| Document number: | PL22.16/12-0020 = WG21 N3330 |
| Date: | 2012-01-17 |
| Project: | Programming Language C++ |
| Reference: | ISO/IEC IS 14882:2003 |
| Reply to: | William M. Miller |
| Edison Design Group, Inc. | |
| wmm@edg.com |
This document contains the C++ core language issues on which the Committee (J16 + WG21) has not yet acted, that is, issues with status "Ready," "Tentatively Ready," "Review," "Drafting," and "Open."
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:
Section references in this document reflect the section numbering of document WG21 N3290.
The purpose of these documents is to record the disposition of issues that have come before the Core Language Working Group of the ANSI (INCITS PL22.16) and ISO (WG21) C++ Standard Committee.
Some issues represent potential defects in the ISO/IEC IS 14882:2003 document and corrected defects in the earlier ISO/IEC 14882:1998 document; others refer to text in the working draft for the next revision of the C++ language, informally known as C++0x, and not to any Standard text. Issues are not necessarily formal ISO Defect Reports (DRs). While some issues will eventually be elevated to DR status, others will be disposed of in other ways. (See Issue Status below.)
The most current public version of this document can be found at http://www.open-std.org/jtc1/sc22/wg21. Requests for further information about these documents should include the document number, reference ISO/IEC 14882:2003, and be submitted to the InterNational Committee for Information Technology Standards (INCITS), 1250 Eye Street NW, Suite 200, Washington, DC 20005, USA.
Information regarding how to obtain a copy of the C++ Standard, join the Standard Committee, or submit an issue can be found in the C++ FAQ at http://www.comeaucomputing.com/csc/faq.html. Public discussion of the C++ Standard and related issues occurs on newsgroup comp.std.c++.
Issues progress through various statuses as the Core Language Working Group and, ultimately, the full PL22.16 and WG21 committees deliberate and act. For ease of reference, issues are grouped in these documents by their status. Issues have one of the following statuses:
Open: The issue is new or the working group has not yet formed an opinion on the issue. If a Suggested Resolution is given, it reflects the opinion of the issue's submitter, not necessarily that of the working group or the Committee as a whole.
Drafting: Informal consensus has been reached in the working group and is described in rough terms in a Tentative Resolution, although precise wording for the change is not yet available.
Review: Exact wording of a Proposed Resolution is now available for an issue on which the working group previously reached informal consensus.
Ready: The working group has reached consensus that the issue is a defect in the Standard, the Proposed Resolution is correct, and the issue is ready to forward to the full Committee for ratification as a proposed defect report.
Tentatively Ready: Like "ready" except that the resolution was produced and approved by a subset of the working group membership between meetings. Persons not participating in these betwee-meeting activities are encouraged to review such resolutions carefully and to alert the working group with any problems that may be found.
DR: The full Committee has approved the item as a proposed defect report. The Proposed Resolution in an issue with this status reflects the best judgment of the Committee at this time regarding the action that will be taken to remedy the defect; however, the current wording of the Standard remains in effect until such time as a Technical Corrigendum or a revision of the Standard is issued by ISO.
TC1: A DR issue included in Technical Corrigendum 1. TC1 is a revision of the Standard issued in 2003.
CD1: A DR issue not resolved in TC1 but included in Committee Draft 1. CD1 was advanced for balloting at the September, 2008 WG21 meeting.
CD2: A DR issue not resolved in CD1 but included in the Final Committee Draft advanced for balloting at the March, 2010 WG21 meeting.
FDIS: A DR issue not resolved in the FCD but included in the Final Draft International Standard advanced for balloting at the March, 2011 WG21 meeting.
WP: A DR issue whose resolution is reflected in the current Working Paper. The Working Paper is a draft for a future version of the Standard.
Dup: The issue is identical to or a subset of another issue, identified in a Rationale statement.
NAD: The working group has reached consensus that the issue is not a defect in the Standard. A Rationale statement describes the working group's reasoning.
Extension: The working group has reached consensus that the issue is not a defect in the Standard but is a request for an extension to the language. The working group expresses no opinion on the merits of an issue with this status; however, the issue will be maintained on the list for possible future consideration as an extension proposal.
Concepts: The issue relates to the “Concepts” proposal that was removed from the working paper at the Frankfurt (July, 2009) meeting and hence is no longer under consideration.
In describing static data members initialized inside the class definition, 9.4.2 [class.static.data] paragraph 3 says,
The member shall still be defined in a namespace scope if it is used in the program...
The definition of “used” is in 3.2 [basic.def.odr] paragraph 1:
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 a constant expression (5.19 [expr.const]) and the lvalue-to-rvalue conversion (4.1 [conv.lval]) is immediately applied.
Now consider the following example:
struct S {
static const int a = 1;
static const int b = 2;
};
int f(bool x) {
return x ? S::a : S::b;
}
According to the current wording of the Standard, this example requires that S::a and S::b be defined in a namespace scope. The reason for this is that, according to 5.16 [expr.cond] paragraph 4, the result of this conditional-expression is an lvalue and the lvalue-to-rvalue conversion is applied to that, not directly to the object, so this fails the “immediately applied” requirement. This is surprising and unfortunate, since only the values and not the addresses of the static data members are used. (This problem also applies to the proposed resolution of issue 696.)
Proposed resolution (August, 2011):
Divide 3.2 [basic.def.odr] paragraph 2 into two paragraphs and change as follows:
An expression is potentially evaluated unless it is an unevaluated operand (Clause 5) or a subexpression thereof. The set of potential results of an expression e is defined as:
if e is an id-expression (5.1.1 [expr.prim.general]), the set whose sole member is e,
if e is a class member access (5.2.5 [expr.ref]), the set of potential results of the object expression,
if e is a pointer-to-member expression (5.5 [expr.mptr.oper]) whose second operand is a constant expression, the set of potential results of the object expression,
if e has the form (e1), the set of potential results of e1,
if e is a glvalue conditional expression (5.16 [expr.cond]), the union of the sets of potential results of the second and third operands,
if e is a comma expression (5.18 [expr.comma]), the set of potential results of the right operand,
otherwise, the empty set.
A variable x whose name appears as a potentially-evaluated expression ex is odr-used unless it x is an object that satisfies the requirements for appearing in a constant expression (5.19 [expr.const]) and ex is an element of the set of potential results of an expression e, where either the lvalue-to-rvalue conversion (4.1 [conv.lval]) is immediately applied to e, or e is a discarded-value expression (Clause 5 [expr]). this is odr-used...
[Drafting note: this wording requires S::a to be defined if it is used in an expression like *&S::a.Proposed resolution (August, 2011):
Change 3.2 [basic.def.odr] paragraph 2 as follows:
...A non-overloaded function whose name appears as a potentially-evaluated expression is odr-used if it is the unique lookup result or it is the selected or a member of a set of candidate overloaded functions (3.4 [basic.lookup], 13.3 [over.match], 13.4 [over.over]), if selected by overload resolution when referred to from a potentially-evaluated expression, is odr-used, unless it is a pure virtual function and its name is not explicitly qualified...
The requirement in 3.2 [basic.def.odr] paragraph 4 that a type T must be complete if an expression is implicitly converted to a pointer to T or reference to T inadvertently applies to user-defined conversions, although it was intended only to refer to built-in conversions.
Proposed resolution (August, 2011):
Change the indicated bullet of 3.2 [basic.def.odr] paragraph 4 as follows:
an expression that is not a null pointer constant, and has type other than cv void*, is converted to the type pointer to T or reference to T using an implicit a standard conversion (Clause 4 [conv]), a dynamic_cast (5.2.7 [expr.dynamic.cast]) or a static_cast (5.2.9 [expr.static.cast]), or
The rules regarding class scope and when the class is considered to be complete (normally implemented by deferred parsing of portions of class member declarations) are inconsistent and need to be clarified.
Proposed resolution (August, 2011):
Change 3.3.7 [basic.scope.class] paragraph 1 as follows:
Change 3.4.1 [basic.lookup.unqual] pargraph 7 as follows:
A name used in the definition of a class X outside of a member function body, default argument, brace-or-equal-initializer of a non-static data member, or nested class definition29 shall be declared in one of the following ways:...
Change 3.4.1 [basic.lookup.unqual] paragraph 8 as follows:
A For the members of a class X, a name used in a member function body, in a default argument, in the brace-or-equal-initializer of a non-static data member (9.2 [class.mem]), or in the definition of a class member function (9.3 [class.mfct]) of class X outside of the definition of X, following the function's member's declarator-id [Footnote: That is, an unqualified name that occurs, for instance, in a type or default argument in the parameter-declaration-clause or in the function body exception-specification. —end footnote], or in the brace-or-equal-initializer of a non-static data member (9.2 [class.mem]) of class X shall be declared in one of the following ways:...
The specification of the forms of the definition of main that an impliementation is required to accept is clear in C99 that the parameter names and the exact syntactic form of the types can vary. Although it is reasonable to assume that a C++ implementation would accept a definition like
int main(int foo, char** bar) { /* ... */ }
instead of the canonical
int main(int argc, char* argv[]) { /* ... */ }
it might be a good idea to clarify the intent using wording similar to C99's.
Proposed resolution (August, 2011):
Change 3.6.1 [basic.start.main] paragraph 2 as follows:
...All implementations shall allow both of the following definitions of main:
int main() { /* ... */ }and
int main(int argc, char* argv[]) { /* ... */ }
function of () returning int and
function of (int, pointer to pointer to char) returning int
as the type of main (8.3.5 [dcl.fct]. In the latter form, for purposes of exposition, the first function parameter is called argc and the second function parameter is called argv, where argc shall be the number of arguments...
An lvalue referring to an out-of-lifetime non-POD class objects can be used in limited ways, subject to the restrictions in 3.8 [basic.life] paragraph 6:
if the original object will be or was of a non-POD class type, the program has undefined behavior if:
the lvalue is used to access a non-static data member or call a non-static member function of the object, or
the lvalue is implicitly converted (4.10 [conv.ptr]) to a reference to a base class type, or
the lvalue is used as the operand of a static_cast (5.2.9 [expr.static.cast]) except when the conversion is ultimately to cv char& or cv unsigned char& ), or
the lvalue is used as the operand of a dynamic_cast (5.2.7 [expr.dynamic.cast]) or as the operand of typeid.
There are at least a couple of questionable things in this list. First, there is no “implicit conversion to a reference to a base class,” as assumed by the second bullet. Presumably this is intended to say that the lvalue is bound to a reference to a base class, and the cross-reference should be to 8.5.3 [dcl.init.ref], not to 4.10 [conv.ptr] (which deals with pointer conversions). However, even given that adjustment, it is not clear why it is forbidden to bind a reference to a non-virtual base class of an out-of-lifetime object, as that is just an address offset calculation. (Binding to a virtual base, of course, would require access to the value of the object and thus cannot be done outside the object's lifetime.)
The third bullet also appears questionable. It's not clear why static_cast is discussed at all here, as the only permissible static_cast conversions involving reference types and non-POD classes are to references to base or derived classes and to the same type, modulo cv-qualification; if implicit “conversion” to a base class reference is forbidden in the second bullet, why would an explicit conversion be permitted in the third? Was this intended to refer to reinterpret_cast? Also, is there a reason to allow char types but disallow array-of-char types (which are more likely to be useful than a single char)?
Proposed resolution (March, 2008):
Change 3.8 [basic.life] paragraph 5 as follows:
...If the object will be or was of a non-trivial class type, the program has undefined behavior if:
the pointer is used to access a non-static data member or call a non-static member function of the object, or
the pointer is implicitly converted (
4.10 [conv.ptr] ) to a pointer to a virtual base class type, orthe pointer is used as the operand of a static_cast (5.2.9 [expr.static.cast]) (except when the conversion is to void*, or to void* and subsequently to char*, or unsigned char*). pointer to cv void, or to pointer to cv void and subsequently to pointer to cv char or pointer to cv unsigned char, or
the pointer is used as the operand of a dynamic_cast (5.2.7 [expr.dynamic.cast])...
Change 3.8 [basic.life] paragraph 6 as follows:
...if the original object will be or was of a non-trivial class type, the program has undefined behavior if:
the lvalue is used to access a non-static data member or call a non-static member function of the object, or
the lvalue is implicitly converted (4.10 [conv.ptr]) bound to a reference to a virtual base class type (8.5.3 [dcl.init.ref]), or
the lvalue is used as the operand of a static_cast (5.2.9 [expr.static.cast]) except when the conversion is ultimately to cv char& or cv unsigned char&, or
the lvalue is used as the operand of a dynamic_cast (5.2.7 [expr.dynamic.cast]) or as the operand of typeid.
[Drafting notes: Paragraph 5 was changed to track the changes to paragraph 6. See also the resolution for issue 658.]
There is no normative requirement on the ranges of the integral types, although the footnote in 3.9.1 [basic.fundamental] paragraph 2 indicates the intent (for int, at least) that they match the values given in the <climits> header. Should there be an explicit requirement of some sort?
(See also paper N1693.)
Proposed resolution (August, 2011):
Change 3.9.1 [basic.fundamental] paragraph 3 as follows:
...collectively called the extended integer types. The signed and unsigned integral types shall satisfy the constraints given in ISO C 5.2.4.2.1.
There does not appear to be any technical difficulty that would require the restriction in 5.1.2 [expr.prim.lambda] paragraph 5 against default arguments in lambda-expressions.
Proposed resolution (August, 2011):
Delete the following sentence from 5.1.2 [expr.prim.lambda] paragraph 5:
Default arguments (8.3.6 [dcl.fct.default]) shall not be specified in the parameter-declaration-clause of a lambda-declarator.
There does not appear to be any technical difficulty that would require the current restriction that the return type of a lambda can be deduced only if the body of the lambda consists of a single return statement. In particular, multiple return statements could be permitted if they all return the same type.
Proposed resolution (August, 2011):
Change 5.1.2 [expr.prim.lambda] paragraph 4 as follows:
...If a lambda-expression does not include a trailing-return-type, it is as if the trailing-return-type denotes the following type:
if the compound-statement is of the form
{ attribute-specifier-seqopt return expression ;
the type of the returned expression after lvalue-to-rvalue conversion (4.1 [conv.lval]), array-to-pointer conversion (4.2 [conv.array]), and function-to-pointer conversion (4.3 [conv.func]);
otherwise, void.
if there are no return statements in the compound-statement, or all return statements return either an expression of type void or no expression or braced-init-list, the type void;
otherwise, if all return statements return an expression and the types of the returned expressions after lvalue-to-rvalue conversion (4.1 [conv.lval]), array-to-pointer conversion (4.2 [conv.array]), and function-to-pointer conversion (4.3 [conv.func]) are the same, that common type;
otherwise, the program is ill-formed.
[Example:
auto x1 = [](int i){ return i; }; // OK: return type is int auto x2 = []{ return { 1, 2 }; }; // error: the return type is void (a // braced-init-list is not an expression) struct A { int fn1(); const int& fn2(); }; template <class T> void f () { [](T t, bool b){ if (b) return t.fn1(); else return t.fn2(); }; } template void f<A>(); // OK: lambda return type is int—end example]
According to the C++ Standard section 5.3.4 [expr.new] paragraph 21 it is unspecified whether the allocation function is called before evaluating the constructor arguments or after evaluating the constructor arguments but before entering the constructor.
On top of that paragraph 17 of the same section insists that
If any part of the object initialization described above [Footnote: This may include evaluating a new-initializer and/or calling a constructor.] terminates by throwing an exception and a suitable deallocation function is found, the deallocation function is called to free the memory in which the object was being constructed... If no unambiguous matching deallocation function can be found, propagating the exception does not cause the object's memory to be freed...
Now suppose we have:
struct copy_throw {
copy_throw(const copy_throw&)
{ throw std::logic_error("Cannot copy!"); }
copy_throw(long, copy_throw)
{ }
copy_throw()
{ }
};
int main()
try {
copy_throw an_object, /* undefined behaviour */
* a_pointer = ::new copy_throw(0, an_object);
return 0;
}
catch(const std::logic_error&)
{ }
Here the new-expression '::new copy_throw(0, an_object)' throws an exception when evaluating the constructor's arguments and before the allocation function is called. However, 5.3.4 [expr.new] paragraph 17 prescribes that in such a case the implementation shall call the deallocation function to free the memory in which the object was being constructed, given that a matching deallocation function can be found.
So a call to the Standard library deallocation function '::operator delete(void*)' shall be issued, but what argument is an implementation supposed to supply to the deallocation function? As per 5.3.4 [expr.new] paragraph 17 - the argument is the address of the memory in which the object was being constructed. Given that no memory has yet been allocated for the object, this will qualify as using an invalid pointer value, which is undefined behaviour by virtue of 3.7.4.2 [basic.stc.dynamic.deallocation] paragraph 4.
Suggested resolution:
Change the first sentence of 5.3.4 [expr.new] paragraph 17 to read:
If the memory for the object being created has already been successfully allocated and any part of the object initialization described above...
Proposed resolution (March, 2008):
Change 5.3.4 [expr.new] paragraph 18 as follows:
If any part of the object initialization described above [Footnote: ...] terminates by throwing an exception, storage has been obtained for the object, and a suitable deallocation function can be found, the deallocation function is called...
Proposed resolution (August, 2011):
Change 5.3.6 [expr.alignof] paragraph 1 as follows:
An alignof expression yields the alignment requirement of its operand type. The operand shall be a type-id representing a complete object type or an array thereof or a reference to a complete object type one of those types.
Proposed resolution (August, 2011):
Change 5.5 [expr.mptr.oper] paragraph 2 as follows:
The binary operator .* binds its second operand, which shall be of type “pointer to member of T” (where T is a completely-defined class type) to its first operand...
Change 5.5 [expr.mptr.oper] paragraph 3 as follows:
The binary operator ->* binds its second operand, which shall be of type “pointer to member of T” (where T is a completely-defined class type) to its first operand...
Proposed resolution (August, 2011):
This issue is resolved by the resolution of issue 1369.
Proposed resolution (August, 2011):
Change 5.19 [expr.const] paragraph 2 as follows:
an lvalue-to-rvalue conversion (4.1 [conv.lval]) unless it is applied to
a glvalue of integral or enumeration type that refers to a non-volatile const object with a preceding initialization, initialized with a constant expression [Note: a string literal (2.14.5 [lex.string]) corresponds to an array of such objects. —end note], or
...
Proposed resolution (August, 2011):
Change 5.19 [expr.const] paragraph 2 as follows:
an lvalue-to-rvalue conversion (4.1 [conv.lval]) unless it is applied to
a non-volatile glvalue of integral or enumeration type that refers to a non-volatile const object with a preceding initialization, initialized with a constant expression, or
a non-volatile glvalue of literal type that refers to a non-volatile object defined with constexpr, or that refers to a sub-object of such an object, or
a non-volatile glvalue of literal type that refers to a non-volatile temporary object whose lifetime has not ended, initialized with a constant expression;
Proposed resolution (August, 2011):
Change 5.19 [expr.const] paragraph 2 as follows:
...
a result that is not mathematically defined or not in the range of representable values for its type;
an operation that would have undefined behavior [Note: including, for example, signed integer overflow (Clause 5 [expr]), certain pointer arithmetic (5.7 [expr.add]), division by zero (5.6 [expr.mul]), or certain shift operations (5.8 [expr.shift]) —end note];
...
a subtraction (5.7 [expr.add]) where both operands are pointers;
...
Use of a parameter in a constexpr function appears to be ill-formed, because the lvalue-to-rvalue conversion on the parameter is not one of those permitted in a constant expression.
Proposed resolution (August, 2011):
Change the indicated bullet of 5.19 [expr.const] paragraph 2 as follows:
an invocation of a constexpr constructor with arguments that, when substituted by function invocation substitution (7.1.5 [dcl.constexpr]), do not produce all constant expressions for the constructor calls and full-expressions in the mem-initializers (including conversions); [Example:...
Delete the final bullet of 7.1.5 [dcl.constexpr] paragraph 3 and move the deleted "." to the preceding sub-bullet:
every constructor call and implicit conversion used in initializing the return value (6.6.3 [stmt.return], 8.5 [dcl.init]) shall be one of those allowed in a constant expression (5.19 [expr.const]).
Delete the final bullet of 7.1.5 [dcl.constexpr] paragraph 4 and change the preceding bullet as follows:
every assignment-expression that is an initializer-clause appearing directly or indirectly within a brace-or-equal-initializer for a non-static data member that is not named by a mem-initializer-id shall be a constant expression; and.
every implicit conversion used in converting a constructor argument to the corresponding parameter type and converting a full-expression to the corresponding member type shall be one of those allowed in a constant expression.
The current wording appears to allow calling a constexpr function that is never defined within the body of a constexpr function. (The wording was intended to allow mutually-recursive constexpr functions but require that the not-yet-defined function be defined before it would be needed in an actual constant expression.)
Proposed resolution (August, 2011):
Change the indicated bullet of 5.19 [expr.const] paragraph 2 as follows:
an invocation of an undefined constexpr function or an undefined constexpr constructor outside the definition of a constexpr function or a constexpr constructor;
The provisions allowing the use of this in a constant expression appear to be unnecessary, as any uses of this in a constant expression that are valid will be replaced by function invocation substitution.
Proposed resolution (August, 2011):
This issue is resolved by the resolution of issue 1369.
The requirement that a class with a constexpr constructor cannot have a virtual base only applies to constructors with non-deleted and non-defaulted function-bodys. This seems like an oversight.
Proposed resolution (August, 2011):
Change 7.1.5 [dcl.constexpr] paragraph 4 as follows:
In a The definition of a constexpr constructor, each of the parameter types shall be a literal type. In addition, either its function-body shall be = delete or = default or it shall satisfy the following constraints:
the class shall not have any virtual base classes;
each of the parameter types shall be a literal type;
its function-body shall not be a function-try-block;
In addition, either its function-body shall be = delete or it shall satisfy the following constraints:
either its function-body shall be = default or the compound-statement of its function-body shall contain only...
Function invocation substitution (7.1.5 [dcl.constexpr] paragraph 5) seems underspecified with respect to this.
Proposed resolution (August, 2011):
Change the indicated bullet of 5.19 [expr.const] paragraph 2 as follows:
this (5.1 [expr.prim] 5.1.1 [expr.prim.general]) unless it appears as the postfix-expression in a class member access expression, including the result of the implicit transformation in the body of a non-static member function (9.3.1 [class.mfct.non-static]) [Note: when evaluating a constant expression, function invocation substitution (7.1.5 [dcl.constexpr]) replaces each occurrence of this in a constexpr member function with a pointer to the class object. —end note];
Change 7.1.5 [dcl.constexpr] paragraph 5 as follows (converting the running text into a bulleted list):
Function invocation substitution for a call of a constexpr function or of a constexpr constructor means:
implicitly converting each argument to the corresponding parameter type as if by copy-initialization,91
substituting that converted expression for each use of the corresponding parameter in the function-body,
in a member function, substituting for each use of this (9.3.2 [class.this]) a prvalue pointer whose value is the address of the object for which the member function is called, and
for in a constexpr functions, implicitly converting the resulting returned expression or braced-init-list to the return type of the function as if by copy-initialization.
Such substitution...
This resolution also resolves issues 1264 and 1367.
The constraints on type-specifiers given in 7.1.6 [dcl.type] paragraphs 2 and 3 (at most one type-specifier except as specified, at least one type-specifier, no redundant cv-qualifiers) are couched in terms of decl-specifier-seqs and declarations. However, they should also apply to constructs that are not syntactically declarations and that are defined to use type-specifier-seqs, including 5.3.4 [expr.new], 6.6 [stmt.jump], 8.1 [dcl.name], and 12.3.2 [class.conv.fct].
Proposed resolution (August, 2011):
Change 7.1.6 [dcl.type] paragraph 3 as follows:
At Except in a declaration of a constructor, destructor, or conversion function, 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 shall appear in a complete type-specifier-seq or a complete decl-specifier-seq.92 A type-specifier-seq shall not define...
(Note: paper N2546, voted into the Working Draft in February, 2008, addresses part of this issue.)
The current wording of 7.1.6.4 [dcl.spec.auto] does not appear to forbid using the auto specifier for both a function declaration with a trailing return type and a variable definition in the same declaration, e.g.,
auto f() -> int, i = 0;
(See also issue 1347.)
Proposed resolution (August, 2011):
Change 7.1.6.4 [dcl.spec.auto] paragraph 7 as follows:
If the list of declarators contains more than one declarator, they shall all form declarations of variables. The the type of each declared variable is determined as described above. If, and if the type deduced for the template parameter U is not the same in each deduction, the program is ill-formed. [Example:...
7.1.6.4 [dcl.spec.auto] does not address the case when the initializer for an auto variable is a parenthesized expression-list.
Proposed resolution (August, 2011):
Change 7.1.6.4 [dcl.spec.auto] paragraph 3 as follows:
...auto shall appear as one of the decl-specifiers in the decl-specifier-seq and the decl-specifier-seq shall be followed by one or more init-declarators, each of which shall have a non-empty initializer. In an initializer of the form
( expression-list )
the expression-list shall be a single assignment-expression. [Example:...
The intent of 7.1.6.4 [dcl.spec.auto] paragraph 7 appears to have been that the type represented by auto should be the same for each declarator in the declaration. However, the current wording does not achieve that goal. For example, in
auto a = 0, b = { 1, 2, 2 };
the auto specifier represents int in the first declarator and std::initializer_list<int> in the second. (See also issue 1265.)
Proposed resolution (August, 2011):
Move the example in 7.1.6.4 [dcl.spec.auto] paragraph 7 into that of paragraph 6 and change paragraph 7 as follows:
...[Example:
auto x1 = { 1, 2 }; // decltype(x1) is std::initializer_list<int> auto x2 = { 1, 2.0 }; // error: cannot deduce element type const auto &i = expr;The type of i is the deduced type of the parameter u in the call f(expr) of the following invented function template:
template <class U> void f(const U& u);—end example]
If the list of declarators init-declarator-list contains more than one declarator init-declarator, the type of each declared variable is determined as described above. If the type deduced for the template parameter U that replaces the occurrence of auto is not the same in each deduction, the program is ill-formed.
[Example:
const auto &i = expr;The type of i is the deduced type of the parameter u in the call f(expr) of the following invented function template:
template <class U> void f(const U& u); auto x = 5, *y = &x; // OK: auto is int auto a = 5, b = { 1, 2 }; // error: different types for auto—end example]
The Standard does not appear to specify what happens for code like the following:
namespace one {
template<typename T> void fun(T);
}
using one::fun;
template<typename T> void fun(T);
7.3.3 [namespace.udecl] paragraph 13 does not appear to apply because it deals only with functions, not function templates:
If a function declaration in namespace scope or block scope has the same name and the same parameter types as a function introduced by a using-declaration, and the declarations do not declare the same function, the program is ill-formed.
John Spicer: For function templates I believe the rule should be that if they have the same function type (parameter types and return type) and have identical template parameter lists, the program is ill-formed.
Proposed resolution (August, 2011):
Change 7.3.3 [namespace.udecl] paragraph 14 as follows:
If a function declaration in namespace scope or block scope has the same name and the same parameter types parameter-type-list (8.3.5 [dcl.fct]) as a function introduced by a using-declaration, and the declarations do not declare the same function, the program is ill-formed. If a function template declaration in namespace scope has the same name, parameter-type-list, return type, and template parameter list as a function template introduced by a using-declaration, the program is ill-formed. [Note: Two using-declarations may introduce functions with the same name and the same parameter types parameter-type-list. If, for a call to an unqualified function name, function overload resolution selects the functions introduced by such using-declarations, the function call is ill-formed. [Example:...
Proposed resolution (August, 2011):
Change the grammar in 8 [dcl.decl] paragraph 4 as follows:
Change the grammar snippet in 13.3.1.1.2 [over.call.object] paragraph 2 as follows:
According to 8.3 [dcl.meaning] paragraph 1,
A declarator-id shall not be qualified except for the definition of a member function (9.3 [class.mfct]) or static data member (9.4 [class.static]) outside of its class, the definition or explicit instantiation of a function or variable member of a namespace outside of its namespace, or the definition of a previously declared explicit specialization outside of its namespace, or the declaration of a friend function that is a member of another class or namespace (11.3 [class.friend]). When the declarator-id is qualified, the declaration shall refer to a previously declared member of the class or namespace to which the qualifier refers...
This restriction prohibits examples like the following:
void f();
void ::f(); // error: qualified declarator
namespace N {
void f();
void N::f() { } // error: qualified declarator
}
There doesn't seem to be any good reason for disallowing such declarations, and a number of implementations accept them in spite of the Standard's prohibition. Should the Standard be changed to allow them?
Notes from the April, 2006 meeting:
In discussing issue 548, the CWG agreed that the prohibition of qualified declarators inside their namespace should be removed.
Proposed resolution (October, 2006):
Remove the indicated words from 8.3 [dcl.meaning] paragraph 1:
...An unqualified-id occurring in a declarator-id shall be a simple identifier except for the declaration of some special functions (12.3 [class.conv], 12.4 [class.dtor], 13.5 [over.oper]) and for the declaration of template specializations or partial specializations (). A declarator-id shall not be qualified except for the definition of a member function (9.3 [class.mfct]) or static data member (9.4 [class.static]) outside of its class, the definition or explicit instantiation of a function or variable member of a namespace outside of its namespace, or the definition of a previously declared explicit specialization outside of its namespace, or the declaration of a friend function that is a member of another class or namespace (11.3 [class.friend]). When the declarator-id is qualified, the declaration shall refer to a previously declared member of the class or namespace to which the qualifier 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 of the declarator-id...
[Drafting note: The omission of “outside of its class” here does not give permission for redeclaration of class members; that is still prohibited by 9.2 [class.mem] paragraph 1. The removal of the enumeration of the kinds of declarations in which a qualified-id can appear does allow a typedef declaration to use a qualified-id, which was not permitted before; if that is undesirable, the prohibition can be reinstated here.]
8.3.5 [dcl.fct]/2 restricts the use of void as parameter type, but does not mention CV qualified versions. Since void f(volatile void) isn't a callable function anyway, 8.3.5 [dcl.fct] should also ban cv-qualified versions. (BTW, this follows C)
Suggested resolution:
A possible resolution would be to add (cv-qualified) before void in
The parameter list (void) is equivalent to the empty parameter list. Except for this special case, (cv-qualified) void shall not be a parameter type (though types derived from void, such as void*, can).
Proposed resolution (August, 2011):
This issue is resolved by the resolution of issue 577.
8.3.5 [dcl.fct] paragraph 2 says,
The parameter list (void) is equivalent to the empty parameter list.
This special case is intended for C compatibility, but C99 describes it differently (6.7.5.3 paragraph 10):
The special case of an unnamed parameter of type void as the only item in the list specifies that the function has no parameters.
The C99 formulation allows typedefs for void, while C++ (and C90) accept only the keyword itself in this role. Should the C99 approach be adopted?
Notes from the October, 2006 meeting:
The CWG did not take a formal position on this issue; however, there was some concern expressed over the treatment of function templates and member functions of class templates if the C++ rule were changed: for a template parameter T, would a function taking a single parameter of type T become a no-parameter function if it were instantiated with T = void?
Proposed resolution (August, 2011):
Change 8.3.5 [dcl.fct] paragraph 4 as follows:
...If the parameter-declaration-clause is empty, the function takes no arguments. The parameter list (void) A parameter list consisting of a single unnamed parameter of non-dependent type void is equivalent to the an empty parameter list. Except for this special case, a parameter shall not have type cv void shall not be a parameter type (though types derived from void, such as void*, can). If the parameter-declaration-clause terminates...
This resolution also resolves issue 332.
According to the new wording of 8.3.6 [dcl.fct.default] paragraph 5,
A default argument is implicitly converted (Clause 4 [conv]) to the parameter type.
This is incorrect when the default argument is a braced-init-list. That sentence doesn't seem to be necessary, but if it is kept, it should be recast in terms of initialization rather than conversion.
Proposed resolution (August, 2011):
Delete the first sentence of 8.3.6 [dcl.fct.default] paragraph 5:
A default argument is implicitly converted (Clause 4 [conv]) to the parameter type.
Proposed resolution (August, 2011):
Change 8.4.2 [dcl.fct.def.default] paragraph 2 as follows:
...If a function is explicitly defaulted on its first declaration,
...
in the case of a copy constructor, move constructor, copy assignment operator, or move assignment operator, it shall have the same parameter type as if it had been implicitly declared.
Change 12.8 [class.copy] paragraph 12 as follows:
A copy/move constructor for class X is trivial if it is not user-provided, its declared parameter type is the same as if it had been implicitly declared, and if...
Change 12.8 [class.copy] paragraph 25 as follows:
A copy/move assignment operator for class X is trivial if it is not user-provided, its declared parameter type is the same as if it had been implicitly declared, and if...
The definition of “user-provided” given in 8.4.2 [dcl.fct.def.default] paragraph 4 applies only to special member functions, while the definition of an aggregate in 8.5.1 [dcl.init.aggr] paragraph 1 relies on that term in identifying constructors that make a class a non-aggregate. As a result, a class with a non-special constructor is considered an aggregate.
Proposed resolution (August, 2011):
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 or deleted on its first declaration...
[Drafting note: This makes a class with only a deleted initializer-list constructor an aggregate.]
8.5 [dcl.init] paragraph 7 only describes how to initialize objects:
To value-initialize an object of type T means:
However, 5.2.3 [expr.type.conv] paragraph 2 calls for value-initializing prvalues, which in the case of scalar types are not objects:
The expression T(), where T is a simple-type-specifier or typename-specifier for a non-array complete object type or the (possibly cv-qualified) void type, creates a prvalue of the specified type, which is value-initialized (8.5 [dcl.init]; no initialization is done for the void() case).
Proposed resolution (August, 2011):
Change 5.2.3 [expr.type.conv] paragraph 2 as follows:
The expression T(), where T is a simple-type-specifier or typename-specifier for a non-array complete object type or the (possibly cv-qualified) void type, creates a prvalue of the specified type,which is value-initialized (8.5 [dcl.init] type, whose value is that produced by value-initializing (8.5 [dcl.init]) an object of type T; no initialization is done for the void() case). [Note:...
Proposed resolution (August, 2011):
Change 8.5 [dcl.init] paragraph 7 as follows:
To value-initialize an object of type T means:
if T is a (possibly cv-qualified) class type (Clause 9 [class]) with either no default constructor (12.1 [class.ctor]) or a default constructor that is user-provided or deleted constructor (12.1 [class.ctor]), then the object is default-initialized default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
if T is a (possibly cv-qualified) non-union class type without a user-provided or deleted default constructor, then the object is zero-initialized and, if T's implicitly-declared default constructor is T has a non-trivial default constructor, that constructor is called. default-initialized;
...
Change 8.5.4 [dcl.init.list] paragraph 3 as follows:
List-initialization of an object or reference of type T is defined as follows:
If the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.
Otherwise, if If T is an aggregate, aggregate initialization is performed (8.5.1 [dcl.init.aggr]). [Example:...
Otherwise, if the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.
...
This resolution also resolves issues 1324 and 1368.
Proposed resolution (August, 2011):
This issue is resolved by the resolution of issue 1301.
Proposed resolution (August, 2011):
This issue is resolved by the resolution of issue 1301.
Proposed resolution (August, 2011):
Change the indicated sub-bullet of 8.5.3 [dcl.init.ref] paragraph 5 as follows:
is an xvalue (but is not a bit-field), class prvalue, array prvalue or function lvalue and “cv1 T1” is reference-compatible with “cv2 T2”, or
Proposed resolution (August, 2011):
Change 8.5.4 [dcl.init.list] paragraph 3 as follows:
...
Otherwise, if T is a class type, constructors are considered...
Otherwise, if T is a reference type, a prvalue temporary of the type referenced by T is list-initialized, and the reference is bound to that temporary. [Note: As usual, the binding will fail and the program is ill-formed if the reference type is an lvalue reference to a non-const type. —end note] [Example: ... —end example]
Otherwise, if the initializer list has a single element of type E and either T is not a reference type or its referenced type is reference-related to E, the object or reference is initialized from that element; if a narrowing conversion (see below) is required to convert the element to T, the program is ill-formed. [Example:...
Otherwise, if T is a reference type, a prvalue temporary of the type referenced by T is list-initialized, and the reference is bound to that temporary. [Note: As usual, the binding will fail and the program is ill-formed if the reference type is an lvalue reference to a non-const type. —end note] [Example: ... —end example]
Otherwise, if the initializer list has no elements...
...
Proposed resolution (August, 2011):
Change 9.2 [class.mem] paragraph 2 as follows:
A class is considered a completely-defined object type (3.9 [basic.types]) (or complete type) at the closing } of the class-specifier. Within the class member-specification, the class is regarded as complete within function bodies, default arguments, exception-specifications, and brace-or-equal-initializers for non-static data members (including such things in nested classes). Otherwise it is regarded as incomplete within its own class member-specification.
Change 15.4 [except.spec] paragraph 2 as follows:
...A type denoted in an exception-specification shall not denote an incomplete type other than a class currently being defined. A type denoted in an exception-specification shall not denote a pointer or reference to an incomplete type, other than cv void*, const void*, volatile void*, or const volatile void* or a pointer or reference to a class currently being defined. A type cv T, “array of T”, or “function returning T” denoted in an exception-specification is adjusted to type T, “pointer to T”, or “pointer to function returning T”, respectively.
Proposed resolution (August, 2011):
Change 8.4.2 [dcl.fct.def.default] paragraph 1 as follows:
A function definition of the form:
attribute-specifier-seqopt decl-specifier-seqopt declarator virt-specifier-seqopt = default ;
is called an explicitly-defaulted definition...
Change 8.4.3 [dcl.fct.def.delete] paragraph 1 as follows:
A function definition of the form:
attribute-specifier-seqopt decl-specifier-seqopt declarator virt-specifier-seqopt = delete ;
is called a deleted definition...
The grammar allows a brace-or-equal-initializer for any class member with a member-declarator, including typedef members and member function declarations, and there is no semantic restriction forbidding those forms, either.
Proposed resolution (August, 2011):
In 9.2 [class.mem], delete pargraph 4 and change paragraph 5 as follows:
A member can be initialized using a constructor; see 12.1 [class.ctor]. [Note: See Clause 12 [special] for a description of constructors and other special member functions. —end note]
A member can be initialized using a brace-or-equal-initializer shall appear only in the declaration of a data member. (For static data members, see 9.4.2 [class.static.data]; for non-static data members, see 12.6.2 [class.base.init]).
According to 9.3.2 [class.this] paragraph 2,
In a const member function, the object for which the function is called is accessed through a const access path; therefore, a const member function shall not modify the object and its non-static data members.
This is clearly overstating the case: mutable members can be modified, a const_cast can be used, and class member access expressions not involving this can also allow the object to be modified. The effect of the cv-qualification of a member function on the type of *this is clear from the preceding paragraph; this statement appears both unnecessary and incorrect.
Proposed resolution (August, 2011):
Merge 9.3.2 [class.this] paragraphs 1 and 2 and change the text as follows:
In the body of a non-static (9.3 [class.mfct]) member function, the keyword this is a prvalue expression whose value is the address of the object for which the function is called. The type of this in a member function of a class X is X*. If the member function is declared const, the type of this is const X*, if the member function is declared volatile, the type of this is volatile X*, and if the member function is declared const volatile, the type of this is const volatile X*. In [Note: thus in a const member function, the object for which the function is called is accessed through a const access path; therefore, a const member function shall not modify the object and its non-static data members. —end note] [Example:...
According to 10.3 [class.virtual] paragraph 8,
If the return type of D::f differs from the return type of B::f, the class type in the return type of D::f shall be complete at the point of declaration of D::f or shall be the class type D.
This provision was intended to deal with covariant return types but inadvertently affects types that vary only in cv-qualification:
struct A;
struct B {
virtual const A* f();
};
struct D : B {
A* f(); // ill-formed
};
Proposed resolution (August, 2011):
Change 10.3 [class.virtual] paragraph 8 as follows:
If the class type in the covariant return type of D::f differs from the return type that of B::f, the class type in the return type of D::f shall be complete at the point of declaration of D::f or shall be the class type D. When the overriding function...
Split off from issue 86.
Should binding a reference to the result of a "," operation whose second operand is a temporary extend the lifetime of the temporary?
const SFileName &C = ( f(), SFileName("abc") );
Notes from the March 2004 meeting:
We think the temporary should be extended.
Proposed resolution (October, 2004):
Change 12.2 [class.temporary] paragraph 2 as indicated:
... In all these cases, the temporaries created during the evaluation of the expression initializing the reference, except the temporary that is the overall result of the expression [Footnote: For example, if the expression is a comma expression (5.18 [expr.comma]) and the value of its second operand is a temporary, the reference is bound to that temporary.] and 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...
[Note: this wording partially resolves issue 86. See also issue 446.]
Notes from the April, 2005 meeting:
The CWG suggested a different approach from the 10/2004 resolution, leaving 12.2 [class.temporary] unchanged and adding normative wording to 5.18 [expr.comma] specifying that, if the result of the second operand is a temporary, that temporary is the result of the comma expression as well.
Proposed Resolution (November, 2006):
Add the indicated wording to 5.18 [expr.comma] paragraph 1:
... The type and value of the result are the type and value of the right operand; the result is of the same value category as its right operand, and is a bit-field if its right operand is a glvalue and a bit-field. If the value of the right operand is a temporary (12.2 [class.temporary]), the result is that temporary.
Proposed resolution (August, 2011):
Change the example in 12.3.1 [class.conv.ctor] paragraph 1 as follows:
struct X {
X(int);
X(const char*, int =0);
X(int, int);
};
void f(X arg) {
X a = 1; // a = X(1)
X b = "Jessie"; // b = X("Jessie",0)
a = 2; // a = X(2)
f(3); // f(X(3))
f({1, 2}); // f(X(1,2))
}
Change the example in 12.3.1 [class.conv.ctor] paragraph 2 as follows:
struct Z {
explicit Z();
explicit Z(int);
explicit Z(int, int);
};
Z a; // OK: default-initialization performed
Z a1 = 1; // error: no implicit conversion
Z a3 = Z(1); // OK: direct initialization syntax used
Z a2(1); // OK: direct initialization syntax used
Z* p = new Z(1); // OK: direct initialization syntax used
Z a4 = (Z)1; // OK: explicit cast used
Z a5 = static_cast<Z>(1); // OK: explicit cast used
Z a6 = { 3, 4 }; // error: no implicit conversion
12.6.2 [class.base.init] paragraph 8 appears to indicate that a class member that is an anonymous union is to be default initilized.
Proposed resolution (August, 2011):
Change the indicated bullet of 12.6.2 [class.base.init] paragraph 8 as follows:
Footnote 112 (12.8 [class.copy] paragraph 2) says,
Because a template constructor is never a copy constructor, the presence of such a template does not suppress the implicit declaration of a copy constructor. Template constructors participate in overload resolution with other constructors, including copy constructors, and a template constructor may be used to copy an object if it provides a better match than other constructors.
However, many of the stipulations about copy construction are phrased to refer only to “copy constructors.” For example, 12.8 [class.copy] paragraph 14 says,
A program is ill-formed if the copy constructor... for an object is implicitly used and the special member function is not accessible (clause 11 [class.access]).
Does that mean that using an inaccessible template constructor to copy an object is permissible, because it is not a “copy constructor?” Obviously not, but each use of the term “copy constructor” in the Standard should be examined to determine if it applies strictly to copy constructors or to any constructor used for copying. (A similar issue applies to “copy assignment operators,” which have the same relationship to assignment operator function templates.)
Proposed Resolution (August, 2011):
Change 3.2 [basic.def.odr] paragraph 2 as follows:
...[Note: This covers calls to named functions (5.2.2 [expr.call]), operator overloading (Clause 13 [over]), user-defined conversions (12.3.2 [class.conv.fct]), allocation function for placement new (5.3.4 [expr.new]), as well as non-default initialization (8.5 [dcl.init]). A copy constructor or move constructor selected to copy or move an object of class type is odr-used even if the call is actually elided by the implementation (12.8 [class.copy]). —end note] ... A copy-assignment function for a class An assignment operator function in a class is odr-used by an implicitly-defined copy-assignment or move-assignment function for another class as specified in 12.8 [class.copy]. A move-assignment function for a class is odr-used by an implicitly-defined move-assignment function for another class as specified in 12.8 [class.copy]. A default constructor...
Delete 12.1 [class.ctor] paragraph 9:
A copy constructor (12.8 [class.copy]) is used to copy objects of class type. A move constructor (12.8 [class.copy]) is used to move the contents of objects of class type.
Change 12.2 [class.temporary] paragraph 1 as follows:
...[Note: even if there is no call to the destructor or copy/move constructor, all the semantic restrictions, such as accessibility (Clause 11 [class.access]) and whether the function is deleted (8.4.3 [dcl.fct.def.delete]), shall be satisfied. this includes accessibility (11 [class.access]) and whether it is deleted, for the constructor selected and for the destructor. However, in the special case of a function call...
Change 12.8 [class.copy] paragraph 13 as follows:
A copy/move constructor that is defaulted and not defined as deleted is implicitly defined if it is odr-used (3.2 [basic.def.odr]) to initialize an object of its class type from a copy of an object of its class type or of a class type derived from its class type [Footnote: See 8.5 [dcl.init] for more details on direct and copy initialization. —end footnote] or when it is explicitly defaulted...
When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the copy/move constructor selected for the copy/move operation and/or the destructor for the object have side effects...
Change 13.3.3.1.2 [over.ics.user] paragraph 4 as follows:
A conversion of an expression of class type to the same class type is given Exact Match rank, and a conversion of an expression of class type to a base class of that type is given Conversion rank, in spite of the fact that a copy/move constructor (i.e., a user-defined conversion function) is called for those cases.
Change 15.1 [except.throw] paragraph 3 as follows:
A throw-expression copy-initializes (8.5 [dcl.init]) a temporary object, called the exception object...
Change 15.1 [except.throw] paragraph 5 as follows:
When the thrown object is a class object, the copy/move constructor selected for the copy-initialization and the destructor shall be accessible, even if the copy/move operation is elided (12.8 [class.copy]).
[Drafting note: 5.17 [expr.ass] paragraph 4, 9 [class] paragraph 4, 9.5 [class.union] paragraph 1, 12.2 [class.temporary] paragraph 2, 12.8 [class.copy] paragraphs 1-2, and 15.4 [except.spec] paragraph 14 do not require any changes.]
According to 12.9 [class.inhctor] paragraph 3, the exception specification for an inheriting constructor has the same exception specification as the inherited constructor. This ignores the exception specifications of default constructors for base classes and nonstatic data members and of functions called in brace-or-equals-initializers of nonstatic data members.
Proposed resolution (August, 2011):
Delete the indicated bullet of 12.9 [class.inhctor] paragraph 2:
Change 12.9 [class.inhctor] paragraph 3 as follows:
...[Note: Default arguments are not inherited. An exception-specification is implied as specified in 15.4 [except.spec]. —end note]
Change 15.4 [except.spec] paragraph 14 as follows:
An inheriting constructor (12.9 [class.inhctor]) and an implicitly declared special member function (Clause 12 [special]) shall have an exception-specification. If f is an inheriting constructor or an implicitly declared default constructor, copy constructor, move constructor, destructor, copy assignment operator, or move assignment operator, its implicit exception-specification specifies the type-id T if and only if T is allowed by the exception-specification of a function directly invoked by f's implicit definition; f shall allow all exceptions if any function it directly invokes allows all exceptions, and f shall allow no exceptions if every function it directly invokes allows no exceptions. [Note: an instantiation of an inheriting constructor template has an implied exception-specification as if it were a non-template inheriting constructor. —end note] [Example:...
Proposed resolution (August, 2011):
Change the example in 13.3.3.2 [over.ics.rank] paragraph 3 bullet 1 sub-bullet 5 as follows:
template<class T> int f(T&); template<class T> int f(T&&); int f(void(&)()); // #1 int f(void(&&)()); // #2 void g(); int i1 = f(g); // calls f(T&) #1
Proposed resolution (August, 2011):
Change 14.1 [temp.param] paragraph 11 as follows:
...A template parameter pack of a function template shall not be followed by another template parameter unless that template parameter can be deduced from the parameter-type-list of the function template or has a default argument (14.8.2 [temp.deduct]). [Example:
template<class T1 = int, class T2> class B; // error // U cannot be deduced from the parameter-type-list or specified template<class... T, class... U> void f() { } // error template<class... T, class U> void g() { } // error—end example]
Proposed resolution (August, 2011):
Change 14.6 [temp.res] paragraph 8 as follows:
Knowing which names are type names allows the syntax of every template definition to be checked. No diagnostic shall be issued for a template definition for which a valid specialization can be generated. If no valid specialization can be generated for a template definition, and that template is not instantiated, the template definition is ill-formed, no diagnostic required. If every valid specialization of a variadic template requires an empty template parameter pack, the template definition is ill-formed, no diagnostic required. If a type used...
Proposed resolution (August, 2011):
Change 14.8.2 [temp.deduct] paragraph 5 as follows:
The resulting substituted and adjusted function type is used as the type of the function template for template argument deduction. If a template argument has not been deduced, its default template argument, if any, is used and its corresponding template parameter has a default argument, the template argument is determined by substituting the template arguments determined for preceding template parameters into the default argument. If the substitution results in an invalid type, as described above, type deduction fails. [Example:...
Proposed resolution (August, 2011):
Change B [implimits] as follows:
Recursively nested template instantiations, including substitution during template argument deduction (14.8.2 [temp.deduct]) [1 024].
I have a question about exception handling with respect to derived to base conversions of pointers caught by reference.
What should the result of this program be?
struct S {};
struct SS : public S {};
int main()
{
SS ss;
int result = 0;
try
{
throw &ss; // throw object has type SS*
// (pointer to derived class)
}
catch (S*& rs) // (reference to pointer to base class)
{
result = 1;
}
catch (...)
{
result = 2;
}
return result;
}
The wording of 15.3 [except.handle] paragraph 3 would seem to say that the catch of S*& does not match and so the catch ... would be taken.
All of the compilers I tried (EDG, g++, Sun, and Microsoft) used the catch of S*& though.
What do we think is the desired behavior for such cases?
My initial reaction is that this is a bug in all of these compilers, but the fact that they all do the same thing gives me pause.
On a related front, if the handler changes the parameter using the reference, what is caught by a subsequent handler?
extern "C" int printf(const char *, ...);
struct S {};
struct SS : public S {};
SS ss;
int f()
{
try
{
throw &ss;
}
catch (S*& rs) // (reference to pointer to base class)
{
rs = 0;
throw;
}
catch (...)
{
}
return 0;
}
int main()
{
try { f(); }
catch (S*& rs) {
printf("rs=%p, &ss=%p\n", rs, &ss);
}
}
EDG, g++, and Sun all catch the original (unmodified) value. Microsoft catches the modified value. In some sense the EDG/g++/Sun behavior makes sense because the later catch could catch the derived class instead of the base class, which would be difficult to do if you let the catch clause update the value to be used by a subsequent catch.
But on this non-pointer case, all of the compilers later catch the modified value:
extern "C" int printf(const char *, ...);
int f()
{
try
{
throw 1;
}
catch (int& i)
{
i = 0;
throw;
}
catch (...)
{
}
return 0;
}
int main()
{
try { f(); }
catch (int& i) {
printf("i=%p\n", i);
}
}
To summarize:
(See also issue 729.)
Notes from the October, 2009 meeting:
The consensus of the CWG was that it should not be possible to catch a pointer to a derived class using a reference to a base class pointer, and that a handler that takes a reference to non-const pointer should allow the pointer to be modified by the handler.
Proposed resolution (March, 2010):
Change 15.3 [except.handle] paragraph 3 as follows:
A handler is a match for an exception object of type E if
The handler is of type cv T or cv T& and E and T are the same type (ignoring the top-level cv-qualifiers), or
the handler is of type cv T or cv T& and T is an unambiguous public base class of E, or
the handler is of type cv1 T* cv2 or const T& where T is a pointer type and E is a pointer type that can be converted to the type of the handler T by either or both of
a standard pointer conversion (4.10 [conv.ptr]) not involving conversions to pointers to private or protected or ambiguous classes
a qualification conversion
the handler is of type cv T or const T& where T is a pointer or pointer to member type and E is std::nullptr_t.
(This resolution also resolves issue 729.)
Notes from the March, 2011 meeting:
This resolution would require an ABI change and was thus deferred for further consideration.
Given the following example:
int f() {
try { /* ... */ }
catch(const int*&) {
return 1;
}
catch(int*&) {
return 2;
}
return 3;
}
can f() return 2? That is, does an int* exception object match a const int*& handler?
According to 15.3 [except.handle] paragraph 3, it does not:
A handler is a match for an exception object of type E if
The handler is of type cv T or cv T& and E and T are the same type (ignoring the top-level cv-qualifiers), or
the handler is of type cv T or cv T& and T is an unambiguous public base class of E, or
the handler is of type cv1 T* cv2 and E is a pointer type that can be converted to the type of the handler by either or both of
a standard pointer conversion (4.10 [conv.ptr]) not involving conversions to pointers to private or protected or ambiguous classes
a qualification conversion
the handler is a pointer or pointer to member type and E is std::nullptr_t.
Only the third bullet allows qualification conversions, but only the first bullet applies to a handler of reference-to-pointer type. This is consistent with how other reference bindings work; for example, the following is ill-formed:
int* p;
const int*& r = p;
(The consistency is not complete; the reference binding would be permitted if r had type const int* const &, but a handler of that type would still not match an int* exception object.)
However, implementation practice seems to be in the other direction; both EDG and g++ do match an int* with a const int*&, and the Microsoft compiler issues an error for the presumed hidden handler in the code above. Should the Standard be changed to reflect existing practice?
(See also issue 388.)
Notes from the October, 2009 meeting:
The CWG agreed that matching the exception object with a handler should, to the extent possible, mimic ordinary reference binding in cases like this.
Proposed resolution (February, 2010):
This issue is resolved by the resolution of issue 388.
Proposed resolution (August, 2011):
Change 15.4 [except.spec] paragraph 14 as follows:
An implicitly declared special member function (Clause 12 [special]) shall have has an exception-specification. If f is an implicitly declared default constructor, copy constructor, move constructor, destructor, copy assignment operator, or move assignment operator, its implicit exception-specification specifies the type-id T if and only if T is allowed by the exception-specification of a function directly invoked by f's implicit definition; f shall allow allows all exceptions if any function it directly invokes allows all exceptions, and f shall allow no exceptions has the exception-specification noexcept(true) if every function it directly invokes allows no exceptions. [Example:
struct A { A(); A(const A&) throw(); A(A&&) throw(); ~A() throw(X); }; struct B { B() throw(); B(const B&) throw(); B(B&&) throw(Y); ~B() throw(Y); }; struct D : public A, public B { // Implicit declaration of D::D(); // Implicit declaration of D::D(const D&) throw() noexcept(true); // Implicit declaration of D::D(D&&) throw(Y); // Implicit declaration of D::~D() throw(X, Y); };Furthermore, if...
The incompatibility described appears not to exist.
Proposed resolution (August, 2011):
Delete the second entry in C.1.3 [diff.conv], i.e., the one headed by
Change: Only pointers to non-const and non-volatile objects may be implicitly converted to void*
In spite of the resolution of issue 112, the exact relationship between cv-qualifiers and array types is not clear. There does not appear to be a definitive normative statement answering the question of whether an array with a const-qualified element type is itself const-qualified; the statement in 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.
hints at an answer but is hardly decisive. For example, is the following example well-formed?
template <class T> struct is_const {
static const bool value = false;
};
template <class T> struct is_const<const T> {
static const bool value = true;
};
template <class T> void f(T &) {
char checker[is_const<T>::value];
}
int const arr[1] = {};
int main() {
f(arr);
}
Also, when 3.10 [basic.lval] paragraph 4 says,
Class prvalues can have cv-qualified types; non-class prvalues always have cv-unqualified types.
does this apply to array rvalues, as it appears? That is, given
struct S {
const int arr[10];
};
is the array rvalue S().arr an array of int or an array of const int?
(The more general question is, when the Standard refers to non-class types, should it be considered to include array types? Or perhaps only arrays of non-class types?)
Proposed resolution (December, 2011):
Change 3.9.3 [basic.type.qualifier] paragraph 5 as follows:
...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. An array type whose elements are cv-qualified is also considered to have the same cv-qualification as its elements. [Example:typedef char CA[5]; typedef const char CC; CC arr1[5] = { 0 }; const CA arr2 = { 0 };The type of both arr1 and arr2 is “array of 5 const char,” and the array type is considered to be const-qualified. —end example]
Change 3.10 [basic.lval] paragraph 4 as follows:
Class and array prvalues can have cv-qualified types; non-class other prvalues always have cv-unqualified types. Unless otherwise indicated...
Proposed resolution (December, 2011):
Change 3.10 [basic.lval] paragraph 4 as follows (supersedes the corresponding change in the resolution of issue 1059):
Class prvalues can have cv-qualified types; non-class prvalues always have cv-unqualified types. Unless otherwise indicated (5.2.2 [expr.call]), prvalues shall always have complete types or the void type; in addition to these types, glvalues can also have incomplete types. [Note: class and array prvalues can have cv-qualified types; other prvalues always have cv-unqualified types. See Clause 5 [expr]. —end note]
Add a new paragraph following 5 [expr] paragraph 5:
If an expression initially has the type “reference to T”...
If a prvalue initially has the type “cv T,” where T is a cv-unqualified non-class, non-array type, the type of the expression is adjusted to T prior to any further analysis.
Change 5.2.2 [expr.call] paragraph 3 as follows:
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 return type shall be an object type, a reference type or the type cv void.
Change 5.2.3 [expr.type.conv] paragraph 2 as follows:
...[Note: if T is a non-class type that is cv-qualified, the cv-qualifiers are ignored discarded when determining the type of the resulting prvalue (3.10 [basic.lval] Clause 5 [expr]). —end note]
Change 5.4 [expr.cast] paragraph 1 as follows:
...[Note: if T is a non-class type that is cv-qualified cv-qualified, the cv-qualifiers are ignored discarded when determining the type of the resulting prvalue; see 3.10 [basic.lval] Clause 5 [expr]. —end note]
Proposed resolution (August, 2011):
Change 13.3.1.6 [over.match.ref] paragraph 1 as follows:
The conversion functions of S and its base classes are considered, except that for copy-initialization, only the non-explicit conversion functions are considered. Those that are not hidden within S and yield type “lvalue reference to cv2 T2” (when 8.5.3 [dcl.init.ref] requires an lvalue result initializing an lvalue reference or an rvalue reference to function) or “cv2 T2” or “rvalue reference to cv2 T2” (when 8.5.3 [dcl.init.ref] requires an rvalue result initializing an rvalue reference or an lvalue reference to function), where “cv1 T” is reference-compatible (8.5.3 [dcl.init.ref]) with “cv2 T2”, are candidate functions.
Change 13.3.3 [over.match.best] paragraph 1 as follows:
...
the context is an initialization by user-defined conversion... or if not that,
the context is an initialization by conversion function for direct reference binding (13.3.1.6 [over.match.ref]) of a reference to function type, the return type of F1 is the same kind of reference (i.e. lvalue or rvalue) as the reference being initialized, and the return type of F2 is not [Example:
template <class T> struct A {
operator T&(); // #1
operator T&&(); // #2
};
typedef int Fn();
A<Fn> a;
Fn& lf = a; // calls #1
Fn&& rf = a; // calls #2
—end example] or, if not that,
F1 is a non-template function...
Proposed resolution (August, 2011):
Change 9 [class] paragraph 3 as follows:
If a class is marked with the class-virt-specifier final and it appears as a base-type-specifier in a base-clause (Clause 10 [class.derived]), the program is ill-formed. An ambiguity between a class-specifier and a simple-declaration or member-declaration using an elaborated-type-specifier arises when the identifier final is followed by a { or : token. The resolution is to consider final a class-virt-specifier instead of a declarator-id in these cases.
Proposed resolution (December, 2011):
Change 14.5.6.1 [temp.over.link] paragraph 5 as follows:
Two expressions involving template parameters are considered equivalent if two function definitions containing the expressions would satisfy the one definition rule (3.2 [basic.def.odr]), except that the tokens used to name the template parameters may differ as long as a token used to name a template parameter in one expression is replaced by another token that names the same template parameter in the other expression. For determining whether two dependent names (14.6.2 [temp.dep]) are equivalent, only the name itself is considered, not the result of name lookup in the context of the template. If multiple declarations of the same function template differ in the result of this name lookup, the result for the first declaration is used. [Example:
template <int I, int J> void f(A<I+J>); // #1 template <int K, int L> void f(A<K+L>); // same as #1 template <class T> decltype(g(T())) h(); int g(int); template <class T> decltype(g(T())) h() // redeclaration of h() uses the earlier lookup { return g(T()); } // ...although the lookup here does find g(int) int i = h<int>(); // template argument substitution fails; g(int) // was not in scope at the first declaration of h()—end example] Two expressions...
Change 14.6.2 [temp.dep] paragraph 1 as follows:
...In an expression of the form:
postfix-expression ( expression-listopt )
where the postfix-expression is an id-expression unqualified-id, the id-expression unqualified-id denotes a dependent name if
any of the expressions in the expression-list is a pack expansion (14.5.3 [temp.variadic]),
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...
Change 14.6.4.2 [temp.dep.candidate] paragraph 1 as follows:
For a function call that depends on a template parameter where the postfix-expression is a dependent name, 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:
For the part of the lookup using unqualified name lookup (3.4.1 [basic.lookup.unqual]) or qualified name lookup (3.4.3 [basic.lookup.qual]), only function declarations from the template definition context are found.
For the part of the lookup using associated namespaces (3.4.2 [basic.lookup.argdep]), only function declarations found in either the template definition context or the template instantiation context are found.
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.
Consider the following example:
void f(int*);
void f(...);
template <int N> void g() {
f(N);
}
int main() {
g<0>();
g<1>();
}
The call to f in g is not type-dependent, so the overload resolution must be done at definition time rather than at instantiation time. As a result, both of the calls to g will result in calls to f(...), i.e., N will not be a null pointer constant, even if the value of N is 0.
It would be most consistent to adopt a rule that a value-dependent expression can never be a null pointer constant, even in cases like
template <int N> void g() {
int* p = N;
}
This would always be ill-formed, even when N is 0.
John Spicer: It's clear that this treatment is required for overload resolution, but it seems too expansive given that there are other cases in which the value of a template parameter can affect the validity of the program, and an implementation is forbidden to issue a diagnostic on a template definition unless there are no possible valid specializations.
Notes from the July, 2009 meeting:
There was a strong consensus among the CWG that only the literal 0 should be considered a null pointer constant, not any arbitrary zero-valued constant expression as is currently specified.
Proposed resolution (August, 2011):
Change 4.10 [conv.ptr] paragraph 1 as follows:
A null pointer constant is an integral constant expression (5.19 [expr.const]) prvalue of integer type that evaluates to integer literal (2.14.2 [lex.icon]), possibly parenthesized, with value zero or a prvalue of type std::nullptr_t. A null pointer constant can be converted...
Change 5.19 [expr.const] paragraph 3 as follows:
...[Note: Such expressions may be used as array bounds (8.3.4 [dcl.array], 5.3.4 [expr.new]), as bit-field lengths (9.6 [class.bit]), as enumerator initializers if the underlying type is not fixed (7.2 [dcl.enum]), as null pointer constants (4.10 [conv.ptr]), and as alignments (7.6.2 [dcl.align]). —end note]...
Change 8.5 [dcl.init] paragraph 5 as follows:
To zero-initialize an object or reference of type T means:
if T is a scalar type (3.9 [basic.types]), the object is set to the value 0 (zero), taken as an integral constant expression integer literal, converted to T; [Footnote: As specified in 4.10 [conv.ptr], converting an integral constant expression integer literal whose value is 0 to a pointer type results in a null pointer value. —end footnote]
...
Change 14.3.2 [temp.arg.nontype] paragraph 5 as follows:
...
for a non-type template-parameter of type pointer to object, qualification conversions (4.4 [conv.qual]) and the array-to-pointer conversion (4.2 [conv.array]) are applied; if the template-argument is of type std::nullptr_t, the null pointer conversion (4.10 [conv.ptr]) is applied. [Note: In particular, neither the null pointer conversion for a zero-valued integral constant expression integer literal (4.10 [conv.ptr]) nor the derived-to-base conversion (4.10 [conv.ptr]) are applied. Although 0 is...
...
Change 15.3 [except.handle] paragraph 3 as follows:
...[Note: A throw-expression whose operand is an integral constant expression of integer type that evaluates to integer literal with value zero does not match a handler of pointer or pointer to member type. —end note]. [Example: ...
Add a new section C.3 as follows:
C.3 C++ and ISO C++ 2011 [diff.cpp11] This subclause lists the differences between C++ and ISO C++ 2011 (ISO/IEC 14882:2011, Programming Languages — C++), by the chapters of this document.
C.3.1 Clause 4: standard conversions [diff.cpp11.conv] 4.10 [conv.ptr]
Change: Only literals are integer null pointer constants
Rationale: Removing surprising interactions with templates and constant expressions
Effect on original feature: Valid C++ 2011 code may fail to compile or produce different results in this International Standard, as the following example illustrates:
void f(void *); // #1 void f(int); // #2 template<int N> void g() { f(0*N); // calls #2; used to call #1 }
The C++ Standard uses the phrase “indeterminate value” without defining it. C99 defines it as “either an unspecified value or a trap representation.” Should C++ follow suit?
In addition, 4.1 [conv.lval] paragraph 1 says that applying the lvalue-to-rvalue conversion to an “object [that] is uninitialized” results in undefined behavior; this should be rephrased in terms of an object with an indeterminate value.
Proposed resolution (August, 2011):
Change 4.1 [conv.lval] paragraphs 1-2 as follows (converting the bulk of the running text of paragraph 2 into a bulleted list):
A glvalue (3.10 [basic.lval]) of a non-function, non-array type T can be converted to a prvalue.53 If T is an incomplete type, a program that necessitates this conversion is ill-formed. If the object to which the glvalue refers is not an object of type T and is not an object of a type derived from T, or if the object is uninitialized, a program that necessitates this conversion has undefined behavior. If T is a non-class type, the type of the prvalue is the cv-unqualified version of T. Otherwise, the type of the prvalue is T.54
When an lvalue-to-rvalue conversion occurs in an unevaluated operand or a subexpression thereof (Clause 5 [expr]) the value contained in the referenced object is not accessed. In all other cases, the result of the conversion is determined according to the following rules:
if T is (possibly cv-qualified) std::nullptr_t, the result is a null pointer constant (4.10 [conv.ptr]).
Otherwise, if the glvalue has T is a class type, the conversion copy-initializes a temporary of type T from the glvalue and the result of the conversion is a prvalue for the temporary.
Otherwise, if the object to which the glvalue refers contains an invalid pointer value (3.7.4.2 [basic.stc.dynamic.deallocation], 3.7.4.3 [basic.stc.dynamic.safety]), the behavior is undefined.
Otherwise, if T is a (possibly cv-qualified) unsigned character type (3.9.1 [basic.fundamental]), and the object to which the glvalue refers contains an indeterminate value (5.3.4 [expr.new], 8.5 [dcl.init], 12.6.2 [class.base.init]), and that object is not a local variable or the glvalue was the operand of a unary & operator or it was bound to a reference, the result is an unspecified value. [Footnote: The value may be different each time the lvalue-to-rvalue conversion is applied to the object. —end footnote]
Otherwise, if the object to which the glvalue refers contains an indeterminate value, the behavior is undefined.
Otherwise, if the glvalue has (possibly cv-qualified) type std::nullptr_t, the prvalue result is a null pointer constant (4.10 [conv.ptr]).
Otherwise, the value contained in the object indicated by the glvalue is the prvalue result.
[Drafting note: this implements the idea of C99 DR338, i.e., an uninitialized unsigned char object allocated to a register might still trap, for example on Itanium with its “NaT” state.]
Change 5.2.5 [expr.ref] paragraph 4 as follows:
...If E1 is an lvalue, then E1.E2 is an lvalue; otherwise, if E1 is an xvalue, then E1.E2 is an xvalue; otherwise, it is a prvalue. Let the notation...
Change 5.5 [expr.mptr.oper] paragraph 6 as follows:
...The result of a .* expression whose second operand is a pointer to a data member is of the same value category (3.10 [basic.lval]) as its first operand an lvalue if the first operand is an lvalue and an xvalue otherwise. The result...
[Drafting note: this change addresses issue 240, the third item, in a minimally-intrusive way. Also, a class member access for a class prvalue is in fact conceptually referring to memory and thus an lvalue-to-rvalue conversion should be applied before further processing. We might want to restrict the xvalue result to cases where the member has non-class type.
This resolution also resolves issues 129, 240, 312, and 1013.
Does the Standard require that an uninitialized auto variable have a stable (albeit indeterminate) value? That is, does the Standard require that the following function return true?
bool f() {
unsigned char i; // not initialized
unsigned char j = i;
unsigned char k = i;
return j == k; // true iff "i" is stable
}
3.9.1 [basic.fundamental]
paragraph 1
requires that uninitialized unsigned char variables have a
valid value, so the initializations of j and k are
well-formed and required not to trap. The question here is whether
the value of i is allowed to change between those
initializations.
Mike Miller: 1.9 [intro.execution] paragraph 10 says,
An instance of each object with automatic storage duration (3.7.3 [basic.stc.auto] ) is associated with each entry into its block. Such an object exists and retains its last-stored value during the execution of the block and while the block is suspended...I think that the most reasonable way to read this is that the only thing that is allowed to change the value of an automatic (non-volatile?) value is a "store" operation in the abstract machine. There are no "store" operations to i between the initializations of j and k, so it must retain its original (indeterminate but valid) value, and the result of the program is well-defined.
The quibble, of course, is whether the wording "last-stored value" should be applied to a "never-stored" value. I think so, but others might differ.
Tom Plum: 7.1.6.1 [dcl.type.cv] paragraph 8 says,
[Note: volatile is a hint to the implementation to avoid aggressive optimization involving the object because the value of the object might be changed by means undetectable by an implementation. See 1.9 [intro.execution] for detailed semantics. In general, the semantics of volatile are intended to be the same in C++ as they are in C. ]>From this I would infer that non-volatile means "shall not be changed by means undetectable by an implementation"; that the compiler is entitled to safely cache accesses to non-volatile objects if it can prove that no "detectable" means can modify them; and that therefore i shall maintain the same value during the example above.
Nathan Myers: This also has practical code-generation consequences. If the uninitialized auto variable lives in a register, and its value is really unspecified, then until it is initialized that register can be used as a temporary. Each time it's "looked at" the variable has the value that last washed up in that register. After it's initialized it's "live" and cannot be used as a temporary any more, and your register pressure goes up a notch. Fixing the uninit'd value would make it "live" the first time it is (or might be) looked at, instead.
Mike Ball: I agree with this. I also believe that it was certainly never my intent that an uninitialized variable be stable, and I would have strongly argued against such a provision. Nathan has well stated the case. And I am quite certain that it would be disastrous for optimizers. To ensure it, the frontend would have to generate an initializer, because optimizers track not only the lifetimes of variables, but the lifetimes of values assigned to those variables. This would put C++ at a significant performance disadvantage compared to other languages. Not even Java went this route. Guaranteeing defined behavior for a very special case of a generally undefined operation seems unnecessary.
Proposed resolution (August, 2011):
This issue is resolved by the resolution of issue 616.
3.7.4.2 [basic.stc.dynamic.deallocation] paragraph 4 mentions that the effect of using an invalid pointer value is undefined. However, the standard never says what it means to 'use' a value.
There are a number of possible interpretations, but it appears that each of them leads to undesired conclusions:
int *x = new int(0); delete x; x = 0;into undefined behaviour. As this is a common idiom, this is clearly undesirable.
int *x = new int(0); delete x; x->~int();into undefined behaviour; according to 5.2.4 [expr.pseudo], the variable x is 'evaluated' as part of evaluating the pseudo destructor call. This, in turn, would mean that all containers (23 [containers]) of pointers show undefined behaviour, e.g. 23.3.5.4 [list.modifiers] requires to invoke the destructor as part of the clear() method of the container.
If any other meaning was intended for 'using an expression', that meaning should be stated explicitly.
(See also issue 623.)
Proposed resolution (August, 2011):
This issue is resolved by the resolution of issue 616.
In 3.9 [basic.types] paragraph 10, the standard makes it quite clear that volatile qualified types are PODs:
Arithmetic types (3.9.1 [basic.fundamental]), enumeration types, pointer types, and pointer to member types (3.9.2 [basic.compound]), and cv-qualified versions of these types (3.9.3 [basic.type.qualifier]) are collectively called scalar types. Scalar types, POD-struct types, POD-union types (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.
However in 3.9 [basic.types] paragraph 3, the standard makes it clear that PODs can be copied “as if” they were a collection of bytes by memcpy:
For any POD type T, if two pointers to T point to distinct T objects obj1 and obj2, where neither obj1 nor obj2 is a base-class subobject, if the value of obj1 is copied into obj2, using the std::memcpy library function, obj2 shall subsequently hold the same value as obj1.
The problem with this is that a volatile qualified type may need to be copied in a specific way (by copying using only atomic operations on multithreaded platforms, for example) in order to avoid the “memory tearing” that may occur with a byte-by-byte copy.
I realise that the standard says very little about volatile qualified types, and nothing at all (yet) about multithreaded platforms, but nonetheless this is a real issue, for the following reason:
The forthcoming TR1 will define a series of traits that provide information about the properties of a type, including whether a type is a POD and/or has trivial construct/copy/assign operations. Libraries can use this information to optimise their code as appropriate, for example an array of type T might be copied with a memcpy rather than an element-by-element copy if T is a POD. This was one of the main motivations behind the type traits chapter of the TR1. However it's not clear how volatile types (or POD's which have a volatile type as a member) should be handled in these cases.
Notes from the April, 2005 meeting:
It is not clear whether the volatile qualifier actually guarantees atomicity in this way. Also, the work on the memory model for multithreading being done by the Evolution Working Group seems at this point likely to specify additional semantics for volatile data, and that work would need to be considered before resolving this issue.
Proposed resolution, August, 2011:
Change 3.9 [basic.types] paragraph 9 as follows:
...Scalar types, trivially copyable class types (Clause 9 [class]), arrays of such types, and cv-qualified non-volatile const-qualified versions of these types (3.9.3 [basic.type.qualifier]) are collectively called trivially copyable types. Scalar types, trivial class types...
Change 7.1.6.1 [dcl.type.cv] paragraphs 6-7 as follows:
What constitutes an access to an object that has volatile-qualified type is implementation-defined. If an attempt is made to refer to an object defined with a volatile-qualified type through the use of a glvalue with a non-volatile-qualified type, the program behavior is undefined.
[Note: volatile is a hint to the implementation to avoid aggressive optimization involving the object because the value of the object might be changed by means undetectable by an implementation. Furthermore, for some implementations, volatile might indicate that special hardware instructions are required to access the object. See 1.9 [intro.execution] for detailed semantics. In general, the semantics of volatile are intended to be the same in C++ as they are in C. —end note]
Change 9 [class] paragraph 6 as follows:
A trivially copyable class is a class that:
has no non-static data members or base classes of non-trivially copyable type,
...
Change 29.5 [atomics.types.generic] paragraph 1 as follows:
There is a generic class template atomic<T>. The type of the template argument T shall be cv-unqualified and trivially copyable (3.9).
Note, January, 2012:
Additional changes to the proposed resolution have been suggested, so the issue has been moved back to "review" status.
4.1 [conv.lval] paragraph 1 says,
If the object to which the lvalue refers is not an object of type T and is not an object of a type derived from T, or if the object is uninitialized, a program that necessitates this conversion has undefined behavior.
I think there are at least three related issues around this specification:
Presumably assigning a valid value to an uninitialized object allows it to participate in the lvalue-to-rvalue conversion without undefined behavior (otherwise the number of programs with defined behavior would be vanishingly small :-). However, the wording here just says "uninitialized" and doesn't mention assignment.
There's no exception made for unsigned char types. The wording in 3.9.1 [basic.fundamental] was carefully crafted to allow use of unsigned char to access uninitialized data so that memcpy and such could be written in C++ without undefined behavior, but this statement undermines that intent.
It's possible to get an uninitialized rvalue without invoking the lvalue-to-rvalue conversion. For instance:
struct A {
int i;
A() { } // no init of A::i
};
int j = A().i; // uninitialized rvalue
There doesn't appear to be anything in the current IS wording that says that this is undefined behavior. My guess is that we thought that in placing the restriction on use of uninitialized objects in the lvalue-to-rvalue conversion we were catching all possible cases, but we missed this one.
In light of the above, I think the discussion of uninitialized objects ought to be removed from 4.1 [conv.lval] paragraph 1. Instead, something like the following ought to be added to 3.9 [basic.types] paragraph 4 (which is where the concept of "value" is introduced):
Any use of an indeterminate value (5.3.4 [expr.new], 8.5 [dcl.init], 12.6.2 [class.base.init]) of any type other than char or unsigned char results in undefined behavior.
John Max Skaller:
A().i had better be an lvalue; the rules are wrong. Accessing a member of a structure requires it be converted to an lvalue, the above calculation is 'as if':
struct A {
int i;
A *get() { return this; }
};
int j = (*A().get()).i;
and you can see the bracketed expression is an lvalue.
A consequence is:
int &j= A().i; // OK, even if the temporary evaporates
j now refers to a 'destroyed' value. Any use of j is an error. But the binding at the time is valid.
Proposed Resolution (November, 2006):
Add the indicated words to 3.9 [basic.types] paragraph 4:
... For trivial types, the value representation is a set of bits in the object representation that determines a value, which is one discrete element of an implementation-defined set of values. Any use of an indeterminate value (5.3.4 [expr.new], 8.5 [dcl.init], 12.6.2 [class.base.init]) of a type other than unsigned char results in undefined behavior.
Change 4.1 [conv.lval] paragraph 1 as follows:
If the object to which the lvalue refers is not an object of type T and is not an object of a type derived from T, or if the object is uninitialized, a program that necessitates this conversion has undefined behavior.
Additional note (May, 2008):
The C committee is dealing with a similar issue in their DR336. According to this analysis, they plan to take almost the opposite approach to the one described above by augmenting the description of their version of the lvalue-to-rvalue conversion. The CWG did not consider that access to an unsigned char might still trap if it is allocated in a register and needs to reevaluate the proposed resolution in that light. See also issue 129.
Proposed resolution (August, 2011):
This issue is resolved by the resolution of issue 616.
According to 4.1 [conv.lval] paragraph 1, an lvalue-to-rvalue conversion on an uninitialized object produces undefined behavior. Since there is only one “value” of type std::nullptr_t, an lvalue-to-rvalue conversion on a std::nullptr_t glvalue does not need to fetch the value from storage. Is there any need for undefined behavior in this case?
Proposed resolution (August, 2011):
This issue is resolved by the resolution of issue 616.
5.2.7 [expr.dynamic.cast] paragraph 2 allows an expression of any value category when the target type is an rvalue reference. However, paragraph 6 requires that the operand be an lvalue if the runtime check is to be applied. This requirement should presumably be relaxed to require only a glvalue when the target type is an rvalue reference.
Proposed resolution (August, 2011):
Change 5.2.7 [expr.dynamic.cast] paragraph 6 as follows:
Otherwise, v shall be a pointer to or an lvalue a glvalue of a polymorphic type (10.3 [class.virtual]).
Additional note, January, 2012:
An objection has been raised to the proposed resolution on the basis that it unnecessarily weakens the distinction between rvalues and lvalues, making it easier to create dangling references. Its status has therefore been changed back to "review" to allow further discussion.
5.2.10 [expr.reinterpret.cast] paragraph 11, dealing with casting to reference types, only allows an lvalue operand. Presumably it should allow a glvalue operand when the target is an rvalue reference type.
Proposed resolution (August, 2011):
Change 5.2.10 [expr.reinterpret.cast] paragraph 11:
An lvalue A glvalue expression of type T1 can be cast to the type “reference to T2” if an expression of type “pointer to T1” can be explicitly converted to the type “pointer to T2” using a reinterpret_cast. The result refers to the same object as the source glvalue, but with the specified type. [Note: That is, for lvalues, a reference cast reinterpret_cast<T&>(x) has the same effect as the conversion *reinterpret_cast<T*>(&x) with the built-in & and * operators (and similarly for reinterpret_cast<T&&>(x)). —end note] The result refers to the same object as the source lvalue, but with a different type. The result is an lvalue for an lvalue reference type or an rvalue reference to function type and an xvalue for an rvalue reference to object type. No temporary is created,...
Additional note, January, 2012:
An objection has been raised to the proposed resolution on the basis that it unnecessarily weakens the distinction between rvalues and lvalues, making it easier to create dangling references. Its status has therefore been changed back to "review" to allow further discussion.
The result of the noexcept operator does not consider possible exceptions thrown by the destructors for temporaries created in the operand expression.
Proposed resolution (August, 2011):
Change 1.9 [intro.execution] paragraph 10 as follows:
A full-expression is an expression that is not a subexpression of another expression. [Note: in some contexts such as unevaluated operands, a syntactic subexpression is considered a full-expression (Clause 5 [expr]). —end note] If a language construct...
Change 5 [expr] paragraph 7 as follows:
...An unevaluated operand is not evaluated. An unevaluated operand is considered a full-expression. [Note:...
[Drafting note: This uniformly handles sizeof(A()), noexcept(A()), typeid(A()), and decltype(A()) with regard to the semantic requirements on ~A (accessible and not deleted), which might be checked via SFINAE. A programmer can use decltype(new A) to avoid considering the destructor. If this is undesired, an alternative change just addresses the noexecept issue:]
Change 5.3.7 [expr.unary.noexcept] paragraph 3 as follows:
The result of the noexcept operator is false if in a potentially-evaluated context the expression considered as a full-expression (1.9 [intro.execution]) would contain
a potentially evaluated call [Footnote: This includes implicit calls such as the call to an allocation function in a new-expression or the destructor call to destroy a temporary. —end footnote] to a function, member function, function pointer, or member function pointer that does not have a non-throwing exception-specification (15.4 [except.spec]), unless the call is a constant expression (5.19 [expr.const]),
...
[Editing note: all the occurrences of “potentially evaluated” in 5.3.7 [expr.unary.noexcept] paragraph 3 should be hyphenated.]
Proposed resolution (August, 2011):
Change 5.19 [expr.const] paragraph 2 as follows:
an lvalue-to-rvalue conversion (4.1 [conv.lval]) unless it is applied to...
an lvalue-to-rvalue conversion (4.1 [conv.lval]) that is applied to a glvalue of type cv1 T that refers to an object of type cv2 U, where T and U are neither the same type nor similar types (4.4 [conv.qual]);
an lvalue-to-rvalue conversion (4.1 [conv.lval]) that is applied to a glvalue that refers to a non-active member...
Note, January, 2012:
Additional discussion has occurred, so this issue has been returned to "review" status to allow further consideration.
Proposed resolution (August, 2011):
Change the two indicated (not contiguous) sub-bullets of 8.5.3 [dcl.init.ref] paragraph 5 as follows:
has a class type (i.e., T2 is a class type), where T1 is not reference-related to T2, and can be implicitly converted to an lvalue of type “cv3 T3,”...
has a class type (i.e., T2 is a class type), where T1 is not reference-related to T2, and can be implicitly converted to an xvalue, class prvalue, or function lvalue of type “cv3 T3”, where “cv1 T1” is reference-compatible with “cv3 T3” (see 13.3.1.6 [over.match.ref]),
Additional note, January, 2012:
Questions have been raised regarding the consistency of the treatment of class prvalues in this resolution with other types . The issue is thus being returned to "review" status for additional discussion.
Is the signedness of x in the following example implementation-defined?
template <typename T> struct A {
T x : 7;
};
template struct A<long>;
A similar example could be created with a typedef.
Lawrence Crowl: According to 9.6 [class.bit] paragraph 3,
It is implementation-defined whether a plain (neither explicitly signed nor unsigned) char, short, int or long bit-field is signed or unsigned.
This clause is conspicuously silent on typedefs and template parameters.
Clark Nelson: At least in C, the intention is that the presence or absence of this redundant keyword is supposed to be remembered through typedef declarations. I don't remember discussing it in C++, but I would certainly hope that we don't want to do something different. And presumably, we would want template type parameters to work the same way.
So going back to the original example, in an instantiation of A<long>, the signedness of the bit-field is implementation-defined, but in an instantiation of A<signed long>, the bit-field is definitely signed.
Peter Dimov: How can this work? Aren't A<long> and A<signed long> the same type?
(See also issue 739.)Proposed resolution (August, 2011):
This issue is resolved by the resolution of issue 739.Note, January, 2012:
Returned to "review" status because of additional questions raised regarding the proposed resolution of issue 739.
9.6 [class.bit] paragraph 3 says,
It is implementation-defined whether a plain (neither explicitly signed nor unsigned) char, short, int or long bit-field is signed or unsigned.
The implications of this permission for an implementation that chooses to treat plain bit-fields as unsigned are not clear. Does this mean that the type of such a bit-field is adjusted to the unsigned variant or simply that sign-extension is not performed when the value is fetched? C99 is explicit in specifying the former (6.7.2 paragraph 5: “for bit-fields, it is implementation-defined whether the specifier int designates the same type as signed int or the same type as unsigned int”), while C90 takes the latter approach (6.5.2.1: “Whether the high-order bit position of a (possibly qualified) 'plain' int bit-field is treated as a sign bit is implementation-defined”).
(See also issue 675 and issue 741.)Additional note, May, 2009:
As an example of the implications of this question, consider the following declaration:
struct S {
int i: 2;
signed int si: 2;
unsigned int ui: 2;
} s;
Is it implementation-defined which expression, cond?s.i:s.si or cond?s.i:s.ui, is an lvalue (the lvalueness of the result depends on the second and third operands having the same type, per 5.16 [expr.cond] paragraph 4)?
Proposed resolution (August, 2011):
Change 9.6 [class.bit] paragraph 3 as follows:
A bit-field shall not be a static member. A bit-field shall have integral or enumeration type (3.9.1 [basic.fundamental]). It is implementation-defined whether a plain (neither explicitly signed nor unsigned) char, short, int, long, or long long bit-field is signed or unsigned. For a bit-field with a non-dependent type (14.6.2.1 [temp.dep.type]) that is specified to be plain (neither explicitly signed nor unsigned) short, int, long, or long long or a typename-name that is so defined (possibly through multiple levels of typedefs), it is implementation-defined whether the type of the bit-field is the corresponding signed or unsigned type. [Example:
struct B { long x : 3; typedef signed int si; si y : 1; typedef int i; i z : 1; }; template<class T> struct A { T x : 7; };It is implementation-defined whether B::x has type signed long or unsigned long. B::y has type signed int. It is implementation-defined whether B::z has type signed int or unsigned int. A<int>::x and A<signed int>::x designate the same entity of type signed int. A<unsigned int>:;x has type unsigned int. —end example]
A bool value...
This resolution also resolves issue 675.
Note, January, 2012:
Additional questions have been raised about the proposed resolution, so the status was returned to "review" to allow further discussion.
Is the following example well-formed?
template<class T> struct A {
typedef int M;
struct B {
typedef void M;
struct C;
};
};
template<class T> struct A<T>::B::C : A<T> {
M // A<T>::M or A<T>::B::M?
p[2];
};
14.6.2 [temp.dep] paragraph 3 says the use of M should refer to A<T>::B::M because the base class A<T> is not searched because it's dependent. But in this case A<T> is also the current instantiation (14.6.2.1 [temp.dep.type]) so it seems like it should be searched.
Notes from the August, 2011 meeting:
The recent changes to the handling of the current instantiation may have sufficiently addressed this issue.
The current wording does not indicate that initialization of a non-class object is a full-expression, but presumably should do so.
According to 2.3 [lex.charset] paragraph 2,
The character designated by the universal-character-name \UNNNNNNNN is that character whose character short name in ISO/IEC 10646 is NNNNNNNN; the character designated by the universal-character-name \uNNNN is that character whose character short name in ISO/IEC 10646 is 0000NNNN. If the hexadecimal value for a universal-character-name corresponds to a surrogate code point (in the range 0xD800-0xDFFF, inclusive), the program is ill-formed. 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 0x00-0x1F or 0x7F-0x9F, both inclusive) or to a character in the basic source character set, the program is ill-formed.
It is not specified what should happen if the hexadecimal value does not designate a Unicode code point: is that undefined behavior or does it make the program ill-formed?
As an aside, a note should be added explaining why these requirements apply to to an r-char-sequence when, as the footnote at the end of the paragraph explains,
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.
2.5 [lex.pptoken] paragraph 2 specifies that there are 5 categories of tokens in phases 3 to 6. With 2.13 [lex.operators] paragraph 1, it is unclear whether new is an identifier or a preprocessing-op-or-punc; likewise for delete. This is relevant to answer the question whether
#define delete foo
is a well-formed control-line, since that requires an identifier after the define token.
(See also issue 189.)
The nonterminals operator and punctuator in 2.7 [lex.token] are not defined. There is a definition of the nonterminal operator in 13.5 [over.oper] paragraph 1, but it is apparent that the two nonterminals are not the same: the latter includes keywords and multi-token operators and does not include the nonoverloadable operators mentioned in paragraph 3.
There is a definition of preprocessing-op-or-punc in 2.13 [lex.operators] , with the notation that
Each preprocessing-op-or-punc is converted to a single token in translation phase 7 (2.1).However, this list doesn't distinguish between operators and punctuators, it includes digraphs and keywords (can a given token be both a keyword and an operator at the same time?), etc.
Suggested resolution:
Additional note (April, 2005):
The resolution for this problem should also address the fact that sizeof and typeid (and potentially others like decltype that may be added in the future) are described in some places as “operators” but are not listed in 13.5 [over.oper] paragraph 3 among the operators that cannot be overloaded.
(See also issue 369.)
The various uses of the term “declarative region” throughout the Standard indicate that the term is intended to refer to the entire block, class, or namespace that contains a given declaration. For example, 3.3 [basic.scope] paragraph 2 says, in part:
[Example: in
int j = 24; int main() { int i = j, j; j = 42; }The declarative region of the first j includes the entire example... The declarative region of the second declaration of j (the j immediately before the semicolon) includes all the text between { and }...
However, the actual definition given for “declarative region” in 3.3 [basic.scope] paragraph 1 does not match this usage:
Every name is introduced in some portion of program text called a declarative region, which is the largest part of the program in which that name is valid, that is, in which that name may be used as an unqualified name to refer to the same entity.
Because (except in class scope) a name cannot be used before it is declared, this definition contradicts the statement in the example and many other uses of the term throughout the Standard. As it stands, this definition is identical to that of the scope of a name.
The term “scope” is also misused. The scope of a declaration is defined in 3.3 [basic.scope] paragraph 1 as the region in which the name being declared is valid. However, there is frequent use of the phrase “the scope of a class,” not referring to the region in which the class's name is valid but to the declarative region of the class body, and similarly for namespaces, functions, exception handlers, etc. There is even a mention of looking up a name “in the scope of the complete postfix-expression” (3.4.5 [basic.lookup.classref] paragraph 3), which is the exact inverse of the scope of a declaration.
This terminology needs a thorough review to make it logically consistent. (Perhaps a discussion of the scope of template parameters could also be added to section 3.3 [basic.scope] at the same time, as all other kinds of scopes are described there.)
Proposed resolution (November, 2006):
Change 3.3 [basic.scope] paragraph 1 as follows:
Every name is introduced in some portion of program text called a declarative region, which is the largest part of the program in which that name is valid, that is, in which that name may be used as an unqualified name to refer to the same entity a statement, block, function declarator, function-definition, class, handler, template-declaration, template-parameter-list of a template template-parameter, or namespace. In general, each particular name is valid may be used as an unqualified name to refer to the entity of its declaration or to the label only within some possibly discontiguous portion of program text called its scope. To determine the scope of a declaration...
Change 3.3 [basic.scope] paragraph 3 as follows:
The names declared by a declaration are introduced into the scope in which the declaration occurs declarative region that directly encloses the declaration, except that declaration-statements, function parameter names in the declarator of a function-definition, exception-declarations (3.3.3 [basic.scope.local]), the presence of a friend specifier (11.3 [class.friend]), certain uses of the elaborated-type-specifier (7.1.6.3 [dcl.type.elab]), and using-directives (7.3.4 [namespace.udir]) alter this general behavior.
Change 3.3.3 [basic.scope.local] paragraphs 1-3 and add a new paragraph 4 before the existing paragraph 4 as follows:
A name declared in a block (6.3 [stmt.block]) is local to that block. Its potential scope begins at its point of declaration (3.3.2 [basic.scope.pdecl]) and ends at the end of its declarative region. The declarative region of a name declared in a declaration-statement is the directly enclosing block (6.3 [stmt.block]). Such a name is local to the block.
The potential scope declarative region of a function parameter name (including one appearing in the declarator of a function-definition or in a lambda-parameter-declaration-clause) or of a function-local predefined variable in a function definition (8.4 [dcl.fct.def]) begins at its point of declaration. If the function has a function-try-block the potential scope of a parameter or of a function-local predefined variable ends at the end of the last associated handler, otherwise it ends at the end of the outermost block of the function definition. A parameter name is the entire function definition or lambda-expression. Such a name is local to the function definition and shall not be redeclared in the any outermost block of the function definition nor in the outermost block of any handler associated with a function-try-block function-body (including handlers of a function-try-block) or lambda-expression.
The name in a catch exception-declaration The declarative region of a name declared in an exception-declaration is its entire handler. Such a name is local to the handler and shall not be redeclared in the outermost block of the handler.
The potential scope of any local name begins at its point of declaration (3.3.2 [basic.scope.pdecl]) and ends at the end of its declarative region.
Change 3.3.5 [basic.funscope] as indicated:
Labels (6.1 [stmt.label]) have function scope and may be used anywhere in the function in which they are declared except in members of local classes (9.8 [class.local]) of that function. Only labels have function scope.
Change 6.7 [stmt.dcl] paragraph 1 as follows:
A declaration statement introduces one or more new identifiers names into a block; it has the form
declaration-statement:
block-declaration
[Note: If an identifier a name introduced by a declaration was previously declared in an outer block, the outer declaration is hidden for the remainder of the block, after which it resumes its force (3.3.10 [basic.scope.hiding]). —end note]
[Drafting notes: This resolution deals almost exclusively with the unclear definition of “declarative region.” I've left the ambiguous use of “scope” alone for now. However sections 3.3.x all have headings reading “xxx scope,” but they don't mean the scope of a declaration but the different kinds of declarative regions and their effects on the scope of declarations contained therein. To me, it looks like most of 3.4 should refer to “declarative region” and not to “scope.”
The change to 6.7 fixes an “identifier” misuse (e.g., extern T operator+(T,T); at block scope introduces a name but not an identifier) and removes normative redundancy.]
The Standard does not completely specify how to look up the type-name(s) in a pseudo-destructor-name (5.2 [expr.post] paragraph 1, 5.2.4 [expr.pseudo]), and what information it does have is incorrect and/or in the wrong place. Consider, for instance, 3.4.5 [basic.lookup.classref] paragraphs 2-3:
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. If the type of the object expression is of pointer to scalar type, the unqualified-id is looked up in the context of the complete postfix-expression.
If the unqualified-id is ~type-name, and the type of the object expression is of a class type C (or of pointer to a class type C), the type-name is 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.
There are at least three things wrong with this passage with respect to pseudo-destructors:
A pseudo-destructor call (5.2.4 [expr.pseudo]) is not a “class member access”, so the statements about scalar types in the object expressions are vacuous: the object expression in a class member access is required to be a class type or pointer to class type (5.2.5 [expr.ref] paragraph 2).
On a related note, the lookup for the type-name(s) in a pseudo-destructor name should not be described in a section entitled “Class member access.”
Although the class member access object expressions are carefully allowed to be either a class type or a pointer to a class type, paragraph 2 mentions only a “pointer to scalar type” (disallowing references) and paragraph 3 deals only with a “scalar type,” presumably disallowing pointers (although it could possibly be a very subtle way of referring to both non-class pointers and references to scalar types at once).
The other point at which lookup of pseudo-destructors is mentioned is 3.4.3 [basic.lookup.qual] paragraph 5:
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.
Again, this specification is in the wrong location (a pseudo-destructor-name is not a qualified-id and thus should not be treated in the “Qualified name lookup” section).
Finally, there is no place in the Standard that describes the lookup for pseudo-destructor calls of the form p->T::~T() and r.T::~T(), where p and r are a pointer and reference to scalar, respectively. To the extent that it gives any guidance at all, 3.4.5 [basic.lookup.classref] deals only with the case where the ~ immediately follows the . or ->, and 3.4.3 [basic.lookup.qual] deals only with the case where the pseudo-destructor-name contains a nested-name-specifier that designates a scope in which names can be looked up.
See document J16/06-0008 = WG21 N1938 for further discussion of this and related issues, including 244, 305, 399, and 414.
Proposed resolution (June, 2008):
Add a new paragraph following 5.2 [expr.post] paragraph 2 as follows:
When a postfix-expression is followed by a dot . or arrow -> operator, the interpretation depends on the type T of the expression preceding the operator. If the operator is ., T shall be a scalar type or a complete class type; otherwise, T shall be a pointer to a scalar type or a pointer to a complete class type. When T is a (pointer to) a scalar type, the postfix-expression to which the operator belongs shall be a pseudo-destructor call (5.2.4 [expr.pseudo]); otherwise, it shall be a class member access (5.2.5 [expr.ref]).
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 The type of the expression preceding the dot operator, or the type to which the expression preceding the arrow operator points, is the object type...
Change 5.2.5 [expr.ref] paragraph 2 as follows:
For the first option (dot) the type of the first expression (the object expression) shall be “class object” (of a complete type) is a class type. For the second option (arrow) the type of the first expression (the pointer expression) shall be “pointer to class object” (of a complete type) is a pointer to a class type. In these cases, the id-expression shall name a member of the class or of one of its base classes.
Add a new paragraph following 3.4 [basic.lookup] paragraph 2 as follows:
In a pseudo-destructor-name that does not include a nested-name-specifier, the type-names are looked up as types in the context of the complete expression.
Delete the last sentence of 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, the unqualified-id is looked up in the scope of class C. If the type of the object expression is of pointer to scalar type, the unqualified-id is looked up in the context of the complete postfix-expression.
Notes from the August, 2011 meeting:
The proposed resolution must be updated with respect to the current wording of the WP.
The description of name lookup in the parameter-declaration-clause of member functions in 3.4.1 [basic.lookup.unqual] paragraphs 7-8 is flawed in at least two regards.
First, both paragraphs 7 and 8 apply to the parameter-declaration-clause of a member function definition and give different rules for the lookup. Paragraph 7 applies to names "used in the definition of a class X outside of a member function body...," which includes the parameter-declaration-clause of a member function definition, while paragraph 8 applies to names following the function's declarator-id (see the proposed resolution of issue 41), including the parameter-declaration-clause.
Second, paragraph 8 appears to apply to the type names used in the parameter-declaration-clause of a member function defined inside the class definition. That is, it appears to allow the following code, which was not the intent of the Committee:
struct S {
void f(I i) { }
typedef int I;
};
Additional note, January, 2012:
brace-or-equal-initializers for non-static data members are intended effectively as syntactic sugar for mem-initializers in constructor definitions; the lookup should be the same.
Paragraph 7 of 3.4.5 [basic.lookup.classref] says,
If the id-expression is a conversion-function-id, its conversion-type-id shall denote the same type in both the context in which the entire postfix-expression occurs and in the context of the class of the object expression (or the class pointed to by the pointer expression).Does this mean that the following example is ill-formed?
struct A { operator int(); } a;
void foo() {
typedef int T;
a.operator T(); // 1) error T is not found in the context
// of the class of the object expression?
}
The second bullet in paragraph 1 of
3.4.3.1 [class.qual]
says,
a conversion-type-id of an operator-function-id is looked up both in the scope of the class and in the context in which the entire postfix-expression occurs and shall refer to the same type in both contextsHow about:
struct A { typedef int T; operator T(); };
struct B : A { operator T(); } b;
void foo() {
b.A::operator T(); // 2) error T is not found in the context
// of the postfix-expression?
}
Is this interpretation correct? Or was the intent for
this to be an error only if
T was found in both scopes and referred to different entities?
If the intent was for these to be errors, how do these rules apply to template arguments?
template <class T1> struct A { operator T1(); }
template <class T2> struct B : A<T2> {
operator T2();
void foo() {
T2 a = A<T2>::operator T2(); // 3) error? when instantiated T2 is not
// found in the scope of the class
T2 b = ((A<T2>*)this)->operator T2(); // 4) error when instantiated?
}
}
(Note bullets 2 and 3 in paragraph 1 of 3.4.3.1 [class.qual] refer to postfix-expression. It would be better to use qualified-id in both cases.)
Erwin Unruh: The intent was that you look in both contexts. If you find it only once, that's the symbol. If you find it in both, both symbols must be "the same" in some respect. (If you don't find it, its an error).
Mike Miller: What's not clear to me in these examples is whether what is being looked up is T or int. Clearly the T has to be looked up somehow, but the "name" of a conversion function clearly involves the base (non-typedefed) type, not typedefs that might be used in a definition or reference (cf 3 [basic] paragraph 7 and 12.3 [class.conv] paragraph 5). (This is true even for types that must be written using typedefs because of the limited syntax in conversion-type-ids — e.g., the "name" of the conversion function in the following example
typedef void (*pf)();
struct S {
operator pf();
};
is S::operator void(*)(), even though you can't write its name
directly.)
My guess is that this means that in each scope you look up the type named in the reference and form the canonical operator name; if the name used in the reference isn't found in one or the other scope, the canonical name constructed from the other scope is used. These names must be identical, and the conversion-type-id in the canonical operator name must not denote different types in the two scopes (i.e., the type might not be found in one or the other scope, but if it's found in both, they must be the same type).
I think this is all very vague in the current wording.
3.4.5 [basic.lookup.classref] does not mention template aliases as the possible result of the lookup but should do so.
In an example like
template<typename T> void f(T p)->decltype(p.T::x);
The nested-name-specifier T:: looks like it refers to the template parameter. However, if this is instantiated with a type like
struct T { int x; };
struct S: T { };
the reference will be ambiguous, since it is looked up in both the context of the expression, finding the template parameter, and in the class, finding the base class injected-class-name, and this could be a deduction failure. As a result, the same declaration with a different parameter name
template<typename U> void f(U p)->decltype(p.U::x);
is, in fact, not a redeclaration because the two can be distinguished by SFINAE.
It would be better to add a new lookup rule that says that if a name in a template definition resolves to a template parameter, that name is not subject to further lookup at instantiation time.
The Standard talks about looking up a conversion-type-id as if it were an identifier (3.4.5 [basic.lookup.classref] paragraph 7), but that is not exactly accurate. Presumably it should talk instead about looking up names (if any) appearing in the type-specifier-seq of the conversion-type-id.
An example in 3.5 [basic.link] paragraph 6 creates two file-scope variables with the same name, one with internal linkage and one with external.
static void f();
static int i = 0; //1
void g() {
extern void f(); // internal linkage
int i; //2: i has no linkage
{
extern void f(); // internal linkage
extern int i; //3: external linkage
}
}
Is this really what we want? C99 has 6.2.2.7/7, which gives undefined behavior for having an identifier appear with internal and external linkage in the same translation unit. C++ doesn't seem to have an equivalent.
Notes from October 2003 meeting:
We agree that this is an error. We propose to leave the example but change the comment to indicate that line //3 has undefined behavior, and elsewhere add a normative rule giving such a case undefined behavior.
Proposed resolution (October, 2005):
Change 3.5 [basic.link] paragraph 6 as indicated:
...Otherwise, if no matching entity is found, the block scope entity receives external linkage. If, within a translation unit, the same entity is declared with both internal and external linkage, the behavior is undefined.
[Example:
static void f(); static int i = 0; // 1 void g () { extern void f (); // internal linkage int i; // 2: i has no linkage { extern void f (); // internal linkage extern int i; // 3: external linkage } }There are three objects named i in this program. The object with internal linkage introduced by the declaration in global scope (line //1 ), the object with automatic storage duration and no linkage introduced by the declaration on line //2, and the object with static storage duration and external linkage introduced by the declaration on line //3. Without the declaration at line //2, the declaration at line //3 would link with the declaration at line //1. But because the declaration with internal linkage is hidden, //3 is given external linkage, resulting in a linkage conflict. —end example]
Notes frum the April 2006 meeting:
According to 3.5 [basic.link] paragraph 9, the two variables with linkage in the proposed example are not “the same entity” because they do not have the same linkage. Some other formulation will be needed to describe the relationship between those two variables.
Notes from the October 2006 meeting:
The CWG decided that it would be better to make a program with this kind of linkage mismatch ill-formed instead of having undefined behavior.
Is the following well-formed?
int f() {
int i = 3;
new (&i) float(1.2);
return i;
}
The wording that is intended to prevent such shenanigans, 3.8 [basic.life] paragraphs 7-9, doesn't quite apply here. In particular, paragraph 7 reads,
If, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, a new object is created at the storage location which the original object occupied, a pointer that pointed to the original object, a reference that referred to the original object, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new object, if:
the storage for the new object exactly overlays the storage location which the original object occupied, and
the new object is of the same type as the original object (ignoring the top-level cv-qualifiers), and...
The problem here is that this wording only applies “after the lifetime of an object has ended and before the storage which the object occupied is reused;” for an object of a scalar type, its lifetime only ends when the storage is reused or released (paragraph 1), so it appears that these restrictions cannot apply to such objects.
Proposed resolution (August, 2010):
This issue is resolved by the resolution of issue 1116.
Related to issue 1027, consider:
int f() {
union U { double d; } u1, u2;
(int&)u1.d = 1;
u2 = u1;
return (int&)u2.d;
}
Does this involve undefined behavior? 3.8 [basic.life] paragraph 4 seems to say that it's OK to clobber u1 with an int object. Then union assignment copies the object representation, possibly creating an int object in u2 and making the return statement well-defined. If this is well-defined, compilers are significantly limited in the assumptions they can make about type aliasing. On the other hand, the variant where U has an array of unsigned char member must be well-defined in order to support std::aligned_storage.
Suggested resolution: Clarify that this case is undefined, but that adding an array of unsigned char to union U would make it well-defined — if a storage location is allocated with a particular type, it should be undefined to create an object in that storage if it would be undefined to access the stored value of the object through the allocated type.
Proposed resolution (August, 2010):
Change 3.8 [basic.life] paragraph 1 as follows:
...The lifetime of an object of type T begins when storage with the proper alignment and size for type T is obtained, and either:
- storage with the proper alignment and size for type T is obtained, and
if the object has non-trivial initialization, its initialization is complete., or
if T is trivially copyable, the object representation of another T object is copied into the storage.
The lifetime of an object of type T ends...
Change 3.8 [basic.life] paragraph 4 as follows:
A program may end the lifetime of any object by reusing the storage which the object occupies or by explicitly calling the destructor for an object of a class type with a non-trivial destructor. For an object of a class type with a non-trivial destructor, the program is not required to call the destructor explicitly before the storage which the object occupies is reused or released; however, if there is no explicit call to the destructor or if a delete-expression (5.3.5 [expr.delete]) is not used to release the storage, the destructor shall not be implicitly called and any program that depends on the side effects produced by the destructor has undefined behavior. If a program obtains storage for an object of a particular type A (e.g. with a variable definition or new-expression) and later reuses that storage for an object of another type B such that accessing the stored value of the B object through a glvalue of type A would have undefined behavior (3.10 [basic.lval]), the behavior is undefined. [Example:
int i; (double&)i = 1.0; // undefined behavior struct S { unsigned char alignas(double) ar[sizeof (double)]; } s; (double&)s = 1.0; // OK, can access stored double through s because it has an unsigned char subobject—end example]
Change 3.10 [basic.lval] paragraph 10 as follows:
If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined52:
the dynamic type of the object,
a cv-qualified version of the dynamic type of the object,
a type similar (as defined in 4.4 [conv.qual]) to the dynamic type of the object,
a type that is the signed or unsigned type corresponding to the dynamic type of the object,
a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object,
a char or unsigned char type,
an aggregate or union type that includes one of the aforementioned types among its elements, bases, or non-static data members (including, recursively, an element, base, or non-static data member of a subaggregate, base, or contained union),.
a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,
a char or unsigned char type.
This resolution also resolves issue 1027.
The requirement in 3.8 [basic.life] paragraph 10 that
is mostly redundant with the constexpr constructor requirements in 7.1.5 [dcl.constexpr] paragraph 4 (although 12.6.2 [class.base.init] does not establish a strict equivalence between brace-or-equal-initializers and mem-initializers).
The aliasing rules given in 3.10 [basic.lval] paragraph 10 rely on the concept of “dynamic type.” The problem is that the dynamic type of an object often cannot be determined (or even sufficiently constrained) at the point at which an optimizer needs to be able to determine whether aliasing might occur or not. For example, consider the function
void foo(int* p, double* q) {
*p = 42;
*q = 3.14;
}
An optimizer, on the basis of the existing aliasing rules, might decide that an int* and a double* cannot refer to the same object and reorder the assignments. This reordering, however, could result in undefined behavior if the function foo is called as follows:
void goo() {
union {
int i;
double d;
} t;
t.i = 12;
foo(&t.i, &t.d);
cout << t.d << endl;
};
Here, the reference to t.d after the call to foo will be valid only if the assignments in foo are executed in the order in which they were written; otherwise, the union will contain an int object rather than a double.
One possibility would be to require that if such aliasing occurs, it be done only via member names and not via pointers.
Notes from the July, 2007 meeting:
This is the same issue as C's DR236. The CWG expressed a desire to address the issue the same way C99 does. The issue also occurs in C++ when placement new is used to end the lifetime of one object and start the lifetime of a different object occupying the same storage.
3.11 [basic.align] speaks of “alignment requirements,” and 3.7.4.1 [basic.stc.dynamic.allocation] requires the result of an allocation function to point to “suitably aligned” storage, but there is no explicit statement of what happens when these requirements are violated (presumably undefined behavior).
According to 4.1 [conv.lval] paragraph 1, applying the lvalue-to-rvalue conversion to any uninitialized object results in undefined behavior. However, character types are intended to allow any data, including uninitialized objects and padding, to be copied (hence the statements in 3.9.1 [basic.fundamental] paragraph 1 that “For character types, all bits of the object representation participate in the value representation” and in 3.10 [basic.lval] paragraph 15 that char and unsigned char types can alias any object). The lvalue-to-rvalue conversion should be permitted on uninitialized objects of character type without evoking undefined behavior.
The descriptions of explicit (5.2.9 [expr.static.cast] paragraph 9) and implicit (4.11 [conv.mem] paragraph 2) pointer-to-member conversions differ in two significant ways:
(This situation cannot arise in an implicit pointer-to-member conversion where the source value is something like &X::f, since you can only implicitly convert from pointer-to-base-member to pointer-to-derived-member. However, if the source value is the result of an explicit "up-cast," the target type of the conversion might still not contain the member referred to by the source value.)
The first difference seems like an oversight. It is not clear whether the latter difference is intentional or not.
(See also issue 794.)
There are at least a couple of problems in the description of the various id-expressions in 5.1.1 [expr.prim.general]:
Paragraph 4 embodies an incorrect assumption about the syntax of qualified-ids:
The operator :: followed by an identifier, a qualified-id, or an operator-function-id is a primary-expression.
The problem here is that the :: is actually part of the syntax of qualified-id; consequently, “:: followed by... a qualified-id” could be something like “:: ::i,” which is ill-formed. Presumably this should say something like, “A qualified-id with no nested-name-specifier is a primary-expression.”
More importantly, some kinds of id-expressions are not described by 5.1.1 [expr.prim.general]. The structure of this section is that the result, type, and lvalue-ness are specified for each of the cases it covers:
paragraph 4 deals with qualified-ids that have no nested-name-specifier
paragraph 7 deals with bare identifiers and with qualified-ids containing a nested-name-specifier that names a class
paragraph 8 deals with qualified-ids containing a nested-name-specifier that names a namespace
This treatment leaves unspecified all the non-identifier unqualified-ids (operator-function-id, conversion-function-id, and template-id), as well as (perhaps) “:: template-id” (it's not clear whether the “:: followed by a qualified-id” case is supposed to apply to template-ids or not). Note also that the proposed resolution of issue 301 slightly exacerbates this problem by removing the form of operator-function-id that contains a tmeplate-argument-list; as a result, references like “::operator+<X>” are no longer covered in 5.1.1 [expr.prim.general].
Consider the following example:
void f(int i) {
auto l1 = [i] {
auto l2 = [&i] {
++i; // Well-formed?
};
};
}
Because the l1 lambda is not marked as mutable, its operator() is const; however, it is not clear from the wording of 5.1.2 [expr.prim.lambda] paragraph 16 whether the captured member of the enclosing lambda is considered const or not.
Split off from issue 315.
Incidentally, another thing that ought to be cleaned up is the inconsistent use of "indirection" and "dereference". We should pick one.
Proposed resolution (December, 2006):
Change 5.3.1 [expr.unary.op] paragraph 1 as follows:
The unary * operator performs indirection dereferences a pointer value: the expression to which it is applied shall be a pointer...
Change 8.3.4 [dcl.array] paragraph 8 as follows:
The results are added and indirection applied values are added and the result is dereferenced to yield an array (of five integers), which in turn is converted to a pointer to the first of the integers.
Change 8.3.5 [dcl.fct] paragraph 9 as follows:
The binding of *fpi(int) is *(fpi(int)), so the declaration suggests, and the same construction in an expression requires, the calling of a function fpi, and then using indirection through dereferencing the (pointer) result to yield an integer. In the declarator (*pif)(const char*, const char*), the extra parentheses are necessary to indicate that indirection through dereferencing a pointer to a function yields a function, which is then called.
Change the index for * and “dereferencing” no longer to refer to “indirection.”
[Drafting note: 26.6.9 [template.indirect.array] requires no change. Many more places in the current wording use “dereferencing” than “indirection.”]
Notes from the August, 2011 meeting:
CWG prefers use of the term “indirection” instead of “dereferencing.” This would be consistent with the usage in the C Standard and would avoid entanglement with the C++ concept of a reference type.
At least a couple of places in the IS state that indirection through a null pointer produces undefined behavior: 1.9 [intro.execution] paragraph 4 gives "dereferencing the null pointer" as an example of undefined behavior, and 8.3.2 [dcl.ref] paragraph 4 (in a note) uses this supposedly undefined behavior as justification for the nonexistence of "null references."
However, 5.3.1 [expr.unary.op] paragraph 1, which describes the unary "*" operator, does not say that the behavior is undefined if the operand is a null pointer, as one might expect. Furthermore, at least one passage gives dereferencing a null pointer well-defined behavior: 5.2.8 [expr.typeid] paragraph 2 says
If the lvalue expression is obtained by applying the unary * operator to a pointer and the pointer is a null pointer value (4.10 [conv.ptr]), the typeid expression throws the bad_typeid exception (18.7.3 [bad.typeid]).
This is inconsistent and should be cleaned up.
Bill Gibbons:
At one point we agreed that dereferencing a null pointer was not undefined; only using the resulting value had undefined behavior.
For example:
char *p = 0;
char *q = &*p;
Similarly, dereferencing a pointer to the end of an array should be allowed as long as the value is not used:
char a[10];
char *b = &a[10]; // equivalent to "char *b = &*(a+10);"
Both cases come up often enough in real code that they should be allowed.
Mike Miller:
I can see the value in this, but it doesn't seem to be well reflected in the wording of the Standard. For instance, presumably *p above would have to be an lvalue in order to be the operand of "&", but the definition of "lvalue" in 3.10 [basic.lval] paragraph 2 says that "an lvalue refers to an object." What's the object in *p? If we were to allow this, we would need to augment the definition to include the result of dereferencing null and one-past-the-end-of-array.
Tom Plum:
Just to add one more recollection of the intent: I was very happy when (I thought) we decided that it was only the attempt to actually fetch a value that creates undefined behavior. The words which (I thought) were intended to clarify that are the first three sentences of the lvalue-to-rvalue conversion, 4.1 [conv.lval]:
An lvalue (3.10 [basic.lval]) of a non-function, non-array type T can be converted to an rvalue. If T is an incomplete type, a program that necessitates this conversion is ill-formed. If the object to which the lvalue refers is not an object of type T and is not an object of a type derived from T, or if the object is uninitialized, a program that necessitates this conversion has undefined behavior.
In other words, it is only the act of "fetching", of lvalue-to-rvalue conversion, that triggers the ill-formed or undefined behavior. Simply forming the lvalue expression, and then for example taking its address, does not trigger either of those errors. I described this approach to WG14 and it may have been incorporated into C 1999.
Mike Miller:
If we admit the possibility of null lvalues, as Tom is suggesting here, that significantly undercuts the rationale for prohibiting "null references" -- what is a reference, after all, but a named lvalue? If it's okay to create a null lvalue, as long as I don't invoke the lvalue-to-rvalue conversion on it, why shouldn't I be able to capture that null lvalue as a reference, with the same restrictions on its use?
I am not arguing in favor of null references. I don't want them in the language. What I am saying is that we need to think carefully about adopting the permissive approach of saying that it's all right to create null lvalues, as long as you don't use them in certain ways. If we do that, it will be very natural for people to question why they can't pass such an lvalue to a function, as long as the function doesn't do anything that is not permitted on a null lvalue.
If we want to allow &*(p=0), maybe we should change the definition of "&" to handle dereferenced null specially, just as typeid has special handling, rather than changing the definition of lvalue to include dereferenced nulls, and similarly for the array_end+1 case. It's not as general, but I think it might cause us fewer problems in the long run.
Notes from the October 2003 meeting:
See also issue 315, which deals with the call of a static member function through a null pointer.
We agreed that the approach in the standard seems okay: p = 0; *p; is not inherently an error. An lvalue-to-rvalue conversion would give it undefined behavior.
Proposed resolution (October, 2004):
(Note: the resolution of issue 453 also resolves part of this issue.)
Add the indicated words to 3.10 [basic.lval] paragraph 2:
An lvalue refers to an object or function or is an empty lvalue (5.3.1 [expr.unary.op]).
Add the indicated words to 5.3.1 [expr.unary.op] paragraph 1:
The unary * operator performs indirection: the expression to which it is applied shall be a pointer to an object type, or a pointer to a function type and the result is an lvalue referring to the object or function to which the expression points, if any. If the pointer is a null pointer value (4.10 [conv.ptr]) or points one past the last element of an array object (5.7 [expr.add]), the result is an empty lvalue and does not refer to any object or function. An empty lvalue is not modifiable. If the type of the expression is “pointer to T,” the type of the result is “T.” [Note: a pointer to an incomplete type (other than cv void) can be dereferenced. The lvalue thus obtained can be used in limited ways (to initialize a reference, for example); this lvalue must not be converted to an rvalue, see 4.1 [conv.lval].—end note]
Add the indicated words to 4.1 [conv.lval] paragraph 1:
If the object to which the lvalue refers is not an object of type T and is not an object of a type derived from T, or if the object is uninitialized, or if the lvalue is an empty lvalue (5.3.1 [expr.unary.op]), a program that necessitates this conversion has undefined behavior.
Change 1.9 [intro.execution] as indicated:
Certain other operations are described in this International Standard as undefined (for example, the effect of dereferencing the null pointer division by zero).
Note (March, 2005):
The 10/2004 resolution interacts with the resolution of issue 73. We added wording to 3.9.2 [basic.compound] paragraph 3 to the effect that a pointer containing the address one past the end of an array is considered to “point to” another object of the same type that might be located there. The 10/2004 resolution now says that it would be undefined behavior to use such a pointer to fetch the value of that object. There is at least the appearance of conflict here; it may be all right, but it at needs to be discussed further.
Notes from the April, 2005 meeting:
The CWG agreed that there is no contradiction between this direction and the resolution of issue 73. However, “not modifiable” is a compile-time concept, while in fact this deals with runtime values and thus should produce undefined behavior instead. Also, there are other contexts in which lvalues can occur, such as the left operand of . or .*, which should also be restricted. Additional drafting is required.
(See also issue 1102.)
It is not clear from 5.3.4 [expr.new] whether a deleted operator delete is referenced by a new-expression in which there is no initialization or in which the initialization cannot throw an exception, rendering the program ill-formed. (The question also arises as to whether such a new-expression constitutes a “use” of the deallocation function in the sense of 3.2 [basic.def.odr].)
Notes from the July, 2009 meeting:
The rationale for defining a deallocation function as deleted would presumably be to prevent such objects from being freed. Treating the new-expression as a use of such a deallocation function would mean that such objects could not be created in the first place. There is already an exemption from freeing an object if “a suitable deallocation function [cannot] be found;” a deleted deallocation function should be treated similarly.
Because the restriction that a trailing-return-type can appear only in a declaration with “the single type-specifier auto” (8.3.5 [dcl.fct] paragraph 2) is a semantic, not a syntactic, restriction, it does not influence disambiguation, which is “purely syntactic” (6.8 [stmt.ambig] paragraph 3). Consequently, some previously unambiguous expressions are now ambiguous. For example:
struct A {
A(int *);
A *operator()(void);
int B;
};
int *p;
typedef struct BB { int C[2]; } *B, C;
void foo() {
// The following line becomes invalid under C++0x:
A (p)()->B; // ill-formed function declaration
// In the following,
// - B()->C is either type-id or class member access expression
// - B()->C[1] is either type-id or subscripting expression
// N3126 subclause 8.2 [dcl.ambig.res] does not mention an ambiguity
// with these forms of expression
A a(B ()->C); // function declaration or object declaration
sizeof(B ()->C[1]); // sizeof(type-id) or sizeof on an expression
}
Notes from the March, 2011 meeting:
CWG agreed that the presence of auto should be considered in disambiguation, even though it is formally handled semantically rather than syntactically.
With the resolution of issue 1044, there is no need to say that the name of the alias cannot appear in the type-id of the declaration.
The permission granted implementations in 7.1.5 [dcl.constexpr] paragraph 5 to diagnose definitions of constexpr functions that can never be used in a constant expression should not apply to an instantiated constexpr function template.
Notes from the August, 2011 meeting:
int f(); // not constexpr
struct A {
int m;
constexpr A(int i = f()) : m(i) { }
};
struct B {
A a;
} b;
This is ill-formed, no diagnostic required, because the defaulted default constructor of B will be declared constexpr but can never be invoked in a constant expression. See issue 1360.
A constexpr constructor is required to initialize all non-static data members (7.1.5 [dcl.constexpr] paragraph 4), which conflicts with the requirement that a constructor for a union is permitted to initialize only a single non-static data member (12.6.2 [class.base.init] paragraph 8).
It is not clear whether the auto specifier can appear in a trailing-return-type.
7.3.1.2 [namespace.memdef] paragraph 3 says,
If a friend declaration in a non-local class first declares a class or function the friend class or function is a member of the innermost enclosing namespace... When looking for a prior declaration of a class or a function declared as a friend, scopes outside the innermost enclosing namespace scope are not considered.It is not clear from this passage how to determine whether an entity is "first declared" in a friend declaration. One question is whether a using-declaration influences this determination. For instance:
void foo();
namespace A{
using ::foo;
class X{
friend void foo();
};
}
Is the friend declaration a reference to ::foo or
a different foo?
Part of the question involves determining the meaning of the word "synonym" in 7.3.3 [namespace.udecl] paragraph 1:
A using-declaration introduces a name into the declarative region in which the using-declaration appears. That name is a synonym for the name of some entity declared elsewhere.Is "using ::foo;" the declaration of a function or not?
More generally, the question is how to describe the lookup of the name in a friend declaration.
John Spicer: When a declaration specifies an unqualified name, that name is declared, not looked up. There is a mechanism in which that declaration is linked to a prior declaration, but that mechanism is not, in my opinion, via normal name lookup. So, the friend always declares a member of the nearest namespace scope regardless of how that name may or may not already be declared there.
Mike Miller: 3.4.1 [basic.lookup.unqual] paragraph 7 says:
A name used in the definition of a class X outside of a member function body or nested class definition shall be declared in one of the following ways:... [Note: when looking for a prior declaration of a class or function introduced by a friend declaration, scopes outside of the innermost enclosing namespace scope are not considered.]The presence of this note certainly implies that this paragraph describes the lookup of names in friend declarations.
John Spicer: It most certainly does not. If that section described the friend lookup it would yield the incorrect results for the friend declarations of f and g below. I don't know why that note is there, but it can't be taken to mean that that is how the friend lookup is done.
void f(){}
void g(){}
class B {
void g();
};
class A : public B {
void f();
friend void f(); // ::f not A::f
friend void g(); // ::g not B::g
};
Mike Miller: If so, the lookups for friend functions and classes behave differently. Consider the example in 3.4.4 [basic.lookup.elab] paragraph 3:
struct Base {
struct Data; // OK: declares nested Data
friend class Data; // OK: nested Data is a friend
};
If the friend declaration is not a reference to ::foo, there is a related but separate question: does the friend declaration introduce a conflicting (albeit "invisible") declaration into namespace A, or is it simply a reference to an as-yet undeclared (and, in this instance, undeclarable) A::foo? Another part of the example in 3.4.4 [basic.lookup.elab] paragraph 3 is related:
struct Data {
friend struct Glob; // OK: Refers to (as yet) undeclared Glob
// at global scope.
};
John Spicer: You can't refer to something that has not yet been declared. The friend is a declaration of Glob, it just happens to declare it in a such a way that its name cannot be used until it is redeclared.
(A somewhat similar question has been raised in connection with issue 36. Consider:
namespace N {
struct S { };
}
using N::S;
struct S; // legal?
According to 9.1 [class.name] paragraph 2,
A declaration consisting solely of class-key identifier ; is either a redeclaration of the name in the current scope or a forward declaration of the identifier as a class name.
Should the elaborated type declaration in this example be considered a redeclaration of N::S or an invalid forward declaration of a different class?)
(See also issues 95, 136, 139, 143, 165, and 166, as well as paper J16/00-0006 = WG21 N1229.)
The following came up recently on comp.lang.c++.moderated (edited for brevity):
namespace N1 {
template<typename T> void f( T* x ) {
// ... other stuff ...
delete x;
}
}
namespace N2 {
using N1::f;
template<> void f<int>( int* ); // A: ill-formed
class Test {
~Test() { }
friend void f<>( Test* x ); // B: ill-formed?
};
}
I strongly suspect, but don't have standardese to prove, that the friend declaration in line B is ill-formed. Can someone show me the text that allows or disallows line B?
Here's my reasoning: Writing "using" to pull the name into namespace N2 merely allows code in N2 to use the name in a call without qualification (per 7.3.3 [namespace.udecl]). But just as declaring a specialization must be done in the namespace where the template really lives (hence line A is ill-formed), I suspect that declaring a specialization as a friend must likewise be done using the original namespace name, not obliquely through a "using". I see nothing in 7.3.3 [namespace.udecl] that would permit this use. Is there?
Andrey Tarasevich: 14.5.4 [temp.friend] paragraph 2 seems to get pretty close: "A friend declaration that is not a template declaration and in which the name of the friend is an unqualified 'template-id' shall refer to a specialization of a function template declared in the nearest enclosing namespace scope".
Herb Sutter: OK, thanks. Then the question in this is the word "declared" -- in particular, we already know we cannot declare a specialization of a template in any other namespace but the original one.
John Spicer: This seems like a simple question, but it isn't.
First of all, I don't think the standard comments on this usage one way or the other.
A similar example using a namespace qualified name is ill-formed based on 8.3 [dcl.meaning] paragraph 1:
namespace N1 {
void f();
}
namespace N2 {
using N1::f;
class A {
friend void N2::f();
};
}
Core issue 138 deals with this example:
void foo();
namespace A{
using ::foo;
class X{
friend void foo();
};
}
The proposed resolution (not yet approved) for issue 138 is that the friend declares a new foo that conflicts with the using-declaration and results in an error.
Your example is different than this though because the presence of the explicit argument list means that this is not declaring a new f but is instead using a previously declared f.
One reservation I have about allowing the example is the desire to have consistent rules for all of the "declaration like" uses of template functions. Issue 275 (in DR status) addresses the issue of unqualified names in explicit instantiation and explicit specialization declarations. It requires that such declarations refer to templates from the namespace containing the explicit instantiation or explicit specialization. I believe this rule is necessary for those directives but is not really required for friend declarations -- but there is the consistency issue.
Notes from April 2003 meeting:
This is related to issue 138. John Spicer is supposed to update his paper on this topic. This is a new case not covered in that paper. We agreed that the B line should be allowed.
8.3.2 [dcl.ref] paragraph 4 says:
A reference shall be initialized to refer to a valid object or function. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the "object" obtained by dereferencing a null pointer, which causes undefined behavior ...]
What is a "valid" object? In particular the expression "valid object" seems to exclude uninitialized objects, but the response to Core Issue 363 clearly says that's not the intent. This is an example (overloading construction on constness of *this) by John Potter, which I think is supposed to be legal C++ though it binds references to objects that are not initialized yet:
struct Fun {
int x, y;
Fun (int x, Fun const&) : x(x), y(42) { }
Fun (int x, Fun&) : x(x), y(0) { }
};
int main () {
const Fun f1 (13, f1);
Fun f2 (13, f2);
cout << f1.y << " " << f2.y << "\n";
}
Suggested resolution: Changing the final part of 8.3.2 [dcl.ref] paragraph 4 to:
A reference shall be initialized to refer to an object or function.
From its point of declaration on (see 3.3.2 [basic.scope.pdecl])
its name is an lvalue
which refers to that object or function. The reference may be
initialized to refer to an uninitialized object but, in that case,
it is usable in limited ways (3.8 [basic.life], paragraph 6)
[Note: On the other hand, a declaration like this:
int & ref = *(int*)0;
is ill-formed because ref will not refer to any object or function
]
I also think a "No diagnostic is required." would better be added (what about something like int& r = r; ?)
Proposed Resolution (October, 2004):
(Note: the following wording depends on the proposed resolution for issue 232.)
Change 8.3.2 [dcl.ref] paragraph 4 as follows:
A reference shall be initialized to refer to a valid object or function. If an lvalue to which a reference is directly bound designates neither an existing object or function of an appropriate type (8.5.3 [dcl.init.ref]), nor a region of memory of suitable size and alignment to contain an object of the reference's type (1.8 [intro.object], 3.8 [basic.life], 3.9 [basic.types]), the behavior is undefined. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “object” empty lvalue obtained by dereferencing a null pointer, which causes undefined behavior. As does not designate an object or function. Also, as described in 9.6 [class.bit], a reference cannot be bound directly to a bit-field. ]
The name of a reference shall not be used in its own initializer. Any other use of a reference before it is initialized results in undefined behavior. [Example:
int& f(int&); int& g(); extern int& ir3; int* ip = 0; int& ir1 = *ip; // undefined behavior: null pointer int& ir2 = f(ir3); // undefined behavior: ir3 not yet initialized int& ir3 = g(); int& ir4 = f(ir4); // ill-formed: ir4 used in its own initializer—end example]
Rationale: The proposed wording goes beyond the specific concerns of the issue. It was noted that, while the current wording makes cases like int& r = r; ill-formed (because r in the initializer does not "refer to a valid object"), an inappropriate initialization can only be detected, if at all, at runtime and thus "undefined behavior" is a more appropriate treatment. Nevertheless, it was deemed desirable to continue to require a diagnostic for obvious compile-time cases.
It was also noted that the current Standard does not say anything about using a reference before it is initialized. It seemed reasonable to address both of these concerns in the same wording proposed to resolve this issue.
Notes from the April, 2005 meeting:
The CWG decided that whether to require an implementation to diagnose initialization of a reference to itself should be handled as a separate issue (504) and also suggested referring to “storage” instead of “memory” (because 1.8 [intro.object] defines an object as a “region of storage”).
Proposed Resolution (April, 2005):
(Note: the following wording depends on the proposed resolution for issue 232.)
Change 8.3.2 [dcl.ref] paragraph 4 as follows:
A reference shall be initialized to refer to a valid object or function. If an lvalue to which a reference is directly bound designates neither an existing object or function of an appropriate type (8.5.3 [dcl.init.ref]), nor a region of storage of suitable size and alignment to contain an object of the reference's type (1.8 [intro.object], 3.8 [basic.life], 3.9 [basic.types]), the behavior is undefined. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “object” empty lvalue obtained by dereferencing a null pointer, which causes undefined behavior. As does not designate an object or function. Also, as described in 9.6 [class.bit], a reference cannot be bound directly to a bit-field. ]
Any use of a reference before it is initialized results in undefined behavior. [Example:
int& f(int&); int& g(); extern int& ir3; int* ip = 0; int& ir1 = *ip; // undefined behavior: null pointer int& ir2 = f(ir3); // undefined behavior: ir3 not yet initialized int& ir3 = g(); int& ir4 = f(ir4); // undefined behavior: ir4 used in its own initializer—end example]
Note (February, 2006):
The word “use” in the last paragraph of the proposed resolution was intended to refer to the description in 3.2 [basic.def.odr] paragraph 2. However, that section does not define what it means for a reference to be “used,” dealing only with objects and functions. Additional drafting is required to extend 3.2 [basic.def.odr] paragraph 2 to apply to references.
Additional note (May, 2008):
The proposed resolution for issue 570 adds wording to define “use” for references.
Note, January, 2012:
The resolution should also probably deal with the fact that the “one-past-the-end” address of an array does not designate a valid object (even if such a pointer might “point to” an object of the correct type, per 3.9.2 [basic.compound]) and thus is not suuitable for the lvalue-to-rvalue conversion.
EDG rejects this code:
template <typename T>
struct S {};
void f (S<int (*)[]>);
G++ accepts it.
This is another case where the standard isn't very clear:
The language from 8.3.5 [dcl.fct] is:
If the type of a parameter includes a type of the form "pointer to array of unknown bound of T" or "reference to array of unknown bound of T," the program is ill-formed.Since "includes a type" is not a term defined in the standard, we're left to guess what this means. (It would be better if this were a recursive definition, the way a type theoretician would do it:
Notes from April 2003 meeting:
We agreed that the example should be allowed.
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.
What if a const POD object has no non-static data members? This wording requires an empty initializer for such cases:
struct Z {
// no data members
operator int() const { return 0; }
};
void f() {
const Z z1; // ill-formed: no initializer
const Z z2 = { }; // well-formed
}
Similar comments apply to a non-POD const object, all of whose non-static data members and base class subobjects have default constructors. Why should the class of such an object be required to have a user-declared default constructor?
(See also issue 78.)
Additional note (February, 2011):
This issue should be brought up again in light of constexpr constructors and non-static data member initializers.
Notes from the August, 2011 meeting:
If the implicit default constructor initializes all subobjects, no initializer should be required.
There is an inconsistency in the handling of references vs pointers in user defined conversions and overloading. The reason for that is that the combination of 8.5.3 [dcl.init.ref] and 4.4 [conv.qual] circumvents the standard way of ranking conversion functions, which was probably not the intention of the designers of the standard.
Let's start with some examples, to show what it is about:
struct Z { Z(){} };
struct A {
Z x;
operator Z *() { return &x; }
operator const Z *() { return &x; }
};
struct B {
Z x;
operator Z &() { return x; }
operator const Z &() { return x; }
};
int main()
{
A a;
Z *a1=a;
const Z *a2=a; // not ambiguous
B b;
Z &b1=b;
const Z &b2=b; // ambiguous
}
So while both classes A and B are structurally equivalent, there is a difference in operator overloading. I want to start with the discussion of the pointer case (const Z *a2=a;): 13.3.3 [over.match.best] is used to select the best viable function. Rule 4 selects A::operator const Z*() as best viable function using 13.3.3.2 [over.ics.rank] since the implicit conversion sequence const Z* -> const Z* is a better conversion sequence than Z* -> const Z*.
So what is the difference to the reference case? Cv-qualification conversion is only applicable for pointers according to 4.4 [conv.qual]. According to 8.5.3 [dcl.init.ref] paragraphs 4-7 references are initialized by binding using the concept of reference-compatibility. The problem with this is, that in this context of binding, there is no conversion, and therefore there is also no comparing of conversion sequences. More exactly all conversions can be considered identity conversions according to 13.3.3.1.4 [over.ics.ref] paragraph 1, which compare equal and which has the same effect. So binding const Z* to const Z* is as good as binding const Z* to Z* in terms of overloading. Therefore const Z &b2=b; is ambiguous. [13.3.3.1.4 [over.ics.ref] paragraph 5 and 13.3.3.2 [over.ics.rank] paragraph 3 rule 3 (S1 and S2 are reference bindings ...) do not seem to apply to this case]
There are other ambiguities, that result in the special treatment of references: Example:
struct A {int a;};
struct B: public A { B() {}; int b;};
struct X {
B x;
operator A &() { return x; }
operator B &() { return x; }
};
main()
{
X x;
A &g=x; // ambiguous
}
Since both references of class A and B are reference compatible with references of class A and since from the point of ranking of implicit conversion sequences they are both identity conversions, the initialization is ambiguous.
So why should this be a defect?
So overall I think this was not the intention of the authors of the standard.
So how could this be fixed? For comparing conversion sequences (and only for comparing) reference binding should be treated as if it was a normal assignment/initialization and cv-qualification would have to be defined for references. This would affect 8.5.3 [dcl.init.ref] paragraph 6, 4.4 [conv.qual] and probably 13.3.3.2 [over.ics.rank] paragraph 3.
Another fix could be to add a special case in 13.3.3 [over.match.best] paragraph 1.
Issue 1232 extended the language to allow creation of array temporaries using initializer lists. However, such initializer lists must be “completely braced;” the elision of braces described in 8.5.1 [dcl.init.aggr] paragraph 11 applies only
In a declaration of the form
T x = { a };
This restriction prevents plausible uses like
array<int, 3> f() {
return { 1, 2, 3 };
}
The requirements for a trivial class include having “a trivial default constructor” (9 [class] paragraph 6). However, with an explicitly-defaulted default constructor and other constructors with default arguments, it is possible to have multiple default constructors. Such a class cannot be default-initialized and thus should probably be considered non-trivial.
According to 9.4.2 [class.static.data] paragraph 4,
Unnamed classes and classes contained directly or indirectly within unnamed classes shall not contain static data members.
There is no such restriction on member functions, and there is no rationale for this difference, given that both static data members and member functions can be defined outside a unnamed class with a typedef name for linkage purposes. (Issue 406 acknowledged the lack of rationale by removing the specious note in 9.4.2 [class.static.data] that attempted to explain the restriction but left the normative prohibition in place.)
It would be more consistent to remove the restriction for classes with a typedef name for linkage purposes.
The access rules in 11.2 [class.access.base] do not appear to handle references in nested classes and outside of nonstatic member functions correctly. For example,
struct A {
typedef int I; // public
};
struct B: private A { };
struct C: B {
void f() {
I i1; // error: access violation
}
I i2; // OK
struct D {
I i3; // OK
void g() {
I i4; // OK
}
};
};
The reason for this discrepancy is that the naming class in the reference to I is different in these cases. According to 11.2 [class.access.base] paragraph 5,
The access to a member is affected by the class in which the member is named. This naming class is the class in which the member name was looked up and found.
In the case of i1, the reference to I is subject to the transformation described in 9.3.1 [class.mfct.non-static] paragraph 3:
Similarly during name lookup, when an unqualified-id (5.1 [expr.prim]) used in the definition of a member function for class X resolves to a static member, an enumerator or a nested type of class X or of a base class of X, the unqualified-id is transformed into a qualified-id (5.1 [expr.prim]) in which the nested-name-specifier names the class of the member function.
As a result, the reference to I in the declaration of i1 is transformed to C::I, so that the naming class is C, and I is inacessible in C. In the remaining cases, however, the transformation does not apply. Thus, the naming class of I in these references is A, and I is publicly accessible in A.
Presumably either the definition of “naming class” must be changed or the transformation of unqualified-ids must be broadened to include all uses within the scope of a class and not just within nonstatic member functions (and following the declarator-id in the definition of a static member, per 9.4 [class.static] paragraph 4).
Does the restriction in 11.4 [class.protected] apply to upcasts across protected inheritance, too? For instance,
struct B {
int i;
};
struct I: protected B { };
struct D: I {
void f(I* ip) {
B* bp = ip; // well-formed?
bp->i = 5; // aka "ip->i = 5;"
}
};
I think the rationale for the 11.4 [class.protected] restriction applies equally well here — you don't know whether ip points to a D object or not, so D::f can't be trusted to treat the protected B subobject consistently with the policies of its actual complete object type.
The current treatment of “accessible base class” in 11.2 [class.access.base] paragraph 4 clearly makes the conversion from I* to B* well-formed. I think that's wrong and needs to be fixed. The rationale for the accessibility of a base class is whether “an invented public member” of the base would be accessible at the point of reference, although we obscured that a bit in the reformulation; it seems to me that the invented member ought to be considered a non-static member for this purpose and thus subject to 11.4 [class.protected].
(See also issues 385 and 471.).Notes from October 2004 meeting:
The CWG tentatively agreed that casting across protective inheritance should be subject to the additional restriction in 11.4 [class.protected].
Proposed resolution (April, 2011)
Change 11.2 [class.access.base] paragraph 4 as follows:
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 (but not a protected [Footnote: A protected invented member is disallowed here for the same reason the additional check of 11.4 [class.protected] is applied to member access: it would allow casting a pointer to a derived class to a protected base class that might be a subobject of an object of a class that is different from the class context in which the reference occurs. —end footnote]) 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. } }; class N2: protected B { }; class P2: public N2 { void f2(N2* n2p) { B* bp = n2p; // error: invented member would be protected and naming // class N2 not the same as or derived from the referencing // class P2 n2p->m = 0; // error (cf 11.4 [class.protected]) for the same reason } };—end example]
The specification of when a defaulted special member function is to be defined as deleted sometimes overlooks variant and array members.
According to 12.1 [class.ctor] paragraph 6, a defaulted default constructor is constexpr if the corresponding user-written constructor would satisfy the constexpr requirements. However, the requirements apply to the definition of a constructor, and a defaulted constructor is defined only if it is odr-used, leaving it indeterminate at declaration time whether the defaulted constructor is constexpr or not.
(See also issue 1358.)
Mark Mitchell raised a number of issues related to the resolution of issue 244 and of destructor lookup in general.
Issue 244 says:
... 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.
But if the reference is "p->X::~X()", the first class-name is looked up in two places (normal lookup and a lookup in the class of p). Does the new wording mean:
This is a test case that illustrates the issue:
struct A {
typedef A C;
};
typedef A B;
void f(B* bp) {
bp->B::~B(); // okay B found by normal lookup
bp->C::~C(); // okay C found by class lookup
bp->B::~C(); // B found by normal lookup C by class -- okay?
bp->C::~B(); // C found by class lookup B by normal -- okay?
}
A second issue concerns destructor references when the class involved is a template class.
namespace N {
template <typename T> struct S {
~S();
};
}
void f(N::S<int>* s) {
s->N::S<int>::~S();
}
The issue here is that the grammar uses "~class-name" for destructor names, but in this case S is a template name when looked up in N.
Finally, what about cases like:
template <typename T> void f () {
typename T::B x;
x.template A<T>::template B<T>::~B();
}
When parsing the template definition, what checks can be done on "~B"?
Sandor Mathe adds :
The standard correction for issue 244 (now in DR status) is still incomplete.
Paragraph 5 of 3.4.3 [basic.lookup.qual] is not applicable for p->T::~T since there is no nested-name-specifier. Section 3.4.5 [basic.lookup.classref] describes the lookup of p->~T but p->T::~T is still not described. There are examples (which are non-normative) that illustrate this sort of lookup but they still leave questions unanswered. The examples imply that the name after ~ should be looked up in the same scope as the name before the :: but it is not stated. The problem is that the name to the left of the :: can be found in two different scopes. Consider the following:
struct S {
struct C { ~C() { } };
};
typedef S::C D;
int main() {
D* p;
p->C::~D(); // valid?
}
Should the destructor call be valid? If there were a nested name specifier, then D should be looked for in the same scope as C. But here, C is looked for in 2 different ways. First, it is searched for in the type of the left hand side of -> and it is also looked for in the lexical context. It is found in one or if both, they must match. So, C is found in the scope of what p points at. Do you only look for D there? If so, this is invalid. If not, you would then look for D in the context of the expression and find it. They refer to the same underlying destructor so this is valid. The intended resolution of the original defect report of the standard was that the name before the :: did not imply a scope and you did not look for D inside of C. However, it was not made clear whether this was to be resolved by using the same lookup mechanism or by introducing a new form of lookup which is to look in the left hand side if that is where C was found, or in the context of the expression if that is where C was found. Of course, this begs the question of what should happen when it is found in both? Consider the modification to the above case when C is also found in the context of the expression. If you only look where you found C, is this now valid because it is in 1 of the two scopes or is it invalid because C was in both and D is only in 1?
struct S {
struct C { ~C() { } };
};
typedef S::C D;
typedef S::C C;
int main() {
D* p;
p->C::~D(); // valid?
}
I agree that the intention of the committee is that the original test case in this defect is broken. The standard committee clearly thinks that the last name before the last :: does not induce a new scope which is our current interpretation. However, how this is supposed to work is not defined. This needs clarification of the standard.
Martin Sebor adds this example (September 2003), along with errors produced by the EDG front end:
namespace N {
struct A { typedef A NA; };
template <class T> struct B { typedef B NB; typedef T BT; };
template <template <class> class T> struct C { typedef C NC; typedef T<A> CA; };
}
void foo (N::A *p)
{
p->~NA ();
p->NA::~NA ();
}
template <class T>
void foo (N::B<T> *p)
{
p->~NB ();
p->NB::~NB ();
}
template <class T>
void foo (typename N::B<T>::BT *p)
{
p->~BT ();
p->BT::~BT ();
}
template <template <class> class T>
void foo (N::C<T> *p)
{
p->~NC ();
p->NC::~NC ();
}
template <template <class> class T>
void foo (typename N::C<T>::CA *p)
{
p->~CA ();
p->CA::~CA ();
}
Edison Design Group C/C++ Front End, version 3.3 (Sep 3 2003 11:54:55)
Copyright 1988-2003 Edison Design Group, Inc.
"t.cpp", line 16: error: invalid destructor name for type "N::B<T>"
p->~NB ();
^
"t.cpp", line 17: error: qualifier of destructor name "N::B<T>::NB" does not
match type "N::B<T>"
p->NB::~NB ();
^
"t.cpp", line 30: error: invalid destructor name for type "N::C<T>"
p->~NC ();
^
"t.cpp", line 31: error: qualifier of destructor name "N::C<T>::NC" does not
match type "N::C<T>"
p->NC::~NC ();
^
4 errors detected in the compilation of "t.cpp".
John Spicer: The issue here is that we're unhappy with the destructor names when doing semantic analysis of the template definitions (not during an instantiation).
My personal feeling is that this is reasonable. After all, why would you call p->~NB for a class that you just named as N::B<T> and you could just say p->~B?
Additional note (September, 2004)
The resolution for issue 244 removed the discussion of p->N::~S, where N is a namespace-name. However, the resolution did not make this construct ill-formed; it simply left the semantics undefined. The meaning should either be defined or the construct made ill-formed.
Moving to always doing overload resolution for determining exception specifications and implicit deletion creates some unfortunate cycles:
template<typename T> struct A {
T t;
};
template <typename T> struct B {
typename T::U u;
};
template <typename T> struct C {
C(const T&);
};
template <typename T> struct D {
C<B<T> > v;
};
struct E {
typedef A<D<E> > U;
};
extern A<D<E> > a;
A<D<E> > a2(a);
If declaring the copy constructor for A<D<E>> is part of instantiating the class, then we need to do overload resolution on D<E>, and thus C<B<E>>. We consider C(const B<E>&), and therefore look to see if there's a conversion from C<B<E>> to B<E>, which instantiates B<E>, which fails because it has a field of type A<D<E>> which is already being instantiated.
Even if we wait until A<D<E>> is considered complete before finalizing the copy constructor declaration, declaring the copy constructor for B<E> will want to look at the copy constructor for A<D<E>>, so we still have the cycle.
I think that to avoid this cycle we need to short-circuit consideration of C(const T&) somehow. But I don't see how we can do that without breaking
struct F {
F(F&);
};
struct G;
struct G2 {
G2(const G&);
};
struct G {
G(G&&);
G(const G2&);
};
struct H: F, G { };
extern H h;
H h2(h);
Here, since G's move constructor suppresses the implicit copy constructor, the defaulted H copy constructor calls G(const G2&) instead. If the move constructor did not suppress the implicit copy constructor, I believe the implicit copy constructor would always be viable, and therefore a better match than a constructor taking a reference to another type.
So perhaps the answer is to reconsider that suppression and then disqualify any constructor taking (a reference to) a type other than the constructor's class from consideration when looking up a subobject constructor in an implicitly defined constructor. (Or assignment operator, presumably.)
Another possibility would be that when we're looking for a conversion from C<B<E>> to B<E> we could somehow avoid considering, or even declaring, the B<E> copy constructor. But that seems a bit dodgy.
Additional note (October, 2010):
An explicitly declared move constructor/op= should not suppress the implicitly declared copy constructor/op=; it should cause it to be deleted instead. This should prevent a member function taking a (reference to) an un-reference-related type from being chosen by overload resolution in a defaulted member function.
And we should clarify that member functions taking un-reference-related types are not even considered during overload resolution in a defaulted member function, to avoid requiring their parameter types to be complete.
If default arguments added in the out-of-class definition of a constructor make it a special member function, this can affect the overload resolution and thus the implicit exception specification and the triviality of an implicitly-declared special member function in a derived class.
The Standard does not allow overloading of member functions that differ only in their return type (cf enable_if).
It's not clear how overloading and partial ordering handle non-deduced pairs of corresponding arguments. For example:
template<typename T>
struct A { typedef char* type; };
template<typename T> char* f1(T, typename A<T>::type); // #1
template<typename T> long* f1(T*, typename A<T>::type*); // #2
long* p1 = f1(p1, 0); // #3
I thought that #3 is ambiguous but different compilers disagree on that. Comeau C/C++ 4.3.3 (EDG 3.0.3) accepted the code, GCC 3.2 and BCC 5.5 selected #1 while VC7.1+ yields ambiguity.
I intuitively thought that the second pair should prevent overloading from triggering partial ordering since both arguments are non-deduced and has different types - (char*, char**). Just like in the following:
template<typename T> char* f2(T, char*); // #3 template<typename T> long* f2(T*, char**); // #4 long* p2 = f2(p2, 0); // #5
In this case all the compilers I checked found #5 to be ambiguous. The standard and DR 214 is not clear about how partial ordering handle such cases.
I think that overloading should not trigger partial ordering (in step 13.3.3 [over.match.best]/1/5) if some candidates have non-deduced pairs with different (specialized) types. In this stage the arguments are already adjusted so no need to mention it (i.e. array to pointer). In case that one of the arguments is non-deuced then partial ordering should only consider the type from the specialization:
template<typename T> struct B { typedef T type; };
template<typename T> char* f3(T, T); // #7
template<typename T> long* f3(T, typename B<T>::type); // #8
char* p3 = f3(p3, p3); // #9
According to my reasoning #9 should yield ambiguity since second pair is (T, long*). The second type (i.e. long*) was taken from the specialization candidate of #8. EDG and GCC accepted the code. VC and BCC found an ambiguity.
John Spicer: There may (or may not) be an issue concerning whether nondeduced contexts are handled properly in the partial ordering rules. In general, I think nondeduced contexts work, but we should walk through some examples to make sure we think they work properly.
Rani's description of the problem suggests that he believes that partial ordering is done on the specialized types. This is not correct. Partial ordering is done on the templates themselves, independent of type information from the specialization.
Notes from October 2004 meeting:
John Spicer will investigate further to see if any action is required.
(See also issue 885.)
According to the Standard (although not implemented this way in most implementations), the following code exhibits non-intuitive behavior:
struct T {
operator short() const;
operator int() const;
};
short s;
void f(const T& t) {
s = t; // surprisingly calls T::operator int() const
}
The reason for this choice is 13.6 [over.built] paragraph 18:
For every triple (L, VQ, R), where L is an arithmetic type, VQ is either volatile or empty, and R is a promoted arithmetic type, there exist candidate operator functions of the form
VQ L& operator=(VQ L&, R);
Because R is a "promoted arithmetic type," the second argument to the built-in assignment operator is int, causing the unexpected choice of conversion function.
Suggested resolution: Provide built-in assignment operators for the unpromoted arithmetic types.
Related to the preceding, but not resolved by the suggested resolution, is the following problem. Given:
struct T {
operator int() const;
operator double() const;
};
I believe the standard requires the following assignment to be ambiguous (even though I expect that would surprise the user):
double x;
void f(const T& t) { x = t; }
The problem is that both of these built-in operator=()s exist (13.6 [over.built] paragraph 18):
double& operator=(double&, int);
double& operator=(double&, double);
Both are an exact match on the first argument and a user conversion on the second. There is no rule that says one is a better match than the other.
The compilers that I have tried (even in their strictest setting) do not give a peep. I think they are not following the standard. They pick double& operator=(double&, double) and use T::operator double() const.
I hesitate to suggest changes to overload resolution, but a possible resolution might be to introduce a rule that, for built-in operator= only, also considers the conversion sequence from the second to the first type. This would also resolve the earlier question.
It would still leave x += t etc. ambiguous -- which might be the desired behavior and is the current behavior of some compilers.
Notes from the 04/01 meeting:
The difference between initialization and assignment is disturbing. On the other hand, promotion is ubiquitous in the language, and this is the beginning of a very slippery slope (as the second report above demonstrates).
Additional note (August, 2010):
See issue 507 for a similar example involving comparison operators.
Static data members of template classes and of nested classes of template classes are not themselves templates but receive much the same treatment as template. For instance, 14 [temp] paragraph 1 says that templates are only "classes or functions" but implies that "a static data member of a class template or of a class nested within a class template" is defined using the template-declaration syntax.
There are many places in the clause, however, where static data members of one sort or another are overlooked. For instance, 14 [temp] paragraph 6 allows static data members of class templates to be declared with the export keyword. I would expect that static data members of (non-template) classes nested within class templates could also be exported, but they are not mentioned here.
Paragraph 8, however, overlooks static data members altogether and deals only with "templates" in defining the effect of the export keyword; there is no description of the semantics of defining a static data member of a template to be exported.
These are just two instances of a systematic problem. The entire clause needs to be examined to determine which statements about "templates" apply to static data members, and which statements about "static data members of class templates" also apply to static data members of non-template classes nested within class templates.
(The question also applies to member functions of template classes; see issue 217, where the phrase "non-template function" in 8.3.6 [dcl.fct.default] paragraph 4 is apparently intended not to include non-template member functions of template classes. See also issue 108, which would benefit from understanding nested classes of class templates as templates. Also, see issue 249, in which the usage of the phrase "member function template" is questioned.)
Notes from the 4/02 meeting:
Daveed Vandevoorde will propose appropriate terminology.
The EDG front-end accepts:
template <typename T>
struct A {
template <typename U>
struct B {};
};
template <typename T>
struct C : public A<T>::template B<T> {
};
It rejects this code if the base-specifier is spelled A<T>::B<T>.
However, the grammar for a base-specifier does not allow the template keyword.
Suggested resolution:
It seems to me that a consistent approach to the solution that looks like it will be adopted for issue 180 (which deals with the typename keyword in similar contexts) would be to assume that B is a template if it is followed by a "<". After all, an expression cannot appear in this context.Notes from the 4/02 meeting:
We agreed that template must be allowed in this context. The syntax needs to be changed. We also opened the related issue 343.
Additional note (August, 2010):
The same considerations apply to mem-initializer-ids, as noted in issue 1019.
In the following example, the template parameter in the partial specialization is non-deducible:
template <class T> struct A { typedef T U; };
template <class T> struct C { };
template <class T> struct C<typename A<T>::U> { };
Several compilers issue errors for this case, but there appears to be nothing in the Standard that would make this ill-formed; it simply seems that the partial specialization will never be matched, so the primary template will be used for all specializations. Should it be ill-formed?
(See also issue 1246.)
Notes from the April, 2006 meeting:
It was noted that there are similar issues for constructors and conversion operators with non-deducible parameters, and that they should probably be dealt with similarly.
There appears to be no requirement that a redeclaration of an alias template must be equivalent to the earlier one. An alias-declaration is not a definition (3.1 [basic.def] paragraph 2), so presumably an alias template declaration is also not a definition and thus the ODR does not apply.
Consider the following example:
template <class T> struct Outer {
struct Inner {
Inner* self();
};
};
template <class T> Outer<T>::Inner*
Outer<T>::Inner::self() { return this; }
According to 14.6 [temp.res] paragraph 3 (before the salient wording was inadvertently removed, see issue 559),
A qualified-id that refers to a type and in which the nested-name-specifier depends on a 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]) shall be prefixed by the keyword typename to indicate that the qualified-id denotes a type, forming a typename-specifier.
Because Outer<T>::Inner is a member of the current instantiation, the Standard does not currently require that it be prefixed with typename when it is used in the return type of the definition of the self() member function. However, it is difficult to parse this definition correctly without knowing that the return type is, in fact, a type, which is what the typename keyword is for. Should the Standard be changed to require typename in such contexts?
According to 14.6 [temp.res] paragraph 3,
When a qualified-id is intended to refer to a type that is not a member of the current instantiation (14.6.2.1 [temp.dep.type]) and its nested-name-specifier refers to a dependent type, it shall be prefixed by the keyword typename, forming a typename-specifier. If the qualified-id in a typename-specifier does not denote a type, the program is ill- formed.
The intent of the programmer cannot form the basis for a compiler determining whether to issue a diagnostic or not.
Suggested resolution:Let N be a qualified-id with a nested-name-specifier that denotes a dependent type. If N is not prefixed by the keyword typename, N shall refer to a member of the current instantiation or it shall not refer to a type.
typename-specifier:
typename nested-name-specifier identifier
typename nested-name-specifier templateopt simple-template-idIf the qualified-id in a typename-specifier does not denote a type, the program is ill-formed.
(See also issues 590 and 591.)
The discussion of issue 1233 revealed that the dependency of function calls involving a braced-init-list containing a pack expansion is not adequately addressed by the existing wording.
Proposed resolution (August, 2011):
Change 14.6.2.1 [temp.dep.type] paragraph 7 as follows:
...If the result of this lookup differs from the result of name lookup in the template definition context, name lookup is ambiguous. [Note: the result of name lookup differs only when the member of the current instantiation was found in a non-dependent base class of the current instantiation and a member with the same name is also introduced by the substitution for a dependent base class of the current instantiation. —end note] [Example:
struct A { int m; }; struct B { int m; }; template<typename T> struct C : A, T { int f() { return this->m; } // finds A::m in the template definition context }; int g(C<B> cb) { return cb.f(); // error: finds both A::m and B::m in the template instantiation context }—end example]
template <class T> class Foo {
public:
typedef int Bar;
Bar f();
};
template <class T> typename Foo<T>::Bar Foo<T>::f() { return 1;}
--------------------
In the class template definition, the declaration of the member function
is interpreted as:
int Foo<T>::f();In the definition of the member function that appears outside of the class template, the return type is not known until the member function is instantiated. Must the return type of the member function be known when this out-of-line definition is seen (in which case the definition above is ill-formed)? Or is it OK to wait until the member function is instantiated to see if the type of the return type matches the return type in the class template definition (in which case the definition above is well-formed)?
Suggested resolution: (John Spicer)
My opinion (which I think matches several posted on the reflector recently) is that the out-of-class definition must match the declaration in the template. In your example they do match, so it is well formed.
I've added some additional cases that illustrate cases that I think either are allowed or should be allowed, and some cases that I don't think are allowed.
template <class T> class A { typedef int X; };
template <class T> class Foo {
public:
typedef int Bar;
typedef typename A<T>::X X;
Bar f();
Bar g1();
int g2();
X h();
X i();
int j();
};
// Declarations that are okay
template <class T> typename Foo<T>::Bar Foo<T>::f()
{ return 1;}
template <class T> typename Foo<T>::Bar Foo<T>::g1()
{ return 1;}
template <class T> int Foo<T>::g2() { return 1;}
template <class T> typename Foo<T>::X Foo<T>::h() { return 1;}
// Declarations that are not okay
template <class T> int Foo<T>::i() { return 1;}
template <class T> typename Foo<T>::X Foo<T>::j() { return 1;}
In general, if you can match the declarations up using only information
from the template, then the declaration is valid.
Declarations like Foo::i and Foo::j are invalid because for a given instance of A<T>, A<T>::X may not actually be int if the class is specialized.
This is not a problem for Foo::g1 and Foo::g2 because for any instance of Foo<T> that is generated from the template you know that Bar will always be int. If an instance of Foo is specialized, the template member definitions are not used so it doesn't matter whether a specialization defines Bar as int or not.
Implementations differ in their treatment of the following code:
template <class T>
struct A {
typename T::X x;
};
template <class T>
struct B {
typedef T* X;
A<B> a;
};
int main ()
{
B<int> b;
}
Some implementations accept it. At least one rejects it because the instantiation of A<B<int> > requires that B<int> be complete, and it is not at the point at which A<B<int> > is being instantiated.
Erwin Unruh:
In my view the programm is ill-formed. My reasoning:
So each class needs the other to be complete.
The problem can be seen much easier if you replace the typedef with
typedef T (*X) [sizeof(B::a)];
Now you have a true recursion. The compiler cannot easily distinguish between a true recursion and a potential recursion.
John Spicer:
Using a class to form a qualified name does not require the class to be complete, it only requires that the named member already have been declared. In other words, this kind of usage is permitted:
class A {
typedef int B;
A::B ab;
};
In the same way, once B has been declared in A, it is also visible to any template that uses A through a template parameter.
The standard could be more clear in this regard, but there are two notes that make this point. Both 3.4.3.1 [class.qual] and 5.1.1 [expr.prim.general] paragraph 7 contain a note that says "a class member can be referred to using a qualified-id at any point in its potential scope (3.3.7 [basic.scope.class])." A member's potential scope begins at its point of declaration.
In other words, a class has three states: incomplete, being completed, and complete. The standard permits a qualified name to be used once a name has been declared. The quotation of the notes about the potential scope was intended to support that.
So, in the original example, class A does not require the type of T to be complete, only that it have already declared a member X.
Bill Gibbons:
The template and non-template cases are different. In the non-template case the order in which the members become declared is clear. In the template case the members of the instantiation are conceptually all created at the same time. The standard does not say anything about trying to mimic the non-template case during the instantiation of a class template.
Mike Miller:
I think the relevant specification is 14.6.4.1 [temp.point] paragraph 3, dealing with the point of instantiation:
For a class template specialization... if the specialization is implicitly instantiated because it is referenced from within another template specialization, if the context from which the specialization is referenced depends on a template parameter, and if the specialization is not instantiated previous to the instantiation of the enclosing template, the point of instantiation is immediately before the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the namespace scope declaration or definition that refers to the specialization.
That means that the point of instantiation of A<B<int> > is before that of B<int>, not in the middle of B<int> after the declaration of B::X, and consequently a reference to B<int>::X from A<B<int> > is ill-formed.
To put it another way, I believe John's approach requires that there be an instantiation stack, with the results of partially-instantiated templates on the stack being available to instantiations above them. I don't think the Standard mandates that approach; as far as I can see, simply determining the implicit instantiations that need to be done, rewriting the definitions at their respective points of instantiation with parameters substituted (with appropriate "forward declarations" to allow for non-instantiating references), and compiling the result normally should be an acceptable implementation technique as well. That is, the implicit instantiation of the example (using, e.g., B_int to represent the generated name of the B<int> specialization) could be something like
struct B_int;
struct A_B_int {
B_int::X x; // error, incomplete type
};
struct B_int {
typedef int* X;
A_B_int a;
};
Notes from 10/01 meeting:
This was discussed at length. The consensus was that the template case should be treated the same as the non-template class case it terms of the order in which members get declared/defined and classes get completed.
Proposed resolution:
In 14.6.4.1 [temp.point] paragraph 3 change:
the point of instantiation is immediately before the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the namespace scope declaration or definition that refers to the specialization.
To:
the point of instantiation is the same as the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the nearest enclosing declaration. [Note: The point of instantiation is still at namespace scope but any declarations preceding the point of instantiation, even if not at namespace scope, are considered to have been seen.]
Add following paragraph 3:
If an implicitly instantiated class template specialization, class member specialization, or specialization of a class template references a class, class template specialization, class member specialization, or specialization of a class template containing a specialization reference that directly or indirectly caused the instantiation, the requirements of completeness and ordering of the class reference are applied in the context of the specialization reference.
and the following example
template <class T> struct A {
typename T::X x;
};
struct B {
typedef int X;
A<B> a;
};
template <class T> struct C {
typedef T* X;
A<C> a;
};
int main ()
{
C<int> c;
}
Notes from the October 2002 meeting:
This needs work. Moved back to drafting status.
Three points have been raised where the wording in 14.7.1 [temp.inst] may not be sufficiently clear.
A class template specialization is implicitly instantiated... if the completeness of the class type affects the semantics of the program...
It is not clear what it means for the "completeness... [to affect] the semantics." Consider the following example:
template<class T> struct A;
extern A<int> a;
void *foo() { return &a; }
template<class T> struct A
{
#ifdef OPTION
void *operator &() { return 0; }
#endif
};
The question here is whether it is necessary for template class A to declare an operator & for the semantics of the program to be affected. If it does not do so, the meaning of &a will be the same whether the class is complete or not and thus arguably the semantics of the program are not affected.
Presumably what was intended is whether the presence or absence of certain member declarations in the template class might be relevant in determining the meaning of the program. A clearer statement may be desirable.
If the overload resolution process can determine the correct function to call without instantiating a class template definition, it is unspecified whether that instantiation actually takes place.
The intent of this wording, as illustrated in the example in that paragraph, is to allow a "smart" implementation not to instantiate class templates if it can determine that such an instantiation will not affect the result of overload resolution, even though the algorithm described in clause 13 [over] requires that all the viable functions be enumerated, including functions that might be found as members of specializations.
Unfortunately, the looseness of the wording allowing this latitude for implementations makes it unclear what "the overload resolution process" is — is it the algorithm in 13 [over] or something else? — and what "the correct function" is.
If an implicit instantiation of a class template specialization is required and the template is declared but not defined, the program is ill-formed.
Here, it is not clear what conditions "require" an implicit instantiation. From the context, it would appear that the intent is to refer to the conditions in paragraph 4 that cause a specialization to be instantiated.
This interpretation, however, leads to different treatment of template and non-template incomplete classes. For example, by this interpretation,
class A;
template <class T> struct TA;
extern A a;
extern TA<int> ta;
void f(A*);
void f(TA<int>*);
int main()
{
f(&a); // well-formed; undefined if A
// has operator &() member
f(&ta); // ill-formed: cannot instantiate
}
A different approach would be to understand "required" in paragraph 6 to mean that a complete type is required in the expression. In this interpretation, if an incomplete type is acceptable in the context and the class template definition is not visible, the instantiation is not attempted and the program is well-formed.
The meaning of "required" in paragraph 6 must be clarified.
Notes on 10/01 meeting:
It was felt that item 1 is solved by addition of the word "might" in the resolution for issue 63; item 2 is not much of a problem; and item 3 could be solved by changing "required" to "required to be complete".
Paragraph 17 of 14.7.3 [temp.expl.spec] says,
A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a template<> for each enclosing class template that is explicitly specialized.
This is curious, because paragraph 3 only allows explicit specialization of members of implicitly-instantiated class specializations, not explicit specializations. Furthermore, paragraph 4 says,
Definitions of members of an explicitly specialized class are defined in the same manner as members of normal classes, and not using the explicit specialization syntax.
Paragraph 18 provides a clue for resolving the apparent contradiction:
In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well. In such explicit specialization declaration, the keyword template followed by a template-parameter-list shall be provided instead of the template<> preceding the explicit specialization declaration of the member.
It appears from this and the following example that the phrase “explicitly specialized” in paragraphs 17 and 18, when referring to enclosing class templates, does not mean that explicit specializations have been declared for them but that their names in the qualified-id are followed by template argument lists. This terminology is confusing and should be changed.
Proposed resolution (October, 2005):
Change 14.7.3 [temp.expl.spec] paragraph 17 as indicated:
A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a template<> for each enclosing class template that is explicitly specialized specialization. [Example:...
Change 14.7.3 [temp.expl.spec] paragraph 18 as indicated:
In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well that is, the template-id naming the template may be composed of template parameter names rather than template-arguments. In For each unspecialized template in such an explicit specialization declaration, the keyword template followed by a template-parameter-list shall be provided instead of the template<> preceding the explicit specialization declaration of the member. The types of the template-parameters in the template-parameter-list shall be the same as those specified in the primary template definition. In such declarations, an unspecialized template-id shall not precede the name of a template specialization in the qualified-id naming the member. [Example:...
Notes from the April, 2006 meeting:
The revised wording describing “unspecialized” templates needs more work to ensure that the parameter names in the template-id are in the correct order; the distinction between template argyments and parameters is also probably not clear enough. It might be better to replace this paragraph completely and avoid the “unspecialized” wording altogether.
Proposed resolution (February, 2010):
Change 14.7.3 [temp.expl.spec] paragraph 17 as follows:
A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a template<> for each enclosing class template that is explicitly specialized specialization. [Example:...
Change 14.7.3 [temp.expl.spec] paragraph 18 as follows:
In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well. In such explicit specialization declaration, the keyword template followed by a template-parameter-list shall be provided instead of the template<> preceding the explicit specialization declaration of the member. The types of the template-parameters in the template-parameter-list shall be the same as those specified in the primary template definition. that is, the corresponding template prefix may specify a template-parameter-list instead of template<> and the template-id naming the template be written using those template-parameters as template-arguments. In such a declaration, the number, kinds, and types of the template-parameters shall be the same as those specified in the primary template definition, and the template-parameters shall be named in the template-id in the same order that they appear in the template-parameter-list. An unspecialized template-id shall not precede the name of a template specialization in the qualified-id naming the member. [Example:...
There are certain constructs that are not covered by the existing categories of “type dependent” and “value dependent.” For example, the expression sizeof(sizeof(T())) is neither type-dependent nor value-dependent, but its validity depends on whether T can be value-constructed. We should be able to overload on such characteristics and select via deduction failure, but we need a term like “instantiation-dependent” to describe these cases in the Standard. The phrase “expression involving a template parameter” seems to come pretty close to capturing this idea.
Notes from the November, 2010 meeting:
The CWG favored extending the concepts of “type-dependent” and “value-dependent” to cover these additional cases, rather than adding a new concept.
Notes from the March, 2011 meeting:
The CWG reconsidered the direction from the November, 2010 meeting, as it would make more constructs dependent, thus requiring more template and typename keywords, resulting in worse error messages, etc.
Notes from the August, 2011 meeting:
The following example (from issue 1273) was deemed relevant for this issue:
template <class T> struct C;
class A
{
int i;
friend struct C<int>;
} a;
class B
{
int i;
friend struct C<float>;
} b;
template <class T>
struct C
{
template <class U> decltype (a.i) f() { } // #1
template <class U> decltype (b.i) f() { } // #2
};
int main()
{
C<int>().f<int>(); // calls #1
C<float>().f<float>(); // calls #2
}
Consider the following example:
template <int> struct X {
typedef int type;
};
template <class T> struct Y { };
template <class T> struct Z {
static int const value = Y<T>::value;
};
template <class T> typename X<Y<T>::value + Z<T>::value>::type f(T);
int f(...);
int main() {
sizeof f(0);
}
The problem here is that there is a combination of an invalid expression in the immediate context (Y<T>::value) and in the non-immediate context (within Z<T> when evaluating Z<T>::value). The Standard does not appear to state clearly whether this program is well-formed (because the error in the immediate context causes deduction failure) or ill-formed (because of the error in the non-immediate context).
Notes from the March, 2011 meeting:
Some members expressed a desire to allow implementations latitude in whether examples like this should be deduction failure or a diagnosable error, just as the order of evaluation of arithmetic operands is largely unconstrained. Others felt that specifying something like a depth-first left-to-right traversal of the expression or declaration would be better. Another possibility suggested was to enforce ordering only at comma operators. No consensus was achieved.
CWG agreed that the arguments should be processed in left-to-right order. Some popular existing code (e.g., Boost) depends on this ordering.
The determination of the exception-specification for an implicitly-declared special member function, as described in 15.4 [except.spec] paragraph 14, does not take into account the fact that nonstatic data member initializers and default arguments in default constructors can contain throw-expressions, which are not part of the exception-specification of any function that is “directly invoked” by the implicit definition. Also, the reference to “directly invoked” functions is not restricted to potentially-evaluated expressions, thus possibly including irrelevant exception-specifications.
It is unspecified if an implicitly-defined copy assignment operator directly invokes the copy assignment operators of virtual bases. The exception-specification of such a copy assignment operator is thus also unspecified. The specification in 15.4 [except.spec] paragraph 14 should explicitly include the exceptions from the copy assignment operators of virtual base classes, regardless of whether the implicit definition actually invokes the virtual base assignment operators or not.
Is the following well-formed?
auto concept HasDestructor<typename T> {
T::~T();
}
concept_map HasDestructor<int&> { }
According to _N2914_.14.10.2.1 [concept.map.fct] paragraph 4, the destructor requirement in the concept map results in an expression x.~X(), where X is the type int&. According to 5.2.4 [expr.pseudo], this expression is ill-formed because the object type and the type-name must be the same type, but the object type cannot be a reference type (references are dropped from types used in expressions, 5 [expr] paragraph 5).
It is not clear whether this should be addressed by changing 5.2.4 [expr.pseudo] or _N2914_.14.10.2.1 [concept.map.fct].
The definition of an argument does not seem to cover many assumed use cases, and we believe that is not intentional. There should be answers to questions such as: Are lambda-captures arguments? Are type names in a throw-spec arguments? “Argument” to casts, typeid, alignof, alignas, decltype and sizeof? why in x[arg] arg is not an argument, but the value forwarded to operator[]() is? Does not apply to operators as call-points not bounded by parentheses? Similar for copy initialization and conversion? What are deduced template “arguments?” what are “default arguments?” can attributes have arguments? What about concepts, requires clauses and concept_map instantiations? What about user-defined literals where parens are not used?
According to 1.4 [intro.compliance] paragraph 7,
A freestanding implementation is one in which execution may take place without the benefit of an operating system, and has an implementation-defined set of libraries that includes certain language-support libraries (17.6.1.3 [compliance]).
This definition links two relatively separate topics: the lack of an operating system and the minimal set of libraries. Furthermore, 3.6.1 [basic.start.main] paragraph 1 says:
[Note: in a freestanding environment, start-up and termination is implementation-defined; start-up contains the execution of constructors for objects of namespace scope with static storage duration; termination contains the execution of destructors for objects with static storage duration. —end note]
It would be helpful if the two characteristics (lack of an operating system and restricted set of libraries) were named separately and if these statements were clarified to identify exactly what is implementation-defined.
Notes from the October, 2009 meeting:
The CWG felt that it needed a specific proposal in a paper before attempting to resolve this issue.
According to 1.9 [intro.execution] paragraph 14, “sequenced before” is a relation between “evaluations.” However, 3.6.3 [basic.start.term] paragraph 3 says,
If the completion of the initialization of a non-local object with static storage duration is sequenced before a call to std::atexit (see <cstdlib>, 18.5 [support.start.term]), the call to the function passed to std::atexit is sequenced before the call to the destructor for the object. If a call to std::atexit is sequenced before the completion of the initialization of a non-local object with static storage duration, the call to the destructor for the object is sequenced before the call to the function passed to std::atexit. If a call to std::atexit is sequenced before another call to std::atexit, the call to the function passed to the second std::atexit call is sequenced before the call to the function passed to the first std::atexit call.
Except for the calls to std::atexit, these events do not correspond to “evaluation” of expressions that appear in the program. If the “sequenced before” relation is to be applied to them, a more comprehensive definition is needed.
The wording of 1.9 [intro.execution] paragraph 6 is intended to describe the values of objects upon entry to and exit from the handler — i.e., that signal handler cannot rely on non-atomic objects being in a consistent state upon entry, nor can it reliably set the value of non-atomic objects and expect that they will continue to have those values after the handler exits. However, the wording could be read as saying even during the execution of the handler it cannot set and use non-atomic objects. The wording should be clarified.
According to 2.2 [lex.phases] paragraph 1, in translation phase 1,
Any source file character not in the basic source character set (2.3 [lex.charset]) is replaced by the universal-character-name that designates that character.
If a character that is not in the basic character set is preceded by a backslash character, for example
"\á"
the result is equivalent to
"\\u00e1"
that is, a backslash character followed by the spelling of the universal-character-name. This is different from the result in C99, which accepts characters from the extended source character set without replacing them with universal-character-names.
According to
If the hexadecimal value for a universal-character-name corresponds to a surrogate code point (in the range 0xD800-0xDFFF, inclusive), the program is ill-formed. 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 0x00-0x1F or 0x7F-0x9F, both inclusive) or to a character in the basic source character set, the program is ill-formed.
These restrictions should not apply to comment text. Arguably the prohibitions of control characters and characters in the basic character set already do not apply, as they require that the preprocessing tokens for literals have already been recognized; this occurs in phase 3, which also replaces comments with single spaces. However, the prohibition of surrogate code points is not so limited and might conceivably be applied within comments.
Probably the most straightforward way of addressing this problem would be simply to state in 2.8 [lex.comment] that character sequences that resemble universal-character-names are not recognized as such within comment text.
According to 2.14.3 [lex.ccon] paragraph 1,
A character literal that does not begin with u, U, or L is an ordinary character literal, also referred to as a narrow-character literal. An ordinary character literal that contains a single c-char has type char, with value equal to the numerical value of the encoding of the c-char in the execution character set.
However, the definition of c-char includes as one possibility a universal-character-name. The value of a universal-character-name cannot, in general, be represented as a char, so this specification is impossible to satisfy.
(See also issue 411 for related questions.)
There is no limit placed on the number of c-chars in a multicharacter literal or a wide-character literal containing multiple c-chars, either in 2.14.3 [lex.ccon] paragraphs 1-2 or in Annex B [implimits]. Presumably this means that an implementation must accept arbitrarily long literals.
An alternative approach might be to state that these literals are conditionally supported with implementation-defined semantics, allowing an implementation to impose a documented limit that makes sense for the particular architecture.
The treatment of character literals containing universal-character-names is not clear. It is reasonable to conclude from 2.14.5 [lex.string] paragraph 15 that if a character named by a UCN cannot be represented by a single character in the runtime character set, it becomes a multibyte character and thus such a character literal is a multicharacter literal, with type int and an implementation-defined value. It would be nice if 2.14.3 [lex.ccon] had the complete story by itself or at least a reference to 2.14.5 [lex.string] for the details.
2.14.5 [lex.string] paragraph 5 reads
Escape sequences and universal-character-names in string literals have the same meaning as in character literals, except that the single quote ' is representable either by itself or by the escape sequence \', and the double quote " shall be preceded by a \. In a narrow string literal, a universal-character-name may map to more than one char element due to multibyte encoding.
The first sentence refers us to 2.14.3 [lex.ccon], where we read in the first paragraph that "An ordinary character literal that contains a single c-char has type char [...]." Since the grammar shows that a universal-character-name is a c-char, something like '\u1234' must have type char (and thus be a single char element); in paragraph 5, we read that "A universal-character-name is translated to the encoding, in the execution character set, of the character named. If there is no such encoding, the universal-character-name is translated to an implemenation-defined encoding."
This is in obvious contradiction with the second sentence. In addition, I'm not really clear what is supposed to happen in the case where the execution (narrow-)character set is UTF-8. Consider the character \u0153 (the oe in the French word oeuvre). Should '\u0153' be a char, with an "error" value, say '?' (in conformance with the requirement that it be a single char), or an int, with the two char values 0xC5, 0x93, in an implementation defined order (in conformance with the requirement that a character representable in the execution character set be represented). Supposing the former, should "\u0153" be the equivalent of "?" (in conformance with the first sentence), or "\xC5\x93" (in conformance with the second).
Notes from October 2003 meeting:
We decided we should forward this to the C committee and let them resolve it. Sent via e-mail to John Benito on November 14, 2003.
Reply from John Benito:
I talked this over with the C project editor, we believe this was handled by the C committee before publication of the current standard.
WG14 decided there needed to be a more restrictive rule for one-to-one mappings: rather than saying "a single c-char" as C++ does, the C standard says "a single character that maps to a single-byte execution character"; WG14 fully expect some (if not many or even most) UCNs to map to multiple characters.
Because of the fundamental differences between C and C++ character types, I am not sure the C committee is qualified to answer this satisfactorily for WG21. WG14 is willing to review any decision reached for compatibility.
I hope this helps.
(See also issue 912 for a related question.)
The decimal-literal in a user-defined-integer-literal might be too large for an unsigned long long to represent (in implementations with extended integer types). In such cases, the original intent appears to have been to call a raw literal operator or a literal operator template; however, the existing wording of 2.14.8 [lex.ext] paragraph 3 always calls the unsigned long long literal operator if it exists, regardless of the value of the decimal-literal.
Consider the following complete program:
void f();
template<typename T> void g() { f(); }
int main() { }
Must f() be defined to make this program well-formed? The current wording of 3.2 [basic.def.odr] does not make any special provision for expressions that appear only in uninstantiated template definitions.
Additional note, January, 2012:
Further discussion indicates that this is not a defect and should be closed as such.
This seems like it should be well-formed:
template <class T> T list(T x);
template <class H, class ...T>
auto list(H h, T ...args) -> decltype(list(args...));
auto list3 = list(1, 2, 3);
but it isn't, because the second list isn't in scope in its own trailing-return-type; the point of declaration is after the declarator, which includes the trailing-return-type. And since int has no associated namespaces, the call in the return type only sees the first list. G++, EDG and Clang all reject the testcase on this basis.
But this seems like a natural pattern for writing variadic function templates, and we could support it by moving the point of declaration to the ->. This would mean having to deal with a function that only has a placeholder for a return type, but I think we can handle that.
Consider:
struct A {
struct B { typedef int X; };
};
template<class B> struct C : A {
B::X q; // Ok: A::B.
struct U { typedef int X; };
template<class U>
struct D;
};
template<class B>
template<class U>
struct C<B>::D {
typename U::X r; // which U?
};
C<int>::D<double> y;
In the definition of D, U definitely needs to be in scope as soon as it's declared because it might have been used in subsequent template parameter declarations, or it might have been used in the id-expression that names the declared entity — just as B is used in C<B>::D. (So 3.3.9 [basic.scope.temp] does the right thing for that purpose.)
But it would be nice if the result of lookup did not depend on whether D's body appears lexically inside C's body; currently, we don't seem to have the wording that makes it so.
The current description of unqualified name lookup in 3.4.1 [basic.lookup.unqual] paragraph 8 does not correctly handle complex cases of nesting. The Standard currently reads,
A name used in the definition of a function that is a member function (9.3) of a class X shall be declared in one of the following ways:In particular, this formulation does not handle the following example:
- before its use in the block in which it is used or in an enclosing block (6.3), or
- shall be a member of class X or be a member of a base class of X (10.2), or
- if X is a nested class of class Y (9.7), shall be a member of Y, or shall be a member of a base class of Y (this lookup applies in turn to Y's enclosing classes, starting with the innermost enclosing class), or
- if X is a local class (9.8) or is a nested class of a local class, before the definition of class X in a block enclosing the definition of class X, or
- if X is a member of namespace N, or is a nested class of a class that is a member of N, or is a local class or nested class within a local class of a function that is a member of N, before the member function definition, in namespace N or in one of N's enclosing namespaces.
struct outer {
static int i;
struct inner {
void f() {
struct local {
void g() {
i = 5;
}
};
}
};
};
Here the reference to i is from a member function of a local
class of a member function of a nested class. Nothing in the rules
allows outer::i to be found, although intuitively it should
be found.
A more comprehensive formulation is needed that allows traversal of any combination of blocks, local classes, and nested classes. Similarly, the final bullet needs to be augmented so that a function need not be a (direct) member of a namespace to allow searching that namespace when the reference is from a member function of a class local to that function. That is, the current rules do not allow the following example:
int j; // global namespace
struct S {
void f() {
struct local2 {
void g() {
j = 5;
}
};
}
};
There seems to be some confusion in the Standard regarding the relationship between 3.4.1 [basic.lookup.unqual] (Unqualified name lookup) and 3.4.2 [basic.lookup.argdep] (Argument-dependent lookup). For example, 3.4.1 [basic.lookup.unqual] paragraph 3 says,
The lookup for an unqualified name used as the postfix-expression of a function call is described in 3.4.2 [basic.lookup.argdep].
In other words, nothing in 3.4.1 [basic.lookup.unqual] applies to function names; the entire lookup is described in 3.4.2 [basic.lookup.argdep].
3.4.2 [basic.lookup.argdep] does not appear to share this view of its responsibility. The closest it comes is in 3.4.2 [basic.lookup.argdep] paragraph 2a:
...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.
Presumably, "ordinary unqualified lookup" is a reference to the processing described in 3.4.1 [basic.lookup.unqual], but, as noted above, 3.4.1 [basic.lookup.unqual] explicitly precludes applying that processing to function names. The details of "ordinary unqualified lookup" of function names are not described anywhere.
The other clauses that reference 3.4.2 [basic.lookup.argdep], clauses 13 [over] and 14 [temp], are split over the question of the relationship between 3.4.1 [basic.lookup.unqual] and 3.4.2 [basic.lookup.argdep]. 13.3.1.1.1 [over.call.func] paragraph 3, for instance, says
The name is looked up in the context of the function call following the normal rules for name lookup in function calls (3.4.2 [basic.lookup.argdep]).
I.e., this reference assumes that 3.4.2 [basic.lookup.argdep] is self-contained. The same is true of 13.3.1.2 [over.match.oper] paragraph 3, second bullet:
The set of non-member candidates is the result of the unqualified lookup of operator@ in the context of the expression according to the usual rules for name lookup in unqualified function calls (3.4.2 [basic.lookup.argdep]), except that all member functions are ignored.
On the other hand, however, 14.6.4.2 [temp.dep.candidate] paragraph 1 explicitly assumes that 3.4.1 [basic.lookup.unqual] and 3.4.2 [basic.lookup.argdep] are both involved in function name lookup and do different things:
For a function call that depends on a template parameter, if the function name is an unqualified-id but not a template-id, the candidate functions are found using the usual lookup rules (3.4.1 [basic.lookup.unqual], 3.4.2 [basic.lookup.argdep]) except that:
- For the part of the lookup using unqualified name lookup (3.4.1 [basic.lookup.unqual]), only function declarations with external linkage from the template definition context are found.
- For the part of the lookup using associated namespaces (3.4.2 [basic.lookup.argdep]), only function declarations with external linkage found in either the template definition context or the template instantiation context are found.
Suggested resolution:
Change 3.4.1 [basic.lookup.unqual] paragraph 1 from
...name lookup ends as soon as a declaration is found for the name.
to
...name lookup ends with the first scope containing one or more declarations of the name.
Change the first sentence of 3.4.1 [basic.lookup.unqual] paragraph 3 from
The lookup for an unqualified name used as the postfix-expression of a function call is described in 3.4.2 [basic.lookup.argdep].
to
An unqualified name used as the postfix-expression of a function call is looked up as described below. In addition, argument-dependent lookup (3.4.2 [basic.lookup.argdep]) is performed on this name to complete the resulting set of declarations.
Although 3.3.9 [basic.scope.temp] now describes the scope of a template parameter, the description of unqualified name lookup in 3.4.1 [basic.lookup.unqual] do not cover uses of template parameter names. The note in 3.4.1 [basic.lookup.unqual] paragraph 16 says,
the rules for name lookup in template definitions are described in 14.6 [temp.res].
but the rules there cover dependent and non-dependent names, not template parameters themselves.
The last bullet of the second paragraph of section 3.4.2 [basic.lookup.argdep] says that:
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.
The first problem with this wording is that it is misleading, since one cannot get such a function argument whose type would be a template-id. The bullet should be speaking about template specializations instead.
The second problem is owing to the use of the word "defined" in the phrases "are the namespace in which the template is defined", "in which any template template arguments are defined", and "as template template arguments are defined". The bullet should use the word "declared" instead, since scenarios like the one below are possible:
namespace A {
template<class T>
struct test {
template<class U>
struct mem_templ { };
};
// declaration in namespace 'A'
template<> template<>
struct test<int>::mem_templ<int>;
void foo(test<int>::mem_templ<int>&)
{ }
}
// definition in the global namespace
template<> template<>
struct A::test<int>::mem_templ<int> {
};
int main()
{
A::test<int>::mem_templ<int> inst;
// According to the current definition of 3.4.2
// foo is not found.
foo(inst);
}
In addition, the bullet doesn't make it clear whether a T which is a class template specialization must also be treated as a class type, i.e. if the contents of the second bullet of the second paragraph of section 3.4.2 [basic.lookup.argdep].
must apply to it or not. The same stands for a T which is a function template specialization. This detail can make a difference in an example such as the one below:
- If T is a class type (including unions), its associated classes are: the class itself; the class of which it is a member, if any; and its direct and indirect base classes. Its associated namespaces are the namespaces in which its associated classes are defined. [This wording is as updated by core issue 90.]
template<class T>
struct slist_iterator {
friend bool operator==(const slist_iterator& x, const slist_iterator& y)
{ return true; }
};
template<class T>
struct slist {
typedef slist_iterator<T> iterator;
iterator begin()
{ return iterator(); }
iterator end()
{ return iterator(); }
};
int main()
{
slist<int> my_list;
slist<int>::iterator mi1 = my_list.begin(), mi2 = my_list.end();
// Must the the friend function declaration
// bool operator==(const slist_iterator<int>&, const slist_iterator<int>&);
// be found through argument dependent lookup? I.e. is the specialization
// 'slist<int>' the associated class of the arguments 'mi1' and 'mi2'. If we
// apply only the contents of the last bullet of 3.4.2/2, then the type
// 'slist_iterator<int>' has no associated classes and the friend declaration
// is not found.
mi1 == mi2;
}
Suggested resolution:
Replace the last bullet of the second paragraph of section 3.4.2 [basic.lookup.argdep]
with
- 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.
- If T is a class template specialization, its associated namespaces and classes are those associated with T when T is regarded as a class type; 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 the primary templates making template template arguments are declared; and the classes in which any primary member templates used as template template arguments are declared.
- If T is a function template specialization, its associated namespaces and classes are those associated with T when T is regarded as a function type; 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 the primary templates making template template arguments are declared; and the classes in which any primary member templates used as template template arguments are declared.
Replace the second bullet of the second paragraph of section 3.4.2 [basic.lookup.argdep]
with
- If T is a class type (including unions), its associated classes are: the class itself; the class of which it is a member, if any; and its direct and indirect base classes. Its associated namespaces are the namespaces in which its associated classes are defined.
- If T is a class type (including unions), its associated classes are: the class itself; the class of which it is a member, if any; and its direct and indirect base classes. Its associated namespaces are the namespaces in which its associated classes are declared [Note: in case of any of the associated classes being a class template specialization, its associated namespace is acually the namespace containing the declaration of the primary class template of the class template specialization].
Both 3.4.3.1 [class.qual] and 3.4.3.2 [namespace.qual] specify that some lookups are to be performed “in the context of the entire postfix-expression,” ignoring the fact that qualified-ids can appear outside of expressions.
It was suggested in document J16/05-0156 = WG21 N1896 that these uses be changed to “the context in which the qualified-id occurs,” but it isn't clear that this formulation adequately covers all the places a qualified-id can occur.
It is unclear to what extent entities without names match across translation units. For example,
struct S {
int :2;
enum { a, b, c } x;
static class {} *p;
};
If this declaration appears in multiple translation units, are all these members "the same" in each declaration?
A similar question can be asked about non-member declarations:
// Translation unit 1:
extern enum { d, e, f } y;
// Translation unit 2:
extern enum { d, e, f } y;
// Translation unit 3:
enum { d, e, f } y;
Is this valid C++? Is it valid C?
James Kanze: S::p cannot be defined, because to do so requires a type specifier and the type cannot be named. ::y is valid C because C only requires compatible, not identical, types. In C++, it appears that there is a new type in each declaration, so it would not be valid. This differs from S::x because the unnamed type is part of a named type — but I don't know where or if the Standard says that.
John Max Skaller: It's not valid C++, because the type is a synthesised, unique name for the enumeration type which differs across translation units, as if:
extern enum _synth1 { d,e,f} y;
..
extern enum _synth2 { d,e,f} y;
had been written.
However, within a class, the ODR implies the types are the same:
class X { enum { d } y; };
in two translation units ensures that the type of member y is the same: the two X's obey the ODR and so denote the same class, and it follows that there's only one member y and one type that it has.
(See also issues 132 and 216.)
The standard says that an unnamed class or enum definition can be given a "name for linkage purposes" through a typedef. E.g.,
typedef enum {} E;
extern E *p;
can appear in multiple translation units.
How about the following combination?
// Translation unit 1:
struct S;
extern S *q;
// Translation unit 2:
typedef struct {} S;
extern S *q;
Is this valid C++?
Also, if the answer is "yes", consider the following slight variant:
// Translation unit 1:
struct S {}; // <<-- class has definition
extern S *q;
// Translation unit 2:
typedef struct {} S;
extern S *q;
Is this a violation of the ODR because two definitions of type S consist of differing token sequences?
The following declarations are allowed within a translation unit:
struct S;
enum { S };
However, 3.5 [basic.link] paragraph 9 seems to say these two declarations cannot appear in two different translation units. That also would mean that the inclusion of a header containing the above in two different translation units is not valid C++.
I suspect this is an oversight and that users should be allowed to have the declarations above appear in different translation units. (It is a fairly common thing to do, I think.)
Mike Miller: I think you meant "enum E { S };" -- enumerators only have external linkage if the enumeration does (3.5 [basic.link] paragraph 4), and 3.5 [basic.link] paragraph 9 only applies to entities with external linkage.
I don't remember why enumerators were given linkage; I don't think it's necessary for mangling non-type template arguments. In any event, I can't think why cross-TU name collisions between enumerators and other entities would cause a problem, so I guess a change here would be okay. I can think of three changes that would have that effect:
Daveed Vandevoorde: I don't think any of these are sufficient in the sense that the problem isn't limited to enumerators. E.g.:
struct X; extern void X();shouldn't create cross-TU collisions either.
Mike Miller: So you're saying that cross-TU collisions should only be prohibited if both names denote entities of the same kind (both functions, both objects, both types, etc.), or if they are both references (regardless of what they refer to, presumably)?
Daveed Vandevoorde: Not exactly. Instead, I'm saying that if two entities (with external linkage) can coexist when they're both declared in the same translation unit (TU), then they should also be allowed to coexist when they're declared in two different translation units.
For example:
int i; void i(); // ErrorThis is an error within a TU, so I don't see a reason to make it valid across TUs.
However, "tag names" (class/struct/union/enum) can sometimes coexist with identically named entities (variables, functions & enumerators, but not namespaces, templates or type names).
There does not appear to be wording that prohibits a block-scope extern object declaration from being a definition.
Is a compiler allowed to interleave constructor calls when performing dynamic initialization of nonlocal objects? What I mean by interleaving is: beginning to execute a particular constructor, then going off and doing something else, then going back to the original constructor. I can't find anything explicit about this in clause 3.6.2 [basic.start.init].
I'll present a few different examples, some of which get a bit wild. But a lot of what this comes down to is exactly what the standard means when it talks about the order of initialization. If it says that some object x must be initialized before a particular event takes place, does that mean that x's constructor must be entered before that event, or does it mean that it must be exited before that event? If object x must be initialized before object y, does that mean that x's constructor must exit before y's constructor is entered?
(The answer to that question might just be common sense, but I couldn't find an answer in clause 3.6.2 [basic.start.init]. Actually, when I read 3.6.2 [basic.start.init] carefully, I find there are a lot of things I took for granted that aren't there.)
OK, so a few specific scenerios.
<runtime gunk>
<Enter A's constructor>
<Enter f>
<runtime gunk>
<Enter B's constructor>
<Enter f>
<Leave f>
<Leave B's constructor>
<Leave f>
<Leave A's constructor>
The implication of a 'yes' answer for users is that any function
called by a constructor, directly or indirectly, must be reentrant.At this point, you might be thinking we could avoid all of this nonsense by removing compilers' freedom to defer initialization until after the beginning of main(). I'd resist that, for two reasons. First, it would be a huge change to make after the standard has been out. Second, that freedom is necessary if we want to have support for dynamic libraries. I realize we don't yet say anything about dynamic libraries, but I'd hate to make decisions that would make such support even harder.
3.6.3 [basic.start.term] paragraph 2 says,
If a function contains a local object of static storage duration that has been destroyed and the function is called during the destruction of an object with static storage duration, the program has undefined behavior if the flow of control passes through the definition of the previously destroyed local object.
I would like to turn this behavior from undefined to well-defined behavior for the purpose of achieving a graceful shutdown, especially in a multi-threaded world.
Background: Alexandrescu describes the “phoenix singleton” in Modern C++ Design. This is a class used as a function local static, that will reconstruct itself, and reapply itself to the atexit chain, if the program attempts to use it after it is destructed in the atexit chain. It achieves this by setting a “destructed flag” in its own state in its destructor. If the object is later accessed (and a member function is called on it), the member function notes the state of the “destructed flag” and does the reconstruction dance. The phoenix singleton pattern was designed to address issues only in single-threaded code where accesses among static objects can have a non-scoped pattern. When we throw in multi-threading, and the possibility that threads can be running after main returns, the chances of accessing a destroyed static significantly increase.
The very least that I would like to see happen is to standardize what I believe is existing practice: When an object is destroyed in the atexit chain, the memory the object occupied is left in whatever state the destructor put it in. If this can only be reliably done for objects with standard layout, that would be an acceptable compromise. This would allow objects to set “I'm destructed” flags in their state and then do something well-defined if accessed, such as throw an exception.
A possible refinement of this idea is to have the compiler set up a 3-state flag around function-local statics instead of the current 2-state flag:
We have the first two states today. We might choose to add the third state, and if execution passes over a function-local static with “destroyed” state, an exception could be thrown. This would mean that we would not have to guarantee memory stability in destroyed objects of static duration.
This refinement would break phoenix singletons, and is not required for the ~mutex()/~condition() I've described and prototyped. But it might make it easier for Joe Coder to apply this kind of guarantee to his own types.
There are several problems with 3.7 [basic.stc]:
3.7 [basic.stc] paragraph 2 says that "Static and automatic storage durations are associated with objects introduced by declarations (3.1 [basic.def]) and implicitly created by the implementation (12.2 [class.temporary])."
In fact, objects "implicitly created by the implementation" are the temporaries described in (12.2 [class.temporary]), and have neither static nor automatic storage duration, but a totally different duration, described in 12.2 [class.temporary].
3.7 [basic.stc] uses the expression "local object" in several places, without ever defining it. Presumably, what is meant is "an object declared at block scope", but this should be said explicitly.
In a recent discussion in comp.lang.c++.moderated, on poster interpreted "local objects" as including temporaries. This would require them to live until the end of the block in which they are created, which contradicts 12.2 [class.temporary]. If temporaries are covered by this section, and the statement in 3.7 [basic.stc] seems to suggest, and they aren't local objects, then they must have static storage duration, which isn't right either.
I propose adding a fourth storage duration to the list after 3.7 [basic.stc] paragraph 1:
And rewriting the second paragraph of this section as follows:
Temporary storage duration is associated with objects implicitly created by the implementation, and is described in 12.2 [class.temporary]. Static and automatic storage durations are associated with objects defined by declarations; in the following, an object defined by a declaration with block scope is a local object. The dynamic storage duration is associated with objects created by the operator new.
Steve Adamczyk: There may well be an issue here, but one should bear in mind the difference between storage duration and object lifetime. As far as I can see, there is no particular problem with temporaries having automatic or static storage duration, as appropriate. The point of 12.2 [class.temporary] is that they have an unusual object lifetime.
Notes from Ocrober 2002 meeting:
It might be desirable to shorten the storage duration of temporaries to allow reuse of them. The as-if rule allows some reuse, but such reuse requires analysis, including noting whether the addresses of such temporaries have been taken.
Notes from the August, 2011 meeting:
The CWG decided that further consideration of this issue would be deferred until someone produces a paper explaining the need for action and proposing specific changes.
The global allocation functions are implicitly declared in every translation unit with exception-specifications (3.7.4 [basic.stc.dynamic] paragraph 2). It is not clear what should happen if a replacement allocation function is declared without an exception-specification. Is that a conflict with the implicitly-declared function (as it would be with explicitly-declared functions, and presumably is if the <new> header is included)? Or does the new declaration replace the implicit one, including the lack of an exception-specification? Or does the implicit declaration prevail? (Regardless of the exception-specification or lack thereof, it is presumably undefined behavior for an allocation function to exit with an exception that cannot be caught by a handler of type std::bad_alloc (3.7.4.1 [basic.stc.dynamic.allocation] paragraph 3).)
When an object is deleted, 3.7.4.2 [basic.stc.dynamic.deallocation] says that the deallocation “[renders] invalid all pointers referring to any part of the deallocated storage.” According to 3.9.2 [basic.compound] paragraph 3, a pointer whose address is one past the end of an array is considered to point to an unrelated object that happens to reside at that address. Does this need to be clarified to specify that the one-past-the-end pointer of an array is not invalidated by deleting the following object? (See also 5.3.5 [expr.delete] paragraph 4, which also mentions that the system deallocation function renders a pointer invalid.)
The current Standard says that any use of an invalid pointer value produces undefined behavior (3.7.4.2 [basic.stc.dynamic.deallocation] paragraph 4). This includes not only dereferencing the pointer but even just fetching its value. The reason for this draconian restriction is that some architectures in the past used dedicated address registers for pointer loads and stores and they could fault if, for example, a segment number in a pointer was not currently mapped.
It is not clear whether such restrictions are necessary with architectures currently in use or reasonably foreseen. This should be investigated to see if the restriction can be loosened to apply only to dereferencing the pointer.
Consider
extern "C" int printf (const char *,...);
struct Base { Base();};
struct Derived: virtual public Base {
Derived() {;}
};
Derived d;
extern Derived& obj = d;
int i;
Base::Base() {
if ((Base *) &obj) i = 4;
printf ("i=%d\n", i);
}
int main() { return 0; }
12.7 [class.cdtor] paragraph 2 makes this valid, but 3.8 [basic.life] paragraph 5 implies that it isn't valid.
Steve Adamczyk: A second issue:
extern "C" int printf(const char *,...);
struct A { virtual ~A(); int x; };
struct B : public virtual A { };
struct C : public B { C(int); };
struct D : public C { D(); };
int main() { D t; printf("passed\n");return 0; }
A::~A() {}
C::C(int) {}
D::D() : C(this->x) {}
Core issue 52 almost, but not quite, says that in evaluating "this->x" you do a cast to the virtual base class A, which would be an error according to 12.7 [class.cdtor] paragraph 2 because the base class B constructor hasn't started yet. 5.2.5 [expr.ref] should be clarified to say that the cast does need to get done.
James Kanze submitted the same issue via comp.std.c++ on 11 July 2003:
Richard Smith: Nonsense. You can use "this" perfectly happily in a constructor, just be careful that (a) you're not using any members that are not fully initialised, and (b) if you're calling virtual functions you know exactly what you're doing.
In practice, and I think in intent, you are right. However, the standard makes some pretty stringent restrictions in 3.8 [basic.life]. To start with, it says (in paragraph 1):
The lifetime of an object is a runtime property of the object. The lifetime of an object of type T begins when:(Emphasis added.) Then when we get down to paragraph 5, it says:The lifetime of an object of type T ends when:
- storage with the proper alignment and size for type T is obtained, and
- if T is a class type with a non-trivial constructor, the constructor calls has COMPLETED.
- if T is a class type with a non-trivial destructor, the destructor call STARTS, or
- the storage which the object occupies is reused or released.
Before the lifetime of an object has started but after the storage which the object will occupy has been allocated [which sounds to me like it would include in the constructor, given the text above] or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any pointer that refers to the storage location where the object will be or was located may be used but only in limited ways. [...] If the object will be or was of a non-POD class type, the program has undefined behavior if:
[...]
- the pointer is implicitly converted to a pointer to a base class type, or [...]
I can't find any exceptions for the this pointer.
Note that calling a non-static function in the base class, or even constructing the base class in initializer list, involves an implicit conversion of this to a pointer to the base class. Thus undefined behavior. I'm sure that this wasn't the intent, but it would seem to be what this paragraph is saying.
Notes from the August, 2011 meeting:
CWG will need a paper exploring this topic before it can act on the issue.
Sent in by David Abrahams:
Yes, and to add to this tangent, 3.9.1 [basic.fundamental] paragraph 1 states "Plain char, signed char, and unsigned char are three distinct types." Strangely, 3.9 [basic.types] paragraph 2 talks about how "... the underlying bytes making up the object can be copied into an array of char or unsigned char. 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." I guess there's no requirement that this copying work properly with signed chars!
Notes from October 2002 meeting:
We should do whatever C99 does. 6.5p6 of the C99 standard says "array of character type", and "character type" includes signed char (6.2.5p15), and 6.5p7 says "character type". But see also 6.2.6.1p4, which mentions (only) an array of unsigned char.
Proposed resolution (April 2003):
Change 3.8 [basic.life] paragraph 5 bullet 3 from
to
Change 3.8 [basic.life] paragraph 6 bullet 3 from
to
Change the beginning of 3.9 [basic.types] paragraph 2 from
For any object (other than a base-class subobject) of POD 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.
to
For any object (other than a base-class subobject) of POD 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 byte-character type.
Add the indicated text to 3.9.1 [basic.fundamental] paragraph 1:
Objects declared as characters (char) shall be large enough to store any member of the implementation's basic character set. If a character from this set is stored in a character object, the integral value of that character object is equal to the value of the single character literal form of that character. It is implementation-defined whether a char object can hold negative values. Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types, called the byte-character types. A char, a signed char, and an unsigned char occupy the same amount of storage and have the same alignment requirements (3.9 [basic.types]); that is, they have the same object representation. For byte-character types, all bits of the object representation participate in the value representation. For unsigned byte-character types, all possible bit patterns of the value representation represent numbers. These requirements do not hold for other types. In any particular implementation, a plain char object can take on either the same values as a signed char or an unsigned char; which one is implementation-defined.
Change 3.10 [basic.lval] paragraph 15 last bullet from
to
Notes from October 2003 meeting:
It appears that in C99 signed char may have padding bits but no trap representation, whereas in C++ signed char has no padding bits but may have -0. A memcpy in C++ would have to copy the array preserving the actual representation and not just the value.
March 2004: The liaisons to the C committee have been asked to tell us whether this change would introduce any unnecessary incompatibilities with C.
Notes from October 2004 meeting:
The C99 Standard appears to be inconsistent in its requirements. For example, 6.2.6.1 paragraph 4 says:
The value may be copied into an object of type unsigned char [n] (e.g., by memcpy); the resulting set of bytes is called the object representation of the value.
On the other hand, 6.2 paragraph 6 says,
If a value is copied into an object having no declared type using memcpy or memmove, or is copied as an array of character type, then the effective type of the modified object for that access and for subsequent accesses that do not modify the value is the effective type of the object from which the value is copied, if it has one.
Mike Miller will investigate further.
Proposed resolution (February, 2010):
Change 3.8 [basic.life] paragraph 5 bullet 4 as follows:
...The program has undefined behavior if:
...
the pointer is used as the operand of a static_cast (5.2.9 [expr.static.cast]) (except when the conversion is to cv void*, or to cv void* and subsequently to char*, or unsigned char* a pointer to a cv-qualified or cv-unqualified byte-character type (3.9.1 [basic.fundamental])), or
...
Change 3.8 [basic.life] paragraph 6 bullet 4 as follows:
...The program has undefined behavior if:
...
the lvalue is used as the operand of a static_cast (5.2.9 [expr.static.cast]) except when the conversion is ultimately to cv char& or cv unsigned char& a reference to a cv-qualified or cv-unqualified byte-character type (3.9.1 [basic.fundamental]) or an array thereof, or
...
Change 3.9 [basic.types] paragraph 2 as follows:
For any object (other than a base-class subobject) of trivially copyable 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 a byte-character type (3.9.1 [basic.fundamental]).39 If the content of the that array of char or unsigned char is copied back into the object, the object shall subsequently hold its original value. [Example:...
Change 3.9.1 [basic.fundamental] paragraph 1 as follows:
...Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types, called the byte-character types. A char, a signed char, and an unsigned char occupy the same amount of storage and have the same alignment requirements (3.11 [basic.align]); that is, they have the same object representation. For byte-character types, all bits of the object representation participate in the value representation. For unsigned character types unsigned char, all possible bit patterns of the value representation represent numbers...
Change 3.10 [basic.lval] paragraph 15 final bullet as follows:
If a program attempts to access the stored value of an object through an lvalue of other than one of the following types the behavior is undefined 52
...
a char or unsigned char byte-character type (3.9.1 [basic.fundamental]).
Change 3.11 [basic.align] paragraph 6 as follows:
The alignment requirement of a complete type can be queried using an alignof expression (5.3.6 [expr.alignof]). Furthermore, the byte-character types (3.9.1 [basic.fundamental]) char, signed char, and unsigned char shall have the weakest alignment requirement. [Note: this enables the byte-character types to be used as the underlying type for an aligned memory area (7.6.2 [dcl.align]). —end note]
Change 5.3.4 [expr.new] paragraph 10 as follows:
...For arrays of char and unsigned char a byte-character type (3.9.1 [basic.fundamental]), the difference between the result of the new-expression and the address returned by the allocation function shall be an integral multiple of the strictest fundamental alignment requirement (3.11 [basic.align]) of any object type whose size is no greater than the size of the array being created. [Note: Because allocation functions are assumed to return pointers to storage that is appropriately aligned for objects of any type with fundamental alignment, this constraint on array allocation overhead permits the common idiom of allocating byte-character arrays into which objects of other types will later be placed. —end note]
Notes from the March, 2010 meeting:
The CWG was not convinced that there was a need to change the existing specification at this time. Some were concerned that there might be implementation difficulties with giving signed char the requisite semantics; implementations for which that is true can currently make char equivalent to unsigned char and avoid those problems, but the suggested change would undermine that strategy.
Currently, literal class types can have mutable members. It is not clear whether that poses any particular problems with constexpr objects and constant expressions, and if so, what should be done about it.
3.9.1 [basic.fundamental] does not impose a requirement on the floating point types that there be an exact representation of the value zero. This omission is significant in 4.12 [conv.bool] paragraph 1, in which any non-zero value converts to the bool value true.
Suggested resolution: require that all floating point types have an exact representation of the value zero.
3.9.1 [basic.fundamental] paragraph 2 says that
There are four signed integer types: "signed char", "short int", "int", and "long int."
This would indicate that const int is not a signed integer type.
The relationship between the values representable by corresponding signed and unsigned integer types is not completely described, but 3.9 [basic.types] paragraph 4 says,
The value representation of an object is the set of bits that hold the value of type T.
and 3.9.1 [basic.fundamental] paragraph 3 says,
The range of nonnegative values of a signed integer type is a subrange of the corresponding unsigned integer type, and the value representation of each corresponding signed/unsigned type shall be the same.
I.e., the maximum value of eac