type having a single conversion function to a pointer type?

See also issue 348.

Proposed resolution:

Change the indicated sentence of 5.3.5 [expr.delete] paragraph 4 as follows:

If the delete-expression calls the implementation deallocation function (3.7.4.2 [basic.stc.dynamic.deallocation]), and if the value of the operand of the delete expression is not the a null pointer constant, the deallocation function will deallocate the storage referenced by the pointer thus rendering the pointer invalid.

Notes from October 2004 meeting:

This wording is superseded by, and this issue will be resolved by, the resolution of issue 348.

Proposed resolution (April, 2005):

This issue is resolved by the resolution of issue 348.




659. Alignment of function types

Section: 5.3.6  [expr.alignof]     Status: CD1     Submitter: Alisdair Meredith     Date: 7 November 2007

[Voted into the WP at the September, 2008 meeting.]

The specification for the alignof operator (5.3.6 [expr.alignof]) does not forbid function types as operands, although it probably should.

Proposed resolution (March, 2008):

The issue, as described, is incorrect. The requirement in 5.3.6 [expr.alignof] is for “a complete object type,” so a function type is already forbidden. However, the existing text does have a problem in this requirement in that it does not allow a reference type, as anticipated by paragraph 3. Consequently, the proposal is to change 5.3.6 [expr.alignof] paragraph 1 as indicated:

An alignof expression yields the alignment requirement of its operand type. The operand shall be a type-id representing a complete object type or a reference to a complete object type.



520. Old-style casts between incomplete class types

Section: 5.4  [expr.cast]     Status: CD1     Submitter: comp.std.c++     Date: 19 May 2005

[Voted into WP at April, 2007 meeting.]

5.4 [expr.cast] paragraph 6 says,

The operand of a cast using the cast notation can be an rvalue of type “pointer to incomplete class type”. The destination type of a cast using the cast notation can be “pointer to incomplete class type”. In such cases, even if there is a inheritance relationship between the source and destination classes, whether the static_cast or reinterpret_cast interpretation is used is unspecified.

The wording seems to allow the following:

  1. casting from void pointer to incomplete type

  2.     struct A;
        struct B;
    
        void *v;
        A *a = (A*)v; // allowed to choose reinterpret_cast
    
  3. variant application of static or reinterpret casting

  4.     B *b = (B*)a;    // compiler can choose static_cast here
        A *aa = (A*)b;   // compiler can choose reinterpret_cast here
        assert(aa == a); // might not hold
    
  5. ability to somehow choose static_cast

  6. It's not entirely clear how a compiler can choose static_cast as 5.4 [expr.cast] paragraph 6 seems to allow. I believe the intent of 5.4 [expr.cast] paragraph 6 is to force the use of reinterpret_cast when either are incomplete class types and static_cast iff the compiler knows both types and there is a non-ambiguous hierarchy-traversal between that cast (or maybe not, core issue 242 talks about this). I cannot see any other interpretation because it isn't intuitive, every compiler I've tried agrees with me, and neither standard pointer conversions (4.10 [conv.ptr] paragraph 3) nor static_cast (5.2.9 [expr.static.cast] paragraph 5) talk about incomplete class types. If the committee agrees with me, I would like to see 4.10 [conv.ptr] paragraph 3 and 5.2.9 [expr.static.cast] paragraph 5 explicitly disallow incomplete class types and the wording of 5.4 [expr.cast] paragraph 6 changed to not allow any other interpretation.

Proposed resolution (April, 2006):

Change 5.4 [expr.cast] paragraph 6 as indicated:

The operand of a cast using the cast notation can be an rvalue of type “pointer to incomplete class type.” The destination type of a cast using the cast notation can be “pointer to incomplete class type.” In such cases, even if there is a inheritance relationship between the source and destination classes, whether the static_cast or reinterpret_cast interpretation is used is unspecified. If both the operand and destination types are class types and one or both are incomplete, it is unspecified whether the static_cast or the reinterpret_cast interpretation is used, even if there is an inheritance relationship between the two classes. [Note: For example, if the classes were defined later in the translation unit, a multi-pass compiler would be permitted to interpret a cast between pointers to the classes as if the class types were complete at that point. —end note]



497. Missing required initialization in example

Section: 5.5  [expr.mptr.oper]     Status: CD1     Submitter: Giovanni Bajo     Date: 03 Jan 2005

[Voted into WP at October 2005 meeting.]

5.5 [expr.mptr.oper] paragraph 5 contains the following example:

    struct S {
        mutable int i;
    };
    const S cs;
    int S::* pm = &S::i;   // pm refers to mutable member S::i
    cs.*pm = 88;           // ill-formed: cs is a const object

The const object cs is not explicitly initialized, and class S does not have a user-declared default constructor. This makes the code ill-formed as per 8.5 [dcl.init] paragraph 9.

Proposed resolution (April, 2005):

Change the example in 5.5 [expr.mptr.oper] paragraph 5 to read as follows:

    struct S {
        S() : i(0) { }
        mutable int i;
    };
    void f()
    {
        const S cs;
        int S::* pm = &S::i;   // pm refers to mutable member S::i
        cs.*pm = 88;           // ill-formed: cs is a const object
    }



614. Results of integer / and %

Section: 5.6  [expr.mul]     Status: CD1     Submitter: Gabriel Dos Reis     Date: 15 January 2007

[Voted into the WP at the September, 2008 meeting as part of paper N2757.]

The current Standard leaves it implementation-defined whether integer division rounds the result toward 0 or toward negative infinity and thus whether the result of % may be negative. C99, apparently reflecting (nearly?) unanimous hardware practice, has adopted the rule that integer division rounds toward 0, thus requiring that the result of -1 % 5 be -1. Should the C++ Standard follow suit?

On a related note, does INT_MIN % -1 invoke undefined behavior? The % operator is defined in terms of the / operator, and INT_MIN / -1 overflows, which by 5 [expr] paragraph 5 causes undefined behavior; however, that is not the “result” of the % operation, so it's not clear. The wording of 5.6 [expr.mul] paragraph 4 appears to allow % to cause undefined behavior only when the second operand is 0.

Proposed resolution (August, 2008):

Change 5.6 [expr.mul] paragraph 4 as follows:

The binary / operator yields the quotient, and the binary % operator yields the remainder from the division of the first expression by the second. If the second operand of / or % is zero the behavior is undefined; otherwise (a/b)*b + a%b is equal to a. If both operands are nonnegative then the remainder is nonnegative; if not, the sign of the remainder is implementation-defined. [Footnote: According to work underway toward the revision of ISO C, the preferred algorithm for integer division follows the rules defined in the ISO Fortran standard, ISO/IEC 1539:1991, in which the quotient is always rounded toward zero. —end footnote]. For integral operands, the / operator yields the algebraic quotient with any fractional part discarded; [Footnote: This is often called “truncation towards zero.” —end footnote] if the quotient a/b is representable in the type of the result, (a/b)*b + a%b is equal to a.

[Drafting note: see C99 6.5.5 paragraph 6.]




661. Semantics of arithmetic comparisons

Section: 5.9  [expr.rel]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 27 November 2007

[Voted into the WP at the June, 2008 meeting.]

The actual semantics of arithmetic comparison — e.g., whether 1 < 2 yields true or false — appear not to be specified anywhere in the Standard. The C Standard has a general statement that

Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false.

There is no corresponding statement in the C++ Standard.

Proposed resolution (February, 2008):

  1. Append the following paragraph to the end of 5.9 [expr.rel]:

  2. If both operands (after conversions) are of arithmetic type, each of the operators shall yield true if the specified relation is true and false if it is false.
  3. Append the following paragraph to the end of 5.10 [expr.eq]:

  4. Each of the operators shall yield true if the specified relation is true and false if it is false.



446. Does an lvalue-to-rvalue conversion on the "?" operator produce a temporary?

Section: 5.16  [expr.cond]     Status: CD1     Submitter: John Potter     Date: 31 Dec 2003

[Voted into WP at October 2005 meeting.]

The problem occurs when the value of the operator is determined to be an rvalue, the selected argument is an lvalue, the type is a class type and a non-const member is invoked on the modifiable rvalue result.

    struct B {
        int v;
        B (int v) : v(v) { }
        void inc () { ++ v; }
        };
    struct D : B {
        D (int v) : B(v) { }
        };

    B b1(42);
    (0 ? B(13) : b1).inc();
    assert(b1.v == 42);

The types of the second and third operands are the same and one is an rvalue. Nothing changes until p6 where an lvalue to rvalue conversion is performed on the third operand. 12.2 [class.temporary] states that an lvalue to rvalue conversion produces a temporary and there is nothing to remove it. It seems clear that the assertion must pass, yet most implementations fail.

There seems to be a defect in p3 b2 b1. First, the conditions to get here and pass the test.

If E1 and E2 have class type, and the underlying class types are the same or one is a base class of the other: E1 can be converted to match E2 if the class of T2 is the same type as, or a base class of, the class of T1, and the cv-qualification of T2 is the same cv-qualification as, or a greater cv-qualification than, the cv-qualification of T1.

If both E1 and E2 are lvalues, passing the conditions here also passes the conditions for p3 b1. Thus, at least one is an rvalue. The case of two rvalues is not interesting and the action is covered by the case when E1 is an rvalue.

    (0 ? D(13) : b1).inc();
    assert(b1.v == 42);
E1 is changed to an rvalue of type T2 that still refers to the original source class object (or the appropriate subobject thereof). [Note: that is, no copy is made. ]

Having changed the rvalue to base type, we are back to the above case where an lvalue to rvalue conversion is required on the third operand at p6. Again, most implementations fail.

The remaining case, E1 an lvalue and E2 an rvalue, is the defect.

    D d1(42);
    (0 ? B(13) : d1).inc();
    assert(d1.v == 42);

The above quote states that an lvalue of type T1 is changed to an rvalue of type T2 without creating a temporary. This is in contradiction to everything else in the standard about lvalue to rvalue conversions. Most implementations pass in spite of the defect.

The usual accessible and unambiguous is missing from the base class.

There seems to be two possible solutions. Following other temporary creations would produce a temporary rvalue of type T1 and change it to an rvalue of type T2. Keeping the no copy aspect of this bullet intact would change the lvalue of type T1 to an lvalue of type T2. In this case the lvalue to rvalue conversion would happen in p6 as usual.

Suggested wording for p3 b2 b1

The base part:

If E1 and E2 have class type, and the underlying class types are the same or one is a base class of the other: E1 can be converted to match E2 if the class of T2 is the same type as, or an accessible and unambiguous base class of, the class of T1, and the cv-qualification of T2 is the same cv-qualification as, or a greater cv-qualification than, the cv-qualification of T1. If the conversion is applied:

The same type temporary version:

If E1 is an lvalue, an lvalue to rvalue conversion is applied. The resulting or original rvalue is changed to an rvalue of type T2 that refers to the same class object (or the appropriate subobject thereof). [Note: that is, no copy is made in changing the type of the rvalue. ]

The never copy version:

The lvalue(rvalue) E1 is changed to an lvalue(rvalue) of type T2 that refers to the original class object (or the appropriate subobject thereof). [Note: that is, no copy is made. ]

The test case was posted to clc++m and results for implementations were reported.

#include <cassert>
struct B {
    int v;
    B (int v) : v(v) { }
    void inc () { ++ v; }
    };
struct D : B {
    D (int v) : B(v) { }
    };
int main () {
    B b1(42);
    D d1(42);
    (0 ? B(13) : b1).inc();
    assert(b1.v == 42);
    (0 ? D(13) : b1).inc();
    assert(b1.v == 42);
    (0 ? B(13) : d1).inc();
    assert(d1.v == 42);
    }

// CbuilderX(EDG301) FFF  Rob Williscroft
// ICC-8.0           FFF  Alexander Stippler
// COMO-4.301        FFF  Alexander Stippler

// BCC-5.4           FFP  Rob Williscroft
// BCC32-5.5         FFP  John Potter
// BCC32-5.65        FFP  Rob Williscroft
// VC-6.0            FFP  Stephen Howe
// VC-7.0            FFP  Ben Hutchings
// VC-7.1            FFP  Stephen Howe
// OpenWatcom-1.1    FFP  Stephen Howe

// Sun C++-6.2       PFF  Ron Natalie

// GCC-3.2           PFP  John Potter
// GCC-3.3           PFP  Alexander Stippler

// GCC-2.95          PPP  Ben Hutchings
// GCC-3.4           PPP  Florian Weimer

I see no defect with regards to lvalue to rvalue conversions; however, there seems to be disagreement about what it means by implementers. It may not be surprising because 5.16 and passing a POD struct to an ellipsis are the only places where an lvalue to rvalue conversion applies to a class type. Most lvalue to rvalue conversions are on basic types as operands of builtin operators.

Notes from the March 2004 meeting:

We decided all "?" operators that return a class rvalue should copy the second or third operand to a temporary. See issue 86.

Proposed resolution (October 2004):

  1. Change 5.16 [expr.cond] paragraph 3 bullet 2 sub-bullet 1 as follows:

    if E1 and E2 have class type, and the underlying class types are the same or one is a base class of the other: E1 can be converted to match E2 if the class of T2 is the same type as, or a base class of, the class of T1, and the cv-qualification of T2 is the same cv-qualification as, or a greater cv-qualification than, the cv-qualification of T1. If the conversion is applied, E1 is changed to an rvalue of type T2 that still refers to the original source class object (or the appropriate subobject thereof). [Note: that is, no copy is made. —end note] by copy-initializing a temporary of type T2 from E1 and using that temporary as the converted operand.
  2. Change 5.16 [expr.cond] paragraph 6 bullet 1 as follows:

    The second and third operands have the same type; the result is of that type. If the operands have class type, the result is an rvalue temporary of the result type, which is copy-initialized from either the second operand or the third operand depending on the value of the first operand.
  3. Change 4.1 [conv.lval] paragraph 2 as follows:

    The value contained in the object indicated by the lvalue is the rvalue result. When an lvalue-to-rvalue conversion occurs within the operand of sizeof (5.3.3 [expr.sizeof]) the value contained in the referenced object is not accessed, since that operator does not evaluate its operand. Otherwise, if the lvalue has a class type, the conversion copy-initializes a temporary of type T from the lvalue and the result of the conversion is an rvalue for the temporary. Otherwise, the value contained in the object indicated by the lvalue is the rvalue result.

[Note: this wording partially resolves issue 86. See also issue 462.]




339. Overload resolution in operand of sizeof in constant expression

Section: 5.19  [expr.const]     Status: CD1     Submitter: Steve Adamczyk     Date: 11 Mar 2002

[Voted into the WP at the June, 2008 meeting as paper N2634.]

I've seen some pieces of code recently that put complex expressions involving overload resolution inside sizeof operations in constant expressions in templates.

5.19 [expr.const] paragraph 1 implies that some kinds of nonconstant expressions are allowed inside a sizeof in a constant expression, but it's not clear that this was intended to extend all the way to things like overload resolution. Allowing such things has some hidden costs. For example, name mangling has to be able to represent all operators, including calls, and not just the operators that can appear in constant expressions.

  template <int I> struct A {};

  char xxx(int);
  char xxx(float);

  template <class T> A<sizeof(xxx((T)0))> f(T){}

  int main()
  {
    f(1);
  }

If complex expressions are indeed allowed, it should be because of an explicit committee decision rather than because of some looseness in this section of the standard.

Notes from the 4/02 meeting:

Any argument for restricting such expressions must involve a cost/benefit ratio: a restriction would be palatable only if it causes minimum hardship for users and allows a substantial reduction in implementation cost. If we propose a restriction, it must be one that library writers can live with.

Lots of these cases fail with current compilers, so there can't be a lot of existing code using them. We plan to find out what cases there are in libraries like Loki and Boost.

We noted that in many cases one can move the code into a class to get the same result. The implementation problem comes up when the expression-in-sizeof is in a template deduction context or part of a template signature. The problem cases are ones where an error causes deduction to fail, as opposed to contexts where an error causes a diagnostic. The latter contexts are easier to handle; however, there are situations where "fail deduction" may be the desired behavior.

Notes from the April 2003 meeting:

Here is a better example:

  extern "C" int printf(const char *, ...);
  char f(int);
  int f(...);
  // Approach 1 -- overload resolution in template class
  // No problem
  template <class T> struct conv_int {
    static const bool value = (sizeof(f(T())) == 1);
  };
  // Approach 2 -- overload resolution in type deduction
  // Difficult
  template <int I> struct A {
    static const int value = I;
  };
  template <class T> bool conv_int2(A<sizeof(f(T()))> p) {
    return p.value == 1;
  }

  template<typename T>
  A<sizeof(f(T()))> make_A() {
    return A<sizeof(f(T()))>();
  }

  int main() {
    printf("short: %d\n", conv_int<short>::value);
    printf("int *: %d\n", conv_int<int *>::value);
    printf("short: %d\n", conv_int2<short>(make_A<short>()));
    printf("int *: %d\n", conv_int2<int *>(make_A<int*>()));
  }

The core working group liked the idea of a restriction that says that expressions inside sizeof in template signature contexts must be otherwise valid as nontype template argument expressions (i.e., integer operations only, limited casts). This of course is subject to whether users can live with that restriction. This topic was brought up in full committee, but there was limited feedback from other groups.

It was also noted that if typeof (whatever it is called) is added, there may be a similar issue there.

Note (March, 2005):

Dave Abrahams (quoting a Usenet posting by Vladimir Marko): The decltype and auto proposal (revision 3: N1607) presents

    template <class T,class U>
    decltype((*(T*)0)+(*(U*)0)) add(const T& t,const U& u);

as a valid declaration (if the proposal is accepted). If [the restrictions in the April, 2003 note] really applied to decltype, the declaration above would be invalid. AFAICT every non-trivial use of decltype in a template function declaration would be invalid. And for me this would render my favorite proposal useless.

I would propose to allow any kind of expression inside sizeof (and decltype) and explicitly add sizeof (and decltype) expressions involving template-parameters to non-deduced contexts (add a bullet to 14.8.2.4 [temp.deduct.partial] paragraph 4).

Jaakko Jarvi: Just reinforcing that this is important and hope for insights. The topic is discussed a bit on page 10 of the latest revision of the proposal (N1705). Here's a quote from the proposal:

However, it is crucial that no restrictions are placed on what kinds of expressions are allowed inside decltype, and therefore also inside sizeof. We suggest that issue 339 is resolved to require the compiler to fail deduction (apply the SFINAE principle), and not produce an error, for as large set of invalid expressions in operands of sizeof or decltype as is possible to comfortably implement. We wish that implementors aid in classifying the kinds of expressions that should produce errors, and the kinds that should lead to failure of deduction.

Notes from the April, 2007 meeting:

The CWG is pursuing a compromise proposal, to which the EWG has tentatively agreed, which would allow arbitrary expressions in the return types of function templates but which would restrict the expressions that participate in the function signature (and thus in overload resolution) to those that can be used as non-type template arguments. During deduction and overload resolution, these complex return types would be ignored; that is, there would be no substitution of the deduced template arguments into the return type at this point. If such a function were selected by overload resolution, however, a substitution failure in the return type would produce a diagnostic rather than a deduction failure.

This approach works when doing overload resolution in the context of a function call, but additional tricks (still being defined) are needed in other contexts such as friend function declaration matching and taking the address of a function, in which the return type does play a part.

Notes from the July, 2007 meeting:

The problem is whether arbitrary expressions (for example, ones that include overload resolution) are allowed in template deduction contexts, and, if so, which expression errors are SFINAE failures and which are hard errors.

This issue deals with arbitrary expressions inside sizeof in deduction contexts. That's a fringe case right now (most compilers don't accept them). decltype makes the problem worse, because the standard use case is one that involves overload resolution. Generalized constant expressions make it worse yet, because they allow overload resolution and class types to show up in any constant expression in a deduction context.

Why is this an issue? Why don't we just say everything is allowed and be done with it?

At the April, 2007 meeting, we were headed toward a solution that imposed a restriction on expressions in deduction contexts, but such a restriction seems to really hamper uses of constexpr functions. So we're now proposing that fully general expressions be allowed, and that most errors in such expressions be treated as SFINAE failures rather than errors.

One issue with writing Standard wording for that is how to define “most.” There's a continuum of errors, some errors being clearly SFINAE failures, and some clearly “real” errors, with lots of unclear cases in between. We decided it's easier to write the definition by listing the errors that are not treated as SFINAE failures, and the list we came up with is as follows:

  1. errors that occur while processing some entity external to the expression, e.g., an instantiation of a template or the generation of the definition of an implicitly-declared copy constructor
  2. errors due to implementation limits
  3. errors due to access violations (this is a judgment call, but the philosophy of access has always been that it doesn't affect visibility)

Everything else produces a SFINAE failure rather than a hard error.

There was broad consensus that this felt like a good solution, but that feeling was mixed with trepidation on several fronts:

We will be producing wording for the Working Draft for the October, 2007 meeting.

(See also issue 657.)




366. String literal allowed in integral constant expression?

Section: 5.19  [expr.const]     Status: CD1     Submitter: Martin v. Loewis     Date: 29 July 2002

[Voted into WP at October 2003 meeting.]

According to 16.1 [cpp.cond] paragraph 1, the if-group

#if "Hello, world"

is well-formed, since it is an integral constant expression. Since that may not be obvious, here is why:

5.19 [expr.const] paragraph 1 says that an integral constant expression may involve literals (2.14 [lex.literal]); "Hello, world" is a literal. It restricts operators to not use certain type conversions; this expression does not use type conversions. It further disallows functions, class objects, pointers, ... - this expression is none of those, since it is an array.

However, 16.1 [cpp.cond] paragraph 6 does not explain what to do with this if-group, since the expression evaluates neither to false(zero) nor true(non-zero).

Proposed resolution (October 2002):

Change the beginning of the second sentence of 5.19 [expr.const] paragraph 1 which currently reads

An integral constant-expression can involve only literals (2.14 [lex.literal]), ...
to say
An integral constant-expression can involve only literals of arithmetic types (2.14 [lex.literal], 3.9.1 [basic.fundamental]), ...




367. throw operator allowed in constant expression?

Section: 5.19  [expr.const]     Status: CD1     Submitter: Martin v. Loewis     Date: 29 July 2002

[Voted into WP at the October, 2006 meeting.]

The following translation unit appears to be well-formed.

int x[true?throw 4:5];

According to 5.19 [expr.const], this appears to be an integral constant expression: it is a conditional expression, involves only literals, and no assignment, increment, decrement, function-call, or comma operators. However, if this is well-formed, the standard gives no meaning to this declaration, since the array bound (8.3.4 [dcl.array] paragraph 1) cannot be computed.

I believe the defect is that throw expressions should also be banned from constant expressions.

Notes from October 2002 meeting:

We should also check on new and delete.

Notes from the April, 2005 meeting:

Although it could be argued that all three of these operators potentially involve function calls — throw to std::terminate, new and delete to the corresponding allocation and deallocation functions — and thus would already be excluded from constant expressions, this reasoning was considered to be too subtle to allow closing the issue with no change. A modification that explicitly clarifies the status of these operators will be drafted.

Proposed resolution (October, 2005):

Change the last sentence of 5.19 [expr.const] as indicated:

In particular, except in sizeof expressions, functions, class objects, pointers, or references shall not be used, and assignment, increment, decrement, function-call function call (including new-expressions and delete-expressions), or comma operators, or throw-expressions shall not be used.

Note: this sentence is also changed by the resolution of issue 530.




457. Wording nit on use of const variables in constant expressions

Section: 5.19  [expr.const]     Status: CD1     Submitter: Mark Mitchell     Date: 03 Feb 2004

[Voted into WP at April 2005 meeting.]

I'm looking at 5.19 [expr.const]. I see:

An integral constant-expression can involve only ... const variables or static data members of integral or enumeration types initialized with constant expressions ...

Shouldn't that be "const non-volatile"?

It seems weird to say that:

  const volatile int i = 3;
  int j[i];
is valid.

Steve Adamczyk: See issue 76, which made the similar change to 7.1.6.1 [dcl.type.cv] paragraph 2, and probably should have changed this one as well.

Proposed resolution (October, 2004):

Change the first sentence in the second part of 5.19 [expr.const] paragraph 1 as follows:

An integral constant-expression can involve only literals of arithmetic types (2.14 [lex.literal], 3.9.1 [basic.fundamental]), enumerators, non-volatile const variables or static data members of integral or enumeration types initialized with constant expressions (8.5 [dcl.init]), non-type template parameters of integral or enumeration types, and sizeof expressions.



530. Nontype template arguments in constant expressions

Section: 5.19  [expr.const]     Status: CD1     Submitter: Mark Mitchell     Date: 21 August 2005

[Voted into the WP at the April, 2007 meeting as part of paper J16/07-0095 = WG21 N2235.]

Consider:

    template <int* p> struct S {
        static const int I = 3;
    };
    int i;
    int a[S<&i>::I];

Clearly this should be valid, but a pedantic reading of 5.19 [expr.const] would suggest that this is invalid because “&i” is not permitted in integral constant expressions.

Proposed resolution (October, 2005):

Change the last sentence of 5.19 [expr.const] paragraph 1 as indicated:

In particular, except in non-type template-arguments or sizeof expressions, functions, class objects, pointers, or references shall not be used, and assignment, increment, decrement, function-call, or comma operators shall not be used.

(Note: the same text is changed by the resolution of issue 367.)

Notes from April, 2006 meeting:

The proposed resolution could potentially be read as saying that the prohibited operations and operators would be permitted in integral constant expressions that are non-type template-arguments. John Spicer is investigating an alternate approach, to say that expressions in non-type template arguments are not part of the expression in which the template-id appears (in contrast to the operand of sizeof, which is part of the containing expression).

Additional note (May, 2008):

This issue is resolved by the rewrite of 5.19 [expr.const] that was done in conjunction with the constexpr proposal, paper N2235.




684. Constant expressions involving the address of an automatic variable

Section: 5.19  [expr.const]     Status: CD1     Submitter: Jens Maurer     Date: 13 March, 2008

[Voted into the WP at the September, 2008 meeting (resolution in paper N2757).]

The expressions that are excluded from being constant expressions in 5.19 [expr.const] paragraph 2 does not address an example like the following:

    void f() {
       int a;
       constexpr int* p = &a;    // should be ill-formed, currently isn't
    }

Suggested resolution:

Add the following bullet to the list in 5.19 [expr.const] paragraph 2:

Proposed resolution (June, 2008):

  1. Change 3.6.2 [basic.start.init] paragraph 1 as follows:

  2. Objects with static storage duration (3.7.1 [basic.stc.static]) or thread storage duration (3.7.2) shall be zero-initialized (8.5 [dcl.init]) before any other initialization takes place. A reference with static or thread storage duration and an object of trivial or literal type with static or thread storage duration can be initialized with a constant expression (5.19 [expr.const]); this is called constant initialization. Constant initialization is performed:
    • if an object of trivial or literal type with static or thread storage duration is initialized with a constant expression (5.19 [expr.const]), or

    • if a reference with static or thread storage duration is initialized with a constant expression that is not an lvalue designating an object with thread or automatic storage duration.

    Together, zero-initialization and constant initialization...
  3. Add the following in 5.19 [expr.const] paragraph 2:

(Note: the change to 3.6.2 [basic.start.init] paragraph 1 needs to be reconciled with the conflicting change in issue 688.)




276. Order of destruction of parameters and temporaries

Section: 6.6  [stmt.jump]     Status: CD1     Submitter: James Kanze     Date: 28 Mar 2001

[Voted into the WP at the June, 2008 meeting.]

According to 6.6 [stmt.jump] paragraph 2,

On exit from a scope (however accomplished), destructors (12.4 [class.dtor]) are called for all constructed objects with automatic storage duration (3.7.3 [basic.stc.auto]) (named objects or temporaries) that are declared in that scope, in the reverse order of their declaration.

This wording is problematic for temporaries and for parameters. First, temporaries are not "declared," so this requirement does not apply to them, in spite of the assertion in the quoted text that it does.

Second, although the parameters of a function are declared in the called function, they are constructed and destroyed in the calling context, and the order of evaluation of the arguments is unspecified (cf 5.2.2 [expr.call] paragraphs 4 and 8). The order of destruction of the parameters might, therefore, be different from the reverse order of their declaration.

Notes from 04/01 meeting:

Any resolution of this issue should be careful not to introduce requirements that are redundant or in conflict with those of other parts of the IS. This is especially true in light of the pending issues with respect to the destruction of temporaries (see issues 86, 124, 199, and 201). If possible, the wording of a resolution should simply reference the relevant sections.

It was also noted that the temporary for a return value is also destroyed "out of order."

Note that issue 378 picks a nit with the wording of this same paragraph.

Proposed Resolution (November, 2006):

Change 6.6 [stmt.jump] paragraph 2 as follows:

On exit from a scope (however accomplished), destructors (12.4 [class.dtor]) are called for all constructed objects with automatic storage duration (3.7.3 [basic.stc.auto]) (named objects or temporaries) that are declared in that scope, in the reverse order of their declaration. variables with automatic storage duration (3.7.3 [basic.stc.auto]) that have been constructed in that scope are destroyed in the reverse order of their construction. [Note: For temporaries, see 12.2 [class.temporary]. —end note] Transfer out of a loop...



378. Wording that says temporaries are declared

Section: 6.6  [stmt.jump]     Status: CD1     Submitter: Gennaro Prota     Date: 07 September 2002

Paragraph 6.6 [stmt.jump] paragraph 2 of the standard says:

On exit from a scope (however accomplished), destructors (12.4 [class.dtor]) are called for all constructed objects with automatic storage duration (3.7.3 [basic.stc.auto]) (named objects or temporaries) that are declared in that scope.

It refers to objects "that are declared" but the text in parenthesis also mentions temporaries, which cannot be declared. I think that text should be removed.

This is related to issue 276.

Proposed Resolution (November, 2006):

This issue is resolved by the resolution of issue 276.




281. inline specifier in friend declarations

Section: 7.1.2  [dcl.fct.spec]     Status: CD1     Submitter: John Spicer     Date: 24 Apr 2001

[Moved to DR at October 2002 meeting.]

There is currently no restriction on the use of the inline specifier in friend declarations. That would mean that the following usage is permitted:

    struct A {
        void f();
    };

    struct B {
        friend inline void A::f();
    };

    void A::f(){}

I believe this should be disallowed because a friend declaration in one class should not be able to change attributes of a member function of another class.

More generally, I think that the inline attribute should only be permitted in friend declarations that are definitions.

Notes from the 04/01 meeting:

The consensus agreed with the suggested resolution. This outcome would be similar to the resolution of issue 136.

Proposed resolution (10/01):

Add to the end of 7.1.2 [dcl.fct.spec] paragraph 3:

If the inline specifier is used in a friend declaration, that declaration shall be a definition or the function shall have previously been declared inline.



317. Can a function be declared inline after it has been called?

Section: 7.1.2  [dcl.fct.spec]     Status: CD1     Submitter: Steve Clamage     Date: 14 Oct 2001

[Voted into WP at October 2005 meeting.]

Steve Clamage: Consider this sequence of declarations:

  void foo() { ... }
  inline void foo();
The non-inline definition of foo precedes the inline declaration. It seems to me this code should be ill-formed, but I could not find anything in the standard to cover the situation.

Bjarne Stroustrup: Neither could I, so I looked in the ARM, which addressed this case (apparently for member function only) in some detail in 7.1.2 (pp103-104).

The ARM allows declaring a function inline after its initial declaration, as long as it has not been called.

Steve Clamage: If the above code is valid, how about this:

  void foo() { ... }    // define foo
  void bar() { foo(); } // use foo
  inline void foo();    // declare foo inline

Bjarne Stroustrup: ... and [the ARM] disallows declaring a function inline after it has been called.

This may still be a good resolution.

Steve Clamage: But the situation in the ARM is the reverse: Declare a function inline, and define it later (with no intervening call). That's a long-standing rule in C++, and allows you to write member function definitions outside the class.

In my example, the compiler could reasonably process the entire function as out-of-line, and not discover the inline declaration until it was too late to save the information necessary for inline generation. The equivalent of another compiler pass would be needed to handle this situation.

Bjarne Stroustrup: I see, and I think your argument it conclusive.

Steve Clamage: I'd like to open a core issue on this point, and I recommend wording along the lines of: "A function defined without an inline specifier shall not be followed by a declaration having an inline specifier."

I'd still like to allow the common idiom

  class T {
    int f();
  };
  inline int T::f() { ... }

Martin Sebor: Since the inline keyword is just a hint to the compiler, I don't see any harm in allowing the construct. Your hypothetical compiler can simply ignore the inline on the second declaration. On the other hand, I feel that adding another special rule will unnecessarily complicate the language.

Steve Clamage: The inline specifier is more than a hint. You can have multiple definitions of inline functions, but only one definition of a function not declared inline. In particular, suppose the above example were in a header file, and included multiple times in a program.

Proposed resolution (October, 2004):

Add the indicated words to 7.1.2 [dcl.fct.spec] paragraph 4:

An inline function shall be defined in every translation unit in which it is used and shall have exactly the same definition in every case (3.2 [basic.def.odr]). [Note: a call to the inline function may be encountered before its definition appears in the translation unit. —end note] If the definition of a function appears in a translation unit before its first declaration as inline, the program is ill-formed. If a function with external linkage is declared inline in one translation unit...



396. Misleading note regarding use of auto for disambiguation

Section: 7.1.2  [dcl.fct.spec]     Status: CD1     Submitter: Herb Sutter     Date: 3 Jan 2003

[Voted into WP at March 2004 meeting.]

BTW, I noticed that the following note in 7.1.1 [dcl.stc] paragraph 2 doesn't seem to have made it onto the issues list or into the TR:

[Note: hence, the auto specifier is almost always redundant and not often used; one use of auto is to distinguish a declaration-statement from an expression-statement (stmt.ambig) explicitly. --- end note]

I thought that this was well known to be incorrect, because using auto does not disambiguate this. Writing:

  auto int f();
is still a declaration of a function f, just now with an error since the function's return type may not use an auto storage class specifier. I suppose an error is an improvement over a silent ambiguity going the wrong way, but it's still not a solution for the user who wants to express the other in a compilable way.

Proposed resolution: Replace that note with the following note:

[Note: hence, the auto specifier is always redundant and not often used. --- end note]

John Spicer: I support the proposed change, but I think the disambiguation case is not the one that you describe. An example of the supposed disambiguation is:

  int i;
  int j;
  int main()
  {
    int(i);  // declares i, not reference to ::i
    auto int(j);  // declares j, not reference to ::j
  }

cfront would take "int(i)" as a cast of ::i, so the auto would force what it would otherwise treat as a statement to be considered a declaration (cfront 3.0 warned that this would change in the future).

In a conforming compiler the auto is always redundant (as you say) because anything that could be considered a valid declaration should be treated as one.

Proposed resolution (April 2003):

Replace 7.1.1 [dcl.stc] paragraph 2

[Note: hence, the auto specifier is almost always redundant and not often used; one use of auto is to distinguish a declaration-statement from an expression-statement (6.8 [stmt.ambig]) explicitly. --- end note]
with
[Note: hence, the auto specifier is always redundant and not often used. One use of auto is to distinguish a declaration-statement from an expression-statement explicitly rather than relying on the disambiguation rules (6.8 [stmt.ambig]), which may aid readers. --- end note]




397. Same address for string literals from default arguments in inline functions?

Section: 7.1.2  [dcl.fct.spec]     Status: CD1     Submitter: Mark Mitchell     Date: 13 Jan 2003

[Voted into WP at April, 2007 meeting.]

Are string literals from default arguments used in extern inlines supposed to have the same addresses across all translation units?

  void f(const char* = "s")
  inline g() {
    f();
  }

Must the "s" strings be the same in all copies of the inline function?

Steve Adamczyk: The totality of the standard's wisdom on this topic is (7.1.2 [dcl.fct.spec] paragraph 4):

A string literal in an extern inline function is the same object in different translation units.

I'd hazard a guess that a literal in a default argument expression is not "in" the extern inline function (it doesn't appear in the tokens of the function), and therefore it need not be the same in different translation units.

I don't know that users would expect such strings to have the same address, and an equally valid (and incompatible) expectation would be that the same string literal would be used for every expansion of a given default argument in a single translation unit.

Notes from April 2003 meeting:

The core working group feels that the address of a string literal should be guaranteed to be the same only if it actually appears textually within the body of the inline function. So a string in a default argument expression in a block extern declaration inside the body of a function would be the same in all instances of the function. On the other hand, a string in a default argument expression in the header of the function (i.e., outside of the body) would not be the same.

Proposed resolution (April 2003):

Change the last sentence and add the note to the end of 7.1.2 [dcl.fct.spec] paragraph 4:

A string literal in the body of an extern inline function is the same object in different translation units. [Note: A string literal that is encountered only in the context of a function call (in the default argument expression of the called function), is not “in” the extern inline function.]

Notes from October 2003 meeting:

We discussed ctor-initializer lists and decided that they are also part of the body. We've asked Clark Nelson to work on syntax changes to give us a syntax term for the body of a function so we can refer to it here. See also issue 452, which could use this term.

(October, 2005: moved to “review” status in concert with issue 452. With that resolution, the wording above needs no further changes.)

Proposed resolution (April, 2006):

Change the last sentence and add the note to the end of 7.1.2 [dcl.fct.spec] paragraph 4:

A string literal in the body of an extern inline function is the same object in different translation units. [Note: A string literal appearing in a default argument expression is not considered to be “in the body” of an inline function merely by virtue of the expression’s use in a function call from that inline function. —end note]



477. Can virtual appear in a friend declaration?

Section: 7.1.2  [dcl.fct.spec]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 23 Sep 2004

[Voted into WP at the October, 2006 meeting.]

I couldn't find wording that makes it invalid to say friend virtual... The closest seems to be 7.1.2 [dcl.fct.spec] paragraph 5, which says:

The virtual specifier shall only be used in declarations of nonstatic class member functions that appear within a member-specification of a class definition; see 10.3 [class.virtual].

I don't think that excludes a friend declaration (which is a valid member-specification by 9.2 [class.mem]).

John Spicer: I agree that virtual should not be allowed on friend declarations. I think the wording in 7.1.2 [dcl.fct.spec] is intended to be the declaration of a function within its class, although I think the wording should be improved to make it clearer.

Proposed resolution (October, 2005):

Change 7.1.2 [dcl.fct.spec] paragraphs 5-6 as indicated:

The virtual specifier shall only be used only in declarations the initial declaration of a non-static class member functions that appear within a member-specification of a class definition function; see 10.3 [class.virtual].

The explicit specifier shall be used only in declarations the declaration of constructors a constructor within a its class definition; see 12.3.1 [class.conv.ctor].




424. Wording problem with issue 56 resolution on redeclaring typedefs in class scope

Section: 7.1.3  [dcl.typedef]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 25 June 2003

[Voted into WP at March 2004 meeting.]

I wonder if perhaps the core issue 56 change in 7.1.3 [dcl.typedef] paragraph 2 wasn't quite careful enough. The intent was to remove the allowance for:

  struct S {
    typedef int I;
    typedef int I;
  };

but I think it also disallows the following:

  class B {
    typedef struct A {} A;
    void f(struct B::A*p);
  };

See also issue 407.

Proposed resolution (October 2003):

At the end of 7.1.3 [dcl.typedef] paragraph 2, add the following:

In a given class scope, a typedef specifier can be used to redefine any class-name declared in that scope that is not also a typedef-name to refer to the type to which it already refers. [Example:
  struct S {
    typedef struct A {} A;  // OK
    typedef struct B B;     // OK
    typedef A A;            // error
  };
]



647. Non-constexpr instances of constexpr constructor templates

Section: 7.1.5  [dcl.constexpr]     Status: CD1     Submitter: Mike Miller     Date: 12 Aug 2007

[Voted into the WP at the September, 2008 meeting.]

According to 7.1.5 [dcl.constexpr] paragraph 5,

If the instantiated template specialization of a constexpr function template would fail to satisfy the requirements for a constexpr function, the constexpr specifier is ignored and the specialization is not a constexpr function.

One would expect to see a similar provision for an instantiated constructor template (because the requirements for a constexpr function [paragraph 3] are different from the requirements for a constexpr constructor [paragraph 4]), but there is none; constexpr constructor templates are not mentioned.

Suggested resolution:

Change the wording of 7.1.5 [dcl.constexpr] paragraph 5 as indicated:

If the instantiated template specialization of a constexpr function template would fail to satisfy the requirements for a constexpr function or constexpr constructor, as appropriate to the function template, the constexpr specifier is ignored and the specialization is not a constexpr function or constexpr constructor.

Proposed resolution (June, 2008):

[Drafting note: This resolution goes beyond the problem described in the issue discussion, which is one aspect of the general failure of the existing wording to deal consistently with the distinctions between constexpr functions and constexpr constructors. The wording below attempts to rectify that problem systematically.]

  1. Change 7.1.5 [dcl.constexpr] paragraph 2 as follows:

  2. A constexpr specifier used in a function declaration the declaration of a function that is not a constructor declares that function to be a constexpr function. Similarly, a constexpr specifier used in a constructor declaration declares that constructor to be a constexpr constructor. Constexpr functions and constexpr constructors are implicitly inline (7.1.2 [dcl.fct.spec]). A constexpr function shall not be virtual (10.3).
  3. Change 7.1.5 [dcl.constexpr] paragraph 3 as follows:

  4. The definition of a constexpr function shall satisfy the following constraints:

    [Example:...

  5. Change 7.1.5 [dcl.constexpr] paragraph 4 as follows:

  6. The definition of a constexpr constructor shall satisfy the following constraints:

    A trivial copy constructor is also a constexpr constructor. [Example: ...

  7. Change 7.1.5 [dcl.constexpr] paragraph 5 as follows:

  8. If the instantiated template specialization of a constexpr function template would fail to satisfy the requirements for a constexpr function or constexpr constructor, the constexpr specifier is ignored and the specialization is not a constexpr function.
  9. Change 7.1.5 [dcl.constexpr] paragraph 6 as follows:

  10. A constexpr specifier used in for a non-static member function definition that is not a constructor declares that member function to be const (9.3.1 [class.mfct.non-static]). [Note: ...



648. Constant expressions in constexpr initializers

Section: 7.1.5  [dcl.constexpr]     Status: CD1     Submitter: Mike Miller     Date: 12 Aug 2007

[Voted into the WP at the September, 2008 meeting.]

The current wording of 7.1.5 [dcl.constexpr] paragraph 7 seems not quite correct. It reads,

A constexpr specifier used in an object declaration declares the object as const. Such an object shall be initialized, and every expression that appears in its initializer (8.5 [dcl.init]) shall be a constant expression.

The phrase “every expression” is intended to cover multiple arguments to a constexpr constructor and multiple expressions in an aggregate initializer. However, it could be read (incorrectly) as saying that non-constant expressions cannot appear as subexpressions in such initializers, even in places where they do not render the full-expression non-constant (i.e., as unevaluated operands and in the unselected branches of &&, ||, and ?:). Perhaps this problem could be remedied by replacing “every expression” with “every full-expression?”

Proposed resolution (June, 2008):

Change 7.1.5 [dcl.constexpr] paragraph 7 as follows:

A constexpr specifier used in an object declaration declares the object as const. Such an object shall be initialized, and every expression that appears in its initializer (8.5) initialized. If it is initialized by a constructor call, the constructor shall be a constexpr constructor and every argument to the constructor shall be a constant expression. Otherwise, every full-expression that appears in its initializer shall be a constant expression. Every implicit conversion used...



283. Template type-parameters are not syntactically type-names

Section: 7.1.6.2  [dcl.type.simple]     Status: CD1     Submitter: Clark Nelson     Date: 01 May 2001

[Voted into WP at April 2003 meeting.]

Although 14.1 [temp.param] paragraph 3 contains an assertion that

A type-parameter defines its identifier to be a type-name (if declared with class or typename)

the grammar in 7.1.6.2 [dcl.type.simple] paragraph 1 says that a type-name is either a class-name, an enum-name, or a typedef-name. The identifier in a template type-parameter is none of those. One possibility might be to equate the identifier with a typedef-name instead of directly with a type-name, which would have the advantage of not requiring parallel treatment of the two in situations where they are treated the same (e.g., in elaborated-type-specifiers, see issue 245). See also issue 215.

Proposed resolution (Clark Nelson, March 2002):

In 14.1 [temp.param] paragraph 3, change "A type-parameter defines its identifier to be a type-name" to "A type-parameter defines its identifier to be a typedef-name"

In 7.1.6.3 [dcl.type.elab] paragraph 2, change "If the identifier resolves to a typedef-name or a template type-parameter" to "If the identifier resolves to a typedef-name".

This has been consolidated with the edits for some other issues. See N1376=02-0034.




516. Use of signed in bit-field declarations

Section: 7.1.6.2  [dcl.type.simple]     Status: CD1     Submitter: comp.std.c++     Date: 25 Apr 2005

[Voted into WP at the October, 2006 meeting.]

7.1.6.2 [dcl.type.simple] paragraph 3 reads,

It is implementation-defined whether bit-fields and objects of char type are represented as signed or unsigned quantities. The signed specifier forces char objects and bit-fields to be signed; it is redundant with other integral types.

The last sentence in that quote is misleading w.r.t. bit-fields. The first sentence in that quote is correct but incomplete.

Proposed fix: change the two sentences to read:

It is implementation-defined whether objects of char type are represented as signed or unsigned quantities. The signed specifier forces char objects signed; it is redundant with other integral types except when declaring bit-fields (9.6 [class.bit]).

Proposed resolution (October, 2005):

Change 7.1.6.2 [dcl.type.simple] paragraph 3 as indicated:

When multiple simple-type-specifiers are allowed, they can be freely intermixed with other decl-specifiers in any order. [Note: It is implementation-defined whether bit-fields and objects of char type and certain bit-fields (9.6 [class.bit]) are represented as signed or unsigned quantities. The signed specifier forces bit-fields and char objects and bit-fields to be signed; it is redundant with other integral types in other contexts. end note]



651. Problems in decltype specification and examples

Section: 7.1.6.2  [dcl.type.simple]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 16 Aug 2007

[Voted into the WP at the September, 2008 meeting.]

The second bullet of 7.1.6.2 [dcl.type.simple] paragraph 4 reads,

The reference to “that function” is imprecise; it is not the actual function called at runtime but the statically chosen function (ignoring covariant return types in virtual functions).

Also, the examples in this paragraph have errors:

  1. The declaration of struct A should end with a semicolon.

  2. The lines of the form decltype(...); are ill-formed; they need a declarator.

Proposed Resolution (October, 2007):

Change 7.1.6.2 [dcl.type.simple] paragraph 4 as follows:

The type denoted by decltype(e) is defined as follows:

The operand of the decltype specifier is an unevaluated operand (clause 5 [expr]).

[Example:

    const int&& foo();
    int i;
    struct A { double x; };
    const A* a = new A();
    decltype(foo()) x1;      // type is const int&&
    decltype(i) x2;          // type is int
    decltype(a->x) x3;       // type is double
    decltype((a->x)) x4;     // type is const double&

end example]




629. auto parsing ambiguity

Section: 7.1.6.4  [dcl.spec.auto]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 14 March 2007

[Voted into the WP at the February, 2008 meeting as paper J16/08-0056 = WG21 N2546.]

We've found an interesting parsing ambiguity with the new meaning of auto. Consider:

    typedef int T;
    void f() {
        auto T = 42;  // Valid or not?
    }

The question here is whether T should be a type specifier or a storage class? 7.1.6.4 [dcl.spec.auto] paragraph 1 says,

The auto type-specifier has two meanings depending on the context of its use. In a decl-specifier-seq that contains at least one type-specifier (in addition to auto) that is not a cv-qualifier, the auto type-specifier specifies that the object named in the declaration has automatic storage duration.

In this case, T is a type-specifier, so the declaration is ill-formed: there is no declarator-id. Many, however, would like to see auto work “just like int,” i.e., forcing T to be redeclared in the inner scope. Concerns cited included hijacking of the name in templates and inline function bodies over the course of time if a program revision introduces a type with that name in the surrounding context. Although it was pointed out that enclosing the name in parentheses in the inner declaration would prevent any such problems, this was viewed as unacceptably ugly.

Notes from the April, 2007 meeting:

The CWG wanted to avoid a rule like, “if auto can be a type-specifier, it is” (similar to the existing “if it can be a declaration, it is” rule) because of the lookahead and backtracking difficulties such an approach would pose for certain kinds of parsing techniques. It was noted that the difficult lookahead cases all involve parentheses, which would not be a problem if only the “=” form of initializer were permitted in auto declarations; only very limited lookahead is required in that case. It was also pointed out that the “if it can be a type-specifier, it is” approach results in a quiet change of meaning for cases like

    typedef int T;
    int n = 3;
    void f() {
        auto T(n);
    }

This currently declares n to be an int variable in the inner scope but would, under the full lookahead approach, declare T to be a variable, quitely changing uses of n inside f() to refer to the outer variable.

The consensus of the CWG was to pursue the change to require the “=” form of initializer for auto.

Notes from the July, 2007 meeting:

See paper J16/07-0197 = WG21 N2337. There was no consensus among the CWG for either of the approaches recommended in the paper; additional input and direction is required.




172. Unsigned int as underlying type of enum

Section: 7.2  [dcl.enum]     Status: CD1     Submitter: Bjarne Stroustrup     Date: 26 Sep 1999

[Moved to DR at October 2002 meeting.]

According to 7.2 [dcl.enum] paragraph 5, the underlying type of an enum is an unspecified integral type, which could potentially be unsigned int. The promotion rules in 4.5 [conv.prom] paragraph 2 say that such an enumeration value used in an expression will be promoted to unsigned int. This means that a conforming implementation could give the value false for the following code:

    enum { zero };
    -1 < zero;       // might be false
This is counterintuitive. Perhaps the description of the underlying type of an enumeration should say that an unsigned underlying type can be used only if the values of the enumerators cannot be represented in the corresponding signed type. This approach would be consistent with the treatment of integral promotion of bitfields (4.5 [conv.prom] paragraph 3).

On a related note, 7.2 [dcl.enum] paragraph 5 says,

the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int.

This specification does not allow for an enumeration like

    enum { a = -1, b = UINT_MAX };

Since each enumerator can fit in an int or unsigned int, the underlying type is required to be no larger than int, even though there is no such type that can represent all the enumerators.

Proposed resolution (04/01; obsolete, see below):

Change 7.2 [dcl.enum] paragraph 5 as follows:

It is implementation-defined which integral type is used as the underlying type for an enumeration except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int neither int nor unsigned int can represent all the enumerator values. Furthermore, the underlying type shall not be an unsigned type if the corresponding signed type can represent all the enumerator values.

See also issue 58.

Notes from 04/01 meeting:

It was noted that 4.5 [conv.prom] promotes unsigned types smaller than int to (signed) int. The signedness chosen by an implementation for small underlying types is therefore unobservable, so the last sentence of the proposed resolution above should apply only to int and larger types. This observation also prompted discussion of an alternative approach to resolving the issue, in which the bmin and bmax of the enumeration would determine the promoted type rather than the underlying type.

Proposed resolution (10/01):

Change 4.5 [conv.prom] paragraph 2 from

An rvalue of type wchar_t (3.9.1 [basic.fundamental]) or an enumeration type (7.2 [dcl.enum]) can be converted to an rvalue of the first of the following types that can represent all the values of its underlying type: int, unsigned int, long, or unsigned long.
to
An rvalue of type wchar_t (3.9.1 [basic.fundamental]) can be converted to an rvalue of the first of the following types that can represent all the values of its underlying type: int, unsigned int, long, or unsigned long. An rvalue of an enumeration type (7.2 [dcl.enum]) can be converted to an rvalue of the first of the following types that can represent all the values of the enumeration (i.e., the values in the range bmin to bmax as described in 7.2 [dcl.enum]): int, unsigned int, long, or unsigned long.




377. Enum whose enumerators will not fit in any integral type

Section: 7.2  [dcl.enum]     Status: CD1     Submitter: Mark Mitchell     Date: 30 August 2002

[Voted into WP at April 2003 meeting.]

7.2 [dcl.enum] defines the underlying type of an enumeration as an integral type "that can represent all the enumerator values defined in the enumeration".

What does the standard say about this code:

  enum E { a = LONG_MIN, b = ULONG_MAX };

?

I think this should be ill-formed.

Proposed resolution:

In 7.2 [dcl.enum] paragraph 5 after

The underlying type of an enumeration is an integral type that can represent all the enumerator values defined in the enumeration.
insert
If no integral type can represent all the enumerator values, the enumeration is ill-formed.




518. Trailing comma following enumerator-list

Section: 7.2  [dcl.enum]     Status: CD1     Submitter: Charles Bryant     Date: 10 May 2005

[Voted into WP at April, 2006 meeting.]

The C language (since C99), and some C++ compilers, accept:

    enum { FOO, };

as syntactically valid. It would be useful

This proposed change is to permit a trailing comma in enum by adding:

enum identifieropt { enumerator-list , }

as an alternative definition for the enum-specifier nonterminal in 7.2 [dcl.enum] paragraph 1.

Proposed resolution (October, 2005):

Change the grammar in 7.2 [dcl.enum] paragraph 1 as indicated:

enum-specifier:



660. Unnamed scoped enumerations

Section: 7.2  [dcl.enum]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 15 November 2007

[Voted into the WP at the September, 2008 meeting.]

The current specification of scoped enumerations does not appear to forbid an example like the following, even though the enumerator e cannot be used:

    enum class { e };

This might be covered by 7 [dcl.dcl] paragraph 3,

In a simple-declaration, the optional init-declarator-list can be omitted only when declaring a class (clause 9 [class]) or enumeration (7.2 [dcl.enum]), that is, when the decl-specifier-seq contains either a class-specifier, an elaborated-type-specifier with a class-key (9.1 [class.name]), or an enum-specifier. In these cases and whenever a class-specifier or enum-specifier is present in the decl-specifier-seq, the identifiers in these specifiers are among the names being declared by the declaration (as class-names, enum-names, or enumerators, depending on the syntax). In such cases, and except for the declaration of an unnamed bit-field (9.6 [class.bit]), the decl-specifier-seq shall introduce one or more names into the program, or shall redeclare a name introduced by a previous declaration.

which, when combined with paragraph 2,

A declaration occurs in a scope (3.3 [basic.scope]); the scope rules are summarized in 3.4 [basic.lookup]. A declaration that declares a function or defines a class, namespace, template, or function also has one or more scopes nested within it. These nested scopes, in turn, can have declarations nested within them. Unless otherwise stated, utterances in clause 7 [dcl.dcl] about components in, of, or contained by a declaration or subcomponent thereof refer only to those components of the declaration that are not nested within scopes nested within the declaration.

appears to rule out the similar class definition,

    struct { int m; };

However, a scoped enumeration is not listed in paragraph 2 among the constructs containing a nested scope (although 3.3.8 [basic.scope.enum] does describe “enumeration scope”); furthermore, an enumerator-definition is not formally a “nested declaration.” If unusable scoped enumeration definitions are to be banned, these shortcomings in 7 [dcl.dcl] paragraph 2 must be addressed. (A note in 7.2 [dcl.enum] mentioning that unnamed scoped enumerations are not allowed would also be helpful.)

Notes from the February, 2008 meeting:

The consensus was to require that the identifier be present in an enum-specifier unless the enum-key is enum.

Proposed resolution (June, 2008):

Change 7.2 [dcl.enum] paragraph 2 as follows:

...The enum-keys enum class and enum struct are semantically equivalent; an enumeration type declared with one of these is a scoped enumeration, and its enumerators are scoped enumerators. The optional identifier shall not be omitted in the declaration of a scoped enumeration. The type-specifier-seq of an enum-base...



540. Propagation of cv-qualifiers in reference-to-reference collapse

Section: 7.3.1  [namespace.def]     Status: CD1     Submitter: Russell Yanofsky     Date: 24 September 2005

[Voted into the WP at the October, 2006 meeting as part of paper J16/06-0188 = WG21 N2118.]

The resolution of issue 106 specifies that an attempt to create a type “reference to cv1 T,” where T is a typedef or template parameter of the type “reference to cv2 S,” actually creates the type “reference to cv12 S,” where cv12 is the union of the two sets of cv-qualifiers.

One objection that has been raised to this resolution is that it is inconsistent with the treatment of cv-qualification and references specified in 8.3.2 [dcl.ref] paragraph 1, which says that cv-qualifiers applied to a typedef or template argument that is a reference type are ignored. For example:

    typedef int& intref;
    const intref r1;       // reference to int
    const intref& r2;      // reference to const int

In fact, however, these two declarations are quite different. In the declaration of r1, const applies to a “top-level” reference, while in the declaration of t2, it occurs under a reference. In general, cv-qualifiers that appear under a reference are preserved, even if the type appears in a context in which top-level cv-qualification is removed, for example, in determining the type of a function from parameter types (8.3.5 [dcl.fct] paragraph 3) and in template argument deduction (14.8.2.1 [temp.deduct.call] paragraph 2).

Another objection to the resolution is that type composition gives different results in a single declaration than it does when separated into two declarations. For example:

    template <class T>
    struct X {
       typedef T const T_const;
       typedef T_const& type1;
       typedef T const& type2;
    };

    X<int&>::type1 t1;    // int&
    X<int&>::type2 t2;    // int const&

The initial motivation for the propagation of cv-qualification during reference-to-reference collapse was to prevent inadvertent loss of cv-qualifiers in contexts in which it could make a difference. For example, if the resolution were changed to discard, rather than propagate, embedded cv-qualification, overload resolution could surprisingly select a non-const version of a member function:

   struct X {
       void g();
       void g() const;
   };

   template <typename T> struct S {
       static void f(const T& t) {
           t.g();    // const or non-const???
       }
   };

   X x;

   void q() {
       S<X>::f(x);    // calls X::g() const
       S<X&>::f(x);   // calls X::g()
   }

Another potentially-surprising outcome of dropping embedded cv-qualifiers would be:

   template <typename T> struct A {
       void f(T&);          // mutating version
       void f(const T&);    // non-mutating version
   };

   A<int&> ai;    // Ill-formed: A<int&> declares f(int&) twice

On the other hand, those who would like to see the resolution changed to discard embedded cv-qualifiers observe that these examples are too simple to be representative of real-world code. In general, it is unrealistic to expect that a template written with non-reference type parameters in mind will automatically work correctly with reference type parameters as a result of applying the issue 106 resolution. Instead, template metaprogramming allows the template author to choose explicitly whether cv-qualifiers are propagated or dropped, according to the intended use of the template, and it is more important to respect the reasonable intuition that a declaration involving a template parameter will not change the type that the parameter represents.

As a sample of real-world code, tr1::tuple was examined. In both cases — the current resolution of issue 106 and one in which embedded cv-qualifiers were dropped — some metaprogramming was required to implement the intended interface, although the version reflecting the revised resolution was somewhat simpler.

Notes from the October, 2005 meeting:

The consensus of the CWG was that the resolution of issue 106 should be revised not to propagate embedded cv-qualification.

Note (February, 2006):

The wording included in the rvalue-reference paper, J16/06-0022 = WG21 N1952, incorporates changes intended to implement the October, 2005 consensus of the CWG.




11. How do the keywords typename/template interact with using-declarations?

Section: 7.3.3  [namespace.udecl]     Status: CD1     Submitter: Bill Gibbons     Date: unknown

[Voted into WP at March 2004 meeting.]

Issue 1:

The working paper is not clear about how the typename/template keywords interact with using-declarations:

     template<class T> struct A {
         typedef int X;
     };
     
     template<class T> void f() {
         typename A<T>::X a;      // OK
         using typename A<T>::X;  // OK
         typename X b;  // ill-formed; X must be qualified
         X c;  // is this OK?
     }
When the rules for typename and the similar use of template were decided, we chose to require that they be used at every reference. The way to avoid typename at every use is to declare a typedef; then the typedef name itself is known to be a type. For using-declarations, we decided that they do not introduce new declarations but rather are aliases for existing declarations, like symbolic links. This makes it unclear whether the declaration "X c;" above should be well-formed, because there is no new name declared so there is no declaration with a "this is a type" attribute. (The same problem would occur with the template keyword when a member template of a dependent class is used). I think these are the main options:
  1. Continue to allow typename in using-declarations, and template (for member templates) too. Attach the "is a type" or "is a template" attribute to the placeholder name which the using-declaration "declares"
  2. Disallow typename and template in using-declarations (just as class-keys are disallowed now). Allow typename and template before unqualified names which refer to dependent qualified names through using-declarations.
  3. Document that this is broken.
Suggested Resolution:

The core WG already resolved this issue according to (1), but the wording does not seem to have been added to the standard. New wording needs to be drafted.

Issue 2:

Either way, one more point needs clarification. If the first option is adopted:

     template<class T> struct A {
         struct X { };
     };
     
     template<class T> void g() {
         using typename A<T>::X;
         X c;    // if this is OK, then X by itself is a  type
         int X;  // is this OK?
     }
When "g" is instantiated, the two declarations of X are compatible (7.3.3 [namespace.udecl] paragraph 10). But there is no way to know this when the definition of "g" is compiled. I think this case should be ill-formed under the first option. (It cannot happen under the second option.) If the second option is adopted:
     template<class T> struct A {
         struct X { };
     };
     
     template<class T> void g() {
         using A<T>::X;
         int X;  // is this OK?
     }
Again, the instantiation would work but there is no way to know that in the template definition. I think this case should be ill-formed under the second option. (It would already be ill-formed under the first option.)

From John Spicer:

The "not a new declaration" decision is more of a guiding principle than a hard and fast rule. For example, a name introduced in a using-declaration can have different access than the original declaration.

Like symbolic links, a using-declaration can be viewed as a declaration that declares an alias to another name, much like a typedef.

In my opinion, "X c;" is already well-formed. Why would we permit typename to be used in a using-declaration if not to permit this precise usage?

In my opinion, all that needs to be done is to clarify that the "typeness" or "templateness" attribute of the name referenced in the using-declaration is attached to the alias created by the using-declaration. This is solution #1.

Tentative Resolution:

The rules for multiple declarations with the same name in the same scope should treat a using-declaration which names a type as a typedef, just as a typedef of a class name is treated as a class declaration. This needs drafting work. Also see Core issue 36.

Rationale (04/99): Any semantics associated with the typename keyword in using-declarations should be considered an extension.

Notes from the April 2003 meeting:

This was reopened because we are now considering extensions again. We agreed that it is desirable for the typename to be "sticky" on a using-declaration, i.e., references to the name introduced by the using-declaration are known to be type names without the use of the typename keyword (which can't be specified on an unqualified name anyway, as of now). The related issue with the template keyword already has a separate issue 109.

Issue 2 deals with the "struct hack." There is an example in 7.3.3 [namespace.udecl] paragraph 10 that shows a use of using-declarations to import two names that coexist because of the "struct hack." After some deliberation, we decided that the template-dependent using-declaration case is different enough that we did not have to support the "struct hack" in that case. A name introduced in such a case is like a typedef, and no other hidden type can be accessed through an elaborated type specifier.

Proposed resolution (April 2003, revised October 2003):

Add a new paragraph to the bottom of 7.3.3 [namespace.udecl]:

If a using-declaration uses the keyword typename and specifies a dependent name (14.6.2 [temp.dep]), the name introduced by the using-declaration is treated as a typedef-name (7.1.3 [dcl.typedef]).



258. using-declarations and cv-qualifiers

Section: 7.3.3  [namespace.udecl]     Status: CD1     Submitter: Liam Fitzpatrick     Date: 2 Nov 2000

[Voted into WP at April 2003 meeting.]

According to 7.3.3 [namespace.udecl] paragraph 12,

When a using-declaration brings names from a base class into a derived class scope, member functions in the derived class override and/or hide member functions with the same name and parameter types in a base class (rather than conflicting).

Note that this description says nothing about the cv-qualification of the hiding and hidden member functions. This means, for instance, that a non-const member function in the derived class hides a const member function with the same name and parameter types instead of overloading it in the derived class scope. For example,

    struct A {
      virtual int f() const;
      virtual int f();
    };
    struct B: A {
      B();
      int f();
      using A::f;
    };

    const B cb;
    int i = cb.f(); // ill-formed: A::f() const hidden in B

The same terminology is used in 10.3 [class.virtual] paragraph 2:

If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name and same parameter list as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf.

Notes on the 04/01 meeting:

The hiding and overriding should be on the basis of the function signature, which includes any cv-qualification on the function.

Proposed resolution (04/02):

In 7.3.3 [namespace.udecl] paragraph 12 change:

When a using-declaration brings names from a base class into a derived class scope, member functions in the derived class override and/or hide member functions with the same name and parameter types in a base class (rather than conflicting).
to read:
When a using-declaration brings names from a base class into a derived class scope, member functions and member function templates in the derived class override and/or hide member functions and member function templates with the same name, parameter-type-list (8.3.5 [dcl.fct]), and cv-qualification in a base class (rather than conflicting).

In 10.3 [class.virtual] paragraph 2 change:

If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name and same parameter list as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf.
to read:
If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name, parameter-type-list (8.3.5 [dcl.fct]), and cv-qualification as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf.

See issue 140 for the definition of parameter-type-list.




460. Can a using-declaration name a namespace?

Section: 7.3.3  [namespace.udecl]     Status: CD1     Submitter: John Spicer     Date: 12 Feb 2004

[Voted into WP at April 2005 meeting.]

Can a using-declaration be used to import a namespace?

namespace my_namespace{
  namespace my_namespace2 {
    int function_of_my_name_space(){ return 2;}
  }
}

int main (){
  using  ::my_namespace::my_namespace2;
  return my_namespace2::function_of_my_name_space();
}

Several popular compilers give an error on this, but there doesn't seem to be anything in 7.3.3 [namespace.udecl] that prohibits it. It should be noted that the user can get the same effect by using a namespace alias:

  namespace my_namespace2 = ::my_namespace::my_namespace2;

Notes from the March 2004 meeting:

We agree that it should be an error.

Proposed resolution (October, 2004):

Add the following as a new paragraph after 7.3.3 [namespace.udecl] paragraph 5:

A using-declaration shall not name a namespace;



4. Does extern "C" affect the linkage of function names with internal linkage?

Section: 7.5  [dcl.link]     Status: CD1     Submitter: Mike Anderson     Date: unknown

[Moved to DR at 4/01 meeting.]

7.5 [dcl.link] paragraph 6 says the following:

Does this apply to static functions as well? For example, is the following well-formed?
        extern "C" {
            static void f(int) {}
            static void f(float) {}
        };
Can a function with internal linkage "have C linkage" at all (assuming that phrase means "has extern "C" linkage"), for how can a function be extern "C" if it's not extern? The function type can have extern "C" linkage — but I think that's independent of the linkage of the function name. It should be perfectly reasonable to say, in the example above, that extern "C" applies only to the types of f(int) and f(float), not to the function names, and that the rule in 7.5 [dcl.link] paragraph 6 doesn't apply.

Suggested resolution: The extern "C" linkage specification applies only to the type of functions with internal linkage, and therefore some of the rules that have to do with name overloading don't apply.

Proposed Resolution:

The intent is to distingush implicit linkage from explicit linkage for both name linkage and language (function type) linkage. (It might be more clear to use the terms name linkage and type linkage to distinguish these concepts. A function can have a name with one kind of linkage and a type with a different kind of linkage. The function itself has no linkage: it has no name, only the declaration has a name. This becomes more obvious when you consider function pointers.)

The tentatively agreed proposal is to apply implicit linkage to names declared in brace-enclosed linkage specifications and to non-top-level names declared in simple linkage specifications; and to apply explicit linkage to top-level names declared in simple linkage specifications.

The language linkage of any function type formed through a function declarator is that of the nearest enclosing linkage-specification. For purposes of determining whether the declaration of a namespace-scope name matches a previous declaration, the language linkage portion of the type of a function declaration (that is, the language linkage of the function itself, not its parameters, return type or exception specification) is ignored.

For a linkage-specification using braces, i.e.

extern string-literal { declaration-seqopt }
the linkage of any declaration of a namespace-scope name (including local externs) which is not contained in a nested linkage-specification, is not declared to have no linkage (static), and does not match a previous declaration is given the linkage specified in the string-literal. The language linkage of the type of any function declaration of a namespace-scope name (including local externs) which is not contained in a nested linkage-specification and which is declared with function declarator syntax is the same as that of a matching previous declaration, if any, else is specified by string-literal.

For a linkage-specification without braces, i.e.

extern string-literal declaration

the linkage of the names declared in the top-level declarators of declaration is specified by string-literal; if this conflicts with the linkage of any matching previous declarations, the program is ill-formed. The language linkage of the type of any top-level function declarator is specified by string-literal; if this conflicts with the language linkage of the type of any matching previous function declarations, the program is ill-formed. The effect of the linkage-specification on other (non top-level) names declared in declaration is the same as that of the brace-enclosed form.

Bill Gibbons: In particular, these should be well-formed:

    extern "C" void f(void (*fp)());   // parameter type is pointer to
                                       // function with C language linkage
    extern "C++" void g(void (*fp)()); // parameter type is pointer to
                                       // function with C++ language linkage

    extern "C++" {                     // well-formed: the linkage of "f"
        void f(void(*fp)());           // and the function type used in the
    }                                  // parameter still "C"

    extern "C" {                       // well-formed: the linkage of "g"
        void g(void(*fp)());           // and the function type used in the
    }                                  // parameter still "C++"

but these should not:

    extern "C++" void f(void(*fp)());  // error - linkage of "f" does not
                                       // match previous declaration
                                       // (linkage of function type used in
                                       // parameter is still "C" and is not
                                       // by itself ill-formed)
    extern "C" void g(void(*fp)());    // error - linkage of "g" does not
                                       // match previous declaration
                                       // (linkage of function type used in
                                       // parameter is still "C++" and is not
                                       // by itself ill-formed)

That is, non-top-level declarators get their linkage from matching declarations, if any, else from the nearest enclosing linkage specification. (As already described, top-level declarators in a brace-enclosed linkage specification get the linkage from matching declarations, if any, else from the linkage specifcation; while top-level declarators in direct linkage specifications get their linkage from that specification.)

Mike Miller: This is a pretty significant change from the current specification, which treats the two forms of language linkage similarly for most purposes. I don't understand why it's desirable to expand the differences.

It seems very unintuitive to me that you could have a top-level declaration in an extern "C" block that would not receive "C" linkage.

In the current standard, the statement in 7.5 [dcl.link] paragraph 4 that

the specified language linkage applies to the function types of all function declarators, function names, and variable names introduced by the declaration(s)

applies to both forms. I would thus expect that in

    extern "C" void f(void(*)());
    extern "C++" {
        void f(void(*)());
    }
    extern "C++" f(void(*)());

both "C++" declarations would be well-formed, declaring an overloaded version of f that takes a pointer to a "C++" function as a parameter. I wouldn't expect that either declaration would be a redeclaration (valid or invalid) of the "C" version of f.

Bill Gibbons: The potential difficulty is the matching process and the handling of deliberate overloading based on language linkage. In the above examples, how are these two declarations matched:

    extern "C" void f(void (*fp1)());

    extern "C++" {
        void f(void(*fp2)());
    }

given that the linkage that is part of fp1 is "C" while the linkage (prior to the matching process) that is part of fp2 is "C++"?

The proposal is that the linkage which is part of the parameter type is not determined until after the match is attempted. This almost always correct because you can't overload "C" and "C++" functions; so if the function names match, it is likely that the declarations are supposed to be the same.

Mike Miller: This seems like more trouble than it's worth. This comparison of function types ignoring linkage specifications is, as far as I know, not found anywhere in the current standard. Why do we need to invent it?

Bill Gibbons: It is possible to construct pathological cases where this fails, e.g.

    extern "C" typedef void (*PFC)();  // pointer to "C" linkage function
    void f(PFC);         // parameter is pointer to "C" function
    void f(void (*)());  // matching declaration or overload based on
                         // difference in linkage type?

It is reasonable to require explicit typedefs in this case so that in the above example the second function declaration gets its parameter type function linkage from the first function declaration.

(In fact, I think you can't get into this situation without having already used typedefs to declare different language linkage for the top-level and parameter linkages.)

For example, if the intent is to overload based on linkage a typedef is needed:

    extern "C" typedef void (*PFC)();  // pointer to "C" linkage function
    void f(PFC);              // parameter is pointer to "C" function
    typedef void (*PFCPP)();  // pointer to "C++" linkage function
    void f(PFCPP);            // parameter is pointer to "C++" function

In this case the two function declarations refer to different functions.

Mike Miller: This seems pretty strange to me. I think it would be simpler to determine the type of the parameter based on the containing linkage specification (implicitly "C++") and require a typedef if the user wants to override the default behavior. For example:

    extern "C" {
        typedef void (*PFC)();    // pointer to "C" function
        void f(void(*)());        // takes pointer to "C" function
    }

    void f(void(*)());            // new overload of "f", taking
                                  // pointer to "C++" function

    void f(PFC);                  // redeclare extern "C" version

Notes from 04/00 meeting:

The following changes were tentatively approved, but because they do not completely implement the proposal above the issue is being kept for the moment in "drafting" status.

Notes from 10/00 meeting:

After further discussion, the core language working group determined that the more extensive proposal described above is not needed and that the following changes are sufficient.

Proposed resolution (04/01):

  1. Change the first sentence of 7.5 [dcl.link] paragraph 1 from

    All function types, function names, and variable names have a language linkage.

    to

    All function types, function names with external linkage, and variable names with external linkage have a language linkage.
  2. Change the following sentence of 7.5 [dcl.link] paragraph 4:
    In a linkage-specification, the specified language linkage applies to the function types of all function declarators, function names, and variable names introduced by the declaration(s).

    to

    In a linkage-specification, the specified language linkage applies to the function types of all function declarators, function names with external linkage, and variable names with external linkage declared within the linkage-specification.
  3. Add at the end of the final example on 7.5 [dcl.link] paragraph 4:

        extern "C" {
          static void f4();    // the name of the function f4 has
                               // internal linkage (not C language
                               // linkage) and the function's type
                               // has C language linkage
        }
        extern "C" void f5() {
          extern void f4();    // Okay -- name linkage (internal)
                               // and function type linkage (C
                               // language linkage) gotten from
                               // previous declaration.
        }
        extern void f4();      // Okay -- name linkage (internal)
                               // and function type linkage (C
                               // language linkage) gotten from
                               // previous declaration.
        void f6() {
          extern void f4();    // Okay -- name linkage (internal)
                               // and function type linkage (C
                               // language linkage) gotten from
                               // previous declaration.
        }
    
  4. Change 7.5 [dcl.link] paragraph 7 from

    Except for functions with internal linkage, a function first declared in a linkage-specification behaves as a function with external linkage. [Example:

        extern "C" double f();
        static double f();     // error
    

    is ill-formed (7.1.1 [dcl.stc]). ] The form of linkage-specification that contains a braced-enclosed declaration-seq does not affect whether the contained declarations are definitions or not (3.1 [basic.def]); the form of linkage-specification directly containing a single declaration is treated as an extern specifier (7.1.1 [dcl.stc]) for the purpose of determining whether the contained declaration is a definition. [Example:

        extern "C" int i;      // declaration
        extern "C" {
    	  int i;           // definition
        }
    

    end example] A linkage-specification directly containing a single declaration shall not specify a storage class. [Example:

        extern "C" static void f(); // error
    

    end example]

    to

    A declaration directly contained in a linkage-specification is treated as if it contains the extern specifier (7.1.1 [dcl.stc]) for the purpose of determining the linkage of the declared name and whether it is a definition. Such a declaration shall not specify a storage class. [Example:
        extern "C" double f();
        static double f();     // error
        extern "C" int i;      // declaration
        extern "C" {
    	    int i;         // definition
        }
        extern "C" static void g(); // error
    

    end example]




29. Linkage of locally declared functions

Section: 7.5  [dcl.link]     Status: CD1     Submitter: Mike Ball     Date: 19 Mar 1998

[Moved to DR at October 2002 meeting. This was incorrectly marked as having DR status between 4/01 and 4/02. It was overlooked when issue 4 was moved to DR at the 4/01 meeting; this one should have been moved as well, because it's resolved by the changes there.]

Consider the following:

    extern "C" void foo()
    {
        extern void bar();
        bar();
    }
Does "bar()" have "C" language linkage?

The ARM is explicit and says

A linkage-specification for a function also applies to functions and objects declared within it.
The DIS says
In a linkage-specification, the specified language linkage applies to the function types of all function declarators, function names, and variable names introduced by the declaration(s).
Is the body of a function definition part of the declaration?

From Mike Miller:

Yes: from 7 [dcl.dcl] paragraph 1,

and 8.4 [dcl.fct.def] paragraph 1: At least that's how I'd read it.

From Dag Brück:

Consider the following where extern "C" has been moved to a separate declaration:

    extern "C" void foo();
    
    void foo() { extern void bar(); bar(); }
I think the ARM wording could possibly be interpreted such that bar() has "C" linkage in my example, but not the DIS wording.

As a side note, I have always wanted to think that placing extern "C" on a function definition or a separate declaration would produce identical programs.

Proposed Resolution (04/01):

See the proposed resolution for Core issue 4, which covers this case.

The ODR should also be checked to see whether it addresses name and type linkage.




686. Type declarations/definitions in type-specifier-seqs and type-ids

Section: 8.1  [dcl.name]     Status: CD1     Submitter: Jens Maurer     Date: 21 March, 2008

[Voted into the WP at the September, 2008 meeting.]

The restrictions on declaring and/or defining classes inside type-specifier-seqs and type-ids are inconsistent throughout the Standard. This is probably due to the fact that nearly all of the sections that deal with them attempt to state the restriction afresh. There are three cases:

  1. 5.3.4 [expr.new], 6.4 [stmt.select], and 12.3.2 [class.conv.fct] prohibit “declarations” of classes and enumerations. That means that

        while (struct C* p = 0) ;
    

    is ill-formed unless a prior declaration of C has been seen. These appear to be cases that should have been fixed by issue 379, changing “class declaration” to “class definition,” but were overlooked.

  2. 5.1.2 [expr.prim.lambda], 7 [dcl.dcl], and 8.3.5 [dcl.fct] (late-specified return types) do not contain any restriction at all.

  3. All the remaining cases prohibit “type definitions,” apparently referring to classes and enumerations.

Suggested resolution:

Add something like, “A class or enumeration shall not be defined in a type-specifier-seq or in a type-id,” to a single place in the Standard and remove all other mentions of that restriction (allowing declarations via elaborated-type-specifier).

Mike Miller:

An alias-declaration is just a different syntax for a typedef declaration, which allows definitions of a class in the type; I would expect the same to be true of an alias-declaration. I don't have any particularly strong attachment to allowing a class definition in an alias-declaration. My only concern is introducing an irregularity into what are currently exact-match semantics with typedefs.

There's a parallel restriction in many (but not all?) of these places on typedef declarations.

Jens Maurer:

Those are redundant, as typedef is not a type-specifier, and should be removed as well.

Proposed resolution (March, 2008):

  1. Delete the indicated words from 5.2.7 [expr.dynamic.cast] paragraph 1:

  2. ...Types shall not be defined in a dynamic_cast....
  3. Delete the indicated words from 5.2.8 [expr.typeid] paragraph 4:

  4. ...Types shall not be defined in the type-id....
  5. Delete the indicated words from 5.2.9 [expr.static.cast] paragraph 1:

  6. ...Types shall not be defined in a static_cast....
  7. Delete the indicated words from 5.2.10 [expr.reinterpret.cast] paragraph 1:

  8. ...Types shall not be defined in a reinterpret_cast....
  9. Delete the indicated words from 5.2.11 [expr.const.cast] paragraph 1:

  10. ...Types shall not be defined in a const_cast....
  11. Delete paragraph 5 of 5.3.3 [expr.sizeof]:

  12. Types shall not be defined in a sizeof expression.
  13. Delete paragraph 5 of 5.3.4 [expr.new]:

  14. The type-specifier-seq shall not contain class declarations, or enumeration declarations.
  15. Delete paragraph 4 of 5.3.6 [expr.alignof]:

  16. A type shall not be defined in an alignof expression.
  17. Delete paragraph 3 of 5.4 [expr.cast]:

  18. Types shall not be defined in casts.
  19. Delete the indicated words from 6.4 [stmt.select] paragraph 2:

  20. ...The type-specifier-seq shall not contain typedef and shall not declare a new class or enumeration....
  21. Add the indicated words to 7.1.6 [dcl.type] paragraph 3:

  22. At least one type-specifier that is not a cv-qualifier is required in a declaration unless it declares a constructor, destructor or conversion function. [Footnote: ... ] A type-specifier-seq shall not define a class or enumeration unless it appears in the type-id of an alias-declaration (7.1.3 [dcl.typedef]).
  23. Delete the indicated words from 12.3.2 [class.conv.fct] paragraph 1:

  24. ...Classes, enumerations, and typedef-names shall not be declared in the type-specifier-seq....
  25. Delete the indicated words from 15.3 [except.handle] paragraph 1:

  26. ...Types shall not be defined in an exception-declaration.
  27. Delete paragraph 6 of 15.4 [except.spec]:

  28. Types shall not be defined in exception-specifications.

[Drafting note: no changes are required to 5.1.2 [expr.prim.lambda], 7.1.3 [dcl.typedef], 7.6.2 [dcl.align], 7.2 [dcl.enum], 8.3.5 [dcl.fct], 14.1 [temp.param], or 14.2 [temp.names].]




160. Missing std:: qualification

Section: 8.2  [dcl.ambig.res]     Status: CD1     Submitter: Al Stevens     Date: 23 Aug 1999

[Moved to DR at 10/01 meeting.]

8.2 [dcl.ambig.res] paragraph 3 shows an example that includes <cstddef> with no using declarations or directives and refers to size_t without the std:: qualification.

Many references to size_t throughout the document omit the std:: namespace qualification.

This is a typical case. The use of std:: is inconsistent throughout the document.

In addition, the use of exception specifications should be examined for consistency.

(See also issue 282.)

Proposed resolution:

In 1.9 [intro.execution] paragraph 9, replace all two instances of "sig_atomic_t" by "std::sig_atomic_t".

In 3.1 [basic.def] paragraph 4, replace all three instances of "string" by "std::string" in the example and insert "#include <string>" at the beginning of the example code.

In 3.6.1 [basic.start.main] paragraph 4, replace

Calling the function
void exit(int);
declared in <cstdlib>...

by

Calling the function std::exit(int) declared in <cstdlib>...

and also replace "exit" by "std::exit" in the last sentence of that paragraph.

In 3.6.1 [basic.start.main] first sentence of paragraph 5, replace "exit" by "std::exit".

In 3.6.2 [basic.start.init] paragraph 4, replace "terminate" by "std::terminate".

In 3.6.3 [basic.start.term] paragraph 1, replace "exit" by "std::exit" (see also issue 28).

In 3.6.3 [basic.start.term] paragraph 3, replace all three instances of "atexit" by "std::atexit" and both instances of "exit" by "std::exit" (see also issue 28).

In 3.6.3 [basic.start.term] paragraph 4, replace

Calling the function
void abort();
declared in <cstdlib>...

by

Calling the function std::abort() declared in <cstdlib>...
and "atexit" by "std::atexit" (see also issue 28).

In 3.7.4.1 [basic.stc.dynamic.allocation] paragraph 1 third sentence, replace "size_t" by "std::size_t".

In 3.7.4.1 [basic.stc.dynamic.allocation] paragraph 3, replace "new_handler" by "std::new_handler". Furthermore, replace "set_new_handler" by "std::set_new_handler" in the note.

In 3.7.4.1 [basic.stc.dynamic.allocation] paragraph 4, replace "type_info" by "std::type_info" in the note.

In 3.7.4.2 [basic.stc.dynamic.deallocation] paragraph 3, replace all four instances of "size_t" by "std::size_t".

In 3.8 [basic.life] paragraph 5, replace "malloc" by "std::malloc" in the example code and insert "#include <cstdlib>" at the beginning of the example code.

In 3.9 [basic.types] paragraph 2, replace "memcpy" by "std::memcpy" (the only instance in the footnote and both instances in the example) and replace "memmove" by "std::memmove" in the footnote (see also issue 43).

In 3.9 [basic.types] paragraph 3, replace "memcpy" by "std::memcpy", once in the normative text and once in the example (see also issue 43).

In 3.9.1 [basic.fundamental] paragraph 8 last sentence, replace "numeric_limits" by "std::numeric_limits".

In 5.2.7 [expr.dynamic.cast] paragraph 9 second sentence, replace "bad_cast" by "std::bad_cast".

In 5.2.8 [expr.typeid] paragraph 2, replace "type_info" by "std::type_info" and "bad_typeid" by "std::bad_typeid".

In 5.2.8 [expr.typeid] paragraph 3, replace "type_info" by "std::type_info".

In 5.2.8 [expr.typeid] paragraph 4, replace both instances of "type_info" by "std::type_info".

In 5.3.3 [expr.sizeof] paragraph 6, replace both instances of "size_t" by "std::size_t".

In 5.3.4 [expr.new] paragraph 11 last sentence, replace "size_t" by "std::size_t".

In 5.7 [expr.add] paragraph 6, replace both instances of "ptrdiff_t" by "std::ptrdiff_t".

In 5.7 [expr.add] paragraph 8, replace "ptrdiff_t" by "std::ptrdiff_t".

In 6.6 [stmt.jump] paragraph 2, replace "exit" by "std::exit" and "abort" by "std::abort" in the note.

In 8.2 [dcl.ambig.res] paragraph 3, replace "size_t" by "std::size_t" in the example.

In 8.4 [dcl.fct.def] paragraph 5, replace "printf" by "std::printf" in the note.

In 12.4 [class.dtor] paragraph 13, replace "size_t" by "std::size_t" in the example.

In 12.5 [class.free] paragraph 2, replace all four instances of "size_t" by "std::size_t" in the example.

In 12.5 [class.free] paragraph 6, replace both instances of "size_t" by "std::size_t" in the example.

In 12.5 [class.free] paragraph 7, replace all four instances of "size_t" by "std::size_t" in the two examples.

In 12.7 [class.cdtor] paragraph 4, replace "type_info" by "std::type_info".

In 13.6 [over.built] paragraph 13, replace all five instances of "ptrdiff_t" by "std::ptrdiff_t".

In 13.6 [over.built] paragraph 14, replace "ptrdiff_t" by "std::ptrdiff_t".

In 13.6 [over.built] paragraph 21, replace both instances of "ptrdiff_t" by "std::ptrdiff_t".

In 14.2 [temp.names] paragraph 4, replace both instances of "size_t" by "std::size_t" in the example. (The example is quoted in issue 96.)

In 14.3 [temp.arg] paragraph 1, replace "complex" by "std::complex", once in the example code and once in the comment.

In 14.7.3 [temp.expl.spec] paragraph 8, issue 24 has already corrected the example.

In 15.1 [except.throw] paragraph 6, replace "uncaught_exception" by "std::uncaught_exception".

In 15.1 [except.throw] paragraph 7, replace "terminate" by "std::terminate" and both instances of "unexpected" by "std::unexpected".

In 15.1 [except.throw] paragraph 8, replace "terminate" by "std::terminate".

In 15.2 [except.ctor] paragraph 3, replace "terminate" by "std::terminate".

In 15.3 [except.handle] paragraph 9, replace "terminate" by "std::terminate".

In 15.4 [except.spec] paragraph 8, replace "unexpected" by "std::unexpected".

In 15.4 [except.spec] paragraph 9, replace "unexpected" by "std::unexpected" and "terminate" by "std::terminate".

In 15.5 [except.special] paragraph 1, replace "terminate" by "std::terminate" and "unexpected" by "std::unexpected".

In the heading of 15.5.1 [except.terminate], replace "terminate" by "std::terminate".

In 15.5.1 [except.terminate] paragraph 1, footnote in the first bullet, replace "terminate" by "std::terminate". In the same paragraph, fifth bullet, replace "atexit" by "std::atexit". In the same paragraph, last bullet, replace "unexpected_handler" by "std::unexpected_handler".

In 15.5.1 [except.terminate] paragraph 2, replace

In such cases,
void terminate();
is called...

by

In such cases, std::terminate() is called...

and replace all three instances of "terminate" by "std::terminate".

In the heading of 15.5.2 [except.unexpected], replace "unexpected" by "std::unexpected".

In 15.5.2 [except.unexpected] paragraph 1, replace

...the function
void unexpected();
is called...

by

...the function std::unexpected() is called...
.

In 15.5.2 [except.unexpected] paragraph 2, replace "unexpected" by "std::unexpected" and "terminate" by "std::terminate".

In 15.5.2 [except.unexpected] paragraph 3, replace "unexpected" by "std::unexpected".

In the heading of 15.5.3 [except.uncaught], replace "uncaught_exception" by "std::uncaught_exception".

In 15.5.3 [except.uncaught] paragraph 1, replace

The function
bool uncaught_exception()
returns true...

by

The function std::uncaught_exception() returns true...
.

In the last sentence of the same paragraph, replace "uncaught_exception" by "std::uncaught_exception".




112. Array types and cv-qualifiers

Section: 8.3.4  [dcl.array]     Status: CD1     Submitter: Steve Clamage     Date: 4 May 1999

[Moved to DR at 10/01 meeting.]

Steve Clamage: Section 8.3.4 [dcl.array] paragraph 1 reads in part as follows:

Any type of the form "cv-qualifier-seq array of N T" is adjusted to "array of N cv-qualifier-seq T," and similarly for "array of unknown bound of T." [Example:
    typedef int A[5], AA[2][3];
    typedef const A CA;     // type is "array of 5 const int"
    typedef const AA CAA;   // type is "array of 2 array of 3 const int"
end example] [Note: an "array of N cv-qualifier-seq T" has cv-qualified type; such an array has internal linkage unless explicitly declared extern (7.1.6.1 [dcl.type.cv] ) and must be initialized as specified in 8.5 [dcl.init] . ]
The Note appears to contradict the sentence that precedes it.

Mike Miller: I disagree; all it says is that whether the qualification on the element type is direct ("const int x[5]") or indirect ("const A CA"), the array itself is qualified in the same way the elements are.

Steve Clamage: In addition, section 3.9.3 [basic.type.qualifier] paragraph 2 says:

A compound type (3.9.2 [basic.compound] ) is not cv-qualified by the cv-qualifiers (if any) of the types from which it is compounded. Any cv-qualifiers applied to an array type affect the array element type, not the array type (8.3.4 [dcl.array] )."
The Note appears to contradict that section as well.

Mike Miller: Yes, but consider the last two sentences of 3.9.3 [basic.type.qualifier] paragraph 5:

Cv-qualifiers applied to an array type attach to the underlying element type, so the notation "cv T," where T is an array type, refers to an array whose elements are so-qualified. Such array types can be said to be more (or less) cv-qualified than other types based on the cv-qualification of the underlying element types.
I think this says essentially the same thing as 8.3.4 [dcl.array] paragraph 1 and its note: the qualification of an array is (bidirectionally) equivalent to the qualification of its members.

Mike Ball: I find this a very far reach. The text in 8.3.4 [dcl.array] is essentially that which is in the C standard (and is a change from early versions of C++). I don't see any justification at all for the bidirectional equivalence. It seems to me that the note is left over from the earlier version of the language.

Steve Clamage: Finally, the Note seems to say that the declaration

    volatile char greet[6] = "Hello";
gives "greet" internal linkage, which makes no sense.

Have I missed something, or should that Note be entirely removed?

Mike Miller: At least the wording in the note should be repaired not to indicate that volatile-qualification gives an array internal linkage. Also, depending on how the discussion goes, either the wording in 3.9.3 [basic.type.qualifier] paragraph 2 or in paragraph 5 needs to be amended to be consistent regarding whether an array type is considered qualified by the qualification of its element type.

Steve Adamczyk pointed out that the current state of affairs resulted from the need to handle reference binding consistently. The wording is intended to define the question, "Is an array type cv-qualified?" as being equivalent to the question, "Is the element type of the array cv-qualified?"

Proposed resolution (10/00):

Replace the portion of the note in 8.3.4 [dcl.array] paragraph 1 reading

such an array has internal linkage unless explicitly declared extern (7.1.6.1 [dcl.type.cv]) and must be initialized as specified in 8.5 [dcl.init].

with

see 3.9.3 [basic.type.qualifier].



140. Agreement of parameter declarations

Section: 8.3.5  [dcl.fct]     Status: CD1     Submitter: Steve Clamage     Date: 15 Jul 1999

[Moved to DR at 10/01 meeting.]

8.3.5 [dcl.fct] paragraph 3 says,

All declarations for a function with a given parameter list shall agree exactly both in the type of the value returned and in the number and type of parameters.
It is not clear what this requirement means with respect to a pair of declarations like the following:
    int f(const int);
    int f(int x) { ... }
Do they violate this requirement? Is x const in the body of the function declaration?

Tom Plum: I think the FDIS quotation means that the pair of decls are valid. But it doesn't clearly answer whether x is const inside the function definition. As to intent, I know the intent was that if the function definition wants to specify that x is const, the const must appear specifically in the defining decl, not just on some decl elsewhere. But I can't prove that intent from the drafted words.

Mike Miller: I think the intent was something along the following lines:

Two function declarations denote the same entity if the names are the same and the function signatures are the same. (Two function declarations with C language linkage denote the same entity if the names are the same.) All declarations of a given function shall agree exactly both in the type of the value returned and in the number and type of parameters; the presence or absence of the ellipsis is considered part of the signature.
(See 3.5 [basic.link] paragraph 9. That paragraph talks about names in different scopes and says that function references are the same if the "types are identical for purposes of overloading," i.e., the signatures are the same. See also 7.5 [dcl.link] paragraph 6 regarding C language linkage, where only the name is required to be the same for declarations in different namespaces to denote the same function.)

According to this paragraph, the type of a parameter is determined by considering its decl-specifier-seq and declarator and then applying the array-to-pointer and function-to-pointer adjustments. The cv-qualifier and storage class adjustments are performed for the function type but not for the parameter types.

If my interpretation of the intent of the second sentence of the paragraph is correct, the two declarations in the example violate that restriction — the parameter types are not the same, even though the function types are. Since there's no dispensation mentioned for "no diagnostic required," an implementation presumably must issue a diagnostic in this case. (I think "no diagnostic required" should be stated if the declarations occur in different translation units — unless there's a blanket statement to that effect that I have forgotten?)

(I'd also note in passing that, if my interpretation is correct,

    void f(int);
    void f(register int) { }
is also an invalid pair of declarations.)

Proposed resolution (10/00):

  1. In 1.3 [intro.defs] “signature,” change "the types of its parameters" to "its parameter-type-list (8.3.5 [dcl.fct])".

  2. In the third bullet of 3.5 [basic.link] paragraph 9 change "the function types are identical for the purposes of overloading" to "the parameter-type-lists of the functions (8.3.5 [dcl.fct]) are identical."

  3. In the sub-bullets of the third bullet of 5.2.5 [expr.ref] paragraph 4, change all four occurrences of "function of (parameter type list)" to "function of parameter-type-list."

  4. In 8.3.5 [dcl.fct] paragraph 3, change

    All declarations for a function with a given parameter list shall agree exactly both in the type of the value returned and in the number and type of parameters; the presence or absence of the ellipsis is considered part of the function type.
    to
    All declarations for a function shall agree exactly in both the return type and the parameter-type-list.

  5. In 8.3.5 [dcl.fct] paragraph 3, change

    The resulting list of transformed parameter types is the function's parameter type list.
    to
    The resulting list of transformed parameter types and the presence or absence of the ellipsis is the function's parameter-type-list.

  6. In 8.3.5 [dcl.fct] paragraph 4, change "the parameter type list" to "the parameter-type-list."

  7. In the second bullet of 13.1 [over.load] paragraph 2, change all occurrences of "parameter types" to "parameter-type-list."

  8. In 13.3 [over.match] paragraph 1, change "the types of the parameters" to "the parameter-type-list."

  9. In the last sub-bullet of the third bullet of 13.3.1.2 [over.match.oper] paragraph 3, change "parameter type list" to "parameter-type-list."

Note, 7 Sep 2001:

Editorial changes while putting in issue 147 brought up the fact that injected-class-name is not a syntax term and therefore perhaps shouldn't be written with hyphens. The same can be said of parameter-type-list.




262. Default arguments and ellipsis

Section: 8.3.5  [dcl.fct]     Status: CD1     Submitter: Jamie Schmeiser     Date: 13 Nov 2000

[Voted into WP at April 2003 meeting.]

The interaction of default arguments and ellipsis is not clearly spelled out in the current wording of the Standard. 8.3.6 [dcl.fct.default] paragraph 4 says,

In a given function declaration, all parameters subsequent to a parameter with a default argument shall have default arguments supplied in this or previous declarations.

Strictly speaking, ellipsis isn't a parameter, but this could be clearer. Also, in 8.3.5 [dcl.fct] paragraph 2,

If the parameter-declaration-clause terminates with an ellipsis, the number of arguments shall be equal to or greater than the number of parameters specified.

This could be interpreted to refer to the number of arguments after the addition of default arguments to the argument list given in the call expression, but again it could be clearer.

Notes from 04/01 meeting:

The consensus opinion was that an ellipsis is not a parameter and that default arguments should be permitted preceding an ellipsis.

Proposed Resolution (4/02):

Change the following sentence in 8.3.5 [dcl.fct] paragraph 2 from

If the parameter-declaration-clause terminates with an ellipsis, the number of arguments shall be equal to or greater than the number of parameters specified.

to

If the parameter-declaration-clause terminates with an ellipsis, the number of arguments shall be equal to or greater than the number of parameters that do not have a default argument.

As noted in the defect, section 8.3.6 [dcl.fct.default] is correct but could be clearer.

In 8.3.6 [dcl.fct.default], add the following as the first line of the example in paragraph 4.

  void g(int = 0, ...);  // okay, ellipsis is not a parameter so it can follow 
                         // a parameter with a default argument



295. cv-qualifiers on function types

Section: 8.3.5  [dcl.fct]     Status: CD1     Submitter: Nathan Sidwell     Date: 29 Jun 2001

[Moved to DR at October 2002 meeting.]

This concerns the inconsistent treatment of cv qualifiers on reference types and function types. The problem originated with GCC bug report c++/2810. The bug report is available at http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view&pr=2810&database=gcc

8.3.2 [dcl.ref] describes references. Of interest is the statement (my emphasis)

Cv-qualified references are ill-formed except when the cv-qualifiers are introduced through the use of a typedef or of a template type argument, in which case the cv-qualifiers are ignored.

Though it is strange to ignore 'volatile' here, that is not the point of this defect report. 8.3.5 [dcl.fct] describes function types. Paragraph 4 states,

In fact, if at any time in the determination of a type a cv-qualified function type is formed, the program is ill-formed.

No allowance for typedefs or template type parameters is made here, which is inconsistent with the equivalent reference case.

The GCC bug report was template code which attempted to do,

    template <typename T> void foo (T const &);
    void baz ();
    ...
    foo (baz);

in the instantiation of foo, T is `void ()' and an attempt is made to const qualify that, which is ill-formed. This is a surprise.

Suggested resolution:

Replace the quoted sentence from paragraph 4 in 8.3.5 [dcl.fct] with

cv-qualified functions are ill-formed, except when the cv-qualifiers are introduced through the use of a typedef or of a template type argument, in which case the cv-qualifiers are ignored.

Adjust the example following to reflect this.

Proposed resolution (10/01):

In 8.3.5 [dcl.fct] paragraph 4, replace

The effect of a cv-qualifier-seq in a function declarator is not the same as adding cv-qualification on top of the function type, i.e., it does not create a cv-qualified function type. In fact, if at any time in the determination of a type a cv-qualified function type is formed, the program is ill-formed. [Example:
  typedef void F();
  struct S {
    const F f;          // ill-formed
  };
-- end example]
by
The effect of a cv-qualifier-seq in a function declarator is not the same as adding cv-qualification on top of the function type. In the latter case, the cv-qualifiers are ignored. [Example:
  typedef void F();
  struct S {
    const F f;          // ok; equivalent to void f();
  };
-- end example]

Strike the last bulleted item in 14.8.2 [temp.deduct] paragraph 2, which reads

Attempting to create a cv-qualified function type.

Nathan Sidwell comments (18 Dec 2001 ): The proposed resolution simply states attempts to add cv qualification on top of a function type are ignored. There is no mention of whether the function type was introduced via a typedef or template type parameter. This would appear to allow

  void (const *fptr) ();
but, that is not permitted by the grammar. This is inconsistent with the wording of adding cv qualifiers to a reference type, which does mention typedefs and template parameters, even though
  int &const ref;
is also not allowed by the grammar.

Is this difference intentional? It seems needlessly confusing.

Notes from 4/02 meeting:

Yes, the difference is intentional. There is no way to add cv-qualifiers other than those cases.

Notes from April 2003 meeting:

Nathan Sidwell pointed out that some libraries use the inability to add const to a type T as a way of testing that T is a function type. He will get back to us if he has a proposal for a change.




681. Restrictions on declarators with late-specified return types

Section: 8.3.5  [dcl.fct]     Status: CD1     Submitter: Mike Miller     Date: 10 March, 2008

[Voted into the WP at the September, 2008 meeting as part of paper N2757.]

The wording added to 8.3.5 [dcl.fct] for declarators with late-specified return types says,

In a declaration T D where D has the form

and the type of the contained declarator-id in the declaration T D1 is “derived-declarator-type-list T,” T shall be the single type-specifier auto and the derived-declarator-type-list shall be empty.

These restrictions were intended to ensure that the return type of the function is exactly the specified type-id following the ->, not modified by declarator operators and cv-qualification.

Unfortunately, the requirement for an empty derived-declarator-type-list does not achieve this goal but instead forbids declarations like

    auto (*fp)() -> int;    // pointer to function returning int

while allowing declarations like

    auto *f() -> int;       // function returning pointer to int

The reason for this is that, according to the grammar in 8 [dcl.decl] paragraph 4, the declarator *f() -> int is parsed as a ptr-operator applied to the direct-declarator f() -> int; that is, the declarator D1 seen in 8.3.5 [dcl.fct] is just f, and the derived-declarator-type-list is thus empty.

By contrast, the declarator (*fp)() -> int is parsed as the direct-declarator (*fp) followed by the parameter-declaration-clause, etc. In this case, D1 in 8.3.5 [dcl.fct] is (*fp) and the derived-declarator-type-list is “pointer to,” i.e., not empty.

My personal view is that there is no reason to forbid the (*fp)() -> int form, and that doing so is problematic. For example, this restriction would require users desiring the late-specified return type syntax to write function parameters as function types and rely on parameter type transformations rather than writing them as pointer-to-function types, as they will actually turn out to be:

    void f(auto (*fp)() -> int);  // ill-formed
    void f(auto fp() -> int);     // OK (but icky)

It may be helpful in deciding whether to allow this form to consider the example of a function returning a pointer to a function. With the current restriction, only one of the three plausible forms is allowed:

    auto (*f())() -> int;           // Disallowed
    auto f() -> int (*)();          // Allowed
    auto f() -> auto (*)() -> int;  // Disallowed
Suggested resolution:
  1. Delete the words “and the derived-declarator-type-list shall be empty” from 8.3.5 [dcl.fct] paragraph 2.

  2. Add a new paragraph following 8 [dcl.decl] paragraph 4:

  3. A ptr-operator shall not be applied, directly or indirectly, to a function declarator with a late-specified return type (8.3.5 [dcl.fct]).

Proposed resolution (June, 2008):

  1. Change the grammar in 8 [dcl.decl] paragraph 4 as follows:

  2. Change the grammar in 8.1 [dcl.name] paragraph 1 as follows:

  3. Change 8.3.5 [dcl.fct] paragraph 2 as follows:

  4. ... T shall be the single type-specifier auto and the derived-declarator-type-list shall be empty. Then the type...
  5. Change all occurrences of direct-new-declarator in 5.3.4 [expr.new] to noptr-new-declarator. These changes appear in the grammar in paragraph 1 and in the text of paragraphs 6-8, as follows:

  6. When the allocated object is an array (that is, the direct-noptr-new-declarator syntax is used or the new-type-id or type-id denotes an array type), the new-expression yields a pointer to the initial element (if any) of the array. [Note: both new int and new int[10] have type int* and the type of new int[i][10] is int (*)[10]end note]

    Every constant-expression in a direct-noptr-new-declarator shall be an integral constant expression (5.19 [expr.const]) and evaluate to a strictly positive value. The expression in a direct-noptr-new-declarator shall be of integral type, enumeration type, or a class type for which a single non-explicit conversion function to integral or enumeration type exists (12.3 [class.conv]). If the expression is of class type, the expression is converted by calling that conversion function, and the result of the conversion is used in place of the original expression. If the value of the expression is negative, the behavior is undefined. [Example: given the definition int n = 42, new float[n][5] is well-formed (because n is the expression of a direct-noptr-new-declarator), but new float[5][n] is ill-formed (because n is not a constant expression). If n is negative, the effect of new float[n][5] is undefined. —end example]

    When the value of the expression in a direct-noptr-new-declarator is zero, the allocation function is called to allocate an array with no elements.




136. Default arguments and friend declarations

Section: 8.3.6  [dcl.fct.default]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 9 July 1999

[Moved to DR at 10/01 meeting.]

8.3.6 [dcl.fct.default] paragraph 4 says,

For non-template functions, default arguments can be added in later declarations of a function in the same scope. Declarations in different scopes have completely distinct sets of default arguments. That is, declarations in inner scopes do not acquire default arguments from declarations in outer scopes, and vice versa.
It is unclear how this wording applies to friend function declarations. For example,
    void f(int, int, int=0);             // #1
    class C {
        friend void f(int, int=0, int);  // #2
    };
    void f(int=0, int, int);             // #3
Does the declaration at #2 acquire the default argument from #1, and does the one at #3 acquire the default arguments from #2?

There are several related questions involved with this issue:

  1. Is the friend declaration in the scope of class C or in the surrounding namespace scope?

    Mike Miller: 8.3.6 [dcl.fct.default] paragraph 4 is speaking about the lexical location of the declaration... The friend declaration occurs in a different declarative region from the declaration at #1, so I would read [this paragraph] as saying that it starts out with a clean slate of default arguments.

    Bill Gibbons: Yes. It occurs in a different region, although it declares a name in the same region (i.e. a redeclaration). This is the same as with local externs and is intended to work the same way. We decided that local extern declarations cannot add (beyond the enclosing block) new default arguments, and the same should apply to friend declarations.

    John Spicer: The question is whether [this paragraph] does (or should) mean declarations that appear in the same lexical scope or declarations that declare names in the same scope. In my opinion, it really needs to be the latter. It seems somewhat paradoxical to say that a friend declaration declares a function in namespace scope yet the declaration in the class still has its own attributes. To make that work I think you'd have to make friends more like block externs that really do introduce a name into the scope in which the declaration is contained.

  2. Should default arguments be permitted in friend function declarations, and what effect should they have?

    Bill Gibbons: In the absence of a declaration visible in class scope to which they could be attached, default arguments on friend declarations do not make sense. [They should be] ill-formed, to prevent surprises.

    John Spicer: It is important that the following case work correctly:

            class X {
                    friend void f(X x, int i = 1){}
            };
    
            int main()
            {
                    X x;
                    f(x);
            }
    

    In other words, a function first declared in a friend declaration must be permitted to have default arguments and those default arguments must be usable when the function is found by argument dependent lookup. The reason that this is important is that it is common practice to define functions in friend declarations in templates, and that definition is the only place where the default arguments can be specified.

  3. What restrictions should be placed on default argument usage with friend declarations?

    John Spicer: We want to avoid instantiation side effects. IMO, the way to do this would be to prohibit a friend declaration from providing default arguments if a declaration of that function is already visible. Once a function has had a default specified in a friend declaration it should not be possible to add defaults in another declaration be it a friend or normal declaration.

    Mike Miller: The position that seems most reasonable to me is to allow default arguments in friend declarations to be used in Koenig lookup, but to say that they are completely unrelated to default arguments in declarations in the surrounding scope; and to forbid use of a default argument in a call if more than one declaration in the overload set has such a default, as in the proposed resolution for issue 1.

(See also issues 21, 95, 138, 139, 143, 165, and 166.)

Notes from 10/99 meeting:

Four possible outcomes were identified:

  1. If a friend declaration declares a default parameter, allow no other declarations of that function in the translation unit.
  2. Same as preceding, but only allow the friend declaration if it is also a definition.
  3. Disallow default arguments in friend declarations.
  4. Treat the default arguments in each friend declaration as a distinct set, causing an error if the call would be ambiguous.

The core group eliminated the first and fourth options from consideration, but split fairly evenly between the remaining two.

A straw poll of the full committee yielded the following results (given as number favoring/could live with/"over my dead body"):

  1. 0/14/5
  2. 8/13/5
  3. 11/7/14
  4. 7/10/9

Additional discussion is recorded in the "Record of Discussion" for the meeting, J16/99-0036 = WG21 N1212. See also paper J16/00-0040 = WG21 N1263.

Proposed resolution (10/00):

In 8.3.6 [dcl.fct.default], add following paragraph 4:

If a friend declaration specifies a default argument expression, that declaration must be a definition and shall be the only declaration of the function or function template in the translation unit.



5. CV-qualifiers and type conversions

Section: 8.5  [dcl.init]     Status: CD1     Submitter: Josee Lajoie     Date: unknown

[Moved to DR at 4/01 meeting.]

The description of copy-initialization in 8.5 [dcl.init] paragraph 14 says:

Should "destination type" in this last bullet refer to "cv-unqualified destination type" to make it clear that the destination type excludes any cv-qualifiers? This would make it clearer that the following example is well-formed:
     struct A {
       A(A&);
     };
     struct B : A { };
     
     struct C {
       operator B&();
     };
     
     C c;
     const A a = c; // allowed?

The temporary created with the conversion function is an lvalue of type B. If the temporary must have the cv-qualifiers of the destination type (i.e. const) then the copy-constructor for A cannot be called to create the object of type A from the lvalue of type const B. If the temporary has the cv-qualifiers of the result type of the conversion function, then the copy-constructor for A can be called to create the object of type A from the lvalue of type const B. This last outcome seems more appropriate.

Steve Adamczyk:

Because of late changes to this area, the relevant text is now the third sub-bullet of the fourth bullet of 8.5 [dcl.init] paragraph 14:

Otherwise (i.e., for the remaining copy-initialization cases), user-defined conversion sequences that can convert from the source type to the destination type or (when a conversion function is used) to a derived class thereof are enumerated... The function selected is called with the initializer expression as its argument; if the function is a constructor, the call initializes a temporary of the destination type. The result of the call (which is the temporary for the constructor case) is then used to direct-initialize, according to the rules above, the object that is the destination of the copy-initialization.

The issue still remains whether the wording should refer to "the cv-unqualified version of the destination type." I think it should.

Notes from 10/00 meeting:

The original example does not illustrate the remaining problem. The following example does:

    struct C { };
    C c;
    struct A {
        A(const A&);
        A(const C&);
    };
    const volatile A a = c;    // Okay

Proposed Resolution (04/01):

In 8.5 [dcl.init], paragraph 14, bullet 4, sub-bullet 3, change

if the function is a constructor, the call initializes a temporary of the destination type.

to

if the function is a constructor, the call initializes a temporary of the cv-unqualified version of the destination type.



78. Section 8.5 paragraph 9 should state it only applies to non-static objects

Section: 8.5  [dcl.init]     Status: CD1     Submitter: Judy Ward     Date: 15 Dec 1998

Paragraph 9 of 8.5 [dcl.init] says:

If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD class type (or array thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlying class type shall have a user-declared default constructor. Otherwise, if no initializer is specified for an object, the object and its subobjects, if any, have an indeterminate initial value; if the object or any of its subobjects are of const-qualified type, the program is ill-formed.
It should be made clear that this paragraph does not apply to static objects.

Proposed resolution (10/00): In 8.5 [dcl.init] paragraph 9, replace

Otherwise, if no initializer is specified for an object..."
with
Otherwise, if no initializer is specified for a non-static object...



177. Lvalues vs rvalues in copy-initialization

Section: 8.5  [dcl.init]     Status: CD1     Submitter: Steve Adamczyk     Date: 25 October 1999

[Moved to DR at 4/02 meeting.]

Is the temporary created during copy-initialization of a class object treated as an lvalue or an rvalue? That is, is the following example well-formed or not?

    struct B { };
    struct A {
        A(A&);    // not const
        A(const B&);
    };
    B b;
    A a = b;

According to 8.5 [dcl.init] paragraph 14, the initialization of a is performed in two steps. First, a temporary of type A is created using A::A(const B&). Second, the resulting temporary is used to direct-initialize a using A::A(A&).

The second step requires binding a reference to non-const to the temporary resulting from the first step. However, 8.5.3 [dcl.init.ref] paragraph 5 requires that such a reference be bound only to lvalues.

It is not clear from 3.10 [basic.lval] whether the temporary created in the process of copy-initialization should be treated as an lvalue or an rvalue. If it is an lvalue, the example is well-formed, otherwise it is ill-formed.

Proposed resolution (04/01):

  1. In 8.5 [dcl.init] paragraph 14, insert the following after "the call initializes a temporary of the destination type":

    The temporary is an rvalue.
  2. In 15.1 [except.throw] paragraph 3, replace

    The temporary is used to initialize the variable...

    with

    The temporary is an lvalue and is used to initialize the variable...

(See also issue 84.)




277. Zero-initialization of pointers

Section: 8.5  [dcl.init]     Status: CD1     Submitter: Andrew Sawyer     Date: 5 Apr 2001

[Moved to DR at 10/01 meeting.]

The intent of 8.5 [dcl.init] paragraph 5 is that pointers that are zero-initialized will contain a null pointer value. Unfortunately, the wording used,

...set to the value of 0 (zero) converted to T

does not match the requirements for creating a null pointer value given in 4.10 [conv.ptr] paragraph 1:

A null pointer constant is an integral constant expression (5.19 [expr.const]) rvalue of integer type that evaluates to zero. A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type...

The problem is that the "value of 0" in the description of zero-initialization is not specified to be an integral constant expression. Nonconstant expressions can also have the value 0, and converting a nonconst 0 to pointer type need not result in a null pointer value.

Proposed resolution (04/01):

In 8.5 [dcl.init] paragraph 5, change

...set to the value 0 (zero) converted to T;

to

...set to the value 0 (zero), taken as an integral constant expression, converted to T; [footnote: as specified in 4.10 [conv.ptr], converting an integral constant expression whose value is 0 to a pointer type results in a null pointer value.]



302. Value-initialization and generation of default constructor

Section: 8.5  [dcl.init]     Status: CD1     Submitter: Steve Adamczyk     Date: 23 Jul 2001

[Moved to DR at October 2002 meeting.]

We've been looking at implementing value-initialization. At one point, some years back, I remember Bjarne saying that something like X() in an expression should produce an X object with the same value one would get if one created a static X object, i.e., the uninitialized members would be zero-initialized because the whole object is initialized at program startup, before the constructor is called.

The formulation for default-initialization that made it into TC1 (in 8.5 [dcl.init]) is written a little differently (see issue 178), but I had always assumed that it would still be a valid implementation to zero the whole object and then call the default constructor for the troublesome "non-POD but no user-written constructor" cases.

That almost works correctly, but I found a problem case:

    struct A {
      A();
      ~A();
    };
    struct B {
      // B is a non-POD with no user-written constructor.
      // It has a nontrivial generated constructor.
      const int i;
      A a;
    };
    int main () {
      // Value-initializing a "B" doesn't call the default constructor for
      // "B"; it value-initializes the members of B.  Therefore it shouldn't
      // cause an error on generation of the default constructor for the
      // following:
      new B();
    }

If the definition of the B::B() constructor is generated, an error is issued because the const member "i" is not initialized. But the definition of value-initialization doesn't require calling the constructor, and therefore it doesn't require generating it, and therefore the error shouldn't be detected.

So this is a case where zero-initializing and then calling the constructor is not equivalent to value-initializing, because one case generates an error and the other doesn't.

This is sort of unfortunate, because one doesn't want to generate all the required initializations at the point where the "()" initialization occurs. One would like those initializations to be packaged in a function, and the default constructor is pretty much the function one wants.

I see several implementation choices:

  1. Zero the object, then call the default generated constructor. This is not valid unless the standard is changed to say that the default constructor might be generated for value-initialization cases like the above (that is, it's implementation-dependent whether the constructor definition is generated). The zeroing operation can of course be optimized, if necessary, to hit only the pieces of the object that would otherwise be left uninitialized. An alternative would be to require generation of the constructor for value-initialization cases, even if the implementation technique doesn't call the constructor at that point. It's pretty likely that the constructor is going to have to be generated at some point in the program anyway.
  2. Make a new value-initialization "constructor," whose body looks a lot like the usual generated constructor, but which also zeroes other members. No errors would be generated while generating this modified constructor, because it generates code that does full initialization. (Actually, it wouldn't guarantee initialization of reference members, and that might be an argument for generating the constructor, in order to get that error.) This is standard-conforming, but it destroys object-code compatibility.
  3. Variation on (1): Zero first, and generate the object code for the default constructor when it's needed for value-initialization cases, but don't issue any errors at that time. Issue the errors only if it turns out the constructor is "really" referenced. Aside from the essential shadiness of this approach, I fear that something in the generation of the constructor will cause a template instantiation which will be an abservable side effect.

Personally, I find option 1 the least objectionable.

Proposed resolution (10/01):

Add the indicated wording to the third-to-last sentence of 3.2 [basic.def.odr] pararaph 2:

A default constructor for a class is used by default initialization or value initialization as specified in 8.5 [dcl.init].

Add a footnote to the indicated bullet in 8.5 [dcl.init] paragraph 5:

Add the indicated wording to the first sentence of 12.1 [class.ctor] paragraph 7:

An implicitly-declared default constructor for a class is implicitly defined when it is used (3.2 [basic.def.odr]) to create an object of its class type (1.8 [intro.object]).



509. Dead code in the specification of default initialization

Section: 8.5  [dcl.init]     Status: CD1     Submitter: Mike Miller     Date: 18 Mar 2005

[Voted into the WP at the September, 2008 meeting (resolution in paper N2762).]

The definition of default initialization (8.5 [dcl.init] paragraph 5) is:

However, default initialization is invoked only for non-POD class types and arrays thereof (5.3.4 [expr.new] paragraph 15 for new-expressions, 8.5 [dcl.init] paragraph 10 for top-level objects, and 12.6.2 [class.base.init] paragraph 4 for member and base class subobjects — but see issue 510). Consequently, all cases that invoke default initialization are handled by the first two bullets; the third bullet can never be reached. Its presence is misleading, so it should be removed.

Notes from the September, 2008 meeting:

The approach adopted in the resolution in paper N2762 was different from the suggestion above: it changes the definition of default initialization to include POD types and changes the third bullet to specify that “no initialization is performed.”




543. Value initialization and default constructors

Section: 8.5  [dcl.init]     Status: CD1     Submitter: Mike Miller     Date: 27 October 2005

[Voted into the WP at the September, 2008 meeting (resolution in paper N2762).]

The wording resulting from the resolution of issue 302 does not quite implement the intent of the issue. The revised wording of 3.2 [basic.def.odr] paragraph 2 is:

A default constructor for a class is used by default initialization or value initialization as specified in 8.5 [dcl.init].

This sounds as if 8.5 [dcl.init] specifies how and under what circumstances value initialization uses a default constructor (which was, in fact, the case for default initialization in the original wording). However, the normative text there makes it plain that value initialization does not call the default constructor (the permission granted to implementations to call the default constructor for value initialization is in a non-normative footnote).

The example that occasioned this observation raises an additional question. Consider:

    struct POD {
      const int x;
    };

    POD data = POD();

According to the (revised) resolution of issue 302, this code is ill-formed because the implicitly-declared default constructor will be implicitly defined as a result of being used by value initialization (12.1 [class.ctor] paragraph 7), and the implicitly-defined constructor fails to initialize a const-qualified member (12.6.2 [class.base.init] paragraph 4). This seems unfortunate, because the (trivial) default constructor of a POD class is otherwise not used — default initialization applies only to non-PODs — and it is not actually needed in value initialization. Perhaps value initialization should be defined to “use” the default constructor only for non-POD classes? If so, both of these problems would be resolved by rewording the above-referenced sentence of 3.2 [basic.def.odr] paragraph 2 as:

A default constructor for a non-POD class is used by default initialization or value initialization as specified in (8.5 [dcl.init]).

Notes from the April, 2006 meeting:

The approach favored by the CWG was to leave 3.2 [basic.def.odr] unchanged and to add normative wording to 8.5 [dcl.init] indicating that it is unspecified whether the default constructor is called.

Notes from the October, 2006 meeting:

The CWG now prefers that it should not be left unspecified whether programs of this sort are well- or ill-formed; instead, the Standard should require that the default constructor be defined in such cases. Three possibilities of implementing this decision were discussed:

  1. Change 3.2 [basic.def.odr] to state flatly that the default constructor is used by value initialization (removing the implication that 8.5 [dcl.init] determines the conditions under which it is used).

  2. Change 8.5 [dcl.init] to specify that non-union class objects with no user-declared constructor are value-initialized by first zero-initializing the object and then calling the (implicitly-defined) default constructor, replacing the current specification of value-initializing each of its sub-objects.

  3. Add a normative statement to 8.5 [dcl.init] that value-initialization causes the implicitly-declared default constructor to be implicitly defined, even if it is not called.

Proposed resolution (June, 2008):

Change the second bullet of the value-initialization definition in 8.5 [dcl.init] paragraph 5 as follows:

Notes from the September, 2008 meeting:

The resolution supplied in paper N2762 differs from the June, 2008 proposed resolution in that the implicitly-declared default constructor is only called (and thus defined) if it is non-trivial, making the struct POD example above well-formed.




430. Ordering of expression evaluation in initializer list

Section: 8.5.1  [dcl.init.aggr]     Status: CD1     Submitter: Nathan Sidwell     Date: 23 July 2003

[Voted into the WP at the April, 2007 meeting as part of paper J16/07-0099 = WG21 N2239.]

A recent GCC bug report ( http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11633) asks about the validity of

  int count = 23;
  int foo[] = { count++, count++, count++ };
is this undefined or unspecified or something else? I can find nothing in 8.5.1 [dcl.init.aggr] that indicates whether the components of an initializer-list are evaluated in order or not, or whether they have sequence points between them.

6.7.8/23 of the C99 std has this to say

The order in which any side effects occur among the initialization list expressions is unspecified.
I think similar wording is needed in 8.5.1 [dcl.init.aggr]

Steve Adamczyk: I believe the standard is clear that each initializer expression in the above is a full-expression (1.9 [intro.execution]/12-13; see also issue 392) and therefore there is a sequence point after each expression (1.9 [intro.execution]/16). I agree that the standard does not seem to dictate the order in which the expressions are evaluated, and perhaps it should. Does anyone know of a compiler that would not evaluate the expressions left to right?

Mike Simons: Actually there is one, that does not do left to right: gcc/C++. None of the post increment operations take effect until after the statement finishes. So in the sample code gcc stores 23 into all positions in the array. The commercial vendor C++ compilers for AIX, Solaris, Tru64, HPUX (parisc and ia64), and Windows, all do sequence points at each ',' in the initializer list.




491. Initializers for empty-class aggregrate members

Section: 8.5.1  [dcl.init.aggr]     Status: CD1     Submitter: Nathan Sidwell     Date: 15 Dec 2004

[Voted into WP at April, 2007 meeting.]

The current wording of 8.5.1 [dcl.init.aggr] paragraph 8 requires that

An initializer for an aggregate member that is an empty class shall have the form of an empty initializer-list {}.

This is overly constraining. There is no reason that the following should be ill-formed:

    struct S { };
    S s;
    S arr[1] = { s };

Mike Miller: The wording of 8.5.1 [dcl.init.aggr] paragraph 8 is unclear. “An aggregate member” would most naturally mean “a member of an aggregate.” In context, however, I think it must mean “a member [of an aggregate] that is an aggregate”, that is, a subaggregate. Members of aggregates need not themselves be aggregates (cf paragraph 13 and 12.6.1 [class.expl.init]); it cannot be the case that an object of an empty class with a user-declared constructor must be initialized with {} when it is a member of an aggregate. This wording should be clarified, regardless of the decision on Nathan's point.

Proposed resolution (October, 2005):

This issue is resolved by the resolution of issue 413.




632. Brace-enclosed initializer for scalar member of aggregate

Section: 8.5.1  [dcl.init.aggr]     Status: CD1     Submitter: Greg Comeau     Date: 3 May 2007

[Voted into the WP at the June, 2008 meeting as part of paper N2672.]

C (both C90 and C99) appear to allow a declaration of the form

    struct S { int i; } s = { { 5 } };

in which the initializer of a scalar member of an aggregate can itself be brace-enclosed. The relevant wording from the C99 Standard is found in 6.7.8 paragraph 11:

The initializer for a scalar shall be a single expression, optionally enclosed in braces.

and paragraph 16:

Otherwise, the initializer for an object that has aggregate or union type shall be a brace-enclosed list of initializers for the elements or named members.

The “list of initializers” in paragraph 16 must be a recursive reference to paragraph 11 (that's the only place that describes how an initialized item gets its value from the initializer expression), which would thus make the “brace-enclosed” part of paragraph 11 apply to each of the initializers in the list in paragraph 16 as well.

This appears to be an incompatibility between C and C++: 8.5.1 [dcl.init.aggr] paragraph 11 says,

If the initializer-list begins with a left brace, then the succeeding comma-separated list of initializer-clauses initializes the members of a subaggregate....

which clearly leaves the impression that only a subaggregate may be initialized by a brace-enclosed initializer-clause.

Either the specification in 8.5.1 [dcl.init.aggr] should be changed to allow a brace-enclosed initializer of a scalar member of an aggregate, as in C, or this incompatibility should be listed in Appendix C [diff].

Notes from the July, 2007 meeting:

It was noted that implementations differ in their handling of this construct; however, the issue is long-standing and fairly obscure.

Notes from the October, 2007 meeting:

The initializer-list proposal will resolve this issue when it is adopted.




291. Overload resolution needed when binding reference to class rvalue

Section: 8.5.3  [dcl.init.ref]     Status: CD1     Submitter: Andrei Iltchenko     Date: 15 Jun 2001

[Voted into WP at October 2005 meeting.]

There is a place in the Standard where overload resolution is implied but the way that a set of candidate functions is to be formed is omitted. See below.

According to the Standard, when initializing a reference to a non-volatile const class type (cv1 T1) with an rvalue expression (cv2 T2) where cv1 T1 is reference compatible with cv2 T2, the implementation shall proceed in one of the following ways (except when initializing the implicit object parameter of a copy constructor) 8.5.3 [dcl.init.ref] paragraph 5 bullet 2 sub-bullet 1:

While the first case is quite obvious, the second one is a bit unclear as it says "a constructor is called to copy the entire rvalue object into the temporary" without specifying how the temporary is created -- by direct-initialization or by copy-initialization? As stated in DR 152, this can make a difference when the copy constructor is declared as explicit. How should the set of candidate functions be formed? The most appropriate guess is that it shall proceed as per 13.3.1.3 [over.match.ctor].

Another detail worth of note is that in the draft version of the Standard as of 2 December 1996 the second bullet read:

J. Stephen Adamczyk replied that the reason for changing "a copy constructor" to "a constructor" was to allow for member template converting constructors.

However, the new wording is somewhat in conflict with the footnote #93 that says that when initializing the implicit object parameter of a copy constructor an implementation must eventually choose the first alternative (binding without copying) to avoid infinite recursion. This seems to suggest that a copy constructor is always used for initializing the temporary of type "cv1 T2".

Furthermore, now that the set of candidate functions is not limited to only the copy constructors of T2, there might be some unpleasant consequences. Consider a rather contrived sample below:

    int   * pi = ::new(std::nothrow) int;
    const std::auto_ptr<int>   & ri = std::auto_ptr<int>(pi);

In this example the initialization of the temporary of type '<TT>const std::auto_ptr<int>' (to which 'ri' is meant to be subsequently bound) doesn't fail, as it would had the approach with copy constructors been retained, instead, a yet another temporary gets created as the well-known sequence:

    std::auto_ptr<int>::operator std::auto_ptr_ref<int>()
    std::auto_ptr<int>(std::auto_ptr_ref<int>)

is called (assuming, of course, that the set of candidate functions is formed as per 13.3.1.3 [over.match.ctor]). The second temporary is transient and gets destroyed at the end of the initialization. I doubt that this is the way that the committee wanted this kind of reference binding to go.

Besides, even if the approach restricting the set of candidates to copy constructors is restored, it is still not clear how the initialization of the temporary (to which the reference is intended to be bound) is to be performed -- using direct-initialization or copy-initialization.

Another place in the Standard that would benefit from a similar clarification is the creation of an exception object, which is delineated in 15.1 [except.throw].

David Abrahams (February 2004): It appears, looking at core 291, that there may be a need to tighten up 8.5.3 [dcl.init.ref]/5.

Please see the attached example file, which demonstrates "move semantics" in C++98. Many compilers fail to compile test 10 because of the way 8.5.3/5 is interpreted. My problem with that interpretation is that test 20:

    typedef X const XC;
    sink2(XC(X()));
does compile. In other words, it *is* possible to construct the const temporary from the rvalue. IMO, that is the proper test.

8.5.3/5 doesn't demand that a "copy constructor" is used to copy the temporary, only that a constructor is used "to copy the temporary". I hope that when the language is tightened up to specify direct (or copy initialization), that it also unambiguously allows the enclosed test to compile. Not only is it, I believe, within the scope of reasonable interpretation of the current standard, but it's an incredibly important piece of functionality for library writers and users alike.

#include <iostream>
#include <cassert>

template <class T, class X>
struct enable_if_same
{
};

template <class X>
struct enable_if_same<X, X>
{
    typedef char type;
};

struct X
{
    static int cnt;  // count the number of Xs
    
    X()
      : id(++cnt)
      , owner(true)
    {
        std::cout << "X() #" << id << std::endl;
    }
    
    // non-const lvalue - copy ctor
    X(X& rhs)
      : id(++cnt)
      , owner(true)
    {
        std::cout << "copy #" << id << " <- #" << rhs.id << std::endl;
    }

    // const lvalue - T will be deduced as X const
    template <class T>
    X(T& rhs, typename enable_if_same<X const,T>::type = 0)
      : id(++cnt)
      , owner(true)
    {
        std::cout << "copy #" << id << " <- #" << rhs.id << " (const)" << std::endl;
    }

    ~X()
    {
        std::cout << "destroy #" << id << (owner?"":" (EMPTY)") << std::endl;
    }
    
 private:    // Move stuff
    struct ref { ref(X*p) : p(p) {} X* p; };

 public:    // Move stuff
    operator ref() {
        return ref(this);
    }

    // non-const rvalue
    X(ref rhs)
      : id(++cnt)
      , owner(rhs.p->owner)
    {
        std::cout << "MOVE #" << id << " <== #" << rhs.p->id << std::endl;
        rhs.p->owner = false;
        assert(owner);
    }
    
 private:   // Data members
    int id;
    bool owner;
};

int X::cnt;


X source()
{
    return X();
}

X const csource()
{
    return X();
}

void sink(X)
{
    std::cout << "in rvalue sink" << std::endl;
}

void sink2(X&)
{
    std::cout << "in non-const lvalue sink2" << std::endl;
}

void sink2(X const&)
{
    std::cout << "in const lvalue sink2" << std::endl;
}

void sink3(X&)
{
    std::cout << "in non-const lvalue sink3" << std::endl;
}

template <class T>
void tsink(T)
{
    std::cout << "in templated rvalue sink" << std::endl;
}

int main()
{
    std::cout << " ------ test 1, direct init from rvalue ------- " << std::endl;
#ifdef __GNUC__ // GCC having trouble parsing the extra parens
    X z2((0, X() ));
#else
    X z2((X()));
#endif 

    std::cout << " ------ test 2, copy init from rvalue ------- " << std::endl;
    X z4 = X();

    std::cout << " ------ test 3, copy init from lvalue ------- " << std::endl;
    X z5 = z4;

    std::cout << " ------ test 4, direct init from lvalue ------- " << std::endl;
    X z6(z4);
    
    std::cout << " ------ test 5, construct const ------- " << std::endl;
    X const z7;
    
    std::cout << " ------ test 6, copy init from lvalue ------- " << std::endl;
    X z8 = z7;

    std::cout << " ------ test 7, direct init from lvalue ------- " << std::endl;
    X z9(z7);
    
    std::cout << " ------ test 8, pass rvalue by-value ------- " << std::endl;
    sink(source());
    
    std::cout << " ------ test 9, pass const rvalue by-value ------- " << std::endl;
    sink(csource());

    std::cout << " ------ test 10, pass rvalue by overloaded reference ------- " << std::endl;
    // This one fails in Comeau's strict mode due to 8.5.3/5.  GCC 3.3.1 passes it.
    sink2(source());

    std::cout << " ------ test 11, pass const rvalue by overloaded reference ------- " << std::endl;
    sink2(csource());

#if 0    // These two correctly fail to compile, just as desired
    std::cout << " ------ test 12, pass rvalue by non-const reference ------- " << std::endl;
    sink3(source());

    std::cout << " ------ test 13, pass const rvalue by non-const reference ------- " << std::endl;
    sink3(csource());
#endif 

    std::cout << " ------ test 14, pass lvalue by-value ------- " << std::endl;
    sink(z5);
    
    std::cout << " ------ test 15, pass const lvalue by-value ------- " << std::endl;
    sink(z7);

    std::cout << " ------ test 16, pass lvalue by-reference ------- " << std::endl;
    sink2(z4);

    std::cout << " ------ test 17, pass const lvalue by const reference ------- " << std::endl;
    sink2(z7);

    std::cout << " ------ test 18, pass const lvalue by-reference ------- " << std::endl;
#if 0   // correctly fails to compile, just as desired
    sink3(z7);
#endif 

    std::cout << " ------ test 19, pass rvalue by value to template param ------- " << std::endl;
    tsink(source());

    std::cout << " ------ test 20, direct initialize a const A with an A ------- " << std::endl;
    typedef X const XC;
    sink2(XC(X()));
}

Proposed Resolution:

(As proposed by N1610 section 5, with editing.)

Change paragraph 5, second bullet, first sub-bullet, second sub-sub-bullet as follows:

A temporary of type "cv1 T2" [sic] is created, and a constructor is called to copy the entire rvalue object into the temporary via copy-initialization from the entire rvalue object. The reference is bound to the temporary or to a sub-object within the temporary.

The text immediately following that is changed as follows:

The constructor that would be used to make the copy shall be callable whether or not the copy is actually done. The constructor and any conversion function that would be used in the initialization shall be callable whether or not the temporary is actually created.

Note, however, that the way the core working group is leaning on issue 391 (i.e., requiring direct binding) would make this change unnecessary.

Proposed resolution (April, 2005):

This issue is resolved by the resolution of issue 391.




391. Require direct binding of short-lived references to rvalues

Section: 8.5.3  [dcl.init.ref]     Status: CD1     Submitter: Raoul Gough     Date: 14 Nov 2002

[Voted into WP at October 2005 meeting.]

After some email exchanges with Rani Sharoni, I've come up with the following proposal to allow reference binding to non-copyable rvalues in some cases. Rationale and some background appear afterwards.

---- proposal ----

Replace the section of 8.5.3 [dcl.init.ref] paragraph 5 that begins "If the initializer expression is an rvalue" with the following:

---- rationale ----

  1. The intention of the current wording is to provide the implementation freedom to construct an rvalue of class type at an arbitrary location and copy it zero or more times before binding any reference to it.
  2. The standard allows code to call a member function on an rvalue of class type (in 5.2.5 [expr.ref], I guess). This means that the implementation can be forced to bind the reference directly, with no freedom to create any temporary copies. e.g.
       class nc {
         nc (nc const &);  // private, nowhere defined
       public:
         nc ();
         nc const &by_ref () const { return *this; }
       };
    
       void f () {
         void g (nc const &);
    
         g (nc());          // Ill-formed
         g (nc().by_ref()); // Ok - binds directly to rvalue
       }
    
    Forcing a direct binding in this way is possible wherever the lifetime of the reference does not extend beyond the containing full expression, since the reference returned by the member function remains valid for this long.
  3. As demonstrated above, existing implementations must already be capable of constructing an rvalue of class type in the "right" place the first time. Some compilers already silently allow the direct binding of references to non-copyable rvalues.
  4. The change will not break any portable user code. It would break any platform-specific user code that relies on copies being performed by the particular implementation.

---- background ----

The proposal is based on a recent discussion in this group. I originally wanted to leave the implementation free to copy the rvalue if there was a callable copy constructor, and only have to bind directly if none was callable. Unfortunately, a traditional compiler can't always tell whether a function is callable or not, e.g. if the copy constructor is declared but not defined. Rani pointed this out in an example, and suggested that maybe trivial copy constructors should still be allowed (by extension, maybe wherever the compiler can determine callability). I've gone with this version because it's simpler, and I also figure the "as if" rule gives the compiler some freedom with POD types anyway.

Notes from April 2003 meeting:

We agreed generally with the proposal. We were unsure about the need for the restriction regarding long-lived references. We will check with the proposer about that.

Jason Merrill points out that the test case in issue 86 may be a case where we do not want to require direct binding.

Further information from Rani Sharoni (April 2003):

I wasn't aware about the latest suggestion of Raoul as it appears in core issue 391. In our discussions we tried to formulate a different proposal.

The rational, as we understood, behind the implementation freedom to make an extra copying (8.5.3/5/2/12) of the rvalue is to allow return values in registers which on some architectures are not addressable. The example that Raoul and I presented shows that this implementation freedom is not always possible since we can "force" the rvalue to be addressable using additional member function (by_ref). The example only works for short lived rvalues and this is probably why Raoul narrow the suggestion.

I had different rational which was related to the implementation of conditional operator in VC. It seems that when conditional operator is involved VC does use an extra copying when the lifetime of the temporary is extended:

  struct A { /* ctor with side effect */};

  void f(A& x) {
    A const& r = cond ? A(1) : x; // VC actually make an extra copy of
                                  // the rvalue A(1)
  }

I don't know what the consideration behind the VC implementation was (I saw open bug on this issue) but it convinced me to narrow the suggestion.

IMHO such limitation seems to be too strict because it might limit the optimizer since returning class rvalues in registers might be useful (although I'm not aware about any implementation that actually does it). My suggestion was to forbid the extra copying if the ctor is not viable (e.g. A::A(A&) ). In this case the implementation "freedom" doesn't exist (since the code might not compile) and only limits the programmer freedom (e.g. Move Constructors - http://www.cuj.com/experts/2102/alexandr.htm).

Core issue 291 is strongly related to the above issue and I personally prefer to see it resolved first. It seems that VC already supports the resolution I prefer.

Notes from October 2003 meeting:

We ended up feeling that this is just one of a number of cases of optimizations that are widely done by compilers and allowed but not required by the standard. We don't see any strong reason to require compilers to do this particular optimization.

Notes from the March 2004 meeting:

After discussing issue 450, we found ourselves reconsidering this, and we are now inclined to make a change to require the direct binding in all cases, with no restriction on long-lived references. Note that such a change would eliminate the need for a change for issue 291.

Proposed resolution (October, 2004):

Change 8.5.3 [dcl.init.ref] paragraph 5 bullet 2 sub-bullet 1 as follows:

If the initializer expression is an rvalue, with T2 a class type, and "cv1 T1" is reference-compatible with "cv2 T2", the reference is bound to the object represented by the rvalue (see 3.10 [basic.lval]) or to a sub-object within that object. in one of the following ways (the choice is implementation-defined): The constructor that would be used to make the copy shall be callable whether or not the copy is actually done. [Example:
  struct A { };
  struct B : public A { } b;
  extern B f();
  const A& rca = f ();  // Bound Either bound to the A sub-object of the B rvalue,
                        // or the entire B object is copied and the reference
                        // is bound to the A sub-object of the copy
end example]

[This resolution also resolves issue 291.]




450. Binding a reference to const to a cv-qualified array rvalue

Section: 8.5.3  [dcl.init.ref]     Status: CD1     Submitter: Steve Adamczyk     Date: 16 Jan 2004

[Voted into WP at October 2005 meeting.]

It's unclear whether the following is valid:

const int N = 10;
const int M = 20;
typedef int T;
void f(T const (&x)[N][M]){}

struct X {
	int i[10][20];
};

X g();

int main()
{
	f(g().i);
}

When you run this through 8.5.3 [dcl.init.ref], you sort of end up falling off the end of the standard's description of reference binding. The standard says in the final bullet of paragraph 5 that an array temporary should be created and copy-initialized from the rvalue array, which seems implausible.

I'm not sure what the right answer is. I think I'd be happy with allowing the binding in this case. We would have to introduce a special case like the one for class rvalues.

Notes from the March 2004 meeting:

g++ and EDG give an error. Microsoft (8.0 beta) and Sun accept the example. Our preference is to allow the direct binding (no copy). See the similar issue with class rvalues in issue 391.

Proposed resolution (October, 2004):

  1. Insert a new bullet in 8.5.3 [dcl.init.ref] paragraph 5 bullet 2 before sub-bullet 2 (which begins, “Otherwise, a temporary of type ‘cv1 T1’ is created...”):

    If the initializer expression is an rvalue, with T2 an array type, and “cv1 T1” is reference-compatible with “cv2 T2”, the reference is bound to the object represented by the rvalue (see 3.10 [basic.lval]).
  2. Change 3.10 [basic.lval] paragraph 2 as follows:

    An lvalue refers to an object or function. Some rvalue expressions — those of (possibly cv-qualified) class or array type or cv-qualified class type — also refer to objects.



175. Class name injection and base name access

Section: 9  [class]     Status: CD1     Submitter: John Spicer     Date: 21 February 1999

[Moved to DR at 10/01 meeting.]

With class name injection, when a base class name is used in a derived class, the name found is the injected name in the base class, not the name of the class in the scope containing the base class. Consequently, if the base class name is not accessible (e.g., because is is in a private base class), the base class name cannot be used unless a qualified name is used to name the class in the class or namespace of which it is a member.

Without class name injection the following example is valid. With class name injection, A is inaccessible in class C.

    class A { };
    class B: private A { };
    class C: public B {
        A* p;    // error: A inaccessible
    };

At the least, the standard should be more explicit that this is, in fact, ill-formed.

(See paper J16/99-0010 = WG21 N1187.)

Proposed resolution (04/01):

Add to the end of 11.1 [class.access.spec] paragraph 3:

[Note: In a derived class, the lookup of a base class name will find the injected-class-name instead of the name of the base class in the scope in which it was declared. The injected-class-name might be less accessible than the name of the base class in the scope in which it was declared.] [Example:

    class A { };
    class B : private A { };
    class C : public B {
        A* p;    // error: injected-class-name A is inaccessible
        ::A* q;  // OK
    };

end example]




273. POD classes and operator&()

Section: 9  [class]     Status: CD1     Submitter: Andrei Iltchenko     Date: 10 Mar 2001

[Moved to DR at October 2002 meeting.]

I think that the definition of a POD class in the current version of the Standard is overly permissive in that it allows for POD classes for which a user-defined operator function operator& may be defined. Given that the idea behind POD classes was to achieve compatibility with C structs and unions, this makes 'Plain old' structs and unions behave not quite as one would expect them to.

In the C language, if x and y are variables of struct or union type S that has a member m, the following expression are allowed: &x, x.m, x = y. While the C++ standard guarantees that if x and y are objects of a POD class type S, the expressions x.m, x = y will have the same effect as they would in C, it is still possible for the expression &x to be interpreted differently, subject to the programmer supplying an appropriate version of a user-defined operator function operator& either as a member function or as a non-member function.

This may result in surprising effects. Consider:

    // POD_bomb is a POD-struct. It has no non-static non-public data members,
    // no virtual functions, no base classes, no constructors, no user-defined
    // destructor, no user-defined copy assignment operator, no non-static data
    // members of type pointer to member, reference, non-POD-struct, or
    // non-POD-union.
    struct  POD_bomb  {
       int   m_value1;
       int   m_value2;
       int  operator&()
       {   return  m_value1++;   }
       int  operator&() const
       {   return  m_value1 + m_value2;   }
    };

3.9 [basic.types] paragraph 2 states:

For any complete POD object type T, whether or not the object holds a valid value of type T, the underlying bytes (1.7 [intro.memory]) making up the object can be copied into an array of char or unsigned char [footnote: By using, for example, the library functions (17.6.1.2 [headers]) memcpy or memmove]. If the content of the array of char or unsigned char is copied back into the object, the object shall subsequently hold its original value. [Example:
    #define N sizeof(T)
    char buf[N];
    T obj;   // obj initialized to its original value
    memcpy(buf, &obj, N);
		// between these two calls to memcpy,
		// obj might be modified
    memcpy(&obj, buf, N);
		// at this point, each subobject of obj of scalar type
		// holds its original value
end example]

Now, supposing that the complete POD object type T in the example above is POD_bomb, and we cannot any more count on the assertions made in the comments to the example. Given a standard conforming implementation, the code will not even compile. And I see no legal way of copying the contents of an object of a complete object type POD_bomb into an array of char or unsigned char with memcpy or memmove without making use of the unary & operator. Except, of course, by means of an ugly construct like:

    struct  POD_without_ampersand  {
       POD_bomb   a_bomb;
    }  obj;
    #define N sizeof(POD_bomb)
    char buf[N];
    memcpy(buf, &obj, N);
    memcpy(&obj, buf, N);

The fact that the definition of a POD class allows for POD classes for which a user-defined operator& is defined, may also present major obstacles to implementers of the offsetof macro from <cstddef>

18.2 [support.types] paragraph 5 says:

The macro offsetof accepts a restricted set of type arguments in this International Standard. type shall be a POD structure or a POD union (clause 9 [class]). The result of applying the offsetof macro to a field that is a static data member or a function is undefined."

Consider a well-formed C++ program below:

    #include <cstddef>
    #include <iostream>


    struct  POD_bomb  {
       int   m_value1;
       int   m_value2;
       int  operator&()
       {   return  m_value1++;   }
       int  operator&() const
       {   return  m_value1 + m_value2;   }
    };


    // POD_struct is a yet another example of a POD-struct.
    struct  POD_struct  {
       POD_bomb   m_nonstatic_bomb1;
       POD_bomb   m_nonstatic_bomb2;
    };


    int  main()
    {

       std::cout << "offset of m_nonstatic_bomb2: " << offsetof(POD_struct,
           m_nonstatic_bomb2) << '\n';
       return  0;

    }

See Jens Maurer's paper 01-0038=N1324 for an analysis of this issue.

Notes from 10/01 meeting:

A consensus was forming around the idea of disallowing operator& in POD classes when it was noticed that it is permitted to declare global-scope operator& functions, which cause the same problems. After more discussion, it was decided that such functions should not be prohibited in POD classes, and implementors should simply be required to "get the right answer" in constructs such as offsetof and va_start that are conventionally implemented using macros that use the "&" operator. It was noted that one can cast the original operand to char & to de-type it, after which one can use the built-in "&" safely.

Proposed resolution:




284. qualified-ids in class declarations

Section: 9  [class]     Status: CD1     Submitter: Mike Miller     Date: 01 May 2001

[Moved to DR at October 2002 meeting.]

Although 8.3 [dcl.meaning] requires that a declaration of a qualified-id refer to a member of the specified namespace or class and that the member not have been introduced by a using-declaration, it applies only to names declared in a declarator. It is not clear whether there is existing wording enforcing the same restriction for qualified-ids in class-specifiers and elaborated-type-specifiers or whether additional wording is required. Once such wording is found/created, the proposed resolution of issue 275 must be modified accordingly.

Notes from 10/01 meeting:

The sentiment was that this should be required on class definitions, but not on elaborated type specifiers in general (which are references, not declarations). We should also make sure we consider explicit instantiations, explicit specializations, and friend declarations.

Proposed resolution (10/01):

Add after the end of 9.1 [class.name] paragraph 3:

When a nested-name-specifier is specified in a class-head or in an elaborated-type-specifier, the resulting qualified name shall refer to a previously declared member of the class or namespace to which the nested-name-specifier refers, and the member shall not have been introduced by a using-declaration in the scope of the class or namespace nominated by the nested-name-specifier.



327. Use of "structure" without definition

Section: 9  [class]     Status: CD1     Submitter: James Kanze     Date: 9 Dec 2001

[Voted into WP at April, 2007 meeting.]

In 9 [class] paragraph 4, the first sentence says "A structure is a class definition defined with the class-key struct". As far as I know, there is no such thing as a structure in C++; it certainly isn't listed as one of the possible compound types in 3.9.2 [basic.compound]. And defining structures opens the question of whether a forward declaration is a structure or not. The parallel here with union (which follows immediately) suggests that structures and classes are really different things, since the same wording is used, and classes and unions do have some real differences, which manifest themselves outside of the definition. It also suggests that since one can't forward declare union with class and vice versa, the same should hold for struct and class -- I believe that the intent was that one could use struct and class interchangeably in forward declaration.

Suggested resolution:

I suggest something like the following:

If a class is defined with the class-key class, its members and base classes are private by default. If a class is defined with the class-key struct, its members and base classes are public by default. If a class is defined with the class-key union, its members are public by default, and it holds only one data member at a time. Such classes are called unions, and obey a number of additional restrictions, see 9.5 [class.union].

Proposed resolution (April, 2006):

This issue is resolved by the resolution of issue 538.




379. Change "class declaration" to "class definition"

Section: 9  [class]     Status: CD1     Submitter: Jens Maurer     Date: 21 Oct 2002

[Voted into WP at March 2004 meeting.]

The ARM used the term "class declaration" to refer to what would usually be termed the definition of the class. The standard now often uses "class definition", but there are some surviving uses of "class declaration" with the old meaning. They should be found and changed.

Proposed resolution (April 2003):

Replace in 3.1 [basic.def] paragraph 2

A declaration is a definition unless it declares a function without specifying the function's body (8.4 [dcl.fct.def]), it contains the extern specifier (7.1.1 [dcl.stc]) or a linkage-specification [Footnote: Appearing inside the braced-enclosed declaration-seq in a linkage-specification does not affect whether a declaration is a definition. --- end footnote] (7.5 [dcl.link]) and neither an initializer nor a function-body, it declares a static data member in a class declaration definition (9.4 [class.static]), it is a class name declaration (9.1 [class.name]), or it is a typedef declaration (7.1.3 [dcl.typedef]), a using-declaration (7.3.3 [namespace.udecl]), or a using-directive (7.3.4 [namespace.udir]).

Replace in 7.1.2 [dcl.fct.spec] paragraphs 5 and 6

The virtual specifier shall only be used in declarations of nonstatic class member functions that appear within a member-specification of a class declaration definition; see 10.3 [class.virtual].

The explicit specifier shall be used only in declarations of constructors within a class declaration definition; see 12.3.1 [class.conv.ctor].

Replace in 7.1.3 [dcl.typedef] paragraph 4

A typedef-name that names a class is a class-name (9.1 [class.name]). If a typedef-name is used following the class-key in an elaborated-type-specifier (7.1.6.3 [dcl.type.elab]) or in the class-head of a class declaration definition (9 [class]), or is used as the identifier in the declarator for a constructor or destructor declaration (12.1 [class.ctor], 12.4 [class.dtor]), the program is ill-formed.

Replace in 7.3.1.2 [namespace.memdef] paragraph 3

The name of the friend is not found by simple name lookup until a matching declaration is provided in that namespace scope (either before or after the class declaration definition granting friendship).

Replace in 8.3.2 [dcl.ref] paragraph 4

There shall be no references to references, no arrays of references, and no pointers to references. The declaration of a reference shall contain an initializer (8.5.3 [dcl.init.ref]) except when the declaration contains an explicit extern specifier (7.1.1 [dcl.stc]), is a class member (9.2 [class.mem]) declaration within a class declaration definition, or is the declaration of a parameter or a return type (8.3.5 [dcl.fct]); see 3.1 [basic.def].

Replace in 8.5.3 [dcl.init.ref] paragraph 3

The initializer can be omitted for a reference only in a parameter declaration (8.3.5 [dcl.fct]), in the declaration of a function return type, in the declaration of a class member within its class declaration definition (9.2 [class.mem]), and where the extern specifier is explicitly used.

Replace in 9.1 [class.name] paragraph 2

A class definition declaration introduces the class name into the scope where it is defined declared and hides any class, object, function, or other declaration of that name in an enclosing scope (3.3 [basic.scope]). If a class name is declared in a scope where an object, function, or enumerator of the same name is also declared, then when both declarations are in scope, the class can be referred to only using an elaborated-type-specifier (3.4.4 [basic.lookup.elab]).

Replace in 9.4 [class.static] paragraph 4

Static members obey the usual class member access rules (clause 11 [class.access]). When used in the declaration of a class member, the static specifier shall only be used in the member declarations that appear within the member-specification of the class declaration definition.

Replace in 9.7 [class.nest] paragraph 1

A class can be defined declared within another class. A class defined declared within another is called a nested class. The name of a nested class is local to its enclosing class. The nested class is in the scope of its enclosing class. Except by using explicit pointers, references, and object names, declarations in a nested class can use only type names, static members, and enumerators from the enclosing class.

Replace in 9.8 [class.local] paragraph 1

A class can be defined declared within a function definition; such a class is called a local class. The name of a local class is local to its enclosing scope. The local class is in the scope of the enclosing scope, and has the same access to names outside the function as does the enclosing function. Declarations in a local class can use only type names, static variables, extern variables and functions, and enumerators from the enclosing scope.

Replace in 10 [class.derived] paragraph 1

... The class-name in a base-specifier shall not be an incompletely defined class (clause 9 [class]); this class is called a direct base class for the class being declared defined. During the lookup for a base class name, non-type names are ignored (3.3.10 [basic.scope.hiding]). If the name found is not a class-name, the program is ill-formed. A class B is a base class of a class D if it is a direct base class of D or a direct base class of one of D's base classes. A class is an indirect base class of another if it is a base class but not a direct base class. A class is said to be (directly or indirectly) derived from its (direct or indirect) base classes. [Note: See clause 11 [class.access] for the meaning of access-specifier.] Unless redefined redeclared in the derived class, members of a base class are also considered to be members of the derived class. The base class members are said to be inherited by the derived class. Inherited members can be referred to in expressions in the same manner as other members of the derived class, unless their names are hidden or ambiguous (10.2 [class.member.lookup]). [Note: the scope resolution operator :: (5.1.1 [expr.prim.general]) can be used to refer to a direct or indirect base member explicitly. This allows access to a name that has been redefined redeclared in the derived class. A derived class can itself serve as a base class subject to access control; see 11.2 [class.access.base]. A pointer to a derived class can be implicitly converted to a pointer to an accessible unambiguous base class (4.10 [conv.ptr]). An lvalue of a derived class type can be bound to a reference to an accessible unambiguous base class (8.5.3 [dcl.init.ref]).]

Replace in 10.1 [class.mi] paragraph 5

For another example,
class V { /* ... */ };
class A : virtual public V { /* ... */ };
class B : virtual public V { /* ... */ };
class C : public A, public B { /* ... */ };
for an object c of class type C, a single subobject of type V is shared by every base subobject of c that is declared to have has a virtual base class of type V.

Replace in the example in 10.2 [class.member.lookup] paragraph 6 (the whole paragraph was turned into a note by the resolution of core issue 39)

The names defined declared in V and the left hand instance of W are hidden by those in B, but the names defined declared in the right hand instance of W are not hidden at all.

Replace in 10.4 [class.abstract] paragraph 2

... A virtual function is specified pure by using a pure-specifier (9.2 [class.mem]) in the function declaration in the class declaration definition. ...

Replace in the footnote at the end of 11.2 [class.access.base] paragraph 1

[Footnote: As specified previously in clause 11 [class.access], private members of a base class remain inaccessible even to derived classes unless friend declarations within the base class declaration definition are used to grant access explicitly.]

Replace in 11.3 [class.access.dcl] paragraph 1

The access of a member of a base class can be changed in the derived class by mentioning its qualified-id in the derived class declaration definition. Such mention is called an access declaration. ...

Replace in the example in 13.4 [over.over] paragraph 5

The initialization of pfe is ill-formed because no f() with type int(...) has been defined declared, and not because of any ambiguity.

Replace in C.1.5 [diff.dcl] paragraph 1

Rationale: Storage class specifiers don't have any meaning when associated with a type. In C++, class members can be defined declared with the static storage class specifier. Allowing storage class specifiers on type declarations could render the code confusing for users.

Replace in C.1.7 [diff.class] paragraph 3

In C++, a typedef name may not be redefined redeclared in a class declaration definition after being used in the declaration that definition
Drafting notes:

The resolution of core issue 45 (DR) deletes 11.8 [class.access.nest] paragraph 2.

The following occurrences of "class declaration" are not changed:




383. Is a class with a declared but not defined destructor a POD?

Section: 9  [class]     Status: CD1     Submitter: Gennaro Prota     Date: 18 Sep 2002

[Voted into WP at March 2004 meeting.]

The standard (9 [class] par. 4) says that "A POD-struct is an aggregate class that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-defined copy assignment operator and no user-defined destructor."

Note that it says 'user-defined', not 'user-declared'. Is it the intent that if e.g. a copy assignment operator is declared but not defined, this does not (per se) prevent the class to be a POD-struct?

Proposed resolution (April 2003):

Replace in 9 [class] paragraph 4

A POD-struct is an aggregate class that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-defined declared copy assignment operator and no user-defined declared destructor. Similarly, a POD-union is an aggregate union that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-defined declared copy assignment operator and no user-defined declared destructor.

Drafting note: The changes are shown relative to TC1, incorporating the changes from the resolution of core issue 148.




413. Definition of "empty class"

Section: 9  [class]     Status: CD1     Submitter: Pete Becker     Date: 30 Apr 2003

[Voted into WP at April, 2007 meeting.]

The proposal says that value is true if "T is an empty class (10)". Clause 10 doesn't define an empty class, although it has a note that says a base class may "be of zero size (clause 9)" 9/3 says "Complete objects and member subobjects of class type shall have nonzero size." This has a footnote, which says "Base class subobjects are not so constrained."

The standard uses the term "empty class" in two places (8.5.1 [dcl.init.aggr]), but neither of those places defines it. It's also listed in the index, which refers to the page that opens clause 9, i.e. the nonzero size stuff cited above.

So, what's the definition of "empty class" that determines whether the predicate is_empty is true?

The one place where it's used is 8.5.1 [dcl.init.aggr] paragraph 8, which says (roughly paraphrased) that an aggregate initializer for an empty class must be "{}", and when such an initializer is used for an aggregate that is not an empty class the members are default-initialized. In this context it's pretty clear what's meant. In the type traits proposal it's not as clear, and it was probably intended to have a different meaning. The boost implementation, after it eliminates non-class types, determines whether the trait is true by comparing the size of a class derived from T to the size of an otherwise-identical class that is not derived from T.

Howard Hinnant: is_empty was created to find out whether a type could be derived from and have the empty base class optimization successfully applied. It was created in part to support compressed_pair which attempts to optimize away the space for one of its members in an attempt to reduce spatial overhead. An example use is:

  template <class T, class Compare = std::less<T> >
  class SortedVec
  {
  public:
  ...
  private:
    T* data_;
    compressed_pair<Compare, size_type> comp_;

    Compare&       comp()       {return comp_.first();}
    const Compare& comp() const {return comp_.first();}
    size_type&     sz()         {return comp_.second();}
    size_type      sz() const   {return comp_.second();}
  };

Here the compare function is optimized away via the empty base optimization if Compare turns out to be an "empty" class. If Compare turns out to be a non-empty class, or a function pointer, the space is not optimized away. is_empty is key to making this work.

This work built on Nathan's article: http://www.cantrip.org/emptyopt.html.

Clark Nelson: I've been looking at issue 413, and I've reached the conclusion that there are two different kinds of empty class. A class containing only one or more anonymous bit-field members is empty for purposes of aggregate initialization, but not (necessarily) empty for purposes of empty base-class optimization.

Of course we need to add a definition of emptiness for purposes of aggregate initialization. Beyond that, there are a couple of questions:

  1. Should the definition of emptiness used by the is_empty predicate be defined in a language clause or a library clause?
  2. Do we need to open a new core issue pointing out the fact that the section on aggregate initialization does not currently say that unnamed bit-fields are skipped?

Notes from the October, 2005 meeting:

There are only two places in the Standard where the phrase “empty class” appears, both in 8.5.1 [dcl.init.aggr] paragraph 8. Because it is not clear whether the definition of “empty for initialization purposes” is suitable for use in defining the is_empty predicate, it would be better just to avoid using the phrase in the language clauses. The requirements of 8.5.1 [dcl.init.aggr] paragraph 8 appear to be redundant; paragraph 6 says that an initializer-list must have no more initializers than the number of elements to initialize, so an empty class already requires an empty initializer-list, and using an empty initializer-list with a non-empty class is covered adequately by paragraph 7's description of the handling of an initializer-list with fewer initializers than the number of members to initialize.

Proposed resolution (October, 2005):

  1. Change 8.5.1 [dcl.init.aggr] paragraph 5 by inserting the indicated text:

  2. Static data members and anonymous bit fields are not considered members of the class for purposes of aggregate initialization. [Example:

        struct A {
            int i;
            static int s;
            int j;
            int :17;
            int k;
        } a = { 1 , 2 , 3 };
    

    Here, the second initializer 2 initializes a.j and not the static data member A::s, and the third initializer 3 initializes a.k and not the padding before it.end example]

  3. Delete 8.5.1 [dcl.init.aggr] paragraph 8:

  4. An initializer for an aggregate member that is an empty class shall have the form of an empty initializer-list {}. [Example:

        struct S { };
        struct A {
            S s;
            int i;
        } a = { { } , 3 };
    

    end example] An empty initializer-list can be used to initialize any aggregate. If the aggregate is not an empty class, then each member of the aggregate shall be initialized with a value of the form T() (5.2.3 [expr.type.conv]), where T represents the type of the uninitialized member.

This resolution also resolves issue 491.

Additional note (October, 2005):

Deleting 8.5.1 [dcl.init.aggr] paragraph 8 altogether may not be a good idea. It would appear that, in its absence, the initializer elision rules of paragraph 11 would allow the initializer for a in the preceding example to be written { 3 } (because the empty-class member s would consume no initializers from the list).

Proposed resolution (October, 2006):

(Drafting note: this resolution also cleans up incorrect references to syntactic non-terminals in the nearby paragraphs.)

  1. Change 8.5.1 [dcl.init.aggr] paragraph 4 as indicated:

    An array of unknown size initialized with a brace-enclosed initializer-list containing n initializers initializer-clauses, where n shall be greater than zero... An empty initializer list {} shall not be used as the initializer initializer-clause for an array of unknown bound.
  2. Change 8.5.1 [dcl.init.aggr] paragraph 5 by inserting the indicated text:

    Static data members and anonymous bit fields are not considered members of the class for purposes of aggregate initialization. [Example:

        struct A {
            int i;
            static int s;
            int j;
            int :17;
            int k;
        } a = { 1 , 2 , 3 };
    

    Here, the second initializer 2 initializes a.j and not the static data member A::s, and the third initializer 3 initializes a.k and not the anonymous bit field before it.end example]

  3. Change 8.5.1 [dcl.init.aggr] paragraph 6 as indicated:

    An initializer-list is ill-formed if the number of initializers initializer-clauses exceeds the number of members...
  4. Change 8.5.1 [dcl.init.aggr] paragraph 7 as indicated:

    If there are fewer initializers initializer-clauses in the list than there are members...
  5. Replace 8.5.1 [dcl.init.aggr] paragraph 8:

    An initializer for an aggregate member that is an empty class shall have the form of an empty initializer-list {}. [Example:

        struct S { };
        struct A {
            S s;
            int i;
        } a = { { } , 3 };
    

    end example] An empty initializer-list can be used to initialize any aggregate. If the aggregate is not an empty class, then each member of the aggregate shall be initialized with a value of the form T() (5.2.3 [expr.type.conv]), where T represents the type of the uninitialized member.

    with:

    If an aggregate class C contains a subaggregate member m that has no members for purposes of aggregate initialization, the initializer-clause for m shall not be omitted from an initializer-list for an object of type C unless the initializer-clauses for all members of C following m are also omitted. [Example:

        struct S { } s;
        struct A {
            S s1;
            int i1;
            S s2;
            int i2;
            S s3;
            int i3;
        } a = {
            { },   // Required initialization
            0,
            s,     // Required initialization
            0
        };         // Initialization not required for A::s3 because A::i3 is also not initialized
    

    end example]

  6. Change 8.5.1 [dcl.init.aggr] paragraph 10 as indicated:

    When initializing a multi-dimensional array, the initializers initializer-clauses initialize the elements...
  7. Change 8.5.1 [dcl.init.aggr] paragraph 11 as indicated:

    Braces can be elided in an initializer-list as follows. If the initializer-list begins with a left brace, then the succeeding comma-separated list of initializers initializer-clauses initializes the members of a subaggregate; it is erroneous for there to be more initializers initializer-clauses than members. If, however, the initializer-list for a subaggregate does not begin with a left brace, then only enough initializers initializer-clauses from the list are taken to initialize the members of the subaggregate; any remaining initializers initializer-clauses are left to initialize the next member of the aggregate of which the current subaggregate is a member. [Example:...
  8. Change 8.5.1 [dcl.init.aggr] paragraph 12 as indicated:

    All implicit type conversions (clause 4 [conv]) are considered when initializing the aggregate member with an initializer from an initializer-list assignment-expression. If the initializer assignment-expression can initialize a member, the member is initialized. Otherwise, if the member is itself a non-empty subaggregate, brace elision is assumed and the initializer assignment-expression is considered for the initialization of the first member of the subaggregate. [Note: As specified above, brace elision cannot apply to subaggregates with no members for purposes of aggregate initialization; an initializer-clause for the entire subobject is required. —end note] [Example:... Braces are elided around the initializer initializer-clause for b.a1.i...
  9. Change 8.5.1 [dcl.init.aggr] paragraph 15 as indicated:

    When a union is initialized with a brace-enclosed initializer, the braces shall only contain an initializer initializer-clause for the first member of the union...
  10. Change 8.5.1 [dcl.init.aggr] paragraph 16 as indicated:

    [Note: as As described above, the braces around the initializer initializer-clause for a union member can be omitted if the union is a member of another aggregate. —end note]

(Note: this resolution also resolves issue 491.)




538. Definition and usage of structure, POD-struct, POD-union, and POD class

Section: 9  [class]     Status: CD1     Submitter: Alisdair Meredith     Date: 10 August 2005

[Voted into WP at April, 2007 meeting.]

There are several problems with the terms defined in 9 [class] paragraph 4:

A structure is a class defined with the class-key struct; its members and base classes (clause 10 [class.derived]) are public by default (clause 11 [class.access]). A union is a class defined with the class-key union; its members are public by default and it holds only one data member at a time (9.5 [class.union]). [Note: aggregates of class type are described in 8.5.1 [dcl.init.aggr]. —end note] A POD-struct is an aggregate class that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-declared copy assignment operator and no user-declared destructor. Similarly, a POD-union is an aggregate union that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-declared copy assignment operator and no user-declared destructor. A POD class is a class that is either a POD-struct or a POD-union.
  1. Although the term structure is defined here, it is used only infrequently throughout the Standard, often apparently inadvertently and consequently incorrectly:

    There does not appear to be a reason for defining the term structure. The one reference where it is arguably useful, in the note in 5.2.5 [expr.ref], could be rewritten as something like, “'class objects' may be defined using the class, struct, or union class-keys; see clause 9 [class].”

  2. Based on its usage later in the paragraph and elsewhere, “POD-struct” appears to be intended to exclude unions. However, the definition of “aggregate class” in 8.5.1 [dcl.init.aggr] paragraph 1 includes unions. Furthermore, the name itself is confusing, leading to the question of whether it was intended that only classes defined using struct could be POD-structs or if class-classes are included. The definition should probably be rewritten as, “A POD-struct is an aggregate class defined with the class-key struct or the class-key class that has no...

  3. In most references outside clause 9 [class], POD-struct and POD-union are mentioned together and treated identically. These references should be changed to refer to the unified term, “POD class.”

  4. Noted in passing: 18.2 [support.types] paragraph 4 refers to the undefined terms “POD structure” and (unhyphenated) “POD union;” the pair should be replaced by a single reference to “POD class.”

Proposed resolution (April, 2006):

  1. Change 9 [class] paragraph 4 as indicated:

    A structure is a class defined with the class-key struct; its members and base classes (clause 10 [class.derived]) are public by default (clause 11 [class.access]). A union is a class defined with the class-key union; its members are public by default and it holds only one data member at a time (9.5 [class.union]). [Note: aggregates of class type are described in 8.5.1 [dcl.init.aggr]. —end note] A POD-struct is an aggregate class that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-declared copy assignment operator and no user-declared destructor. Similarly, a POD-union is an aggregate union that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-declared copy assignment operator and no user-declared destructor. A POD class is a class that is either a POD-struct or a POD-union. A POD class is an aggregate class that has no non-static data members of non-POD type (or array of such a type) or reference, and has no user-declared copy assignment operator and no user-declared destructor. A POD-struct is a POD class defined with the class-key struct or the class-key class. A POD-union is a POD class defined with the class-key union.
  2. Change 11.2 [class.access.base] paragraph 2 as indicated:

    In the absence of an access-specifier for a base class, public is assumed when the derived class is declared defined with the class-key struct and private is assumed when the class is declared defined with the class-key class. [Example:...
  3. Delete the note in 5.2.5 [expr.ref] paragraph 4:

    [Note: “class objects” can be structures (9.2 [class.mem]) and unions (9.5 [class.union]). Classes are discussed in clause 9 [class]. —end note]
  4. Change the commentary in the example in 9.2 [class.mem] paragraph 11 as indicated:

    ...an integer, and two pointers to similar structures objects of the same type. Once this definition...

    ...the count member of the structure object to which sp points; s.left refers to the left subtree pointer of the structure object s; and...

  5. Change 17.3 [definitions] “iostream class templates” as indicated:

    ...the argument traits is a structure class which defines additional characteristics...
  6. Change 18.6 [support.dynamic] paragraph 4 as indicated:

    If type is not a POD structure or a POD union POD class (clause 9), the results are undefined.
  7. Change the third bullet of B [implimits] paragraph 2 as indicated:

  8. Change the nineteenth bullet of B [implimits] paragraph 2 as indicated:

  9. Change the twenty-first bullet of B [implimits] paragraph 2 as indicated:

  10. Change C.2 [diff.library] paragraph 6 as indicated:

    The C++ Standard library provides 2 standard structures structs from the C library, as shown in Table 126.
  11. Change the last sentence of 3.9 [basic.types] paragraph 10 as indicated:

    Scalar types, POD-struct types, POD-union types POD classes (clause 9 [class]), arrays of such types and cv-qualified versions of these types (3.9.3 [basic.type.qualifier]) are collectively called POD types.

    Drafting note: Do not change 3.9 [basic.types] paragraph 11, because it's a note and the definition of “layout-compatible” is separate for POD-struct and POD-union in 9.2 [class.mem].

(This resolution also resolves issue 327.)




568. Definition of POD is too strict

Section: 9  [class]     Status: CD1     Submitter: Matt Austern     Date: 20 March 2006

[Voted into the WP at the July, 2007 meeting as part of paper J16/07-0202 = WG21 N2342.]

A POD struct (9 [class] paragraph 4) is “an aggregate class that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types), or reference, and that has no user-defined copy assignment operator and no user-defined destructor.” Meanwhile, an aggregate class (8.5.1 [dcl.init.aggr] paragraph 1) must have “no user-declared constructors, no private or protecte non-static data members, no base classes, and no virtual functions.”

This is too strict. The whole reason we define the notion of POD is for the layout compatibility guarantees in 9.2 [class.mem] paragraphs 14-17 and the byte-for-byte copying guarantees of 3.9 [basic.types] paragraph 2. None of those guarantees should be affected by the presence of ordinary constructors, any more than they're affected by the presence of any other member function. It’s silly for the standard to make layout and memcpy guarantees for this class:

 struct A {
    int n;
 };

but not for this one:

  struct B {
    int n;
    B(n_) : n(n_) { }
  };

With either A or B, it ought to be possible to save an array of those objects to disk with a single call to Unix’s write(2) system call or the equivalent. At present the standard says that it’s legal for A but not B, and there isn’t any good reason for that distinction.

Suggested resolution:

The following doesn’t fix all problems (in particular it still doesn’t let us treat pair<int, int> as a POD), but at least it goes a long way toward fixing the problem: in 8.5.1 [dcl.init.aggr] paragraph 1, change “no user-declared constructors” to “no nontrivial default constructor and no user-declared copy constructor.”

(Yes, I’m aware that this proposed change would also allow brace initialization for some types that don't currently allow it. I consider this to be a feature, not a bug.)

Mike Miller: I agree that something needs to be done about “POD,” but I’m not sure that this is it. My own take is that “POD” is used for too many different things — things that are related but not identical — and the concept should be split. The current definition is useful, as is, for issues regarding initialization and lifetime. For example, I wouldn’t want to relax the prohibition of jumping over a constructor call in 6.7 [stmt.dcl] (which is currently phrased in terms of POD types). On the other hand, I agree that the presence of a user-declared constructor says nothing about layout and bitwise copying. This needs (IMHO) a non-trivial amount of further study to determine how many categories we need (instead of just POD versus non-POD), which guarantees and prohibitions go with which category, the interaction of “memcpy initialization” (for want of a better term) with object lifetime, etc.

(See paper J16/06-0172 = WG21 N2102.)

Proposed resolution (April, 2007):

Adoption of the POD proposal (currently J16/07-0090 = WG21 N2230) will resolve this issue.




417. Using derived-class qualified name in out-of-class nested class definition

Section: 9.1  [class.name]     Status: CD1     Submitter: Jon Caves     Date: 19 May 2003

[Voted into WP at October 2004 meeting.]

We had a user complain that our compiler was allowing the following code:

  struct B {
    struct S;
  };

  struct D : B { };

  struct D::S {
  };

We took one look at the code and made the reasonable (I would claim) assumption that this was indeed a bug in our compiler. Especially as we had just fixed a very similar issue with the definition of static data members.

Imagine our surprise when code like this showed up in Boost and that every other compiler we tested accepts this code. So is this indeed legal (it seems like it must be) and if so is there any justification for this beyond 3.4.3.1 [class.qual]?

John Spicer: The equivalent case for a member function is covered by the declarator rules in 8.3 [dcl.meaning] paragraph 1. The committee has previously run into cases where a restriction should apply to both classes and non-classes, but fails to do so because there is no equivalent of 8.3 [dcl.meaning] paragraph 1 for classes.

Given that, by the letter of the standard, I would say that this case is allowed.

Notes from October 2003 meeting:

We feel this case should get an error.

Proposed Resolution (October 2003):

Note that the change here interacts with issue 432.

Add the following as a new paragraph immediately following 3.3.2 [basic.scope.pdecl] paragraph 2:

The point of declaration for a class first declared by a class-specifier is immediately after the identifier or template-id (if any) in its class-head (Clause 9 [class]). The point of declaration for an enumeration is immediately after the identifier (if any) in its enum-specifier (7.2 [dcl.enum]).

Change point 1 of 3.3.7 [basic.scope.class] paragraph 1 to read:

The potential scope of a name declared in a class consists not only of the declarative region following the name's declarator point of declaration, but also of all function bodies, default arguments, and constructor ctor-initializers in that class (including such things in nested classes).

[Note that the preceding change duplicates one of the changes in the proposed resolution of issue 432.]

Change 14.7.2 [temp.explicit] paragraph 2 to read:

If the explicit instantiation is for a member function, a member class or a static data member of a class template specialization, the name of the class template specialization in the qualified-id for the member declarator name shall be a template-id.

Add the following as paragraph 5 of Clause 9 [class]:

If a class-head contains a nested-name-specifier, the class-specifier shall refer to a class that was previously declared directly in the class or namespace to which the nested-name-specifier refers (i.e., neither inherited nor introduced by a using-declaration), and the class-specifier shall appear in a namespace enclosing the previous declaration.

Delete 9.1 [class.name] paragraph 4 (this was added by issue 284):

When a nested-name-specifier is specified in a class-head or in an elaborated-type-specifier, the resulting qualified name shall refer to a previously declared member of the class or namespace to which the nested-name-specifier refers, and the member shall not have been introduced by a using-declaration in the scope of the class or namespace nominated by the nested-name-specifier.



328. Missing requirement that class member types be complete

Section: 9.2  [class.mem]     Status: CD1     Submitter: Michiel Salters     Date: 10 Dec 2001

[Voted into WP at March 2004 meeting.]

Is it legal to use an incomplete type (3.9 [basic.types] paragraph 6) as a class member, if no object of such class is ever created ?

And as a class template member, even if the template is instantiated, but no object of the instantiated class is created?

The consensus seems to be NO, but no wording was found in the standard which explicitly disallows it.

The problem seems to be that most of the restrictions on incomplete types are on their use in objects, but class members are not objects.

A possible resolution, if this is considered a defect, is to add to 3.2 [basic.def.odr] paragraph 4, (situations when T must be complete), the use of T as a member of a class or instantiated class template.

The thread on comp.std.c++ which brought up the issue was "Compiler differences: which is correct?", started 2001 11 30. <3c07c8fb$0$8507$ed9e5944@reading.news.pipex.net>

Proposed Resolution (April 2002, revised April 2003):

Change the first bullet of the note in 3.2 [basic.def.odr] paragraph 4 and add two new bullets following it, as follows:

Replace 9.2 [class.mem] paragraph 8 by:

Non-static (9.4 [class.static]) data members shall not have incomplete types. In particular, a class C shall not contain a non-static member of class C, but it can contain a pointer or reference to an object of class C.

See also 3.9 [basic.types] paragraph 6, which is relevant but not changed by the Proposed Resolution.




437. Is type of class allowed in member function exception specification?

Section: 9.2  [class.mem]     Status: CD1     Submitter: Cary Coutant     Date: 10 Oct 2003

[Voted into WP at April 2005 meeting.]

I've encountered a C++ program in which a member function wants to declare that it may throw an object of its own class, e.g.:

  class Foo {
  private:
     int val;
  public:
     Foo( int &initval ) { val = initval; };
     void throwit() throw(Foo) { throw (*this); };
  };

The compiler is complaining that Foo is an incomplete type, and can't be used in the exception specification.

My reading of the standard [basic.types] is inconclusive. Although it does state that the class declaration is considered complete when the closing brace is read, I believe it also intends that the member function declarations should not be semantically validated until the class has been completely declared.

If this isn't allowed, I don't know how else a member function could be declared to throw an object of its own class.

John Spicer: The type is considered complete within function bodies, but not in their declaration (see 9.2 [class.mem] paragraph 2).

Proposed Resolution:

Change 9.2 [class.mem] paragraph 2 as follows:

Within the class member-specification, the class is regarded as complete within function bodies, default arguments, exception-specifications, and constructor ctor-initializers (including such things in nested classes).

Rationale: Taken with 8.3.5 [dcl.fct] paragraph 6, the exception-specification is the only part of a function declaration/definition in which the class name cannot be used because of its putative incompleteness. There is no justification for singling out exception specifications this way; both in the function body and in a catch clause, the class type will be complete, so there is no harm in allowing the class name to be used in the exception-specification.




613. Unevaluated uses of non-static class members

Section: 9.2  [class.mem]     Status: CD1     Submitter: Herb Sutter     Date: 28 October 2006

[Voted into WP at April, 2007 meeting.]

According to 9.2 [class.mem] paragraph 9, the name of a non-static data member can only be used with an object reference (explicit or implied by the this pointer of a non-static member function) or to form a pointer to member. This restriction applies even in the operand of sizeof, although the operand is not evaluated and thus no object is needed to perform the operation. Consequently, determining the size of a non-static class member often requires a circumlocution like

    sizeof((C*) 0)->m

instead of the simpler and more obvious (but incorrect)

    sizeof(C::m)

The CWG considered this question as part of issue 198 and decided at that time to retain the restriction on consistency grounds: the rule was viewed as applying uniformly to expressions, and making an exception for sizeof would require introducing a special-purpose “wart.”

The issue has recently resurfaced, in part due to the fact that the restriction would also apply to the decltype operator. Like the unary & operator to form a pointer to member, sizeof and decltype need neither an lvalue nor an rvalue, requiring solely the declarative information of the named operand. One possible approach would be to define the concept of “unevaluated operand” or the like, exempt unevaluated operands from the requirement for an object reference in 9.2 [class.mem] paragraph 9, and then define the operands of these operators as “unevaluated.”

Proposed resolution (April, 2007):

The wording is given in paper J16/07-0113 = WG21 N2253.




620. Declaration order in layout-compatible POD structs

Section: 9.2  [class.mem]     Status: CD1     Submitter: Martin Sebor     Date: 1 March 2007

[Voted into the WP at the July, 2007 meeting as part of paper J16/07-0202 = WG21 N2342.]

It should be made clear in 9.2 [class.mem] paragraph 15,

Two POD-struct (clause 9 [class]) types are layout-compatible if they have the same number of non-static data members, and corresponding non-static data members (in order) have layout-compatible types (3.9 [basic.types]).

that “corresponding... (in order)” refers to declaration order and not the order in which the members are laid out in memory.

However, this raises the point that, in cases where an access-specifier is involved, the declaration and layout order can be different (see paragraph 12). Thus, for two POD-struct classes A and B,

    struct A {
        char c;
        int i;
    }
    struct B {
        char c;
      public:
        int i;
    };

a compiler could move B::i before B::c, but A::c must precede A::i. It does not seem reasonable that these two POD-structs would be considered layout-compatible, even though they satisfy the requirement that corresponding members in declaration order are layout-compatible.

One possibility would be to require that neither POD-struct have an access-specifier in order to be considered layout-compatible. (It's not sufficient to require that they have the same access-specifiers, because the compiler is not required to lay out the storage the same way for different classes.)

8.5.1 [dcl.init.aggr] paragraph 2 should also be clarified to make explicit that “increasing... member order” refers to declaration order.

Proposed resolution (April, 2007):

This issue will be resolved by the adoption of the POD proposal (currently J16/07-0090 = WG21 N2230). That paper does not propose a change to the wording of 8.5.1 [dcl.init.aggr] paragraph 2, but the CWG feels that the intent of that paragraph (that the initializers are used in declaration order) is clear enough not to require revision.




452. Wording nit on description of this

Section: 9.3.2  [class.this]     Status: CD1     Submitter: Gennaro Prota     Date: 8 Jan 2004

[Voted into WP at July, 2007 meeting.]

9.3.2 [class.this] paragraph 1, which specifies the meaning of the keyword 'this', seems to limit its usage to the *body* of non-static member functions. However 'this' is also usable in ctor-initializers which, according to the grammar in 8.4 [dcl.fct.def] par. 1, are not part of the body.

Proposed resolution: Changing the first part of 9.3.2 [class.this] par. 1 to:

In the body of a nonstatic (9.3) member function or in a ctor-initializer (12.6.2), the keyword this is a non-lvalue expression whose value is the address of the object for which the function is called.

NOTE: I'm talking of constructors as functions that are "called"; there have been discussions on c.l.c++.m as to whether constructors are "functions" and to whether this terminology is correct or not; I think it is both intuitive and in agreement with the standard wording.

Steve Adamczyk: See also issue 397, which is defining a new syntax term for the body of a function including the ctor-initializers.

Notes from the March 2004 meeting:

This will be resolved when issue 397 is resolved.

Proposed resolution (October, 2005):

  1. Change 8.4 [dcl.fct.def] paragraph 1 as indicated:

  2. Function definitions have the form

    An informal reference to the body of a function should be interpreted as a reference to the nonterminal function-body.

  3. Change the definition of function-try-block in 15 [except] paragraph 1:

  4. Change 3.3.7 [basic.scope.class] paragraph 1, point 1, as indicated:

  5. The potential scope of a name declared in a class consists not only of the declarative region following the name's point of declaration, but also of all function bodies, bodies and default arguments, and constructor ctor-initializers in that class (including such things in nested classes).
  6. Change 3.3.7 [basic.scope.class] paragraph 1, point 5, as indicated:

  7. The potential scope of a declaration that extends to or past the end of a class definition also extends to the regions defined by its member definitions, even if the members are defined lexically outside the class (this includes static data member definitions, nested class definitions, member function definitions (including the member function body and, for constructor functions (12.1 [class.ctor]), the ctor-initializer (12.6.2 [class.base.init])) and any portion of the declarator part of such definitions which follows the identifier, including a parameter-declaration-clause and any default arguments (8.3.6 [dcl.fct.default]). [Example:...
  8. Change footnote 32 in 3.4.1 [basic.lookup.unqual] paragraph 8 as indicated:

  9. That is, an unqualified name that occurs, for instance, in a type or default argument expression in the parameter-declaration-clause, parameter-declaration-clause or in the function body, or in an expression of a mem-initializer in a constructor definition.
  10. Change 5.1.1 [expr.prim.general] paragraph 3 as indicated:

  11. ...The keyword this shall be used only inside a non-static class member function body (9.3 [class.mfct]) or in a constructor mem-initializer (12.6.2 [class.base.init])...
  12. Change 9.2 [class.mem] paragraph 2 as indicated:

  13. ...Within the class member-specification, the class is regarded as complete within function bodies, default arguments, and exception-specifications, and constructor ctor-initializers (including such things in nested classes)...
  14. Change 9.2 [class.mem] paragraph 9 as indicated:

  15. Each occurrence in an expression of the name of a non-static data member or non-static member function of a class shall be expressed as a class member access (5.2.5 [expr.ref]), except when it appears in the formation of a pointer to member (5.3.1 [expr.unary.op]), or or when it appears in the body of a non-static member function of its class or of a class derived from its class (9.3.1 [class.mfct.non-static]), or when it appears in a mem-initializer for a constructor for its class or for a class derived from its class (12.6.2 [class.base.init]).
  16. Change the note in 9.3 [class.mfct] paragraph 5 as indicated:

  17. [Note: a name used in a member function definition (that is, in the parameter-declaration-clause including the default arguments (8.3.6 [dcl.fct.default]), or or in the member function body, or, for a constructor function (12.1 [class.ctor]), in a mem-initializer expression (12.6.2 [class.base.init])) is looked up as described in 3.4 [basic.lookup]. —end note]
  18. Change 9.3.1 [class.mfct.non-static] paragraph 1 as indicated:

  19. ...A non-static member function may also be called directly using the function call syntax (5.2.2 [expr.call], 13.3.1.1 [over.match.call]) from within the body of a member function of its class or of a class derived from its class.

  20. Change 9.3.1 [class.mfct.non-static] paragraph 3 as indicated:

  21. When an id-expression (5.1.1 [expr.prim.general]) that is not part of a class member access syntax (5.2.5 [expr.ref]) and not used to form a pointer to member (5.3.1 [expr.unary.op]) is used in the body of a non-static member function of class X or used in the mem-initializer for a constructor of class X, if name lookup (3.4.1 [basic.lookup.unqual]) resolves the name in the id-expression to a non-static non-type member of class X or of a base class of X, the id-expression is transformed into a class member access expression (5.2.5 [expr.ref]) using (*this) (9.3.2 [class.this]) as the postfix-expression to the left of the . operator...
  22. Change 12.1 [class.ctor] paragraph 7 as indicated:

  23. ...The implicitly-defined default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with an empty mem-initializer-list no ctor-initializer (12.6.2 [class.base.init]) and an empty function body compound-statement...
  24. Change 12.6.2 [class.base.init] paragraph 4 as indicated:

  25. ...After the call to a constructor for class X has completed, if a member of X is neither specified in the constructor’s mem-initializers, nor default-initialized, nor value-initialized, nor given a value during execution of the compound-statement of the body of the constructor, the member has indeterminate value.
  26. Change the last bullet of 12.6.2 [class.base.init] paragraph 5 as indicated:

  27. Change 15 [except] paragraph 4 as indicated:

  28. A function-try-block associates a handler-seq with the ctor-initializer, if present, and the function-body compound-statement. An exception thrown during the execution of the initializer expressions in the ctor-initializer or during the execution of the function-body compound-statement transfers control to a handler in a function-try-block in the same way as an exception thrown during the execution of a try-block transfers control to other handlers. [Example:

        int f(int);
        class C {
            int i;
            double d;
        public:
            C(int, double);
        };
    
        C::C(int ii, double id)
        try
            : i(f(ii)), d(id)
        {
            // constructor function body statements
        }
        catch (...)
        {
            // handles exceptions thrown from the ctor-initializer
            // and from the constructor function body statements
        }
    

    end example]

  29. Change 15.2 [except.ctor] paragraph 2 as indicated:

  30. When an exception is thrown, control is transferred to the nearest handler with a matching type (15.3 [except.handle]); “nearest” means the handler for which the compound-statement, compound-statement or ctor-initializer, or function-body following the try keyword was most recently entered by the thread of control and not yet exited.



406. Static data member in class with name for linkage purposes

Section: 9.4.2  [class.static.data]     Status: CD1     Submitter: Jorgen Bundgaard     Date: 12 Mar 2003

[Voted into WP at March 2004 meeting.]

The following test program is claimed to be a negative C++ test case for "Unnamed classes shall not contain static data members", c.f. ISO/IEC 14882 section 9.4.2 [class.static.data] paragraph 5.

 
  struct B {
         typedef struct {
                 static int i;          // Is this legal C++ ?
         } A;
  };
 
  int B::A::i = 47;      // Is this legal C++ ?

We are not quite sure about what an "unnamed class" is. There is no exact definition in ISO/IEC 14882; the closest we can come to a hint is the wording of section 7.1.3 [dcl.typedef] paragraph 5, where it seems to be understood that a class-specifier with no identifier between "class" and "{" is unnamed. The identifier provided after "}" ( "A" in the test case above) is there for "linkage purposes" only.

To us, class B::A in the test program above seems "named" enough, and there is certainly a mechanism to provide the definition for B::A::i (in contrast to the note in section 9.4.2 [class.static.data] paragraph 5).

Our position is therefore that the above test program is indeed legal C++. Can you confirm or reject this claim?

Herb Sutter replied to the submitter as follows: Here are my notes based on a grep for "unnamed class" in the standard:

So yes, an unnamed class is one where there is no identifier (class name) between the class-key and the {. This is also in harmony with the production for class-name in 9 [class] paragraph 1:

Notes from the October 2003 meeting:

We agree that the example is not valid; this is an unnamed class. We will add wording to define an unnamed class. The note in 9.4.2 [class.static.data] paragraph 5 should be corrected or deleted.

Proposed Resolution (October 2003):

At the end of clause 9 [class], paragraph 1, add the following:

A class-specifier where the class-head omits the optional identifier defines an unnamed class.

Delete the following from 9.4.2 [class.static.data] paragraph 5:

[ Note: this is because there is no mechanism to provide the definitions for such static data members. ]



454. When is a definition of a static data member required?

Section: 9.4.2  [class.static.data]     Status: CD1     Submitter: Gennaro Prota     Date: 18 Jan 2004

[Voted into WP at the October, 2006 meeting.]

As a result of the resolution of core issue 48, the current C++ standard is not in sync with existing practice and with user expectations as far as definitions of static data members having const integral or const enumeration type are concerned. Basically what current implementations do is to require a definition only if the address of the constant is taken. Example:

void f() {

  std::string s;
  ... 

  // current implementations don't require a definition
  if (s.find('a', 3) == std::string::npos) {
   ...
  }

To the letter of the standard, though, the above requires a definition of npos, since the expression std::string::npos is potentially evaluated. I think this problem would be easily solved with simple changes to 9.4.2 [class.static.data] paragraph 4, 9.4.2 [class.static.data] paragraph 5 and 3.2 [basic.def.odr] paragraph 3.

Suggested resolution:

Replace 9.4.2 [class.static.data] paragraph 4 with:

If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be [note1] an integral constant expression (5.19). In that case, the member can appear in integral constant expressions. No definition of the member is required, unless an lvalue expression that designates it is potentially evaluated and either used as operand to the built-in unary & operator [note 2] or directly bound to a reference.

If a definition exists, it shall be at namespace scope and shall not contain an initializer.

In 9.4.2 [class.static.data] paragraph 5 change

There shall be exactly one definition of a static data member that is used in a program; no diagnostic is required; see 3.2.

to

Except as allowed by 9.4.2 par. 4, there shall be exactly one definition of a static data member that is potentially evaluated (3.2) in a program; no diagnostic is required.

In 3.2 [basic.def.odr] paragraph 3 add, at the beginning:

Except for the omission allowed by 9.4.2, par. 4, ...

[note 1] Actually it shall be a "= followed by a constant-expression". This could probably be an editorial fix, rather than a separate DR.

[note 2] Note that this is the case when reinterpret_cast-ing to a reference, like in

struct X { static const int value = 0; };
const char & c = reinterpret_cast<const char&>(X::value);
See 5.2.10 [expr.reinterpret.cast]/10

More information, in response to a question about why issue 48 does not resolve the problem:

The problem is that the issue was settled in a way that solves much less than it was supposed to solve; that's why I decided to file, so to speak, a DR on a DR.

I understand this may seem a little 'audacious' on my part, but please keep reading. Quoting from the text of DR 48 (emphasis mine):

Originally, all static data members still had to be defined outside the class whether they were used or not.

But that restriction was supposed to be lifted [...]

In particular, if an integral/enum const static data member is initialized within the class, and its address is never taken, we agreed that no namespace-scope definition was required.

The corresponding resolution doesn't reflect this intent, with the definition being still required in most situations anyway: it's enough that the constant appears outside a place where constants are required (ignoring the obvious cases of sizeof and typeid) and you have to provide a definition. For instance:

  struct X {
   static const int c = 1;
  };

  void f(int n)
  {
   if (n == X::c)   // <-- potentially evaluated
    ...
  }

<start digression>

Most usages of non-enum BOOST_STATIC_COSTANTs, for instance, are (or were, last time I checked) non-conforming. If you recall, Paul Mensonides pointed out that the following template

// map_integral

template<class T, T V> struct map_integral : identity<T> {
  static const T value = V;
};

template<class T, T V> const T map_integral<T, V>::value;

whose main goal is to map the same couples (type, value) to the same storage, also solves the definition problem. In this usage it is an excellent hack (if your compiler is good enough), but IMHO still a hack on a language defect.

<end digression>

What I propose is to solve the issue according to the original intent, which is also what users expect and all compilers that I know of already do. Or, in practice, we would have a rule that exists only as words in a standard document.

PS: I've sent a copy of this to Mr. Adamczyk to clarify an important doubt that occurred to me while writing this reply:

if no definition is provided for an integral static const data member is that member an object? Paragraph 1.8/1 seems to say no, and in fact it's difficult to think it is an object without assuming/pretending that a region of storage exists for it (an object *is* a region of storage according to the standard).

I would think that when no definition is required we have to assume that it could be a non-object. In that case there's nothing in 3.2 which says what 'used' means for such an entity and the current wording would thus be defective. Also, since the name of the member is an lvalue and 3.10/2 says an lvalue refers to an object we would have another problem.

OTOH the standard could pretend it is always an object (though the compiler can optimize it away) and in this case it should probably make a special case for it in 3.2/2.

Notes from the March 2004 meeting:

We sort of like this proposal, but we don't feel it has very high priority. We're not going to spend time discussing it, but if we get drafting for wording we'll review it.

Proposed resolution (October, 2005):

  1. Change the first two sentences of 3.2 [basic.def.odr] paragraph 2 from:

    An expression is potentially evaluated unless it appears where an integral constant expression is required (see 5.19 [expr.const]), is the operand of the sizeof operator (5.3.3 [expr.sizeof]), or is the operand of the typeid operator and the expression does not designate an lvalue of polymorphic class type (5.2.8 [expr.typeid]). An object or non-overloaded function is used if its name appears in a potentially-evaluated expression.

    to

    An expression that is the operand of the sizeof operator (5.3.3 [expr.sizeof]) is unevaluated, as is an expression that is the operand of the typeid operator if it is not an lvalue of a polymorphic class type (5.2.8 [expr.typeid]); all other expressions are potentially evaluated. An object or non-overloaded function whose name appears as a potentially-evaluated expression is used, unless it is an object that satisfies the requirements for appearing in an integral constant expression (5.19 [expr.const]) and the lvalue-to-rvalue conversion (4.1 [conv.lval]) is immediately applied.
  2. Change the first sentence of 9.4.2 [class.static.data] paragraph 2 as indicated:

  3. If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which whose constant-expression shall be an integral constant expression (5.19 [expr.const]).



58. Signedness of bit fields of enum type

Section: 9.6  [class.bit]     Status: CD1     Submitter: Steve Adamczyk     Date: 13 Oct 1998

[Voted into WP at the October, 2006 meeting.]

Section 9.6 [class.bit] paragraph 4 needs to be more specific about the signedness of bit fields of enum type. How much leeway does an implementation have in choosing the signedness of a bit field? In particular, does the phrase "large enough to hold all the values of the enumeration" mean "the implementation decides on the signedness, and then we see whether all the values will fit in the bit field", or does it require the implementation to make the bit field signed or unsigned if that's what it takes to make it "large enough"?

(See also issue 172.)

Note (March, 2005): Clark Nelson observed that there is variation among implementations on this point.

Notes from April, 2005 meeting:

Although implementations enjoy a great deal of latitude in handling bit-fields, it was deemed more user-friendly to ensure that the example in paragraph 4 will work by requiring implementations to use an unsigned underlying type if the enumeration type has no negative values. (If the implementation is allowed to choose a signed representation for such bit-fields, the comparison against TRUE will be false.)

In addition, it was observed that there is an apparent circularity between 7.2 [dcl.enum] paragraph 7 and 9.6 [class.bit] paragraph 4 that should be resolved.

Proposed resolution (April, 2006):

  1. Replace 7.2 [dcl.enum] paragraph 7, deleting the embedded footnote 85, with the following:

    For an enumeration where emin is the smallest enumerator and emax is the largest, the values of the enumeration are the values in the range bmin to bmax, defined as follows: Let K be 1 for a two's complement representation and 0 for a one's complement or sign-magnitude representation. bmax is the smallest value greater than or equal to max(|emin|-K,|emax|) and equal to 2M-1, where M is a non-negative integer. bmin is zero if emin is non-negative and -(bmax+K) otherwise. The size of the smallest bit-field large enough to hold all the values of the enumeration type is max(M,1) if bmin is zero and M+1 otherwise. It is possible to define an enumeration that has values not defined by any of its enumerators.
  2. Add the indicated text to the second sentence of 9.6 [class.bit] paragraph 4:

    If the value of an enumerator is stored into a bit-field of the same enumeration type and the number of bits in the bit-field is large enough to hold all the values of that enumeration type (7.2 [dcl.enum]), the original enumerator value and the value of the bit-field shall compare equal.



436. Problem in example in 9.6 paragraph 4

Section: 9.6  [class.bit]     Status: CD1     Submitter: Roberto Santos     Date: 10 October 2003

[Voted into WP at October 2004 meeting.]

It looks like the example on 9.6 [class.bit] paragraph 4 has both the enum and function contributing the identifier "f" for the same scope.

  enum BOOL { f=0, t=1 };
  struct A {
    BOOL b:1;
  };
  A a;
  void f() {
    a.b = t;
    if (a.b == t) // shall yield true
    { /* ... */ }
  }

Proposed resolution:

Change the example at the end of 9.6 [class.bit]/4 from:

  enum BOOL { f=0, t=1 };
  struct A {
    BOOL b:1;
  };
  A a;
  void f() {
    a.b = t;
    if (a.b == t) // shall yield true
    { /* ... */ }
  }

To:

  enum BOOL { FALSE=0, TRUE=1 };
  struct A {
    BOOL b:1;
  };
  A a;
  void f() {
    a.b = TRUE;
    if (a.b == TRUE) // shall yield true
    { /* ... */ }
  }




198. Definition of "use" in local and nested classes

Section: 9.8  [class.local]     Status: CD1     Submitter: Erwin Unruh     Date: 27 Jan 2000

[Voted into WP at April 2003 meeting.]

9.8 [class.local] paragraph 1 says,

Declarations in a local class can use only type names, static variables, extern variables and functions, and enumerators from the enclosing scope.
The definition of when an object or function is "used" is found in 3.2 [basic.def.odr] paragraph 2 and essentially says that the operands of sizeof and non-polymorphic typeid operators are not used. (The resolution for issue 48 will add contexts in which integral constant expressions are required to the list of non-uses.)

This definition of "use" would presumably allow code like

    void foo() {
        int i;
        struct S {
            int a[sizeof(i)];
        };
    };
which is required for C compatibility.

However, the restrictions on nested classes in 9.7 [class.nest] paragraph 1 are very similar to those for local classes, and the example there explicitly states that a reference in a sizeof expression is a forbidden use (abbreviated for exposition):

    class enclose {
    public:
        int x;
        class inner {
            void f(int i)
            {
                int a = sizeof(x);  // error: refers to enclose::x
            }
        };
    };

[As a personal note, I have seen real-world code that was exactly like this; it was hard to persuade the author that the required writearound, sizeof(((enclose*) 0)->x), was an improvement over sizeof(x). —wmm]

Similarly, 9.2 [class.mem] paragraph 9 would appear to prohibit examples like the following:

    struct B {
        char x[10];
    };
    struct D: B {
        char y[sizeof(x)];
    };

Suggested resolution: Add cross-references to 3.2 [basic.def.odr] following the word "use" in both 9.7 [class.nest] and 9.8 [class.local] , and change the example in 9.7 [class.nest] to indicate that a reference in a sizeof expression is permitted. In 9.2 [class.mem] paragraph 9, "referred to" should be changed to "used" with a cross_reference to 3.2 [basic.def.odr].

Notes from 10/01 meeting:

It was noted that the suggested resolution did not make the sizeof() example in 9.7 [class.nest] valid. Although the reference to the argument of sizeof() is not regarded as a use, the right syntax must be used nonetheless to reference a non-static member from the enclosing class. The use of the member name by itself is not valid. The consensus within the core working group was that nothing should be done about this case. It was later discovered that 9.4 [class.static] paragraph 3 states that

The definition of a static member shall not use directly the names of the nonstatic members of its class or of a base class of its class (including as operands of the sizeof operator). The definition of a static member may only refer to these members to form pointer to members (5.3.1 [expr.unary.op]) or with the class member access syntax (5.2.5 [expr.ref]).

This seems to reinforce the decision of the working group.

The use of "use" should still be cross-referenced. The statements in 9.7 [class.nest] and 9.8 [class.local] should also be rewritten to state the requirement positively rather than negatively as the list of "can't"s is already missing some cases such as template parameters.

Notes from the 4/02 meeting:

We backed away from "use" in the technical sense, because the requirements on the form of reference are the same whether or not the reference occurs inside a sizeof.

Proposed Resolution (revised October 2002):

In 9.2 [class.mem] paragraph 9, replace

Except when used to form a pointer to member (5.3.1 [expr.unary.op]), when used in the body of a nonstatic member function of its class or of a class derived from its class (9.3.1 [class.mfct.non-static]), or when used in a mem-initializer for a constructor for its class or for a class derived from its class (12.6.2 [class.base.init]), a nonstatic data or function member of a class shall only be referred to with the class member access syntax (5.2.5 [expr.ref]).

with the following paragraph

Each occurrence in an expression of the name of a nonstatic data member or nonstatic member function of a class shall be expressed as a class member access (5.2.5 [expr.ref]), except when it appears in the formation of a pointer to member (5.3.1 [expr.unary.op]), when it appears in the body of a nonstatic member function of its class or of a class derived from its class (9.3.1 [class.mfct.non-static]), or when it appears in a mem-initializer for a constructor for its class or for a class derived from its class (12.6.2 [class.base.init]).

In 9.7 [class.nest] paragraph 1, replace the last sentence,

Except by using explicit pointers, references, and object names, declarations in a nested class can use only type names, static members, and enumerators from the enclosing class.

with the following

[Note: In accordance with 9.2 [class.mem], except by using explicit pointers, references, and object names, declarations in a nested class shall not use nonstatic data members or nonstatic member functions from the enclosing class. This restriction applies in all constructs including the operands of the sizeof operator.]

In the example following 9.7 [class.nest] paragraph 1, change the comment on the first statement of function f to emphasize that sizeof(x) is an error. The example reads in full:

  int x;
  int y;
  class enclose {
  public:
    int x;
    static int s;
    class inner {
      void f(int i)
      {
        int a = sizeof(x);  // error: direct use of enclose::x even in sizeof
        x = i;              // error: assign to enclose::x
        s = i;              // OK: assign to enclose::s
        ::x = i;            // OK: assign to global x
        y = i;              // OK: assign to global y
      }
      void g(enclose* p, int i)
      {
        p->x = i;        // OK: assign to enclose::x
      }
    };
  };
   
  inner* p = 0;             // error: inner not in scope



484. Can a base-specifier name a cv-qualified class type?

Section: 10  [class.derived]     Status: CD1     Submitter: Richard Corden     Date: 21 Oct 2004

[Voted into WP at the October, 2006 meeting.]

Issue 298, recently approved, affirms that cv-qualified class types can be used as nested-name-specifiers. Should the same be true for base-specifiers?

Rationale (April, 2005):

The resolution of issue 298 added new text to 9.1 [class.name] paragraph 5 making it clear that a typedef that names a cv-qualified class type is a class-name. Because the definition of base-specifier simply refers to class-name, it is already the case that cv-qualified class types are permitted as base-specifiers.

Additional notes (June, 2005):

It's not completely clear what it means to have a cv-qualified type as a base-specifier. The original proposed resolution for issue 298 said that “the cv-qualifiers are ignored,” but that wording is not in the resolution that was ultimately approved.

If the cv-qualifiers are not ignored, does that mean that the base-class subobject should be treated as always similarly cv-qualified, regardless of the cv-qualification of the derived-class lvalue used to access the base-class subobject? For instance:

    typedef struct B {
        void f();
        void f() const;
        int i;
    } const CB;

    struct D: CB { };

    void g(D* dp) {
        dp->f();    // which B::f?
        dp->i = 3;  // permitted?
    }

Proposed resolution (October, 2005):

  1. Change 9.1 [class.name] paragraph 5 as indicated:

  2. A typedef-name (7.1.3 [dcl.typedef]) that names a class type, or a cv-qualified version thereof, is also a class-name, but class-name. If a typedef-name that names a cv-qualified class type is used where a class-name is required, the cv-qualifiers are ignored. A typedef-name shall not be used as the identifier in a class-head.
  3. Delete 7.1.3 [dcl.typedef] paragraph 8:

  4. [Note: if the typedef-name is used where a class-name (or enum-name) is required, the program is ill-formed. For example,

        typedef struct {
            S();     // error: requires a return type because S is
                      // an ordinary member function, not a constructor
        } S;
    

    end note]




39. Conflicting ambiguity rules

Section: 10.2  [class.member.lookup]     Status: CD1     Submitter: Neal M Gafter     Date: 20 Aug 1998

[Voted into WP at April 2005 meeting.]

The ambiguity text in 10.2 [class.member.lookup] may not say what we intended. It makes the following example ill-formed:

    struct A {
        int x(int);
    };
    struct B: A {
        using A::x;
        float x(float);
    };
    
    int f(B* b) {
        b->x(3);  // ambiguous
    }
This is a name lookup ambiguity because of 10.2 [class.member.lookup] paragraph 2:
... Each of these declarations that was introduced by a using-declaration is considered to be from each sub-object of C that is of the type containing the declaration designated by the using-declaration. If the resulting set of declarations are not all from sub-objects of the same type, or the set has a nonstatic member and includes members from distinct sub-objects, there is an ambiguity and the program is ill-formed.
This contradicts the text and example in paragraph 12 of 7.3.3 [namespace.udecl] .

Proposed Resolution (10/00):

  1. Replace the two cited sentences from 10.2 [class.member.lookup] paragraph 2 with the following:

    The resulting set of declarations shall all be from sub-objects of the same type, or there shall be a set of declarations from sub-objects of a single type that contains using-declarations for the declarations found in all other sub-object types. Furthermore, for nonstatic members, the resulting set of declarations shall all be from a single sub-object, or there shall be a set of declarations from a single sub-object that contains using-declarations for the declarations found in all other sub-objects. Otherwise, there is an ambiguity and the program is ill-formed.
  2. Replace the examples in 10.2 [class.member.lookup] paragraph 3 with the following:

        struct A {
            int x(int);
            static int y(int);
        };
        struct V {
            int z(int);
        };
        struct B: A, virtual V {
            using A::x;
            float x(float);
            using A::y;
            static float y(float);
            using V::z;
            float z(float);
        };
        struct C: B, A, virtual V {
        };
    
        void f(C* c) {
            c->x(3);    // ambiguous -- more than one sub-object A
            c->y(3);    // not ambiguous
            c->z(3);    // not ambiguous
        }
    

Notes from 04/01 meeting:

The following example should be accepted but is rejected by the wording above:

    struct A { static void f(); };

    struct B1: virtual A {
        using A::f;
    };

    struct B2: virtual A {
        using A::f;
    };

    struct C: B1, B2 { };

    void g() {
        C::f();        // OK, calls A::f()
    }

Notes from 10/01 meeting (Jason Merrill):

The example in the issues list:

    struct A {
        int x(int);
    };
    struct B: A {
        using A::x;
        float x(float);
    };
    
    int f(B* b) {
        b->x(3);  // ambiguous
    }
Is broken under the existing wording:
... Each of these declarations that was introduced by a using-declaration is considered to be from each sub-object of C that is of the type containing the declaration designated by the using-declaration. If the resulting set of declarations are not all from sub-objects of the same type, or the set has a nonstatic member and includes members from distinct sub-objects, there is an ambiguity and the program is ill-formed.
Since the two x's are considered to be "from" different objects, looking up x produces a set including declarations "from" different objects, and the program is ill-formed. Clearly this is wrong. The problem with the existing wording is that it fails to consider lookup context.

The first proposed solution:

The resulting set of declarations shall all be from sub-objects of the same type, or there shall be a set of declarations from sub-objects of a single type that contains using-declarations for the declarations found in all other sub-object types. Furthermore, for nonstatic members, the resulting set of declarations shall all be from a single sub-object, or there shall be a set of declarations from a single sub-object that contains using-declarations for the declarations found in all other sub-objects. Otherwise, there is an ambiguity and the program is ill-formed.
breaks this testcase:
    struct A { static void f(); };

    struct B1: virtual A {
        using A::f;
    };

    struct B2: virtual A {
        using A::f;
    };

    struct C: B1, B2 { };

    void g() {
        C::f();        // OK, calls A::f()
    }
because it considers the lookup context, but not the definition context; under this definition of "from", the two declarations found are the using-declarations, which are "from" B1 and B2.

The solution is to separate the notions of lookup and definition context. I have taken an algorithmic approach to describing the strategy.

Incidentally, the earlier proposal allows one base to have a superset of the declarations in another base; that was an extension, and my proposal does not do that. One algorithmic benefit of this limitation is to simplify the case of a virtual base being hidden along one arm and not another ("domination"); if we allowed supersets, we would need to remember which subobjects had which declarations, while under the following resolution we need only keep two lists, of subobjects and declarations.

Proposed resolution (October 2002):

Replace 10.2 [class.member.lookup] paragraph 2 with:

The following steps define the result of name lookup for a member name f in a class scope C.

The lookup set for f in C, called S(f,C), consists of two component sets: the declaration set, a set of members named f; and the subobject set, a set of subobjects where declarations of these members (possibly including using-declarations) were found. In the declaration set, using-declarations are replaced by the members they designate, and type declarations (including injected-class-names) are replaced by the types they designate. S(f,C) is calculated as follows.

If C contains a declaration of the name f, the declaration set contains every declaration of f in C (excluding bases), the subobject set contains C itself, and calculation is complete.

Otherwise, S(f,C) is initially empty. If C has base classes, calculate the lookup set for f in each direct base class subjobject Bi, and merge each such lookup set S(f,Bi) in turn into S(f,C).

The following steps define the result of merging lookup set S(f,Bi) into the intermediate S(f,C):

The result of name lookup for f in C is the declaration set of S(f,C). If it is an invalid set, the program is ill-formed.

[Example:

    struct A { int x; };                    // S(x,A) = {{ A::x }, { A }}
    struct B { float x; };                  // S(x,B) = {{ B::x }, { B }}
    struct C: public A, public B { };       // S(x,C) = { invalid, { A in C, B in C }}
    struct D: public virtual C { };         // S(x,D) = S(x,C)
    struct E: public virtual C { char x; }; // S(x,E) = {{ E::x }, { E }}
    struct F: public D, public E { };       // S(x,F) = S(x,E)

    int main() {
      F f;
      f.x = 0;   // OK, lookup finds { E::x }
    }
S(x,F) is unambiguous because the A and B base subobjects of D are also base subobjects of E, so S(x,D) is discarded in the first merge step. --end example]

Turn 10.2 [class.member.lookup] paragraphs 5 and 6 into notes.

Notes from October 2003 meeting:

Mike Miller raised some new issues in N1543, and we adjusted the proposed resolution as indicated in that paper.

Further information from Mike Miller (January 2004):

Unfortunately, I've become aware of a minor glitch in the proposed resolution for issue 39 in N1543, so I'd like to suggest a change that we can discuss in Sydney.

A brief review and background of the problem: the major change we agreed on in Kona was to remove detection of multiple-subobject ambiguity from class lookup (10.2 [class.member.lookup]) and instead handle it as part of the class member access expression. It was pointed out in Kona that 11.2 [class.access.base]/5 has this effect:

If a class member access operator, including an implicit "this->," is used to access a nonstatic data member or nonstatic member function, the reference is ill-formed if the left operand (considered as a pointer in the "." operator case) cannot be implicitly converted to a pointer to the naming class of the right operand.

After the meeting, however, I realized that this requirement is not sufficient to handle all the cases. Consider, for instance,

    struct B {
        int i;
    };

    struct I1: B { };
    struct I2: B { };

    struct D: I1, I2 {
        void f() {
            i = 0;    // not ill-formed per 11.2p5
        }
    };

Here, both the object expression ("this") and the naming class are "D", so the reference to "i" satisfies the requirement in 11.2 [class.access.base]/5, even though it involves a multiple-subobject ambiguity.

In order to address this problem, I proposed in N1543 to add a paragraph following 5.2.5 [expr.ref]/4:

If E2 is a non-static data member or a non-static member function, the program is ill-formed if the class of E1 cannot be unambiguously converted (10.2) to the class of which E2 is directly a member.

That's not quite right. It does diagnose the case above as written; however, it breaks the case where qualification is used to circumvent the ambiguity:

    struct D2: I1, I2 {
        void f() {
            I2::i = 0;    // ill-formed per proposal
        }
    };

In my proposed wording, the class of "this" can't be converted to "B" (the qualifier is ignored), so the access is ill-formed. Oops.

I think the following is a correct formulation, so the proposed resolution we discuss in Sydney should contain the following paragraph instead of the one in N1543:

If E2 is a nonstatic data member or a non-static member function, the program is ill-formed if the naming class (11.2) of E2 cannot be unambiguously converted (10.2) to the class of which E2 is directly a member.

This reformulation also has the advantage of pointing readers to 11.2 [class.access.base], where the the convertibility requirement from the class of E1 to the naming class is located and which might otherwise be overlooked.

Notes from the March 2004 meeting:

We discussed this further and agreed with these latest recommendations. Mike Miller has produced a paper N1626 that gives just the final collected set of changes.

(This resolution also resolves isssue 306.)




306. Ambiguity by class name injection

Section: 10.2  [class.member.lookup]     Status: CD1     Submitter: Clark Nelson     Date: 19 Jul 2001

[Voted into WP at April 2005 meeting.]

Is the following well-formed?

    struct A {
        struct B { };
    };
    struct C : public A, public A::B {
        B *p;
    };
The lookup of B finds both the struct B in A and the injected B from the A::B base class. Are they the same thing? Does the standard say so?

What if a struct is found along one path and a typedef to that struct is found along another path? That should probably be valid, but does the standard say so?

This is resolved by issue 39

February 2004: Moved back to "Review" status because issue 39 was moved back to "Review".




390. Pure virtual must be defined when implicitly called

Section: 10.4  [class.abstract]     Status: CD1     Submitter: Daniel Frey     Date: 14 Nov 2002

[Voted into WP at March 2004 meeting.]

In clause 10.4 [class.abstract] paragraph 2, it reads:

A pure virtual function need be defined only if explicitly called with the qualified-id syntax (5.1.1 [expr.prim.general]).

This is IMHO incomplete. A dtor is a function (well, a "special member function", but this also makes it a function, right?) but it is called implicitly and thus without a qualified-id syntax. Another alternative is that the pure virtual function is called directly or indirectly from the ctor. Thus the above sentence which specifies when a pure virtual function need be defined ("...only if...") needs to be extended:

A pure virtual function need be defined only if explicitly called with the qualified-id syntax (5.1.1 [expr.prim.general]) or if implicitly called (12.4 [class.dtor] or 12.7 [class.cdtor]).

Proposed resolution:

Change 10.4 [class.abstract] paragraph 2 from

A pure virtual function need be defined only if explicitly called with the qualified-id syntax (5.1.1 [expr.prim.general]).

to

A pure virtual function need be defined only if explicitly called with, or as if with (12.4 [class.dtor]), the qualified-id syntax (5.1.1 [expr.prim.general]).

Note: 12.4 [class.dtor] paragraph 6 defines the "as if" cited.




8. Access to template arguments used in a function return type and in the nested name specifier

Section: 11  [class.access]     Status: CD1     Submitter: Mike Ball     Date: unknown

[Moved to DR at 4/01 meeting.]

Consider the following example:

    class A {
       class A1{};
       static void func(A1, int);
       static void func(float, int);
       static const int garbconst = 3;
     public:
       template < class T, int i, void (*f)(T, int) > class int_temp {};
       template<> class int_temp<A1, 5, func> { void func1() };
       friend int_temp<A1, 5, func>::func1();
       int_temp<A1, 5, func>* func2();
   };
   A::int_temp<A::A1, A::garbconst + 2, &A::func>* A::func2() {...}
ISSUE 1:

In 11 [class.access] paragraph 5 we have:

This means, if we take the loosest possible definition of "access from a particular scope", that we have to save and check later the following names

      A::int_temp
      A::A1
      A::garbconst (part of an expression)
      A::func (after overloading is done)
I suspect that member templates were not really considered when this was written, and that it might have been written rather differently if they had been. Note that access to the template arguments is only legal because the class has been declared a friend, which is probably not what most programmers would expect.

Rationale:

Not a defect. This behavior is as intended.

ISSUE 2:

Now consider void A::int_temp<A::A1, A::garbconst + 2, &A::func>::func1() {...} By my reading of 11.8 [class.access.nest] , the references to A::A1, A::garbconst and A::func are now illegal, and there is no way to define this function outside of the class. Is there any need to do anything about either of these Issues?

Proposed resolution (04/01):

The resolution for this issue is contained in the resolution for issue 45.




494. Problems with the resolution of issue 45

Section: 11  [class.access]     Status: CD1     Submitter: Lloyd J. Lewins     Date: 17 Dec 2004

[Voted into WP at the October, 2006 meeting.]

The proposed resolution for issue 45 inserts the following sentence after 11 [class.access] paragraph 1:

A member of a class can also access all names as the class of which it is a member.

I don't think that this is correctly constructed English. I see two possibilities:

  1. This is a typo, and the correct change is:

    A member of a class can also access all names of the class of which it is a member.

  2. The intent is something more like:

    A member of a nested class can also access all names accessible by any other member of the class of which it is a member.

[Note: this was editorially corrected at the time defect resolutions were being incorporated into the Working Paper to read, “...can also access all the names declared in the class of which it is a member,” which is essentially the same as the preceding option 1.]

I would prefer to use the language proposed for 11.8 [class.access.nest]:

A nested class is a member and as such has the same access rights as any other member.

A second problem is with the text in 11.4 [class.friend] paragraph 2:

[Note: this means that access to private and protected names is also granted to member functions of the friend class (as if the functions were each friends) and to the static data member definitions of the friend class. This also means that private and protected type names from the class granting friendship can be used in the base-clause of a nested class of the friend class. However, the declarations of members of classes nested within the friend class cannot access the names of private and protected members from the class granting friendship. Also, because the base-clause of the friend class is not part of its member declarations, the base-clause of the friend class cannot access the names of the private and protected members from the class granting friendship. For example,
    class A {
        class B { };
        friend class X;
    };
    class X : A::B {     // ill-formed: A::B cannot be accessed
                         // in the base-clause for X
        A::B mx;         // OK: A::B used to declare member of X
        class Y: A::B {  // OK: A::B used to declare member of X
            A::B my;     // ill-formed: A::B cannot be accessed
                         // to declare members of nested class of X
        };
    };
end note]

This seems to be an oversight. The proposed change to 11.8 [class.access.nest] paragraph 1 would appear to have eliminated the restrictions on nested class access. However, at least one compiler (gcc 3.4.3) doesn't appear to take my view, and continues with the restrictions on access by classes within a friend class, while implementing the rest of the resolution of issue 45.

Note (March, 2005):

Andreas Hommel: I think issue 45 requires an additional change in 9.7 [class.nest] paragraph 4:

Like a member function, a friend function (11.4 [class.friend]) defined within a nested class is in the lexical scope of that class; it obeys the same rules for name binding as a static member function of that class (9.4 [class.static]) and has no special access rights to members of an enclosing class.

I believe the “no special access rights” language should be removed.

Proposed resolution (October, 2005):

This issue is resolved by the resolution of issue 372.




9. Clarification of access to base class members

Section: 11.2  [class.access.base]     Status: CD1     Submitter: unknown     Date: unknown

[Moved to DR at 4/01 meeting.]

11.2 [class.access.base] paragraph 4 says:

A base class is said to be accessible if an invented public member of the base class is accessible. If a base class is accessible, one can implicitly convert a pointer to a derived class to a pointer to that base class.
Given the above, is the following well-formed?
    class D;
     
    class B
    {
     protected:
       int b1;
 
       friend void foo( D* pd );
    };
     
    class D : protected B { };
     
    void foo( D* pd )
    {
       if ( pd->b1 > 0 ); // Is 'b1' accessible?
    }
Can you access the protected member b1 of B in foo? Can you convert a D* to a B* in foo?

1st interpretation:

A public member of B is accessible within foo (since foo is a friend), therefore foo can refer to b1 and convert a D* to a B*.

2nd interpretation:

B is a protected base class of D, and a public member of B is a protected member of D and can only be accessed within members of D and friends of D. Therefore foo cannot refer to b1 and cannot convert a D* to a B*.

(See J16/99-0042 = WG21 N1218.)

Proposed Resolution (04/01):

  1. Add preceding 11.2 [class.access.base] paragraph 4:
    A base class B of N is accessible at R, if
    • an invented public member of B would be a public member of N, or
    • R occurs in a member or friend of class N, and an invented public member of B would be a private or protected member of N, or
    • R occurs in a member or friend of a class P derived from N, and an invented public member of B would be a private or protected member of P, or
    • there exists a class S such that B is a base class of S accessible at R and S is a base class of N accessible at R. [Example:
          class B {
          public:
              int m;
          };
      
          class S: private B {
              friend class N;
          };
      
          class N: private S {
              void f() {
      	    B* p = this;  // OK because class S satisfies the
      			// fourth condition above: B is a base
      			// class of N accessible in f() because
      			// B is an accessible base class of S
      			// and S is an accessible base class of N.
              }
          };
      
      end example]
  2. Delete the first sentence of 11.2 [class.access.base] paragraph 4:
    A base class is said to be accessible if an invented public member of the base class is accessible.
  3. Replace the last sentence ("A member m is accessible...") by the following:
    A member m is accessible at the point R when named in class N if
    • m as a member of N is public, or
    • m as a member of N is private, and R occurs in a member or friend of class N, or
    • m as a member of N is protected, and R occurs in a member or friend of class N, or in a member or friend of a class P derived from N, where m as a member of P is private or protected, or
    • there exists a base class B of N that is accessible at R, and m is accessible at R when named in class B. [Example:...

The resolution for issue 207 modifies this wording slightly.




16. Access to members of indirect private base classes

Section: 11.2  [class.access.base]     Status: CD1     Submitter: unknown     Date: unknown

[Moved to DR at 4/01 meeting.]

The text in 11.2 [class.access.base] paragraph 4 does not seem to handle the following cases:

    class D;
     
    class B {
    private:
        int i;
        friend class D;
    };
     
    class C : private B { };
     
    class D : private C {
        void f() {
            B::i; //1: well-formed?
            i;    //2: well-formed?
        }
    };
The member i is not a member of D and cannot be accessed in the scope of D. What is the naming class of the member i on line //1 and line //2?

Proposed Resolution (04/01): The resolution for this issue is contained in the resolution for issue 9..




207. using-declarations and protected access

Section: 11.2  [class.access.base]     Status: CD1     Submitter: Jason Merrill     Date: 28 Feb 2000

[Moved to DR at 10/01 meeting.]

Consider the following example:

  class A {
  protected:
    static void f() {};
  };

  class B : A {
  public:
    using A::f;
    void g() {
      A::f();
    }
  };

The standard says in 11.2 [class.access.base] paragraph 4 that the call to A::f is ill-formed:

A member m is accessible when named in class N if

Here, m is A::f and N is A.

It seems clear to me that the third bullet should say "public, private or protected".

Steve Adamczyk:The words were written before using-declarations existed, and therefore didn't anticipate this case.

Proposed resolution (04/01):

Modify the third bullet of the third change ("A member m is accessible...") in the resolution of issue 9 to read "public, private, or protected" instead of "private or protected."




77. The definition of friend does not allow nested classes to be friends

Section: 11.4  [class.friend]     Status: CD1     Submitter: Judy Ward     Date: 15 Dec 1998

[Moved to DR at 4/02 meeting.]

The definition of "friend" in 11.4 [class.friend] says:

A friend of a class is a function or class that is not a member of the class but is permitted to use the private and protected member names from the class. ...
A nested class, i.e. INNER in the example below, is a member of class OUTER. The sentence above states that it cannot be a friend. I think this is a mistake.
    class OUTER {
        class INNER;
        friend class INNER;
        class INNER {};
    };

Proposed resolution (04/01):

Change the first sentence of 11.4 [class.friend] as follows:

A friend of a class is a function or class that is not a member of the class but is allowed given permission to use the private and protected member names from the class. The name of a friend is not in the scope of the class, and the friend is not called with the member access operators (5.2.5 [expr.ref]) unless it is a member of another class. A class specifies its friends, if any, by way of friend declarations. Such declarations give special access rights to the friends, but they do not make the nominated friends members of the befriending class.



500. Access in base-specifiers of friend and nested classes

Section: 11.4  [class.friend]     Status: CD1     Submitter: Andreas Hommel     Date: 25 Jan 2005

[Voted into WP at the October, 2006 meeting.]

I don't know the reason for this distinction, but it seems to be surprising that Base::A is legal and D is illegal in this example:

    class D;
    class Base 
    { 
        class A;
        class B;
        friend class D;
    }; 
    class Base::B 
    {
    };
    class Base::A : public Base::B  // OK because of issue 45
    { 
    };
    class D : public Base::B        // illegal because of 11.4p4
    { 
    };

Shouldn't this be consistent (either way)?

Notes from the April, 2005 meeting:

In discussing issue 372, the CWG decided that access in the base-specifiers of a class should be the same as for its members, and that resolution will apply to friend declarations, as well.

Proposed resolution (October, 2005):

This issue is resolved by the resolution of issue 372.




385. How does protected member check of 11.5 interact with using-declarations?

Section: 11.5  [class.protected]     Status: CD1     Submitter: Vincent Korstanje     Date: 24 Sep 2002

[Voted into WP at October 2004 meeting.]

We consider it not unreasonable to do the following

  class A { 
    protected: 
    void g();
  }; 
  class B : public A { 
    public: 
      using A::g; // B::g is a public synonym for A::g 
  }; 

  class C: public A {
    void foo();
  };

  void C::foo() { 
    B b; 
    b.g(); 
  } 

However the EDG front-end does not like and gives the error

  #410-D: protected function "A::g" is not accessible through a "B" pointer or  object 
    b.g();
      ^

Steve Adamczyk: The error in this case is due to 11.5 [class.protected] of the standard, which is an additional check on top of the other access checking. When that section says "a protected nonstatic member function ... of a base class" it doesn't indicate whether the fact that there is a using-declaration is relevant. I'd say the current wording taken at face value would suggest that the error is correct -- the function is protected, even if the using-declaration for it makes it accessible as a public function. But I'm quite sure the wording in 11.5 [class.protected] was written before using-declarations were invented and has not been reviewed since for consistency with that addition.

Notes from April 2003 meeting:

We agreed that the example should be allowed.

Proposed resolution (April 2003, revised October 2003):

Change 11.5 [class.protected] paragraph 1 from

When a friend or a member function of a derived class references a protected nonstatic member function or protected nonstatic data member of a base class, an access check applies in addition to those described earlier in clause 11 [class.access]. [Footnote: This additional check does not apply to other members, e.g. static data members or enumerator member constants.] Except when forming a pointer to member (5.3.1 [expr.unary.op]), the access must be through a pointer to, reference to, or object of the derived class itself (or any class derived from that class (5.2.5 [expr.ref]). If the access is to form a pointer to member, the nested-name-specifier shall name the derived class (or any class derived from that class).

to

An additional access check beyond those described earlier in clause 11 [class.access] is applied when a nonstatic data member or nonstatic member function is a protected member of its naming class (11.2 [class.access.base]). [Footnote: This additional check does not apply to other members, e.g., static data members or enumerator member constants.] As described earlier, access to a protected member is granted because the reference occurs in a friend or member of some class C. If the access is to form a pointer to member (5.3.1 [expr.unary.op]), the nested-name-specifier shall name C or a class derived from C. All other accesses involve a (possibly implicit) object expression (5.2.5 [expr.ref]). In this case, the class of the object expression shall be C or a class derived from C.

Additional discussion (September, 2004):

Steve Adamczyk: I wonder if this wording is incorrect. Consider:

    class A {
    public:
      int p;
    };
    class B : protected A {
      // p is a protected member of B
    };
    class C : public B {
      friend void fr();
    };
    void fr() {
      B *pb = new B;
      pb->p = 1;  // Access okay?  Naming class is B, p is a protected member of B,
                  // the "C" of the issue 385 wording is C, but access is not via
                  // an object of type C or a derived class thereof.
    }

I think the formulation that the member is a protected member of its naming class is not what we want. I think we intended that the member is protected in the declaration that is found, where the declaration found might be a using-declaration.

Mike Miller: I think the proposed wording makes the access pb->p ill-formed, and I think that's the right thing to do.

First, protected inheritance of A by B means that B intends the public and protected members of A to be part of B's implementation, available to B's descendants only. (That's why there's a restriction on converting from B* to A*, to enforce B's intention on the use of members of A.) Consequently, I see no difference in access policy between your example and

    class B {
    protected:
        int p;
    };

Second, the reason we have this rule is that C's use of inherited protected members might be different from their use in a sibling class, say D. Thus members and friends of C can only use B::p in a manner consistent with C's usage, i.e., in C or derived-from-C objects. If we rewrote your example slightly,

    class D: public B { };

    void fr(B* pb) {
        pb->p = 1;
    }

    void g() {
        fr(new D);
    }

it's clear that the intent of this rule is broken — fr would be accessing B::p assuming C's policies when the object in question actually required D's policies.

(See also issues 471 and 472.)




10. Can a nested class access its own class name as a qualified name if it is a private member of the enclosing class?

Section: 11.8  [class.access.nest]     Status: CD1     Submitter: Josee Lajoie     Date: unknown

[Moved to DR at 4/01 meeting.]

Paragraph 1 says: "The members of a nested class have no special access to members of an enclosing class..."

This prevents a member of a nested class from being defined outside of its class definition. i.e. Should the following be well-formed?

    class D {
        class E {
            static E* m;
        };
    };
     
    D::E* D::E::m = 1; // ill-formed
This is because the nested class does not have access to the member E in D. 11 [class.access] paragraph 5 says that access to D::E is checked with member access to class E, but unfortunately that doesn't give access to D::E. 11 [class.access] paragraph 6 covers the access for D::E::m, but it doesn't affect the D::E access. Are there any implementations that are standard compliant that support this?

Here is another example:

    class C {
        class B
        {
            C::B *t; //2 error, C::B is inaccessible
        };
    };
This causes trouble for member functions declared outside of the class member list. For example:
    class C {
        class B
        {
            B& operator= (const B&);
        };
    };
     
    C::B& C::B::operator= (const B&) { } //3
If the return type (i.e. C::B) is access checked in the scope of class B (as implied by 11 [class.access] paragraph 5) as a qualified name, then the return type is an error just like referring to C::B in the member list of class B above (i.e. //2) is ill-formed.

Proposed resolution (04/01):

The resolution for this issue is incorporated into the resolution for issue 45.




45. Access to nested classes

Section: 11.8  [class.access.nest]     Status: CD1     Submitter: Daveed Vandevoorde     Date: 29 Sep 1998

[Moved to DR at 4/01 meeting.]

Example:

    #include <iostream.h>
    
    class C {  // entire body is private
        struct Parent {
            Parent() { cout << "C::Parent::Parent()\n"; }
        };
    
        struct Derived : Parent {
            Derived() { cout << "C::Derived::Derived()\n"; }
        };
    
        Derived d;
    };
    
    
    int main() {
        C c;      //  Prints message from both nested classes
        return 0;
    }
How legal/illegal is this? Paragraphs that seem to apply here are:

11 [class.access] paragraph 1:

A member of a class can be
and 11.8 [class.access.nest] paragraph 1:
The members of a nested class have no special access to members of an enclosing class, nor to classes or functions that have granted friendship to an enclosing class; the usual access rules (clause 11 [class.access] ) shall be obeyed. [...]
This makes me think that the ': Parent' part is OK by itself, but that the implicit call of 'Parent::Parent()' by 'Derived::Derived()' is not.

From Mike Miller:

I think it is completely legal, by the reasoning given in the (non-normative) 11.8 [class.access.nest] paragraph 2. The use of a private nested class as a base of another nested class is explicitly declared to be acceptable there. I think the rationale in the comments in the example ("// OK because of injection of name A in A") presupposes that public members of the base class will be public members in a (publicly-derived) derived class, regardless of the access of the base class, so the constructor invocation should be okay as well.

I can't find anything normative that explicitly says that, though.

(See also papers J16/99-0009 = WG21 N1186, J16/00-0031 = WG21 N1254, and J16/00-0045 = WG21 N1268.)

Proposed Resolution (04/01):

  1. Insert the following as a new paragraph following 11 [class.access] paragraph 1:

    A member of a class can also access all names as the class of which it is a member. A local class of a member function may access the same names that the member function itself may access. [Footnote: Access permissions are thus transitive and cumulative to nested and local classes.]
  2. Delete 11 [class.access] paragraph 6.

  3. In 11.8 [class.access.nest] paragraph 1, change

    The members of a nested class have no special access to members of an enclosing class, nor to classes or functions that have granted friendship to an enclosing class; the usual access rules (clause 11 [class.access]) shall be obeyed.

    to

    A nested class is a member and as such has the same access rights as any other member.

    Change

        B b;       // error: E::B is private
    

    to

        B b;       // Okay, E::I can access E::B
    

    Change

        p->x = i;      // error: E::x is private
    

    to

        p->x = i;      // Okay, E::I can access E::x
    
  4. Delete 11.8 [class.access.nest] paragraph 2.

(This resolution also resolves issues 8 and 10.




263. Can a constructor be declared a friend?

Section: 12.1  [class.ctor]     Status: CD1     Submitter: Martin Sebor     Date: 13 Nov 2000

[Voted into WP at April 2003 meeting.]

According to 12.1 [class.ctor] paragraph 1, a declaration of a constructor has a special limited syntax, in which only function-specifiers are allowed. A friend specifier is not a function-specifier, so one interpretation is that a constructor cannot be declared in a friend declaration.

(It should also be noted, however, that neither friend nor function-specifier is part of the declarator syntax, so it's not clear that anything conclusive can be derived from the wording of 12.1 [class.ctor].)

Notes from 04/01 meeting:

The consensus of the core language working group was that it should be permitted to declare constructors as friends.

Proposed Resolution (revised October 2002):

Change paragraph 1a in 3.4.3.1 [class.qual] (added by the resolution of issue 147) as follows:

If the nested-name-specifier nominates a class C, and the name specified after the nested-name-specifier, when looked up in C, is the injected-class-name of C (clause 9 [class]), the name is instead considered to name the constructor of class C. Such a constructor name shall be used only in the declarator-id of a constructor definition declaration that appears outside of the class definition names a constructor....

Note: the above does not allow qualified names to be used for in-class declarations; see 8.3 [dcl.meaning] paragraph 1. Also note that issue 318 updates the same paragraph.

Change the example in 11.4 [class.friend], paragraph 4 as follows:

class Y {
  friend char* X::foo(int);
  friend X::X(char);   // constructors can be friends
  friend X::~X();      // destructors can be friends
  //...
};



326. Wording for definition of trivial constructor

Section: 12.1  [class.ctor]     Status: CD1     Submitter: James Kanze     Date: 9 Dec 2001

[Voted into WP at October 2003 meeting.]

In 12.1 [class.ctor] paragraph 5, the standard says "A constructor is trivial if [...]", and goes on to define a trivial default constructor. Taken literally, this would mean that a copy constructor can't be trivial (contrary to 12.8 [class.copy] paragraph 6). I suggest changing this to "A default constructor is trivial if [...]". (I think the change is purely editorial.)

Proposed Resolution (revised October 2002):

Change 12.1 [class.ctor] paragraph 5-6 as follows:

A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared user-declared constructor for class X, a default constructor is implicitly declared. An implicitly-declared implicitly-declared default constructor is an inline public member of its class. A default constructor is trivial if it is an implicitly-declared default constructor and if:

Otherwise, the default constructor is non-trivial.

Change 12.4 [class.dtor] paragraphs 3-4 as follows (the main changes are removing italics):

If a class has no user-declared user-declared destructor, a destructor is declared implicitly. An implicitly-declared implicitly-declared destructor is an inline public member of its class. A destructor is trivial if it is an implicitly-declared destructor and if:

Otherwise, the destructor is non-trivial non-trivial.

In 9.5 [class.union] paragraph 1, change "trivial constructor" to "trivial default constructor".

In 12.2 [class.temporary] paragraph 3, add to the reference to 12.1 [class.ctor] a second reference, to 12.8 [class.copy].




331. Allowed copy constructor signatures

Section: 12.1  [class.ctor]     Status: CD1     Submitter: Richard Smith     Date: 8 Jan 2002

[Voted into WP at October 2003 meeting.]

12.1 [class.ctor] paragraph 10 states

A copy constructor for a class X is a constructor with a first parameter of type X & or of type const X &. [Note: see 12.8 [class.copy] for more information on copy constructors.]

No mention is made of constructors with first parameters of types volatile X & or const volatile X &. This statement seems to be in contradiction with 12.8 [class.copy] paragraph 2 which states

A non-template constructor for class X is a copy constructor if its first parameter is of type X &, const X &, volatile X & or const volatile X &, ...

12.8 [class.copy] paragraph 5 also mentions the volatile versions of the copy constructor, and the comparable paragraphs for copy assignment (12.8 [class.copy] paragraphs 9 and 10) all allow volatile versions, so it seems that 12.1 [class.ctor] is at fault.

Proposed resolution (October 2002):

Change 12.1 [class.ctor] paragraph 10 from

A copy constructor for a class X is a constructor with a first parameter of type X& or of type const X&. [Note: see 12.8 [class.copy] for more information on copy constructors. ]
to (note that the dropping of italics is intentional):
A copy constructor (12.8 [class.copy]) is used to copy objects of class type.




86. Lifetime of temporaries in query expressions

Section: 12.2  [class.temporary]     Status: CD1     Submitter: Steve Adamczyk     Date: Jan 1999

[Voted into WP at April, 2006 meeting.]

In 12.2 [class.temporary] paragraph 5, should binding a reference to the result of a "?" operation, each of whose branches is a temporary, extend both temporaries?

Here's an example:

    const SFileName &C = noDir ? SFileName("abc") : SFileName("bcd");

Do the temporaries created by the SFileName conversions survive the end of the full expression?

Notes from 10/00 meeting:

Other problematic examples include cases where the temporary from one branch is a base class of the temporary from the other (i.e., where the implementation must remember which type of temporary must be destroyed), or where one branch is a temporary and the other is not. Similar questions also apply to the comma operator. The sense of the core language working group was that implementations should be required to support these kinds of code.

Notes from the March 2004 meeting:

We decided that the cleanest model is one in which any "?" operation that returns a class rvalue always copies one of its operands to a temporary and returns the temporary as the result of the operation. (Note that this may involve slicing.) An implementation would be free to optimize this using the rules in 12.8 [class.copy] paragraph 15, and in fact we would expect that in many cases compilers would do such optimizations. For example, the compiler could construct both rvalues in the above example into a single temporary, and thus avoid a copy.

See also issue 446.

Proposed resolution (October, 2004):

This issue is resolved by the resolutions of issue 446.

Note (October, 2005):

This issue was overlooked when issue 446 was moved to “ready” status and was thus inadvertently omitted from the list of issues accepted as Defect Reports at the October, 2005 meeting.




124. Lifetime of temporaries in default initialization of class arrays

Section: 12.2  [class.temporary]     Status: CD1     Submitter: Jack Rouse     Date: 3 June 1999

[Moved to DR at 4/01 meeting.]

Jack Rouse: 12.2 [class.temporary] states that temporary objects will normally be destroyed at the end of the full expression in which they are created. This can create some unique code generation requirements when initializing a class array with a default constructor that uses a default argument. Consider the code:

    struct T {
       int i;
       T( int );
       ~T();
    };

    struct S {
       S( int = T(0).i );
       ~S();
    };

    S* f( int n )
    {
       return new S[n];
    }
The full expression allocating the array in f(int) includes the default constructor for S. Therefore according to 1.9 [intro.execution] paragraph 14, it includes the default argument expression for S(int). So evaluation of the full expression should include evaluating the default argument "n" times and creating "n" temporaries of type T. But the destruction of the temporaries must be delayed until the end of the full expression so this requires allocating space at runtime for "n" distinct temporaries. It is unclear how these temporaries are supposed to be allocated and deallocated. They cannot readily be autos because a variable allocation is required.

I believe that many existing implementations will destroy the temporaries needed by the default constructor after each array element is initialized. But I can't find anything in the standard that allows the temporaries to be destroyed early in this case.

I think the standard should allow the early destruction of temporaries used in the default initialization of class array elements. I believe early destruction is the status quo, and I don't think the users of existing C++ compilers have been adversely impacted by it.

Proposed resolution (04/01):

The proposed resolution is contained in the proposal for issue 201.




199. Order of destruction of temporaries

Section: 12.2  [class.temporary]     Status: CD1     Submitter: Alan Nash     Date: 27 Jan 2000

[Voted into the WP at the April, 2007 meeting as part of paper J16/07-0099 = WG21 N2239.]

12.2 [class.temporary] paragraph 3 simply states the requirement that temporaries created during the evaluation of an expression

are destroyed as the last step in evaluating the full-expression (1.9) that (lexically) contains the point where they were created.
There is nothing said about the relative order in which these temporaries are destroyed.

Paragraph 5, dealing with temporaries bound to references, says

the temporaries created during the evaluation of the expression initializing the reference, except the temporary to which the reference is bound, are destroyed at the end of the full-expression in which they are created and in the reverse order of the completion of their construction.
Is this difference intentional? May temporaries in expressions other than those initializing references be deleted in non-LIFO order?

Notes from 04/00 meeting:

Steve Adamczyk expressed concern about constraining implementations that are capable of fine-grained parallelism -- they may be unable to determine the order of construction without adding undesirable overhead.

Proposed resolution (April, 2007):

As specified in paper J16/07-0099 = WG21 N2239.




201. Order of destruction of temporaries in initializers

Section: 12.2  [class.temporary]     Status: CD1     Submitter: Alan Nash     Date: 31 Jan 2000

[Moved to DR at 4/01 meeting.]

According to 12.2 [class.temporary] paragraph 4, an expression appearing as the initializer in an object definition constitutes a context "in which temporaries are destroyed at a different point than the end of the full-expression." It goes on to say that the temporary containing the value of the expression persists until after the initialization is complete (see also issue 117). This seems to presume that the end of the full-expression is a point earlier than the completion of the initialization.

However, according to 1.9 [intro.execution] paragraphs 12-13, the full-expression in such cases is, in fact, the entire initialization. If this is the case, the behavior described for temporaries in an initializer expression is simply the normal behavior of temporaries in any expression, and treating it as an exception to the general rule is both incorrect and confusing.

Proposed resolution (04/01):

[Note: this proposal also addresses issue 124.]

  1. Add to the end of 1.9 [intro.execution] paragraph 12:

    If the initializer for an object or sub-object is a full-expression, the initialization of the object or sub-object (e.g., by calling a constructor or copying an expression value) is considered to be part of the full-expression.
  2. Replace 12.2 [class.temporary] paragraph 4 with:

    There are two contexts in which temporaries are destroyed at a different point than the end of the full-expression. The first context is when a default constructor is called to initialize an element of an array. If the constructor has one or more default arguments, any temporaries created in the default argument expressions are destroyed immediately after return from the constructor.



320. Question on copy constructor elision example

Section: 12.2  [class.temporary]     Status: CD1     Submitter: Steve Clamage     Date: 2 Nov 2001

[Voted into WP at April 2005 meeting.]

Section 12.2 [class.temporary] paragraph 2, abridged:

  X f(X);
  void g()
  {
	X a;
	a = f(a);
  }

a=f(a) requires a temporary for either the argument a or the result of f(a) to avoid undesired aliasing of a.

The note seems to imply that an implementation is allowed to omit copying "a" to f's formal argument, or to omit using a temporary for the return value of f. I don't find that license in normative text.

Function f returns an X by value, and in the expression the value is assigned (not copy-constructed) to "a". I don't see how that temporary can be omitted. (See also 12.8 [class.copy] p 15)

Since "a" is an lvalue and not a temporary, I don't see how copying "a" to f's formal parameter can be avoided.

Am I missing something, or is 12.2 [class.temporary] p 2 misleading?

Proposed resolution (October, 2004):

In 12.2 [class.temporary] paragraph 2, change the last sentence as indicated:

On the other hand, the expression a=f(a) requires a temporary for either the argument a or the result of f(a) to avoid undesired aliasing of a the result of f(a), which is then assigned to a.



392. Use of full expression lvalue before temporary destruction

Section: 12.2  [class.temporary]     Status: CD1     Submitter: Stephen Clamage     Date: 21 Nov 2002

[Voted into WP at March 2004 meeting.]

class C {
public:
    C();
    ~C();
    int& get() { return p; } // reference return
private:
    int p;
};

int main ()
{
    if ( C().get() ) // OK?
}

Section 12.2 [class.temporary] paragraph 3 says a temp is destroyed as the last step in evaluating the full expression. But the expression C().get() has a reference type. Does 12.2 [class.temporary] paragraph 3 require that the dereference to get a boolean result occur before the destructor runs, making the code valid? Or does the code have undefined behavior?

Bill Gibbons: It has undefined behavior, though clearly this wasn't intended. The lvalue-to-rvalue conversion that occurs in the "if" statement is not currently part of the full-expression.

From section 12.2 [class.temporary] paragraph 3:

Temporary objects are destroyed as the last step in evaluating the full-expression (1.9 [intro.execution]) that (lexically) contains the point where they were created.

From section 1.9 [intro.execution] paragraph 12:

A full-expression is an expression that is not a subexpression of another expression. If a language construct is defined to produce an implicit call of a function, a use of the language construct is considered to be an expression for the purposes of this definition.

The note in section 1.9 [intro.execution] paragraph 12 goes on to explain that this covers expressions used as initializers, but it does not discuss lvalues within temporaries.

It is a small point but it is probably worth correcting 1.9 [intro.execution] paragraph 12. Instead of the "implicit call of a function" wording, it might be better to just say that a full-expression includes any implicit use of the expression value in the enclosing language construct, and include a note giving implicit calls and lvalue-to-rvalue conversions as examples.

Offhand the places where this matters include: initialization (including member initializers), selection statements, iteration statements, return, throw

Proposed resolution (April 2003):

Change 1.9 [intro.execution] paragraph 12-13 to read:

A full-expression is an expression that is not a subexpression of another expression. If a language construct is defined to produce an implicit call of a function, a use of the language construct is considered to be an expression for the purposes of this definition. Conversions applied to the result of an expression in order to satisfy the requirements of the language construct in which the expression appears are also considered to be part of the full-expression.

[Note: certain contexts in C++ cause the evaluation of a full-expression that results from a syntactic construct other than expression (5.18 [expr.comma]). For example, in 8.5 [dcl.init] one syntax for initializer is

but the resulting construct is a function call upon a constructor function with expression-list as an argument list; such a function call is a full-expression. For example, in 8.5 [dcl.init], another syntax for initializer is but again the resulting construct might be a function call upon a constructor function with one assignment-expression as an argument; again, the function call is a full-expression. ] [Example:

  struct S {
      S(int i): I(i) { }
      int& v() { return I; }
    private:
      int I;
  };

  S s1(1);           // full-expression is call of S::S(int)
  S s2 = 2;          // full-expression is call of S::S(int)

  void f() {
      if (S(3).v())  // full-expression includes lvalue-to-rvalue and
                     // int to bool conversions, performed before
                     // temporary is deleted at end of full-expression
      { }
  }
end example]



443. Wording nit in description of lifetime of temporaries

Section: 12.2  [class.temporary]     Status: CD1     Submitter: Matthias Hofmann     Date: 2 Dec 2003

[Voted into WP at April 2005 meeting.]

There seems to be a typo in 12.2 [class.temporary]/5, which says "The temporary to which the reference is bound or the temporary that is the complete object TO a subobject OF which the TEMPORARY is bound persists for the lifetime of the reference except as specified below."

I think this should be "The temporary to which the reference is bound or the temporary that is the complete object OF a subobject TO which the REFERENCE is bound persists for the lifetime of the reference except as specified below."

I used upper-case letters for the parts I think need to be changed.

Proposed resolution (October, 2004):

Change 12.2 [class.temporary] paragraph 5 as indicated:

The temporary to which the reference is bound or the temporary that is the complete object to of a subobject of to which the temporary reference is bound persists for the lifetime of the reference except as specified below.



464. Wording nit on lifetime of temporaries to which references are bound

Section: 12.2  [class.temporary]     Status: CD1     Submitter: Allan Odgaard     Date: 21 Feb 2004

[Voted into WP at April, 2006 meeting.]

Section 12.2 [class.temporary] paragraph 5 ends with this "rule":

[...] if obj2 is an object with static or automatic storage duration created after the temporary is created, the temporary shall be destroyed after obj2 is destroyed.

For the temporary to be destroyed after obj2 is destroyed, when obj2 has static storage, I would say that the reference to the temporary should also have static storage, but that is IMHO not clear from the paragraph.

Example:

    void f ()
    {
       const T1& ref = T1();
       static T2 obj2;
       ...
    }

Here the temporary would be destoyed before obj2, contrary to the rule above.

Steve Adamczyk: I agree there's a minor issue here. I think the clause quoted above meant for obj1 and obj2 to have the same storage duration. Replacing "obj2 is an object with static or automatic storage duration" by "obj2 is an object with the same storage duration as obj1" would, I believe, fix the problem.

Notes from October 2004 meeting:

We agreed with Steve Adamczyk's suggestion.

Proposed resolution (October, 2005):

Change 12.2 [class.temporary] paragraph 5 as follows:

... In addition, the destruction of temporaries bound to references shall take into account the ordering of destruction of objects with static or automatic storage duration (3.7.1 [basic.stc.static], 3.7.3 [basic.stc.auto]); that is, if obj1 is an object with static or automatic storage duration created before the temporary is created with the same storage duration as the temporary, the temporary shall be destroyed before obj1 is destroyed; if obj2 is an object with static or automatic storage duration created after the temporary is created with the same storage duration as the temporary, the temporary shall be destroyed after obj2 is destroyed...



296. Can conversion functions be static?

Section: 12.3.2  [class.conv.fct]     Status: CD1     Submitter: Scott Meyers     Date: 5 Jul 2001

[Moved to DR at October 2002 meeting.]

May user-defined conversion functions be static? That is, should this compile?

    class Widget {
    public:
      static operator bool() { return true; }
    };

All my compilers hate it. I hate it, too. However, I don't see anything in 12.3.2 [class.conv.fct] that makes it illegal. Is this a prohibition that arises from the grammar, i.e., the grammar doesn't allow "static" to be followed by a conversion-function-id in a member function declaration? Or am I just overlooking something obvious that forbids static conversion functions?

Proposed Resolution (4/02):

Add to 12.3.2 [class.conv.fct] as a new paragraph 7:

Conversion functions cannot be declared static.



244. Destructor lookup

Section: 12.4  [class.dtor]     Status: CD1     Submitter: John Spicer     Date: 6 Sep 2000

[Moved to DR at October 2002 meeting.]

12.4 [class.dtor] contains this example:

    struct B {
        virtual ~B() { }
    };
    struct D : B {
        ~D() { }
    };

    D D_object;
    typedef B B_alias;
    B* B_ptr = &D_object;

    void f() {
        D_object.B::~B();               // calls B's destructor
        B_ptr->~B();                    // calls D's destructor
        B_ptr->~B_alias();              // calls D's destructor
        B_ptr->B_alias::~B();           // calls B's destructor
        B_ptr->B_alias::~B_alias();     // error, no B_alias in class B
    }

On the other hand, 3.4.3 [basic.lookup.qual] contains this example:

    struct C {
        typedef int I;
    };
    typedef int I1, I2;
    extern int* p;
    extern int* q;
    p->C::I::~I();       // I is looked up in the scope of C
    q->I1::~I2();        // I2 is looked up in the scope of
                         // the postfix-expression
    struct A {
        ~A();
    };
    typedef A AB;
    int main()
    {
        AB *p;
        p->AB::~AB();    // explicitly calls the destructor for A
    }

Note that

     B_ptr->B_alias::~B_alias();

is claimed to be an error, while the equivalent

     p->AB::~AB();

is claimed to be well-formed.

I believe that clause 3 is correct and that clause 12 is in error. We worked hard to get the destructor lookup rules in clause 3 to be right, and I think we failed to notice that a change was also needed in clause 12.

Mike Miller:

Unfortunately, I don't believe 3.4.3 [basic.lookup.qual] covers the case of p->AB::~AB(). It's clearly intended to do so, as evidenced by 3.4.3.1 [class.qual] paragraph 1 ("a destructor name is looked up as specified in 3.4.3 [basic.lookup.qual]"), but I don't think the language there does so.

The relevant paragraph is 3.4.3 [basic.lookup.qual] paragraph 5. (None of the other paragraphs in that section deal with this topic at all.) It has two parts. The first is

If a pseudo-destructor-name (5.2.4 [expr.pseudo]) contains a nested-name-specifier, the type-names are looked up as types in the scope designated by the nested-name-specifier.

This sentence doesn't apply, because ~AB isn't a pseudo-destructor-name. 5.2.4 [expr.pseudo] makes clear that this syntactic production (5.2 [expr.post] paragraph 1) only applies to cases where the type-name is not a class-name. p->AB::~AB is covered by the production using id-expression.

The second part of 3.4.3 [basic.lookup.qual] paragraph 5 says

In a qualified-id of the form:

    ::opt nested-name-specifier ~ class-name

where the nested-name-specifier designates a namespace name, and in a qualified-id of the form:

    ::opt nested-name-specifier class-name :: ~ class-name

the class-names are looked up as types in the scope designated by the nested-name-specifier.

This wording doesn't apply, either. The first one doesn't because the nested-name-specifier is a class-name, not a namespace name. The second doesn't because there's only one layer of qualification.

As far as I can tell, there's no normative text that specifies how the ~AB is looked up in p->AB::~AB(). 3.4.3.1 [class.qual], where all the other class member qualified lookups are handled, defers to 3.4.3 [basic.lookup.qual], and 3.4.3 [basic.lookup.qual] doesn't cover the case.

See also issue 305.

Jason Merrill: My thoughts on the subject were that the name we use in a destructor call is really meaningless; as soon as we see the ~ we know what the user means, all we're doing from that point is testing their ability to name the destructor in a conformant way. I think that everyone will agree that

  anything::B::~B()
should be well-formed, regardless of the origins of the name "B". I believe that the rule about looking up the second "B" in the same context as the first was intended to provide this behavior, but to me this seems much more heavyweight than necessary. We don't need a whole new type of lookup to be able to use the same name before and after the ~; we can just say that if the two names match, the call is well-formed. This is significantly simpler to express, both in the standard and in an implementation.

Anyone writing two different names here is either deliberately writing obfuscated code, trying to call the destructor of a nested class, or fighting an ornery compiler (i.e. one that still wants to see B_alias::~B()). I think we can ignore the first case. The third would be handled by reverting to the old rule (look up the name after ~ in the normal way) with the lexical matching exception described above -- or we could decide to break such code, do no lookup at all, and only accept a matching name. In a good implementation, the second should probably get an error message telling them to write Outer::Inner::~Inner instead.

We discussed this at the meetings, but I don't remember if we came to any sort of consensus on a direction. I see three options:

  1. Stick with the status quo, i.e. the special lookup rule such that if the name before ::~ is a class name, the name after ::~ is looked up in the same scope as the previous one. If we choose this option, we just need better wording that actually expresses this, as suggested in the issue list. This option breaks old B_alias::~B code where B_alias is declared in a different scope from B.
  2. Revert to the old rules, whereby the name after ::~ is looked up just like a name after ::, with the exception that if it matches the name before ::~ then it is considered to name the same class. This option supports old code and code that writes B_alias::~B_alias. It does not support the q->I1::~I2 usage of 3.4.3 [basic.lookup.qual], but that seems like deliberate obfuscation. This option is simpler to implement than #1.
  3. Do no lookup for a name after ::~; it must match the name before. This breaks old code as #1, but supports the most important case where the names match. This option may be slightly simpler to implement than #2. It is certainly easier to teach.

My order of preference is 2, 3, 1.

Incidentally, it seems to me oddly inconsistent to allow Namespace::~Class, but not Outer::~Inner. Prohibiting the latter makes sense from the standpoint of avoiding ambiguity, but what was the rationale for allowing the former?

John Spicer: I agree that allowing Namespace::~Class is odd. I'm not sure where this came from. If we eliminated that special case, then I believe the #1 rule would just be that in A::B1::~B2 you look up B1 and B2 in the same place in all cases.

I don't like #2. I don't think the "old" rules represent a deliberate design choice, just an error in the way the lookup was described. The usage that rule permits p->X::~Y (where Y is a typedef to X defined in X), but I doubt people really do that. In other words, I think that #1 a more useful special case than #2 does, not that I think either special case is very important.

One problem with the name matching rule is handling cases like:

  A<int> *aip;

  aip->A<int>::~A<int>();  // should work
  aip->A<int>::~A<char>(); // should not
I would favor #1, while eliminating the special case of Namespace::~Class.

Proposed resolution (10/01):

Replace the normative text of 3.4.3 [basic.lookup.qual] paragraph 5 after the first sentence with:

Similarly, in a qualified-id of the form:

    ::opt nested-name-specifieropt class-name :: ~ class-name
the second class-name is looked up in the same scope as the first.

In 12.4 [class.dtor] paragraph 12, change the example to

D D_object;
typedef B B_alias;
B* B_ptr = &D_object;

void f() {
  D_object.B::~B();                //  calls  B's destructor
  B_ptr->~B();                    //  calls  D