| Document number: | J16/08-0014 = WG21 N2504 |
| Date: | 2008-02-03 |
| 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," "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 J16/07-0331 = WG21 N2461.
The purpose of these documents is to record the disposition of issues that have come before the Core Language Working Group of the ANSI (J16) 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, commonly 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 J16 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.
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.
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.
According to 10.3 [class.virtual] paragraph 2:
Then in any well-formed class, for each virtual function declared in that class or any of its direct or indirect base classes there is a unique final overrider that overrides that function and every other overrider of that function. The rules for member lookup (10.2 [class.member.lookup]) are used to determine the final overrider for a virtual function in the scope of a derived class but ignoring names introduced by using-declarations.
I think that description is wrong on at least a couple of counts. First, consider the following example:
struct A { virtual void f(); };
struct B: A { };
struct C: A { void f(); };
struct D: B, C { };
What is the “unique final overrider” of A::f() in D? According to 10.3 [class.virtual] paragraph 2, we determine that by looking up f in D using the lookup rules in 10.2 [class.member.lookup]. However, that lookup determines that f in D is ambiguous, so there is no “unique final overrider” of A::f() in D. Consequently, because “any well-formed class” must have such an overrider, D must be ill-formed.
Of course, we all know that D is not ill-formed. In fact, 10.3 [class.virtual] paragraph 10 contains an example that illustrates exactly this point:
struct A { virtual void f(); }; struct B1 : A { // note non-virtual derivation void f(); }; struct B2 : A { void f(); }; struct D : B1, B2 { // D has two separate A subobjects };In class D above there are two occurrences of class A and hence two occurrences of the virtual member function A::f. The final overrider of B1::A::f is B1::f and the final overrider of B2::A::f is B2::f.
It appears that the requirement for a “unique final overrider” in 10.3 [class.virtual] paragraph 2 needs to say something about sub-objects. Whatever that “something” is, you can't just say “look up the name in the derived class using 10.2 [class.member.lookup].”
There's another problem with using the 10.2 [class.member.lookup] lookup to specify the final overrider: name lookup just looks up the name, while the overriding relationship is based not only on the name but on a matching parameter-type-list and cv-qualification. To illustrate this point:
struct X {
virtual void f();
};
struct Y: X {
void f(int);
};
struct Z: Y { };
What is the “unique final overrider” of X::f() in A? Again, 10.3 [class.virtual] paragraph 2 says you're supposed to look up f in Z to find it; however, what you find is Y::f(int), not X::f(), and that's clearly wrong.
Proposed Resolution (December, 2006):
Change 10.3 [class.virtual] paragraph 2 as follows:
Then in any well-formed class, for each virtual function declared in that class or any of its direct or indirect base classes there is a unique final overrider that overrides that function and every other overrider of that function. The rules for member lookup (10.2 [class.member.lookup]) are used to determine the final overrider for a virtual function in the scope of a derived class but ignoring names introduced by using-declaration s.A virtual member function vf of a class C is a final overrider unless the most derived class (1.8 [intro.object]) of which C is a base class (if any) declares or inherits another member function that overrides vf. In a derived class, if a virtual member function of a base class subobject has more than one final overrider, the program is ill-formed.
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 an lvalue if its right operand is an lvalue, and is a bit-field if its right operand is an lvalue and a bit-field. If the value of the right operand is a temporary (12.2 [class.temporary]), the result is that temporary.
12.6 [class.init] paragraph 2 says,
When an array of class objects is initialized (either explicitly or implicitly), the constructor shall be called for each element of the array, following the subscript order;
That implies that, given
struct POD {
int x;
};
POD data[10] = {};
this should call the implicitly declared default ctor 10 times, leaving 10 uninitialized ints, rather than value initialize each member of data, resulting in 10 initialized ints (which is required by 8.5.1 [dcl.init.aggr] paragraph 7).
I suggest rephrasing along the lines:
When an array is initialized (either explicitly or implicitly), each element of the array shall be initialized in turn, following the subscript order;
This would allow for PODs and other classes with a dual nature under value/default initialization, and cover copy initialization for arrays too.
Proposed resolution (October, 2006):
Change 12.6 [class.init] paragraph 3 as follows:
When an array of class objects is initialized (either explicitly or implicitly) and the elements are initialized by constructor, the constructor shall be called for each element of the array, following the subscript order; see 8.3.4 [dcl.array].
Jack Rouse: In 12.8 [class.copy] paragraph 8, the standard includes the following about the copying of class subobjects in such a constructor:
Mike Miller: I'm more concerned about 12.8 [class.copy] paragraph 7, which lists the situations in which an implicitly-defined copy constructor can render a program ill-formed. Inaccessible and ambiguous copy constructors are listed, but not a copy constructor with a cv-qualification mismatch. These two paragraphs taken together could be read as requiring the calling of a copy constructor with a non-const reference parameter for a const data member.
Proposed Resolution (November, 2006):
This issue is resolved by the proposed resolution for issue 535.
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 (November, 2006):
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]). Acopyconstructor selected to copy class objects is used even if the call is actually elided by the implementation (12.8 [class.copy]). —end note] ...A copy-assignment function for a classAn assigment operator function in a class is used by an implicitly-defined copy-assignment function for another class as specified in 12.8 [class.copy]...
Delete 12.1 [class.ctor] paragraphs 10 and 11:
A copy constructor (12.8 [class.copy]) is used to copy objects of class type.
A union member shall not be of a class type (or array thereof) that has a non-trivial constructor.
Replace the “example” in 12.2 [class.temporary] paragraph 1 with a note as follows:
[Example: even if the copy constructor is not called, all the semantic restrictions, such as accessibility (clause 11 [class.access]), shall be satisfied. —end example][Note: This includes accessibility (clause 11 [class.access]) for the constructor selected. —end note]
Change 12.8 [class.copy] paragraph 7 as follows:
A non-user-provided copy constructor is implicitly defined if it is used
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(3.2 [basic.def.odr]). [Footnote: See 8.5 [dcl.init] for more details on direct and copy initialization. —end footnote] [Note: the copy constructor is implicitly defined even if the implementation elidedits use (12.2 [class.temporary])the copy operation (12.8 [class.copy]). —end note]A program is ill-formed if the class for which a copy constructor is implicitly defined or explicitly defaulted has:
a non-static data member of class type (or array thereof) with an inaccessible or ambiguous copy constructor, or
a base class with an inaccessible or ambiguous copy constructor.
Before the non-user-provided copy constructor for a class is implicitly defined...
Change 12.8 [class.copy] paragraph 8 as follows:
...Each subobject is copied in the manner appropriate to its type:
if the subobject is of class type,
the copy constructor for the class is useddirect-initialization (8.5 [dcl.init]) is performed [Note: If overload resolution fails or the constructor selected by overload resolution is inaccessible (11 [class.access]) in the context of X, the program is ill-formed. —end note];if the subobject is an array...
[Drafting note: 8.5 [dcl.init] paragraph 15 requires “unambiguous” and 13.3 [over.match] paragraph 3 requires “accessible,” thus no need for normative text here.]
Change 12.8 [class.copy] paragraph 12 as follows:
A non-user-provided copy assignment operator is implicitly defined when
an object of its class type is assigned a value of its class type or a value of a class type derived from its class typeit is used (3.2 [basic.def.odr]). A program is ill-formed if the class for which a copy assignment operator is implicitly defined or explicitly defaulted has:a non-static data member of const or reference type.
a non-static data member of const type, or
a non-static data member of reference type, or
a non-static data member of class type (or array thereof) with an inaccessible copy assignment operator, or
a base class with an inaccessible copy assignment operator.
Change 12.8 [class.copy] paragraph 13 as follows:
... Each subobject is assigned in the manner appropriate to its type:
if the subobject is of class type,
the copy assignment operator for the classthe assignment operator function selected by overload resolution (13.3 [over.match]) for that class is used (as if by explicit qualification; that is, ignoring any possible virtual overriding functions in more derived classes) [Note: If overload resolution fails or the assignment operator function selected by overload resolution is inaccessible (11 [class.access]) in the context of X, the program is ill-formed. —end note];if the subobject is an array...
Delete 12.8 [class.copy] paragraph 14:
A program is ill-formed if the copy constructor or the copy assignment operator for an object is implicitly used and the special member function is not accessible (clause 11 [class.access]). [Note: Copying one object into another using the copy constructor or the copy assignment operator does not change the layout or size of either object. —end note]
Change 12.8 [class.copy] paragraph 15 as follows:
When certain criteria are met, an implementation is allowed to omit the copy construction of a class object, even if thecopyconstructor selected for the copy operation and/or the destructor for the object have side effects. In such cases, the implementation treats the source and target of the omitted copy operation as simply two different ways of referring to the same object, and the destruction of that object occurs at the later of the times when the two objects would have been destroyed without the optimization. [Footnote: Because only one object is destroyed instead of two, and onecopyconstructor is not executed, there is still one object destroyed for each one constructed. —end footnote] This elision...
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 acopyconverting 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 initializes a temporary object, called the exception object,the type of whichby copy-initialization (8.5 [dcl.init]). The type of that temporary object is determined...
Change 15.1 [except.throw] paragraph 5 as follows:
When the thrown object is a class object, thecopyconstructor selected for the copy-initialization and the destructor shall be accessible, even if the copy operation is elided (12.8 [class.copy]).
Change 15.3 [except.handle] paragraphs 16-17 as follows:
When the exception-declaration specifies a class type,
a copy constructorcopy initialization (8.5 [dcl.init]) is used to initialize either the object declared in the exception-declaration or, if the exception-declaration does not specify a name, a temporary object of that type. The object shall not have an abstract class type. The object is destroyed when the handler exits, after the destruction of any automatic objects initialized within the handler. Thecopyconstructor selected for the copy-initialization and the destructor shall be accessible in the context of the handler, even if the copy operation is elided (12.8 [class.copy]).If the copy constructor and destructor are implicitly declared (12.8 [class.copy]), such a use in the handler causes these functions to be implicitly defined; otherwise, the program shall provide a definition for these functions.
The copy constructor and destructor associated with the object shall be accessible even if the copy operation is elided (12.8 [class.copy]).
Change the footnote in 15.5.1 [except.terminate] paragraph 1 as follows:
[Footnote: For example, if the object being thrown is of a classwith a copy constructortype, std::terminate() will be called ifthat copy constructorthe constructor selected to copy the object exits with an exception during a throw. —end footnote]
(This resolution also resolves issue 111.)
[Drafting note: The following do not require changes: 5.17 [expr.ass] paragraph 4; 9 [class] paragraph 5; 9.5 [class.union] paragraph 1; 12.2 [class.temporary] paragraph 2; 12.8 [class.copy] paragraphs 1-2; 15.4 [except.spec] paragraph 14.]
Is the following a “copy assignment operator?”
struct A {
const A& operator=(const A&) volatile;
};
12.8 [class.copy] paragraph 9 doesn't say one way or the other whether cv-qualifiers on the function are allowed. (A similar question applies to the const case, but I avoided that example because it seems so wrong one tends to jump to a conclusion before seeing what the standard says.)
Since the point of the definition of “copy assignment operator” is to control whether the compiler generates a default version if the user doesn’t, I suspect the correct answer is that neither const nor volatile cv-qualification on operator= should be allowed for a “copy assignment operator.” A user can write an operator= like that, but it doesn't affect whether the compiler generates the default one.
Proposed Resolution (November, 2006):
Change 12.8 [class.copy] paragraph 9 as follows:
A user-declared copy assignment operator X::operator= is a non-static non-template non-volatile non-const member function of class X with exactly one parameter of type X, X&, const X&, volatile X& or const volatile X&.
[Drafting note: If a user-declared volatile operator= prevented the implicit declaration of the copy assignment operator, all assignments for objects of the given class (even to non-volatile objects) would pay the penalty for volatile write accesses in the user-declared operator=, despite not needing it.]
12.3.2 [class.conv.fct] paragraph 1 says,
A conversion function is never used to convert a (possibly cv-qualified) object to the (possibly cv-qualified) same object type (or a reference to it), to a (possibly cv-qualified) base class of that type (or a reference to it), or to (possibly cv-qualified) void.
At what point is this enforced, and how is it enforced?
Consider this test case:
struct abc;
struct xyz {
xyz();
xyz(xyz &);
operator xyz& (); // #1
operator abc& (); // #2
};
struct abc : xyz {};
void foo(xyz &);
void bar() {
foo (xyz ());
}
If such conversion functions are part of the overload set, #1 is a better conversion than #2 to convert the temporary xyz object to a non-const reference required for foo's operand. If such conversion functions are not part of the overload set, then #2 would be selected, and AFAICT the program would be well formed.
If the conversion functions are not part of the overload set, then it would seem one cannot take their address. For instance, adding the following line to the above test case would find no suitable function:
xyz &(xyz::*ptr) () = &xyz::operator xyz &;
Notes from the October, 2007 meeting:
The intent of 12.3.2 [class.conv.fct] paragraph 1 is that overload resolution not be attempted at all for the listed cases; that is, if the target type is void, the object's type, or a base of the object's type, the conversion is done directly without considering any conversion functions. Consequently, the questions about whether the conversion function is part of the overload set or not are moot. The wording will be changed to make this clearer.
Proposed Resolution (October, 2007):
Change the footnote in 12.3.2 [class.conv.fct] paragraph 1 as follows:
A conversion function is never used to convert a (possibly cv-qualified) object to the (possibly cv-qualified) same object type (or a reference to it), to a (possibly cv-qualified) base class of that type (or a reference to it), or to (possibly cv-qualified) void. [Footnote: These conversions are considered as standard conversions for the purposes of overload resolution (13.3.3.1 [over.best.ics], 13.3.3.1.4 [over.ics.ref]) and therefore initialization (8.5 [dcl.init]) and explicit casts (5.2.9 [expr.static.cast]). A conversion to void does not invoke any conversion function (5.2.9 [expr.static.cast]). Even though never directly called to perform a conversion, such conversion functions can be declared and can potentially be reached through a call to a virtual conversion function in a base class —end footnote]
The overload resolution rules for ranking a template against a non-template function differ for conversion functions in a surprising way. 13.3.3 [over.match.best] lists four checks, the last three concern this report. For the non-conversion operator case, checks 2 and 3 are applicable, whereas for the conversion operator case checks 3 and 4 are applicable. Checks 2 and 4 concern the ranking of argument and return value conversion sequences respectively. Check 3 concerns only the templatedness of the functions being ranked, and will prefer a non-template to a template. Notice that this check happens after argument conversion sequence ranking, but before return value conversion sequence ranking. This has the effect of always selecting a non-template conversion operator, as the following example shows:
struct C
{
inline operator int () { return 1; }
template <class T> inline operator T () { return 0; }
};
inline long f (long x) { return x; }
int
main (int argc, char *argv[])
{
return f (C ());
}
The non-templated C::operator int function will be selected, rather than the apparently better C::operator long<long> instantiation. This is a surprise, and resulted in a bug report where the user expected the template to be selected. In addition some C++ compilers have implemented the overload ranking as if checks 3 and 4 were transposed.
Is this ordering accidental, or is there a rationale?
Notes from the April, 2005 meeting:
The CWG agreed that the template/non-template distinction should be the final tie-breaker.
Proposed resolution (March, 2007):
In the second bulleted list of 13.3.3 [over.match.best] paragraph 1, move the second and third bullets to the end of the list, to read as follows:
for some argument j, ICSj(F1) is a better conversion sequence than ICSj(F2), or, if not that,
the context is an initialization by user-defined conversion (see 8.5 [dcl.init], 13.3.1.5 [over.match.conv], and 13.3.1.6 [over.match.ref]) and the standard conversion sequence from the return type of F1 to the destination type (i.e., the type of the entity being initialized) is a better conversion sequence than the standard conversion sequence from the return type of F2 to the destination type, [Example: ... —end example] or, if not that,
- F1 is a non-template function and F2 is a function template specialization, or, if not that,
F1 and F2 are function template specializations, and the function template for F1 is more specialized than the template for F2 according to the partial ordering rules described in 14.5.6.2 [temp.func.order].
One of the requirements for two template-ids to refer to the same class or function (14.4 [temp.type] paragraph 1) is that
If we have some template of the form
template <unsigned char c> struct A;
does this imply that A<'\001'> and A<257> (for an eight-bit char) refer to different specializations?
Jens Maurer: Looks like it should say something like, “their corresponding converted non-type template arguments of integral or enumeration type have identical values.”
Proposed resolution (April, 2007):
The change to 14.4 [temp.type] paragraph 1 shown in document J16/07-0118 = WG21 N2258, in which the syntactic non-terminal template-argument is changed to the English term “template argument” is sufficient to remove the confusion about whether the value before or after conversion is used in matching template-ids.
14.6.2 [temp.dep] paragraph 3 reads,
In the definition of a class template or a member of a class template, if a base class of the class template depends on a template-parameter, the base class scope is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member.
This wording applies only to definitions of class templates and members of class templates. That would make the following program ill-formed (but it probably should be well-formed):
struct B{ void f(int); };
template<class T> struct D: B { };
template<class T> void g() {
struct B{ void f(); };
struct A: D<T> {
B m;
};
A a;
a.m.f(); // Presumably, we want ::g()::B::f(), not ::B::f(int)
}
int main () {
g<int>();
return 0;
}
I suspect the wording should be something like
In the definition of a class template or a class defined (directly or indirectly) within the scope of a class template or function template, if a base class...
That should also include deeply nested classes in templates, local classes of non-template member functions of member classes of class templates, etc.
Proposed resolution (October, 2006):
Change 14.6.2 [temp.dep] paragraph 3 as follows:
In the definition of a class or class templateor a member of a class template, if a base classof the class templatedepends on a template-parameter, the base class scope is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member.
It is not clear how to handle the following example:
struct S {
template <typename T> S(const T&);
};
void f(const S&);
void f(int);
void g() {
enum E { e };
f(e); // ill-formed?
}
Three possibilities suggest themselves:
Fail during overload resolution. In order to perform overload resolution for the call to f, the declaration of the required specialization of the S constructor must be instantiated. This instantiation uses a local type and is thus ill-formed (14.3.1 [temp.arg.type] paragraph 2), rendering the example as a whole ill-formed, as well.
Treat this as a type-deduction failure. Although it is not listed currently among the causes of type-deduction failure in 14.8.2 [temp.deduct] paragraph 2, it could plausibly be argued that instantiating a function declaration with a local type as a template type-parameter falls under the rubric of “If a substitution in a template parameter or in the function type of the function template results in an invalid type” and thus should be a type-deduction failure. The result would be that the example is well-formed because f(const S&) would be removed from the list of viable functions.
Fail only if the function selected by overload resolution requires instantiation with a local type. This approach would require that the diagnostic resulting from the instantiation of the function type during overload resolution be suppressed and either regenerated or regurgitated once overload resolution is complete. (The example would be well-formed under this approach because f(int) would be selected as the best match.)
(See also issue 489.)
Notes from the April, 2005 meeting:
The question in the original example was whether there should be an error, even though the uninstantiable template was not needed for calling the best-matching function. The broader issue is whether a user would prefer to get an error or to call a “worse” non-template function in such cases. For example:
template<typename T> void f(T);
void f(int);
void g() {
enum E { e };
f(e); // call f(int) or get an error?
}
It was observed that the type deduction rules are intended to model, albeit selectively, the other rules of the language. This would argue in favor of the second approach, a type-deduction failure, and the consensus of the group was that the incremental benefit of other approaches was not enough to outweigh the additional complexity of specification and implementation.
Proposed resolution (October, 2005):
Add a new sub-bullet following bullet 3, sub-bullet 7 ("Attempting to give an invalid type to a non-type template parameter") of 14.8.2 [temp.deduct] paragraph 2:
Attempting to use a local or unnamed type as the value of a template type parameter.
Additional note (December, 2005):
The Evolution Working Group is currently considering an extension that would effectively give linkage to some (but perhaps not all) types that currently have no linkage. If the proposed resolution above is adopted and then later a change along the lines that the EWG is considering were also adopted, the result would be a silent change in the result of overload resolution, because the newly-acceptable specializations would become part of the overload set. It is not clear whether that possibility is sufficient reason to delay adoption of this resolution or not.
Notes from the April, 2007 meeting:
The Evolution Working Group is now actively pursuing an extension that would allow local and/or nameless types to be used as template arguments, so this resolution will be held in abeyance until the outcome of that proposal is known.
The last two sentences of 14.8.2 [temp.deduct] paragraph 5 read:
When all template arguments have been deduced or obtained from default template arguments, all uses of template parameters in non-deduced contexts are replaced with the corresponding deduced or default argument values. If the substitution results in an invalid type, as described above, type deduction fails.
Shouldn't the substitution occur for all uses of the parameters, so that any of them could result in deduction failure?
Proposed resolution (October, 2006):
Change 14.8.2 [temp.deduct] paragraph 5 as follows:
...When all template arguments have been deduced or obtained from default template arguments, all uses of template parameters innon-deduced contextsthe function type are replaced with the corresponding deduced or default argument values. If the substitution results in an invalid type, as described above, type deduction fails.
According to 15.1 [except.throw] paragraph 3,
The type of the throw-expression shall not be an incomplete type, or a pointer to an incomplete type other than (possibly cv-qualified) void.
This disallows cases like the following, because str has an incomplete type (an array of unknown size):
extern const char str[];
void f() {
throw str;
}
The array-to-pointer conversion is applied to the operand of throw, so there's no problem creating the exception object, which is the reason for the restriction on incomplete types. I believe this case should be permitted.
Notes from the April, 2005 meeting:
The CWG agreed that the example should be permitted. Note that the reference to throw-expression in the cited text is incorrect; a throw-expression includes the throw keyword and is always of type void. This wording problem is addressed in the proposed resolution for issue 475.
Proposed resolution (October, 2006)
Change 15.1 [except.throw] paragraph 3 as indicated:
...The type of the throw-expression shall notIf the type of the exception object would be an incomplete type, or a pointer to an incomplete type other than (possibly cv-qualified) void the program is ill-formed...
According to 15.2 [except.ctor] paragraph 2,
An object that is partially constructed or partially destroyed will have destructors executed for all of its fully constructed subobjects, that is, for subobjects for which the principal constructor (12.6.2 [class.base.init]) has completed execution and the destructor has not yet begun execution. Similarly, if the non-delegating constructor for an object has completed execution and a delegating constructor for that object exits with an exception, the object's destructor will be invoked. Should a constructor for an element of an automatic array throw an exception, only the constructed elements of that array will be destroyed.
The requirement for destruction of array elements explicitly applies only to automatic arrays, and one might conclude from the context that only automatic class objects are in view as well, although that is not explicitly stated. What about local static arrays and class objects? Are they intended also to be subject to the requirement that fully-constructed subobjects are to be destroyed?
Proposed resolution (October, 2006):
Change 15.2 [except.ctor] paragraph 2 as follows:
An object that is partially constructed or partially destroyed will have destructors executed for all of its fully constructed subobjects, that is, for subobjects for which the principal constructor (12.6.2 [class.base.init]) has completed execution and the destructor has not yet begun execution. Similarly, if the non-delegating constructor for an object has completed execution and a delegating constructor for that object exits with an exception, the object’s destructor will be invoked.Should a constructor for an element of an automatic array throw an exception, only the constructed elements of that array will be destroyed.If the object or array was allocated in a new-expression, the matching deallocation function (3.7.3.2 [basic.stc.dynamic.deallocation], 5.3.4 [expr.new], 12.5 [class.free]), if any, is called to free the storage occupied by the object.
The description of preprocessing expressions in 16.1 [cpp.cond] paragraph 4 says,
The resulting tokens comprise the controlling constant expression which is evaluated according to the rules of 5.19 using arithmetic that has at least the ranges specified in 18.2 [support.limits], except that all signed and unsigned integer types act as if they have the same representation as, respectively, intmax_t or uintmax_t (18.3.2).
However, this does not address the type implicitly assigned to integral literals. For example, in an implementation where int is 32 bits and long long is 64 bits, is a literal like 0xffffffff signed or unsigned? WG14 adopted DR 265 to deal with this issue in the essentially-identical wording in C99; we should probably follow suit for C++.
Proposed Resolution (November, 2006):
Change 16.1 [cpp.cond] paragraph 4 as follows:
...and then each preprocessing token is converted into a token. The resulting tokens comprise the controlling constant expression which is evaluated according to the rules of 5.19 [expr.const] using arithmetic that has at least the ranges specified in 18.2 [support.limits], except that. For the purposes of that token conversion and evaluation all signed and unsigned integer types act as if they have the same representation as, respectively, intmax_t or uintmax_t (18.3.2 [stdinth])[Footnote: Thus on an implementation where std::numeric_limits<int>::max() is 0x7FFF and std::numeric_limits<unsigned int>::max() is 0xFFFF, the integer literal 0x8000 is signed and positive within a #if expression even though it is unsigned in translation phase 7 (2.1 [lex.phases]). —end footnote]. This includes interpreting character literals...
In language imported directly from the C Standard, 16.2 [cpp.include] paragraph 5 says,
The implementation provides unique mappings for sequences consisting of one or more nondigits (2.10 [lex.name]) followed by a period (.) and a single nondigit.
This is clearly intended to support C header names like stdio.h. However, C++ has header names like cstdio that do not conform to this pattern but still presumably require “unique mappings.”
Proposed resolution (April, 2006):
Change 16.2 [cpp.include] paragraph 5 as indicated:
The implementation provides unique mappings between the delimited sequence and the external source file name for sequences consisting of one or more nondigits or digits (2.10 [lex.name]), optionally followed by a period (.) and a single nondigit...
(Clark Nelson will discuss this revision with WG14.)
Additional notes (October, 2006):
WG14 takes no position on this proposed change.
3 [basic] paragraph 8, while not incorrect, does not allow for linkage of operators and conversion functions. It says:
An identifier used in more than one translation unit can potentially refer to the same entity in these translation units depending on the linkage (3.5 [basic.link]) of the identifier specified in each translation unit.
Proposed Resolution (November, 2006):
This issue is resolved by the proposed resolution of issue 485.
Clause 3 [basic] paragraph 4 says:
A name is a use of an identifier (2.10 [lex.name]) that denotes an entity or label (6.6.4 [stmt.goto], 6.1 [stmt.label]).
Just three paragraphs later, it says
Two names are the same if
- they are identifiers composed of the same character sequence; or
- they are the names of overloaded operator functions formed with the same operator; or
- they are the names of user-defined conversion functions formed with the same type.
The last two bullets contradict the definition of name in paragraph 4 because they are not identifiers.
This definition affects other parts of the Standard, as well. For example, in 3.4.2 [basic.lookup.argdep] paragraph 1,
When an unqualified name is used as the postfix-expression in a function call (5.2.2 [expr.call]), other namespaces not considered during the usual unqualified lookup (3.4.1 [basic.lookup.unqual]) may be searched, and in those namespaces, namespace-scope friend function declarations (11.4 [class.friend]) not otherwise visible may be found.
With the current definition of name, argument-dependent lookup apparently does not apply to function-notation calls to overloaded operators.
Another related question is whether a template-id is a name or not and thus would trigger an argument-dependent lookup. Personally, I have always viewed a template-id as a name, just like operator+.
Proposed Resolution (November, 2006):
Change clause 3 [basic] paragraphs 3-8 as follows:
An entity is a value, object,
subobject, base class subobject, array element,variable, reference, function,instance of a function,enumerator, type, class member, template, template specialization, namespace, or parameter pack.A name is a use of an
identifieridentifier (2.10 [lex.name]), operator-function-id (13.5 [over.oper]), conversion-function-id (12.3.2 [class.conv.fct]), or template-id (14.2 [temp.names]) that denotes an entity or label (6.6.4 [stmt.goto], 6.1 [stmt.label]).A variable is introduced by the declaration of an object. The variable’s name denotes the object.Every name that denotes an entity is introduced by a declaration. Every name that denotes a label is introduced either by a goto statement (6.6.4 [stmt.goto]) or a labeled-statement (6.1 [stmt.label]).
A variable is introduced by the declaration of an object. The variable's name denotes the object.
Some names denote types
, classes, enumerations,or templates. In general, it is necessary to determine whether or not a name denotes one of these entities before parsing the program that contains it. The process that determines this is called name lookup (3.4 [basic.lookup]).Two names are the same if
they are
identifiersidentifiers composed of the same character sequence; orthey are
the names of overloaded operator functionsoperator-function-ids formed with the same operator; orthey are
the names of user-defined conversion functionsconversion-function-ids formed with the same type., orthey are template-ids that refer to the same class or function (14.4 [temp.type]).
An identifierA name used in more than one translation unit can potentially refer to the same entity in these translation units depending on the linkage (3.5 [basic.link]) of theidentifiername specified in each translation unit.
Change 3.3.6 [basic.scope.class] paragraph 1 item 5 as follows:
The potential scope of a declaration that extends to or past the end of a class definition also extends to the regions defined by its member definitions, even if the members are defined lexically outside the class (this includes static data member definitions, nested class definitions, member function definitions (including the member function body and any portion of the declarator part of such definitions which follows theidentifierdeclarator-id, including a parameter-declaration-clause and any default arguments (8.3.6 [dcl.fct.default]).
[Drafting note: This last change is not really mandated by the issue, but it's another case of “identifier” confusion.]
(This proposed resolution also resolves issue 309.)
3.4.5 [basic.lookup.classref] paragraph 1 says,
In a class member access expression (5.2.5 [expr.ref] ), if the . or -> token is immediately followed by an identifier followed by a <, the identifier must be looked up to determine whether the < is the beginning of a template argument list (14.2 [temp.names] ) or a less-than operator. The identifier is first looked up in the class of the object expression. If the identifier is not found, it is then looked up in the context of the entire postfix-expression and shall name a class or function template.
There do not seem to be any circumstances in which use of a non-member template function would be well-formed as the id-expression of a class member access expression.
Proposed Resolution (November, 2006):
Change 3.4.5 [basic.lookup.classref] paragraph 1 as follows:
In a class member access expression (5.2.5 [expr.ref]), if the . or -> token is immediately followed by an identifier followed by a <, the identifier must be looked up to determine whether the < is the beginning of a template argument list (14.2 [temp.names]) or a less-than operator. The identifier is first looked up in the class of the object expression. If the identifier is not found, it is then looked up in the context of the entire postfix-expression and shall name a classor functiontemplate...
Is this case valid? G++ compiles it.
namespace X {
namespace Y {
struct X {
void f()
{
using namespace X::Y;
namespace Z = X::Y;
}
};
}
}
The relevant citation from the standard is 3.4.6 [basic.lookup.udir]: "When looking up a namespace-name in a using-directive or namespace-alias-definition, only namespace names are considered." This statement could reasonably be interpreted to apply only to the last element of a qualified name, and that's the way EDG and Microsoft seem to interpret it.
However, since a class can't contain a namespace, it seems to me that this interpretation is, shall we say, sub optimal. If the X qualifiers in the above example are interpreted as referring to the struct X, an error of some sort is inevitable, since there can be no namespace for the qualified name to refer to. G++ apparently interprets 3.4.6 [basic.lookup.udir] as applying to nested-name-specifiers in those contexts as well, which makes a valid interpretation of the test possible.
I'm thinking it might be worth tweaking the words in 3.4.6 [basic.lookup.udir] to basically mandate the more useful interpretation. Of course a person could argue that the difference would matter only to a perverse program. On the other hand, namespaces were invented specifically to enable the building of programs that would otherwise be considered perverse. Where name clashes are concerned, one man's perverse is another man's real world.
Proposed Resolution (November, 2006):
Change 3.4.6 [basic.lookup.udir] paragraph 1 as follows:
When looking up a namespace-name in a using-directive or namespace-alias-definition,In a using-directive or namespace-alias-definition, during the lookup for a namespace-name or for a name in a nested-name-specifier, only namespace names are considered.
The C++ standard has inherited the definition of the 'exit' function more or less unchanged from ISO C.
However, when the 'exit' function is called, objects of static extent which have been initialised, will be destructed if their types posses a destructor.
In addition, the C++ standard has inherited the definition of the 'signal' function and its handlers from ISO C, also pretty much unchanged.
The C standard says that the only standard library functions that may be called while a signal handler is executing, are the functions 'abort', 'signal' and 'exit'.
This introduces a bit of a nasty turn, as it is not at all unusual for the destruction of static objects to have fairly complex destruction semantics, often associated with resource release. These quite commonly involve apparently simple actions such as calling 'fclose' for a FILE handle.
Having observed some very strange behaviour in a program recently which in handling a SIGTERM signal, called the 'exit' function as indicated by the C standard.
But unknown to the programmer, a library static object performed some complicated resource deallocation activities, and the program crashed.
The C++ standard says nothing about the interaction between signals, exit and static objects. My observations, was that in effect, because the destructor called a standard library function other than 'abort', 'exit' or 'signal', while transitively in the execution context of the signal handler, it was in fact non-compliant, and the behaviour was undefined anyway.
This is I believe a plausible judgement, but given the prevalence of this common programming technique, it seems to me that we need to say something a lot more positive about this interaction.
Curiously enough, the C standard fails to say anything about the analogous interaction with functions registered with 'atexit' ;-)
Proposed Resolution (10/98):
The current Committee Draft of the next version of the ISO C standard specifies that the only standard library function that may be called while a signal handler is executing is 'abort'. This would solve the above problem.
[This issue should remain open until it has been decided that the next version of the C++ standard will use the next version of the C standard as the basis for the behavior of 'signal'.]
Notes (November, 2006):
C89 is slightly contradictory here: It allows any signal handler to terminate by calling abort, exit, longjmp, but (for asynchronous signals, i.e. not those produced by abort or raise) then makes calling any library function other than signal with the current signal undefined behavior (C89 7.7.1.1). For synchronous signals, C99 forbids calls to raise, but imposes no other restrictions. For asynchronous signals, C99 allows only calls to abort, _Exit, and signal with the current signal (C99 7.14.1.1). The current C++ WP refers to “plain old functions” and “conforming C programs” (18.8 [support.runtime] paragraph 6).
Proposed Resolution (November, 2006):
Change the footnote in 18.8 [support.runtime] paragraph 6 as follows:
In particular, a signal handler using exception handling is very likely to have problems. Also, invoking std::exit causes destruction of objects with static storage duration (3.6.3 [basic.start.term]), including those of the standard library implementation, which, in general, yields undefined behavior in a signal handler (see 1.9 [intro.execution]).
4 [conv] paragraph 1 says,
Standard conversions are implicit conversions defined for built-in types.
However, enumeration types (which take part in the integral promotions) and class types (which take part in the lvalue-to-rvalue conversion) are not “built-in” types, so the definition of “standard conversions” is wrong.
Proposed resolution (October, 2006):
Change 4 [conv] paragraph 1 as follows:
Standard conversions are implicit conversionsdefined for built-in typeswith built-in meaning...
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.
Martin O'Riordan: Having gone through all the relevant references in the IS, it is not conclusive that a call via a pointer to a virtual member function is polymorphic at all, and could legitimately be interpreted as being static.
Consider 5.2.2 [expr.call] paragraph 1:
The function called in a member function call is normally selected according to the static type of the object expression (clause 10 [class.derived] ), but if that function is virtual and is not specified using a qualified-id then the function actually called will be the final overrider (10.3 [class.virtual] ) of the selected function in the dynamic type of the object expression.Here it is quite specific that you get the polymorphic call only if you use the unqualified syntax. But, the address of a member function is "always" taken using the qualified syntax, which by inference would indicate that call with a PMF is static and not polymorphic! Not what was intended.
Yet other references such as 5.5 [expr.mptr.oper] paragraph 4:
If the dynamic type of the object does not contain the member to which the pointer refers, the behavior is undefined.indicate that the opposite may have been intended, by stating that it is the dynamic type and not the static type that matters. Also, 5.5 [expr.mptr.oper] paragraph 6:
If the result of .* or ->* is a function,
then that result can be used
only as the operand for the function
call operator (). [Example:
(ptr_to_obj->*ptr_to_mfct)(10);
calls the member function denoted by ptr_to_mfct
for the object pointed
to by ptr_to_obj. ]
which also implies that it is the object pointed to that determines both the
validity of the expression (the static type of 'ptr_to_obj'
may not have a
compatible function) and the implicit (polymorphic) meaning. Note too,
that this is stated in the non-normative example text.
Andy Sawyer: Assuming the resolution is what I've assumed it is for the last umpteen years (i.e. it does the polymorphic thing), then the follow on to that is "Should there also be a way of selecting the non-polymorphic behaviour"?
Mike Miller: It might be argued that the current wording of 5.2.2 [expr.call] paragraph 1 does give polymorphic behavior to simple calls via pointers to members. (There is no qualified-id in obj.*pmf, and the IS says that if the function is not specified using a qualified-id, the final overrider will be called.) However, it clearly says the wrong thing when the pointer-to-member itself is specified using a qualified-id (obj.*X::pmf).
Bill Gibbons: The phrase qualified-id in 5.2.2 [expr.call] paragraph 1 refers to the id-expression and not to the "pointer-to-member expression" earlier in the paragraph:
For a member function call, the postfix expression shall be an implicit (9.3.1 [class.mfct.non-static] , 9.4 [class.static] ) or explicit class member access (5.2.5 [expr.ref] ) whose id-expression is a function member name, or a pointer-to-member expression (5.5 [expr.mptr.oper] ) selecting a function member.
Mike Miller: To be clear, here's an example:
struct S {
virtual void f();
};
void (S::*pmf)();
void g(S* sp) {
sp->f(); // 1: polymorphic
sp->S::f(); // 2: non-polymorphic
(sp->S::f)(); // 3: non-polymorphic
(sp->*pmf)(); // 4: polymorphic
(sp->*&S::f)(); // 5: polymorphic
}
Notes from October 2002 meeting:
This was moved back to open for lack of a champion. Martin O'Riordan is not expected to be attending meetings.
Proposed resolution (February, 2008):
Change 5.2.2 [expr.call] paragraph 1 as follows:
... For a member function call, the postfix expression shall be an implicit (9.3.1 [class.mfct.non-static], 9.4 [class.static]) or explicit class member access (5.2.5 [expr.ref]) whose id-expression is a function member name, or a pointer-to-member expression (5.5 [expr.mptr.oper]) selecting a function member. The first expression in the postfix expression is then called the object expression, and; the call is as a member of the object pointed to or referred to by the object expression (5.2.5 [expr.ref], 5.5 [expr.mptr.oper]). In the case of an implicit class member access, the implied object is the one pointed to by this. [Note: a member function call of the form f() is interpreted as (*this).f() (see 9.3.1 [class.mfct.non-static]). —end note] If a function or member function name is used, the name can be overloaded (clause 13 [over]), in which case the appropriate function shall be selected according to the rules in 13.3 [over.match].The function called in a member function call is normally selected according to the static type of the object expression (clause 10 [class.derived]), but if that function is virtual and is not specified using a qualified-id then the function actually called will be the final overrider (10.3 [class.virtual]) of the selected function in the dynamic type of the object expressionIf the selected function is non-virtual, or if the id-expression in the class member access expression is a qualified-id, that function is called. Otherwise, its final overrider (10.3 [class.virtual]) in the dynamic type of the object expression is called. ...
Change 5.5 [expr.mptr.oper] paragraph 4 as follows:
The first operand is called the object expression. If the dynamic type of the object expression does not contain the member to which the pointer refers, the behavior is undefined.
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 * operatorperforms indirectiondereferences a pointer value: the expression to which it is applied shall be a pointer...
Change 8.3.4 [dcl.array] paragraph 8 as follows:
Theresults are added and indirection appliedvalues 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 thenusing indirection throughdereferencing the (pointer) result to yield an integer. In the declarator (*pif)(const char*, const char*), the extra parentheses are necessary to indicate thatindirection throughdereferencing 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: 26.5.9 [template.indirect.array] requires no change. Many more places in the current wording use “dereferencing” than “indirection.”]
For delete expressions, 5.3.5 [expr.delete] paragraph 1 says
The operand shall have a pointer type, or a class type having a single conversion function to a pointer type.
However, paragraph 3 of that same section says:
if the static type of the operand is different from its dynamic type, the static type shall be a base class of the operand's dynamic type and the static type shall have a virtual destructor or the behavior is undefined.
Since the operand must be of pointer type, its static type is necessarily the same as its dynamic type. That clause is clearly referring to the object being pointed at, and not to the pointer operand itself.
Correcting the wording gets a little complicated, because dynamic and static types are attributes of expressions, not objects, and there's no sub-expression of a delete-expression which has the relevant types.
Suggested resolution:
then there is a static type and a dynamic type that the hypothetical expression (* const-expression) would have. If that static type is different from that dynamic type, then that static type shall be a base class of that dynamic type, and that static type shall have a virtual destructor, or the behavior is undefined.
There's precedent for such use of hypothetical constructs: see 5.10 [expr.eq] paragraph 2, and 8.1 [dcl.name] paragraph 1.
10.3 [class.virtual] paragraph 3 has a similar problem. It refers to
the type of the pointer or reference denoting the object (the static type).
The type of the pointer is different from the type of the reference, both of which are different from the static type of '*pointer', which is what I think was actually intended. Paragraph 6 contains the exact same wording, in need of the same correction. In this case, perhaps replacing "pointer or reference" with "expression" would be the best fix. In order for this fix to be sufficient, pointer->member must be considered equivalent to (*pointer).member, in which case the "expression" referred to would be (*pointer).
12.5 [class.free] paragraph 4 says thatif a delete-expression is used to deallocate a class object whose static type has...
This should be changed to
if a delete-expression is used to deallocate a class object through a pointer expression whose dereferenced static type would have...
The same problem occurs later, when it says that the
static and dynamic types of the object shall be identical
In this case you could replace "object" with "dereferenced pointer expression".
Footnote 104 says that
5.3.5 [expr.delete] requires that ... the static type of the delete-expression's operand be the same as its dynamic type.
This would need to be changed to
the delete-expression's dereferenced operand
Proposed resolution (December, 2006):
Change 5.3.5 [expr.delete] paragraph 3 as follows:
In the first alternative (delete object), if the static type of theoperandobject to be deleted is different from its dynamic type, the static type shall be a base class of theoperand’sdynamic type of the object to be deleted and the static type shall have a virtual destructor or the behavior is undefined. In the second alternative (delete array) if the dynamic type of the object to be deleted differs from its static type, the behavior is undefined.
Change the footnote in 12.5 [class.free] paragraph 4 as follows:
A similar provision is not needed for the array version of operator delete because 5.3.5 [expr.delete] requires that in this situation, the static type of thedelete-expression’s operandobject to be deleted be the same as its dynamic type.
Change the footnote in 12.5 [class.free] paragraph 5 as follows:
If the static typein the delete-expressionof the object to be deleted is different from the dynamic type and the destructor is not virtual the size might be incorrect, but that case is already undefined; see 5.3.5 [expr.delete].
[Drafting notes: No change is required for 10.3 [class.virtual] paragraph 7 because “the type of the pointer” includes the pointed-to type. No change is required for 12.5 [class.free] paragraph 4 because “...used to deallocate a class object whose static type...” already refers to the object (and not the operand expression).]
According to 6.6 [stmt.jump] paragraph 2,
On exit from a scope (however accomplished), destructors (12.4 [class.dtor]) are called for all constructed objects with automatic storage duration (3.7.2 [basic.stc.auto]) (named objects or temporaries) that are declared in that scope, in the reverse order of their declaration.
This wording is problematic for temporaries and for parameters. First, temporaries are not "declared," so this requirement does not apply to them, in spite of the assertion in the quoted text that it does.
Second, although the parameters of a function are declared in the called function, they are constructed and destroyed in the calling context, and the order of evaluation of the arguments is unspecified (cf 5.2.2 [expr.call] paragraphs 4 and 8). The order of destruction of the parameters might, therefore, be different from the reverse order of their declaration.
Notes from 04/01 meeting:
Any resolution of this issue should be careful not to introduce requirements that are redundant or in conflict with those of other parts of the IS. This is especially true in light of the pending issues with respect to the destruction of temporaries (see issues 86, 124, 199, and 201). If possible, the wording of a resolution should simply reference the relevant sections.
It was also noted that the temporary for a return value is also destroyed "out of order."
Note that issue 378 picks a nit with the wording of this same paragraph.
Proposed Resolution (November, 2006):
Change 6.6 [stmt.jump] paragraph 2 as follows:
On exit from a scope (however accomplished),destructors (12.4 [class.dtor]) are called for all constructed objects with automatic storage duration (3.7.2 [basic.stc.auto]) (named objects or temporaries) that are declared in that scope, in the reverse order of their declaration.variables with automatic storage duration (3.7.2 [basic.stc.auto]) that have been constructed in that scope are destroyed in the reverse order of their construction. [Note: For temporaries, see 12.2 [class.temporary]. —end note] Transfer out of a loop...
Paragraph 6.6 [stmt.jump] paragraph 2 of the standard says:
On exit from a scope (however accomplished), destructors (12.4 [class.dtor]) are called for all constructed objects with automatic storage duration (3.7.2 [basic.stc.auto]) (named objects or temporaries) that are declared in that scope.
It refers to objects "that are declared" but the text in parenthesis also mentions temporaries, which cannot be declared. I think that text should be removed.
This is related to issue 276.
Proposed Resolution (November, 2006):
This issue is resolved by the resolution of issue 276.
The grammar in 7 [dcl.dcl] paragraph 1 says that a declaration-seq is either declaration or declaration-seq declaration. Some declarations end with semicolons and others (e.g. function definitions and namespace declarations) don't. This means that users who put a semicolon after every declaration are technically writing ill-formed code. The trouble is that in this respect the standard is out of sync with reality. It's convenient to allow semicolons after every declaration, and there's no implementation difficulty in doing so. All existing compilers accept this, except in extra-pedantic mode. When all implementations disagree with the standard, it's time for the standard to change.
Suggested resolution:
In the grammar in 7 [dcl.dcl] paragraph 11, change the second line in the definition of declaration-seq to
Proposed resolution (October, 2006):
Add the indicated lines to the grammar definitions in 7 [dcl.dcl] paragraph 1:
declaration:
...
namespace-definition
empty-declaration
...
static_assert-declaration:
static_assert ( constant-expression , string-literal ) ;
empty-declaration:
;
Add the following as a new paragraph after 7 [dcl.dcl] paragraph 4:
An empty-declaration has no effect.
The second bullet of 7.1.6.2 [dcl.type.simple] paragraph 4 reads,
- otherwise, if e is a function call (5.2.2 [expr.call]) or an invocation of an overloaded operator (parentheses around e are ignored), decltype(e) is the return type of that function;
The reference to “that function” is imprecise; it is not the actual function called at runtime but the statically chosen function (ignoring covariant return types in virtual functions).
Also, the examples in this paragraph have errors:
The declaration of struct A should end with a semicolon.
The lines of the form decltype(...); are ill-formed; they need a declarator.
Proposed Resolution (October, 2007):
Change 7.1.6.2 [dcl.type.simple] paragraph 4 as follows:
The type denoted by decltype(e) is defined as follows:
if e is an id-expression or a class member access (5.2.5 [expr.ref]), decltype(e) is the type of the entity named by e. If there is no such entity, or if e names a set of overloaded functions, the program is ill-formed;
otherwise, if e is a function call (5.2.2 [expr.call]) or an invocation of an overloaded operator (parentheses around e are ignored), decltype(e) is the return type of
thatthe statically chosen function;otherwise, if e is an lvalue, decltype(e) is T&, where T is the type of e;
otherwise, decltype(e) is the type of e.
The operand of the decltype specifier is an unevaluated operand (clause 5 [expr]).
[Example:
const int&& foo(); int i; struct A { double x; }; const A* a = new A(); decltype(foo()) x1; // type is const int&& decltype(i) x2; // type is int decltype(a->x) x3; // type is double decltype((a->x)) x4; // type is const double&—end example]
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 (11.4 [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 (11.4 [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...
[