Document number:  J16/06-0101 = WG21 N2031
Date:  2006-06-23
Project:  Programming Language C++
Reference:  ISO/IEC IS 14882:2003
Reply to:  William M. Miller
 Edison Design Group, Inc.

C++ Standard Core Language Closed Issues, Revision 42

This document contains the C++ core language issues for which the Committee (J16 + WG21) has decided that no action is required, that is, issues with status "NAD" ("Not A Defect"), "dup" (duplicate), and "extension."

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

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

Issues with "NAD" Status

449. Consistency in use of hyphen with names of "non" entities

Section: 1.3  intro.defs     Status: NAD     Submitter: Daveed Vandevoorde     Date: 14 Jan 2004

The standard is inconsistent in its use of a hyphen on the following: nontype vs. non-type, non-dependent vs. nondependent, non-deduced vs. nondeduced, and non-template vs. nontemplate. We should pick a preferred form.

Notes from the March 2004 meeting:

If this isn't a purely editorial issue, nothing is. We're referring this to the editor. We prefer the hyphenated forms.

50. Converting pointer to incomplete type to same type

Section: 3.2  basic.def.odr     Status: NAD     Submitter: Steve Adamczyk     Date: 13 Oct 1998

In 3.2  basic.def.odr paragraph 4 bullet 4, it's presumably the case that a conversion to T* requires that T be complete only if the conversion is from a different type. One could argue that there is no conversion (and therefore the text is accurate as it stands) if a cast does not change the type of the expression, but it's probably better to be more explicit here.

On the other hand, this text is non-normative (it's in a note).

Rationale (04/99): The relevant normative text makes this clear. Implicit conversion and static_cast are defined (in 4  conv and 5.2.9  expr.static.cast , respectively) as equivalent to declaration with initialization, which permits pointers to incomplete types, and dynamic_cast (5.2.7  expr.dynamic.cast ) explicitly prohibits pointers to incomplete types.

42. Redefining names from base classes

Section: 3.3.6  basic.scope.class     Status: NAD     Submitter: Steve Clamage     Date: 15 Sep 1998

Consider this code:

    struct Base {
        enum { a, b, c, next };

    struct Derived : public Base {
        enum { d = Base::next, e, f, next };

The idea is that the enumerator "next" in each class is the next available value for enumerators in further derived classes.

If we had written

    enum { d = next, e, f, next };

I think we would run afoul of 3.3.6  basic.scope.class :
A name N used in a class S shall refer to the same declaration in its context and when re-evaluated in the completed scope of S. No diagnostic is required for a violation of this rule.
But in the original code, we don't have an unqualified "next" that refers to anything but the current scope. I think the intent was to allow the code, but I don't find the wording clear on on that point.

Is there another section that makes it clear whether the original code is valid? Or am I being obtuse? Or should the quoted section say "An unqualified name N used in a class ..."?

Rationale (04/99): It is sufficiently clear that "name" includes qualified names and hence the usual lookup rules make this legal.

231. Visibility of names after using-directives

Section: 3.4.1  basic.lookup.unqual     Status: NAD     Submitter: Jörg Barfurth     Date: 31 May 2000

The wording of 3.4.1  basic.lookup.unqual paragraph 2 is misleading. It says:

The declarations from the namespace nominated by a using-directive become visible in a namespace enclosing the using-directive; see 7.3.4  namespace.udir.

According to 7.3.4  namespace.udir paragraph 1, that namespace is

the nearest enclosing namespace which contains both the using-directive and the nominated namespace.

That would seem to imply the following:

    namespace outer {
        namespace inner {
            int i;
        void f() {
            using namespace inner;
        int j = i;   // inner::i is "visible" in namespace outer

Suggested resolution: Change the first sentence of 3.4.1  basic.lookup.unqual paragraph 2 to read:

The declarations from the namespace nominated by a using-directive become visible in the scope in which the using-directive appears after the using-directive.

Notes from the 4/02 meeting:

After a lot of discussion of possible wording changes, we decided the wording should be left alone. 3.4.1  basic.lookup.unqual paragraph 2 is not intended to be a full specification; that's in 7.3.4  namespace.udir paragraph 1. See also 3.3.5  basic.scope.namespace paragraph 1.

91. A union's associated types should include the union itself

Section: 3.4.2  basic.lookup.argdep     Status: NAD     Submitter: John Spicer     Date: 2 Feb 1999

When a union is used in argument-dependent lookup, the union's type is not an associated class type. Consequently, code like this will fail to work.

    union U {
        friend void f(U);
    int main() {
        U u;
        f(u);  // error: no matching f — U is not an associated class
Is this an error in the description of unions in argument-dependent lookup?

Also, this section is written as if unions were distinct from classes. So adding unions to the "associated classes" requires either rewriting the section so that "associated classes" can include unions, or changing the term to be more inclusive, e.g. "associated classes and unions" or "associated types".

Jason Merrill: Perhaps in both cases, the standard text was intended to only apply to anonymous unions.

Liam Fitzpatrick: One cannot create expressions of an anonymous union type.

Rationale (04/99): Unions are class types, so the example is well-formed. Although the wording here could be improved, it does not rise to the level of a defect in the Standard.

384. Argument-dependent lookup and operator functions

Section: 3.4.2  basic.lookup.argdep     Status: NAD     Submitter: Herb Sutter     Date: 18 Sept 2002

I believe the following code example should unambiguously call the member operator+. Am I right?

  //--- some library header ---
  namespace N1 {
    template<class T> struct Base { };

    template<class T> struct X {
      struct Y : public Base<T> {     // here's a member operator+
        Y operator+( int _Off ) const { return Y(); }

      Y f( unsigned i ) { return Y() + i; } // the "+" in question

  //--- some user code ---
  namespace N2 {
    struct Z { };

    template<typename T>              // here's another operator+
    int* operator+( T , unsigned ) { static int i ; return &i ; }

  int main() {
    N1::X< N2::Z > v;
    v.f( 0 );

My expectation is that 3.4.2  basic.lookup.argdep would govern, specifically:

If the ordinary unqualified lookup of the name finds the declaration of a class member function, the associated namespaces and classes are not considered.
So I think the member should hide the otherwise-better-matching one in the associated namespace. Here's what compilers do:

Agree with me and call the member operator+: Borland 5.5, Comeau, EDG 3.0.1, Metrowerks 8.0, MSVC 6.0

Disagree with me and try to call N2::operator+: gcc 2.95.3, 3.1.1, and 3.2; MSVC 7.0

Simple so far, but someone tells me that  over.match.oper muddies the waters. There, paragraph 10 summarizes that subclause:

[Note: the lookup rules for operators in expressions are different than the lookup rules for operator function names in a function call, ...
In particular, consider the above call to "Y() + unsigned" and please help me step through  over.match.oper paragraph 3:
... for a binary operator @ with a left operand of a type whose cv-unqualified version is T1 and a right operand of a type whose cv-unqualified version is T2,
OK so far, here @ is +, and T1 is N1::X::Y.
three sets of candidate functions, designated member candidates, non-member candidates and built-in candidates, are constructed as follows:
[and later are union'd together to get the candidate list]
If T1 is a class type, the set of member candidates is the result of the qualified lookup of T1::operator@ (; otherwise, the set of member candidates is empty.
So there is one member candidate, N1::X::Y::operator+.
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 (basic.lookup.argdep) except that all member functions are ignored.

*** This is the question: What does that last phrase mean? Does it mean:

a) first apply the usual ADL rules to generate a candidate list, then ignore any member functions in that list (this is what I believe and hope it means, and in particular it means that the presence of a member will suppress names that ADL would otherwise find in the associated namespaces); or

b) something else?

In short, does N2::operator+ make it into the candidate list? I think it shouldn't. Am I right?

John Spicer: I believe that the answer is sort-of "a" above. More specifically, the unqualified lookup consists of a "normal" unqualified lookup and ADL. ADL always deals with only namespace members, so the "ignore members functions" part must affect the normal lookup, which should ignore class members when searching for an operator.

I suspect that the difference between compilers may have to do with details of argument-dependent lookup. In the example given, the argument types are "N1::X<N2::Z>::Y" and "unsigned int". In order for N2::operator+ to be a candidate, N2 must be an associated namespace.

N1::X<N2::Z>::Y is a class type, so 3.4.2  basic.lookup.argdep says that its associated classes are its direct and indirect base classes, and its namespaces are the namespaces of those classes. So, its associated namespace is just N1.

3.4.2  basic.lookup.argdep also says:

If T is a template-id, its associated namespaces and classes are the namespace in which the template is defined; for member templates, the member template's class; the namespaces and classes associated with the types of the template arguments provided for template type parameters (excluding template template parameters); the namespaces in which any template template arguments are defined; and the classes in which any member templates used as template template arguments are defined. [Note: non-type template arguments do not contribute to the set of associated namespaces. ]
First of all, there is a problem with the term "is a template-id". template-id is a syntactic constuct and you can't really talk about a type being a template-id. Presumably, this is intended to mean "If T is the type of a class template specialization ...". But does this apply to N1::X<N2::Z>::Y? Y is a class nested within a class template specialization. In addition, its base class is a class template specialization.

I think this raises two issues:

  1. Should the enclosing class(es) of a class, and their template argument lists (if any) contribute to the associated classes/namespaces for ADL?
  2. Should the template argument lists of base classes contribute to the associated classes/namespaces for ADL?

Notes from the April 2003 meeting:

The ADL rules in the standard sort of look at if they are fully recursive, but in fact they are not; in some cases, enclosing classes and base classes are considered, and in others they are not. Microsoft and g++ did fully-recursive implementations, and EDG and IBM did it the other way. Jon Caves reports that Microsoft saw no noticeable difference (e.g., no complaints from customers internal or external) when they made this change, so we believe that even if the rules are imperfect the way they are in the standard, they are clear and the imperfections are small enough that programmers will not notice them. Given that, it seemed prudent to make no changes and just close this issue.

The template-id issue is spun off as issue 403.

132. Local types and linkage

Section: 3.5     Status: NAD     Submitter: Daveed Vandevoorde     Date: 25 June 1999

3.5 paragraph 8 says,

A name with no linkage (notably, the name of a class or enumeration declared in a local scope (3.3.2  basic.scope.local )) shall not be used to declare an entity with linkage.
This wording does not, but should, prohibit use of an unnamed local type in the declaration of an entity with linkage. For example,
    void f() {
        extern struct { } x;  // currently allowed

Proposed resolution: Change the text in 3.5 paragraph 8 from:

A name with no linkage (notably, the name of a class or enumeration declared in a local scope (3.3.2  basic.scope.local)) shall not be used to declare an entity with linkage.
A name with no linkage (notably, the name of a class or enumeration declared in a local scope (3.3.2  basic.scope.local)) or an unnamed type shall not be used to declare an entity with linkage.
In section 3.5 paragraph 8, add to the example, before the closing brace of function f:
extern struct {} x;    // ill-formed

Rationale (10/00): The proposed change would have introduced an incompatibility with the C language. For example, the global declaration

    static enum { A, B, C } abc;

represents an idiom that is used in C but would be prohibited under this resolution.

269. Order of initialization of multiply-defined static data members of class templates

Section: 3.6.2  basic.start.init     Status: NAD     Submitter: Andrei Iltchenko     Date: 8 Feb 2001

According to 3.2  basic.def.odr paragraph 5, it is possible for a static data member of a class template to be defined more than once in a given program provided that each such definition occurs in a different translation unit and the ODR is met.

Now consider the following example:


    #include <iostream>

    int  initializer()
       static int   counter;
       return  counter++;

    int   g_data1 = initializer();

    template<class T>
    struct  exp  {
       static int   m_data;
    template<class T>
    int  exp<T>::m_data = initializer();

    int   g_data2 = initializer();
    extern int   g_data3;

    int  main()
       std::cout << exp<char>::m_data << ", " << g_data1 << ", "
	  << g_data2 << ", " << g_data3 << std::endl;
       return  0;


    extern int  initializer();
    int  g_data3 = initializer();

    template<class T>
    struct  exp  {
       static int   m_data;
    template<class T>
    int  exp<T>::m_data = initializer();

    void  func()

The specialization exp<char>::m_data is implicitly instaniated in both translation units, hence (14.7.1  temp.inst paragraph 1) its initialization occurs. And for both definitions of exp<T>::m_data the ODR is met. According to 3.6.2  basic.start.init paragraph 1:

Objects with static storage duration defined in namespace scope in the same translation unit and dynamically initialized shall be initialized in the order in which their definition appears in the translation unit.

But for exp<T>::m_data we have two definitions. Does it mean that both g_data1 and g_data3 are guaranteed to be dynamically initialized before exp<char>::m_data?

Suggested Resolution: Insert the following sentence before the last two sentences of 3.2  basic.def.odr paragraph 5:

In the case of D being a static data member of a class template the following shall also hold:

Notes from 10/01 meeting:

It was decided that this issue is not linked to issue 270 and that there is no problem, because there is only one instantiation (see 2.1  lex.phases paragraph 8).

465. May constructors of global objects call exit()?

Section: 3.6.2  basic.start.init     Status: NAD     Submitter: Matt Austern     Date: 26 Feb 2004

The subject line pretty much says it all. It's a possibility that hadn't ever occurred to me. I don't see any prohibition in the standard, and I also don't think the possibility introduces any logical inconsistencies. The proper behavior, presumably, would be to go through the list of already-constructed objects (not including the current one, since its constructor wouldn't have finished executing) and destroy them in reverse order. Not fundamentally hard, and I'm sure lots of existing implementations already do that.

I'm just not sure whether the standard was intended to support this, or whether it's just that nobody else thought of it either. If the former, then a non-normative note somewhere in 3.6.2  basic.start.init might be nice.

Rationale (October 2004):

There is nothing in the Standard to indicate that this usage is prohibited, so it must be presumed to be permitted.

234. Reuse of base class subobjects

Section: 3.8     Status: NAD     Submitter: Bill Wade     Date: 28 Jun 2000

3.8 and 12.4  class.dtor discuss explicit management of object lifetime. It seems clear that most object lifetime issues apply to sub-objects (array elements, and data members) as well. The standard supports

     struct X { T t } x;
     T* pt = &x.t;
     new(pt) T;

and this kind of behavior is useful in allocators.

However the standard does not seem to prohibit the same operations on base sub-objects.

   struct D: B{ ... } d;
   B* pb = &d;
   new(pb) B;

However if B and/or D have virtual member functions or virtual bases, it is unlikely that this code will result in a well-formed D object in current implementations (note that the various lines may be in different functions).

Suggested resolution: 12.4  class.dtor should be modified so that explicit destruction of base-class sub-objects be made illegal, or legal only under some restrictive conditions.

Rationale (04/01):

Reallocation of a base class subobject is already disallowed by 3.8 paragraph 7.

290. Should memcpy be allowed into a POD with a const member?

Section: 3.9  basic.types     Status: NAD     Submitter: Garry Lancaster     Date: 12 Jun 2001

Following the definition in 9  class paragraph 4 the following is a valid POD (actually a POD-struct):

    struct test
        const int i;

The legality of PODs with const members is also implied by the text of 5.3.4 paragraph 15 bullet 1, sub-bullet 2 and 12.6.2  class.base.init paragraph 4 bullet 2.

3.9  basic.types paragraph 3 states that

For any POD type T, if two pointers to T point to distinct objects obj1 and obj2, if the value of obj1 is copied into obj2, using the memcpy library function, obj2 shall subsequently hold the same value as obj1.

[Note: this text was changed by TC1, but the essential point stays the same.]

This implies that the following is required to work:

    test obj1 = { 1 };
    test obj2 = { 2 };
    memcpy( &obj2, &obj1, sizeof(test) );

The memcpy of course changes the value of the const member, surely something that shouldn't be allowed.

Suggested resolution:

It is recommended that 3.9  basic.types paragraph 3 be reworded to exclude PODs which contain (directly or indirectly) members of const-qualified type.

Rationale (October, 2004): paragraph 4 already forbids modifying a const member of a POD struct. The prohibition need not be repeated in 3.9  basic.types.

303. Integral promotions on bit-fields

Section: 4.5  conv.prom     Status: NAD     Submitter: Kiril Avdeiv     Date: 24 Jul 2001

Paragraph 3 of section 4.5  conv.prom contains a statement saying that if a bit-field is larger than int or unsigned int, no integral promotions apply to it. This phrase needs further clarification, as it is hardly possible to fugure out what it means. See below.

Assuming a machine with a size of general-purpose register equal 32 bits (where a byte takes up 8 bits) and a C++ implementation where an int is 32 bits and a long is 64 bits. And the following snippet of code:

    struct ExternalInterface {
      long field1:36, field2:28;
    int main() {
      ExternalInterface  myinstance = { 0x100000001L, 0x12,};
      if(myinstance.field1 < 0x100000002L) { //do something }

Does the standard prohibit the implementation from promoting field1's value into two general purpose registers? And imposes a burden of using shift machine instructions to work with the field's value? What else could that phrase mean?

Either alternative is implementation specific, so I don't understand why the phrase "If the bit-field is larger yet, no integral promotions apply to it" made it to the standard.

Notes from 10/01 meeting:

The standard of course does not dictate what an implementation might do with regard to use of registers or shift instructions in the generated code. The phrase cited means only that a larger bit-field does not undergo integral promotions, and therefore it retains the type with which it was declared (long in the above example). The Core Working Group judged that this was sufficiently clear in the standard.

Note that 9.6  class.bit paragraph 1 indicates that any bits in excess of the size of the underlying type are padding bits and do not participate in the value representation. Therefore the field1 bit field in the above example is not capable of holding the indicated values, which require more than 32 bits.

456. Is initialized const int or const bool variable a null pointer constant?

Section: 4.10  conv.ptr     Status: NAD     Submitter: Lloyd Lewins     Date: 31 Jan 2004

In the following code, I expect both "null" and "FALSE" to be null pointer constants -- and that the code should compile and output the string "int*" twice to cout:

#include <iostream>

using namespace std;

void foo(int* p)
     cout << "int*" << endl;

int main(void)
     const int null = 0;
     const bool FALSE = false;

ISO/IEC 14882-1998 4.10  conv.ptr states:

An integral constant expression rvalue of integer type that evaluates to zero (called a /null pointer constant/) can be converted to a pointer type.

Stroustrup appears to agree with me -- he states (3rd edition page 88):

In C, it has been popular to define a macro NULL to represent the zero pointer. Because of C++`s tighter type checking, the use of plain 0, rather than any suggested NULL macro, leads to fewer problems. If you feel you must define NULL, use:
   const int NULL = 0;

However gcc 3.3.1 rejects this code with the errors: error: invalid conversion from `int' to `int*' error: cannot convert `const bool' to `int*' for argument `1' to `
         void foo(int*)'

I have reported this as a bug (, but the gcc team states that 4.10 requires that a null pointer constant must be an rvalue -- and no implicit conversion from an lvalue to an rvalue is required (

a null pointer constant is an integral constant expression rvalue that evaluates to zero [4.10/1] in this case `null' is an lvalue. The standard does not specify that lvalue->rvalue decay happens here, so `null' is not a null pointer constant.

I disagree with the gcc teams interpretation -- I don't see why 3.10  basic.lval doesn't apply:

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

The insertion of the word rvalue appears to have occurred during standardization -- it is not present in either Stroustrup 2nd edition or the 3rd edition. Does the committee deliberately intend to exclude an lvalue as a null pointer constant by adding the word rvalue? If so, it leads to the rather bizarre fact that "null" is not a null pointer constant, but "null + 0" is!

Notes from the March 2004 meeting:

We think this is just a bug in gcc. The const variable does get converted to an rvalue in this context. This case is not really any different than cases like

  const int null = 0;
  int i = null;
  const int i = 1;
  int a[i];
(which are accepted by gcc). No one would argue that the second lines of those examples are invalid because the variables are lvalues, and yet the conversions to rvalue happen implicitly for the same reason cited above -- the contexts require an rvalue.

71. Incorrect cross reference

Section: expr     Status: NAD     Submitter: Neal Gafter     Date: 15 Oct 1998

An operator expression can, according to 5  expr paragraph 2, require transformation into function call syntax. The reference in that paragraph is to 13.5  over.oper , but it should be to  over.match.oper .

Rationale (04/99): The subsections 13.5.1  over.unary , 13.5.2  over.binary , etc. of the referenced section are in fact relevant.

31. Looking up new/delete

Section: 5.3.4     Status: NAD     Submitter: Daveed Vandevoorde     Date: 23 Jun 1998

Section 12.5 paragraph 4 says:

If a delete-expression begins with a unary :: operator, the deallocation function's name is looked up in global scope. Otherwise, if the delete-expression is used to deallocate a class object whose static type has a virtual destructor, the deallocation function is the one found by the lookup in the definition of the dynamic type's virtual destructor (12.4  class.dtor ). Otherwise, if the delete-expression is used to deallocate an object of class T or array thereof, the static and dynamic types of the object shall be identical and the deallocation function's name is looked up in the scope of T. If this lookup fails to find the name, the name is looked up in the global scope. If the result of the lookup is ambiguous or inaccessible, or if the lookup selects a placement deallocation function, the program is ill-formed.
I contrast that with 5.3.4 paragraphs 16 and 17:
If the new-expression creates an object or an array of objects of class type, access and ambiguity control are done for the allocation function, the deallocation function (12.5 ), and the constructor (12.1  class.ctor ). If the new-expression creates an array of objects of class type, access and ambiguity control are done for the destructor (12.4  class.dtor ).

If any part of the object initialization described above terminates by throwing an exception and a suitable deallocation function can be found, the deallocation function is called to free the memory in which the object was being constructed, after which the exception continues to propagate in the context of the new-expression. If no unambiguous matching deallocation function can be found, propagating the exception does not cause the object's memory to be freed. [Note: This is appropriate when the called allocation function does not allocate memory; otherwise, it is likely to result in a memory leak. ]

I think nothing in the latter paragraphs implies that the deallocation function found is the same as that for a corresponding delete-expression. I suspect that may not have been intended and that the lookup should occur "as if for a delete-expression".


Paragraphs 16 through 18 are sufficiently correct and unambiguous as written.

130. Sequence points and new-expressions

Section: 5.3.4     Status: NAD     Submitter: Herb Sutter     Date: 20 June 1999

Clause 5  expr paragraph 4 appears to grant an implementation the right to generate code for a function call like

    f(new T1, new T2)
in the order However, 5.3.4 paragraph 17 seems to require the deallocation of the storage for an object only if part of the initialization of that object terminates with an exception. Given the ordering above, this specification would appear to allow the memory for the T2 object to be leaked if the T1 constructor throws an exception.

Suggested resolution: either forbid the ordering above or expand the requirement for reclaiming storage to include exceptions thrown in all operations between the allocation and the completion of the constructor.

Rationale (10/99): Even in the "traditional" ordering of the calls to allocation functions and constructors, memory can still leak. For instance, if T1 were successfully constructed and then the construction of T2 were terminated by an exception, the memory for T1 would be lost. Programmers concerned about memory leaks will avoid this kind of construct, so it seems unnecessary to provide special treatment for it to avoid the memory leaks associated with one particular implementation strategy.

55. Adding/subtracting pointer and enumeration value

Section: 5.7  expr.add     Status: NAD     Submitter: Steve Adamczyk     Date: 13 Oct 1998

An expression of the form pointer + enum (see paragraph 5) is not given meaning, and ought to be, given that paragraph 2 of this section makes it valid. Presumably, the enum value should be converted to an integral value, and the rest of the processing done on that basis. Perhaps we want to invoke the integral promotions here.

[Should this apply to (pointer - enum) too?]

Rationale (04/99): Paragraph 1 invokes "the usual arithmetic conversions" for operands of enumeration type.

(It was later pointed out that the builtin operator T* operator+(T*, ptrdiff_t) (13.6  over.built paragraph 13) is selected by overload resolution. Consequently, according to  over.match.oper paragraph 7, the operand of enumeration type is converted to ptrdiff_t before being interpreted according to the rules in 5.7  expr.add .)

97. Use of bool constants in integral constant expressions

Section: 5.19  expr.const     Status: NAD     Submitter: Andy Koenig     Date: 18 Feb 1999


    int* p = false;         // Well-formed?
    int* q = !1;            // What about this?
>From 3.9.1  basic.fundamental paragraph 6: "As described below, bool values behave as integral types."

From 4.10  conv.ptr paragraph 1: "A null pointer constant is an integral constant expression rvalue of integer type that evaluates to zero."

From 5.19  expr.const paragraph 1: "An integral constant-expression can involve only literals, enumerators, const variables or static members of integral or enumeration types initialized with constant expressions, ..."

In 2.13.1  lex.icon : No mention of true or false as an integer literal.

From 2.13.5  lex.bool : true and false are Boolean literals.

So the definition of q is certainly valid, but the validity of p depends on how the sentence in 5.19  expr.const is parsed. Does it mean

Or does it mean Or something else?

If the latter, then (3.0 < 4.0) is a constant expression, which I don't think we ever wanted. If the former, though, we have the anomalous notion that true and false are not constant expressions.

Now, you may argue that you shouldn't be allowed to convert false to a pointer. But what about this?

    static const bool debugging = false;
    // ...
    int table[debugging? n+1: n];
Whether the definition of table is well-formed hinges on whether false is an integral constant expression.

I think that it should be, and that failure to make it so was just an oversight.

Rationale (04/99): A careful reading of 5.19  expr.const indicates that all types of literals can appear in integral constant expressions, but floating-point literals must immediately be cast to an integral type.

487. Operator overloading in constant expressions

Section: 5.19  expr.const     Status: NAD     Submitter: Steve Adamczyk     Date: 24 Nov 2004

According to 5.19  expr.const paragraph 1,

In particular, except in sizeof expressions, functions, class objects, pointers, or references shall not be used, and assignment, increment, decrement, function-call, or comma operators shall not be used.

Given a case like

    enum E { e };
    int operator+(int, E);
    int i[4 + e];

does this mean that the overloaded operator+ is not considered (because it can't be called), or is it selected by overload resolution, thus rendering the program ill-formed?

Rationale (April, 2005):

All expressions, including constant expressions, are subject to overload resolution. The example is ill-formed.

467. Jump past initialization of local static variable

Section: 6.7  stmt.dcl     Status: NAD     Submitter: Kerch Holt     Date: 31 Mar 2004

When jumping past initialization of a local static variable the value of the static becomes indeterminate. Seems like this behavior should be illegal just as it is for local variables with automatic linkage.

Here is an example:

struct X {
    X(int i) : x(i) {}
    int x;
int f(int c) {
    if (c)
        goto ly;    // error here for jumping past next stmt.
    static X a = 1;
    return a.x;  // either 1 or 0 depending on implementation.

6.7  stmt.dcl P3 should be changed to:

A program that jumps from a point where a local variable with automatic or static storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has POD type (3.9) and is declared without an initializer (8.5).
This would imply "static X a = 1;" should be flagged as an error. Note that this behavior a may be a "quality of implementation issue" which may be covered in 6.7 P4. Paragraph 4 seems to make the choice of static/dynamic initialization indeterminate. Making this an error and thus determinate seems the correct thing to do since that is what is already required of automatic variables.

Steve Adamczyk: Some version of this may be appropriate, but it's common to have code that is executed only the first time it is reached, and to have an initialization of a static variable inside such a piece of code. In such a case, on executions after the first there is indeed a jump over the declaration, but the static variable is correctly initialized -- it was initialized the first time the routine was called.

  void f() {
    static bool first_time = true;
    if (!first_time) goto after_init;
    static int i = g();
    first_time = false;

Rationale (October, 2004):

The CWG sees no reason to change this specification. Local static variables are different from automatic variables: automatic variables, if not explicitly initialized, can have indeterminate (“garbage”) values, including trap representations, while local static variables are subject to zero initialization and thus cannot have garbage values.

The latitude granted to implementations regarding performing dynamic initialization of local static objects as if it were static initialization is exactly parallel to namespace scope objects (3.6.2  basic.start.init), as are the restrictions on programmer assumptions.

435. Change "declararation or definition" to "declaration"

Section: dcl.dcl     Status: NAD     Submitter: Jens Maurer     Date: 27 Oct 2003

Because a definition is also a declaration, it might make sense to change uses of "declaration or definition" to simply "declaration".

Notes from the March 2004 meeting:

Jens Maurer prepared drafting for this issue, but we find ourselves reluctant to actually make the changes. Though correct, they seemed more likely to be misread than the existing wording.

Proposed resolution:

Remove in 1.3.9  defns.parameter the indicated words:

an object or reference declared as part of a function declaration or definition, or in the catch clause of an exception handler, that acquires a value on entry to the function or handler; ...

Remove in 14.1  temp.param paragraph 10 the indicated words:

The set of default template-arguments available for use with a template declaration or definition is obtained by merging the default arguments from the definition (if in scope) and all declarations in scope in the same way default function arguments are (...).

Remove in 14.6  temp.res paragraph 2 the indicated words:

A name used in a template declaration or definition and that is dependent on a template-parameter is assumed not to name a type unless the applicable name lookup finds a type name or the name is qualified by the keyword typename.

Remove in  temp.point paragraph 1 the indicated words:

Otherwise, the point of instantiation for such a specialization immediately follows the namespace scope declaration or definition that refers to the specialization.

Remove in  temp.point paragraph 3 the indicated words:

Otherwise, the point of instantiation for such a specialization immediately precedes the namespace scope declaration or definition that refers to the specialization.

Remove in 14.7.3  temp.expl.spec paragraph 21 the indicated words:

Default function arguments shall not be specified in a declaration or a definition for one of the following explicit specializations: [Note: default function arguments may be specified in the declaration or definition of a member function of a class template specialization that is explicitly specialized. ]

Remove in  temp.deduct.type paragraph 18 the indicated words:

[Note: a default template-argument cannot be specified in a function template declaration or definition; ...]

Remove in  lib.using.headers paragraph 3 the indicated words:

A translation unit shall include a header only outside of any external declaration or definition, and shall include the header lexically before the first reference to any of the entities it declares or first defines in that translation unit.

Rationale (October, 2004):

CWG felt that readers might misunderstand “declaration” as meaning “non-definition declaration.”

154. Anonymous unions in unnamed namespaces

Section: 7.1.1     Status: NAD     Submitter: Greg Comeau     Date: 9 Aug 1999

9.5  class.union paragraph 3 implies that anonymous unions in unnamed namespaces need not be declared static (it only places that restriction on anonymous unions "declared in a named namespace or in the global namespace").

However, 7.1.1 paragraph 1 says that "global anonymous unions... shall be declared static." This could be read as prohibiting anonymous unions in unnamed namespaces, which are the preferred alternative to the deprecated use of static.

Rationale (10/99): An anonymous union in an unnamed namespace is not "a global anonymous union," i.e., it is not a member of the global namespace.

376. Class "definition" versus class "declaration"

Section: 7.1.2  dcl.fct.spec     Status: NAD     Submitter: Randy Maddox     Date: 28 August 2002

In clause 7.1.2  dcl.fct.spec, para. 3, the following sentence

A function defined within a class definition is an inline function.

should, if I am not mistaken, instead be:

A function defined within a class declaration is an inline function."

Notes from October 2002 meeting:

This is not a defect. Though there is a long history, going back to the ARM, of use of the term "class declaration" to mean the definition of the class, we believe "class definition" is clearer. We have opened issue 379 to deal with changing all other uses of "class declaration" to "class definition" where appropriate.

412. Can a replacement allocation function be inline?

Section: 7.1.2  dcl.fct.spec     Status: NAD     Submitter: Matt Austern     Date: 23 Apr 2003

A customer reports that when he attempts to replace ::operator new with a user-defined function, the standard library calls the default function by preference if the user-defined function is inline. I believe that our compiler is correct, and that such a replacement function isn't allowed to be inline, but I'm not sure there's sufficiently explicit language in the standard.

In general, of course, the definition of an inline function must be present in every translation unit where the function is called. (7.1.2  dcl.fct.spec, par 4) It could be argued that this requirement doesn't quite address replacement functions: what we're dealing with is the odd case where we've already got one definition and the user is supplying a different one. I'd like to see something specifically addressing the case of a replacement function.

So what do we have? I see discussion of requirement for a replacement ::operator new in three places:  lib.replacement.functions, par 2, and 3.7.3 par 2-3. I don't see anything explicitly saying that the replacement function may not be inline. The closest I can find is par 2, which says that "a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library". One might argue that "with this function signature" rules out inline, but that strikes me as a slight stretch.

Have I missed anything?

Andrew Koenig: I think you've turned up a problem in 7.1.2  dcl.fct.spec paragraph 4. Consider:

  // Translation unit 1
  #include <iostream>

  extern void foo(void (*)());

  inline void bar() {
    std::cout << "Hello, world!" << std::endl;

  int main() {

  // Translation unit 2
  void foo(void (*f)()) { (*f)(); }

Are you really trying to tell me that this program is ill-formed because the definition of bar is not available in translation unit 2?

I think not. The actual words in 7.1.2  dcl.fct.spec par 4 are

An inline function shall be defined in every translation unit in which it is used...
and I think at in this context, ``used'' should be interpreted to mean that foo is used only in translation unit 1, where it is converted to a value of type void(*)().

Notes from October 2003 meeting:

We don't think Andy Koenig's comment requires any action; "used" is already defined appropriately.

We agree that this replacement should not be allowed, but we think it's a library issue (in the rules for allowed replacements). Forwarded to library group; it's issue 404 on the library issues list.

422. Is a typedef redeclaration allowed with a template type that might be the same?

Section: 7.1.3  dcl.typedef     Status: NAD     Submitter: Steve Adamczyk     Date: 18 June 2003

Is the following valid?

  template <class T> void f(T) {
    typedef int x;
    typedef T x;
  int main() {

There is an instantiation where the function is valid. Is an implementation allowed to issue an error on the template declaration because the types on the typedef are not the same (7.1.3  dcl.typedef)?

How about

  typedef T x;
  typedef T2 x;

It can be argued that these cases should be allowed because they aren't necessarily wrong, but it can also be argued that there's no reason to write things like the first case above, and if such a case appears it's more likely to be a mistake than some kind of intentional test that int and T are the same type.

Notes from the October 2003 meeting:

We believe that all these cases should be allowed, and that errors should be required only when an instance of the template is generated. The current standard wording does not seem to disallow such cases, so no change is required.

311. Using qualified name to reopen nested namespace

Section: 7.3.1  namespace.def     Status: NAD     Submitter: Bjarne Stroustrup     Date: 18 Sep 2001

I received an inquiry/complaint that you cannot re-open a namespace using a qualified name. For example, the following program is ok, but if you uncomment the commented lines you get an error:

namespace A {
    namespace N {
	int a;
    int b;
    namespace M {
	int c;

//namespace A::N {
//    int d;

namespace A {
    namespace M {
        int e;

int main()
    A::N::a = 1;
    A::b = 2;
    A::M::c = 3;
//  A::N::d = 4;
    A::M::e = 5;

Andrew Koenig: There's a name lookup issue lurking here. For example:

    int x;

    namespace A {
	int x;
	namespace N {
	   int y;

    namespace A::N {
        int* y = &x;  // which x?

Jonathan Caves: I would assume that any rule would state that:

namespace A::B {
would be equivalent to:
namespace A {
   namespace B {
so in your example 'x' would resolve to A::x

BTW: we have received lots of bug reports about this "oversight".

Lawrence Crowl: Even worse is

    int x;
    namespace A {
      int x;
    namespace B {
      int x;
      namespace ::A {
         int* y = &x;
I really don't think that the benefits of qualified names here is worth the cost.

Notes from April 2003 meeting:

We're closing this because it's on the Evolution working group list.

95. Elaborated type specifiers referencing names declared in friend decls

Section:  namespace.memdef     Status: NAD     Submitter: John Spicer     Date: 9 Feb 1999

A change was introduced into the language that made names first declared in friend declarations "invisible" to normal lookups until such time that the identifier was declared using a non-friend declaration. This is described in  namespace.memdef paragraph 3 and 11.4  class.friend paragraph 9 (and perhaps other places).

The standard gives examples of how this all works with friend declarations, but there are some cases with nonfriend elaborated type specifiers for which there are no examples, and which might yield surprising results.

The problem is that an elaborated type specifier is sometimes a declaration and sometimes a reference. The meaning of the following code changes depending on whether or not friend class names are injected (visibly) into the enclosing namespace scope.

    struct A;
    struct B;
    namespace N {
        class X {
            friend struct A;
            friend struct B;
        struct A *p;     // N::A with friend injection, ::A without
        struct B;        // always N::B
Is this the desired behavior, or should all elaborated type specifiers (and not just those of the form "class-key identifier;") have the effect of finding previously declared "invisible" names and making them visible?

Mike Miller: That's not how I would categorize the effect of "struct B;". That declaration introduces the name "B" into namespace N in exactly the same fashion as if the friend declaration did not exist. The preceding friend declaration simply stated that, if a class N::B were ever defined, it would have friendly access to the members of N::X. In other words, the lookups in both "struct A*..." and "struct B;" ignore the friend declarations.

(The standard is schizophrenic on the issue of whether such friend declarations introduce names into the enclosing namespace. 3.3  basic.scope paragraph 4 says,

while 3.3.1  basic.scope.pdecl paragraph 6 says exactly the opposite: Both of these are just notes; the normative text doesn't commit itself either way, just stating that the name is not found until actually declared in the enclosing namespace scope. I prefer the latter description; I think it makes the behavior you're describing a lot clearer and easier to understand.)

John Spicer: The previous declaration of B is not completely ignored though, because certainly changing "friend struct B;" to "friend union B;" would result in an error when B was later redeclared as a struct, wouldn't it?

Bill Gibbons: Right. I think the intent was to model this after the existing rule for local declarations of functions (which dates back to C), where the declaration is introduced into the enclosing scope but the name is not. Getting this right requires being somewhat more rigorous about things like the ODR because there may be declaration clashes even when there are no name clashes. I suspect that the standard gets this right in most places but I would expect there to be a few that are still wrong, in addition to the one Mike pointed out.

Mike Miller: Regarding would result in an error when B was later redeclared

I don't see any reason why it should. The restriction that the class-key must agree is found in  dcl.type.elab and is predicated on having found a matching declaration in a lookup according to 3.4.4  basic.lookup.elab . Since a lookup of a name declared only (up to that point) in a friend declaration does not find that name (regardless of whether you subscribe to the "does-not-introduce" or "introduces-invisibly" school of thought), there can't possibly be a mismatch.

I don't think that the Standard's necessarily broken here. There is no requirement that a class declared in a friend declaration ever be defined. Explicitly putting an incompatible declaration into the namespace where that friend class would have been defined is, to me, just making it impossible to define — which is no problem, since it didn't have to be defined anyway. The only error would occur if the same-named but unbefriended class attempted to use the nonexisting grant of friendship, which would result in an access violation.

(BTW, I couldn't find anything in the Standard that forbids defining a class with a mismatched class-key, only using one in an elaborated-type-specifier. Is this a hole that needs to be filled?)

John Spicer: This is what  dcl.type.elab paragraph 3 says:

The latter part of this paragraph (beginning "This rule also applies...") is somewhat murky to me, but I think it could be interpreted to say that

            class B;
            union B {};
            union B {};
            class B;
are both invalid. I think this paragraph is intended to say that. I'm not so sure it actually does say that, though.

Mike Miller: Regarding I think the intent was to model this after the existing rule for local declarations of functions (which dates back to C)

Actually, that's not the C (1989) rule. To quote the Rationale from X3.159-1989:

Regarding Getting this right requires being somewhat more rigorous

Yes, I think if this is to be made illegal, it would have to be done with the ODR; the name-lookup-based current rules clearly (IMHO) don't apply. (Although to be fair, the [non-normative] note in 3.3  basic.scope paragraph 4 sounds as if it expects friend invisible injection to trigger the multiple-declaration provisions of that paragraph; it's just that there's no normative text implementing that expectation.)

Bill Gibbons: Nor does the ODR currently disallow:

    translation unit #1    struct A;
    translation unit #2    union A;
since it only refers to class definitions, not declarations.

But the obvious form of the missing rule (all declarations of a class within a program must have compatible struct/class/union keys) would also answer the original question.

The declarations need not be visible. For example:

    translation unit #1    int f() { return 0; }
    translation unit #2:   void g() {
                               extern long f();
is ill-formed even though the second "f" is not a visible declaration.

Rationale (10/99): The main issue (differing behavior of standalone and embedded elaborated-type-specifiers) is as the Committee intended. The remaining questions mentioned in the discussion may be addressed in dealing with related issues.

(See also issues 136, 138, 139, 143, 165, and 166.)

165. Definitions of friends and block-scope externs

Section:  namespace.memdef     Status: NAD     Submitter: Derek Inglis     Date: 7 Sep 1999  namespace.memdef paragraph 2 says,

Members of a named namespace can also be defined outside that namespace by explicit qualification (  namespace.qual ) of the name being defined, provided that the entity being defined was already declared in the namespace...
It is not clear whether block-scope extern declarations and friend declarations are sufficient to permit the named entities to be defined outside their namespace. For example,
    namespace NS {
       struct A { friend struct B; };
       void foo() { extern void bar(); }
    struct NS::B { };   // 1) legal?
    void NS::bar() { }  // 2) legal?

Rationale (10/99): Entities whose names are "invisibly injected" into a namespace as a result of friend declarations are not "declared" in that namespace until an explicit declaration of the entity appears at namespace scope. Consequently, the definitions in the example are ill-formed.

(See also issues 95, 136, 138, 139, 143, and 166.)

553. Problems with friend allocation and deallocation functions

Section:  namespace.memdef     Status: NAD     Submitter: Dmitriy Litskalov     Date: 16 December 2005

Consider the following example:

    class C {
       public: enum E {};

       friend void* operator new(size_t, E);
       friend void  operator delete(void*, E);

    void foo() {
       C::E e;
       C* ptr = new(e) C();

This code, which is valid in global scope, becomes ill-formed when the class definition is moved into a namespace, and there is no way to make it valid:

    namespace N {
       class C {
          public: enum E {};

          friend void* operator new(size_t, E);
          friend void  operator delete(void*, E);

    void foo() {
       N::C::E e;
       N::C* ptr = new(e) N::C();

The reason for this is that non-member allocation and deallocation functions are required to be members of the global scope ( paragraph 1, paragraph 1), unqualified friend declarations declare names in the innermost enclosing namespace (  namespace.memdef paragraph 3), and these functions cannot be declared in global scope at a point where the friend declarations could refer to them using qualified-ids because their second parameter is a member of the class and thus can't be named before the class containing the friend declarations is defined.

Possible solutions for this conundrum include invention of some mechanism to allow a friend declaration to designate a namespace scope other than the innermost enclosing namespace in which the friend class or function is to be declared or to relax the innermost enclosing namespace lookup restriction in  namespace.memdef paragraph 3 for friend declarations that nominate allocation and deallocation functions.

Rationale (April, 2006):

The CWG acknowledged that it is not always possible to move code from the global scope into a namespace but felt that this problem was not severe enough to warrant changing the language to accommodate it. Possible solutions include moving the enumeration outside the class or defining member allocation and deallocation functions.

109. Allowing ::template in using-declarations

Section: 7.3.3  namespace.udecl     Status: NAD     Submitter: Daveed Vandevoorde     Date: 6 Apr 1999

Daveed Vandevoorde : While reading Core issue 11 I thought it implied the following possibility:

    template<typename T>
    struct B {
       template<int> void f(int);

    template<typename T>
    struct D: B<T> {
       using B<T>::template f;
       void g() { this->f<1>(0); } // OK, f is a template

However, the grammar for a using-declaration reads:

and nested-name-specifier never ends in "template".

Is that intentional?

Bill Gibbons :

It certainly appears to be, since we have:

so it would be easier to specify using-declaration as: if the "template" keyword were allowed. There was a discussion about whether a dependent name specified in a using-declaration could be given an "is a type" attribute through the typename keyword; the decision was to allow this. But I don't recall if the "is a template" attribute was discussed.

Rationale (04/99): Any semantics associated with the template keyword in using-declarations should be considered an extension.

Notes from the April 2003 meeting:

See also issues 96 and 11.

We decided to make no change and to close this issue as not-a-defect. This is not needed functionality; the example above, for example, can be written with ->template. This issue has been on the issues list for years as an extension, and there has been no clamor for it.

It was also noted that knowing that something is a template is not enough; there's still the issue of knowing whether it is a class or function template.

169. template-ids in using-declarations

Section: 7.3.3  namespace.udecl     Status: NAD     Submitter: Valentin Bonnard     Date: 16 Sep 1999

7.3.3  namespace.udecl paragraph says,

A using-declaration shall not name a template-id.
It is not clear whether this prohibition applies to the entity for which the using-declaration is a synonym or to any name that appears in the using-declaration. For example, is the following code well-formed?
    template <typename T>
    struct base {
	void bar ();

    struct der : base<int> 
	using base<int>::bar; // ill-formed ?

Rationale (10/99): 7.3.3  namespace.udecl paragraph 1 says, "A using-declaration introduces a name..." It is the name that is thus introduced that cannot be a template-id.

461. Make asm conditionally-supported

Section: 7.4  dcl.asm     Status: NAD     Submitter: Clark Nelson     Date: 24 May 2004

Now that the concept of "conditionally-supported" is available (see N1564), perhaps asm should not be required of every implementation.

Rationale (October, 2004):

This is covered in paper N1627. We would like to keep asm as a keyword for all implementations, however, to enhance portability by preventing programmers from inadvertently using it as an identifier.

14. extern "C" functions and declarations in different namespaces

Section: 7.5     Status: NAD     Submitter: Erwin Unruh     Date: unknown

Issue 1

7.5 paragraph 6 says the following:

Is this only for linkage purposes or for both name look up and linkage purposes:
    extern "C" int f(void);
    namespace A {
         extern "C" int f(void);
    using namespace A;
    int i = f(); // Ok because only one function f() or
                 // ill-formed
For name lookup, both declarations of f are visible and overloading cannot distinguish between them. Has the compiler to check that these functions are really the same function or is the program in error?

Rationale: These are the same function for all purposes.

Issue 2

A similar question may arise with typedefs:

    // vendor A
    typedef unsigned int size_t;
    // vendor B
    namespace std {
            typedef unsigned int size_t;
    using namespace std;
    size_t something(); // error?
Is this valid because the typedef size_t refers to the same type in both namespaces?

Rationale (04/99): In 7.3.4  namespace.udir paragraph 4:

If name lookup finds a declaration for a name in two different namespaces, and the declarations do not declare the same entity and do not declare functions, the use of the name is ill-formed.
The term entity applied to typedefs refers to the underlying type or class (3  basic , paragraph 3); therefore both declarations of size_t declare the same entity and the above example is well-formed.

358. Namespaces and extern "C"

Section: 7.5     Status: NAD     Submitter: Steve Clamage     Date: 28 May 2002

Is this code valid:

  extern "C" void f();

  namespace N
    int var; 
    extern "C" void f(){ var = 10; }

The two declarations of f refer to the same external function, but is this a valid way to declare and define f?

And is the definition of f considered to be in namespace N or in the global namespace?

Notes from October 2002 meeting:

Yes, this example is valid. See 7.5 paragraph 6, which contains a similar example with the definition in the global namespace instead. There is only one f, so the question of whether the definition is in the global namespace or the namespace N is not meaningful. The same function is found by name lookup whether it is found from the declaration in namespace N or the declaration in the global namespace, or both (7.3.4  namespace.udir paragraph 4).

333. Ambiguous use of "declaration" in disambiguation section

Section: 8.2  dcl.ambig.res     Status: NAD     Submitter: Michiel Salters     Date: 14 Jan 2002

In deciding whether a construct is an object declaration or a function declaration, 8.2  dcl.ambig.res contains the following gem: "In that context, the choice is between a function declaration [...] and an object declaration [...] Just as for the ambiguities mentioned in 6.8  stmt.ambig, the resolution is to consider any construct that could possibly be a declaration a declaration."

To what declaration do the last two "declarations" refer? Object, function, or (following from the syntax) possibly parameter declarations?

Notes from the 4/02 meeting:

This is not a defect. Section 8.2  dcl.ambig.res reads:

The ambiguity arising from the similarity between a function-style cast and a declaration mentioned in 6.8  stmt.ambig can also occur in the context of a declaration. In that context, the choice is between a function declaration with a redundant set of parentheses around a parameter name and an object declaration with a function-style cast as the initializer. Just as for the ambiguities mentioned in 6.8  stmt.ambig, the resolution is to consider any construct that could possibly be a declaration a declaration.

The wording "any construct" in the last sentence is not limited to top-level constructs. In particular, the function declaration encloses a parameter declaration, whereas the object declaration encloses an expression. Therefore, in case of ambiguity between these two cases, the declaration is parsed as a function declaration.

340. Unclear wording in disambiguation section

Section: 8.2  dcl.ambig.res     Status: NAD     Submitter: Bart v Ingen Schenau     Date: 27 Feb 2002

Consider the following program:

  struct Point
  struct Lattice 
    Lattice(Point, Point, int){}
  int main(void)
    int a, b;
    Lattice latt(Point(a), Point(b), 3);   /* Line X */

The problem concerns the line marked /* Line X */, which is an ambiguous declarations for either an object or a function. The clause that governs this ambiguity is 8.2  dcl.ambig.res paragraph 1, and reads:

The ambiguity arising from the similarity between a function-style cast and a declaration mentioned in 6.8  stmt.ambig can also occur in the context of a declaration. In that context, the choice is between a function declaration with a redundant set of parentheses around a parameter name and an object declaration with a function-style cast as the initializer. Just as for the ambiguities mentioned in 6.8  stmt.ambig, the resolution is to consider any construct that could possibly be a declaration a declaration. [Note: a declaration can be explicitly disambiguated by a nonfunction-style cast, by a = to indicate initialization or by removing the redundant parentheses around the parameter name. ]

Based on this clause there are two possible interpretations of the declaration in line X:

  1. The declaration of latt declares a function with a return value of the type Lattice and taking three arguments. The type of the first two arguments is Point and each of these arguments is followed by a parameter name in redundant parentheses. The type of the third argument can not be determined, because it is a literal. This will result in a syntax error.
  2. The declaration of latt declares an object, because the other option (a function declaration) would result in a syntax error.

Note that the last sentence before the "[Note:" is not much help, because both options are declarations.

Steve Adamczyk: a number of people replied to this posting on comp.std.c++ saying that they did not see a problem. The original poster replied:

I can't do anything but agree with your argumentation. So there is only one correct interpretation of clause 8.2  dcl.ambig.res paragraph 1, but I have to say that with some rewording, the clause can be made a lot clearer, like stating explicitly that the entire declaration must be taken into account and that function declarations are preferred over object declarations.

I would like to suggest the following as replacement for the current clause 8.2  dcl.ambig.res paragraph 1:

The ambiguity arising from the similarity between a functionstyle cast and a declaration mentioned in 6.8  stmt.ambig can also occur in the context of a declaration. In that context, the choice is between a function declaration with a redundant set of parentheses around a parameter name and an object declaration with a function-style cast as the initializer. The resolution is to consider any construct that could possibly be a function declaration a function declaration. [Note: To disambiguate, the whole declaration might have to be examined to determine if it is an object or a function declaration.] [Note: a declaration can be explicitly disambiguated by a nonfunction-style cast, by a = to indicate initialization or by removing the redundant parentheses around the parameter name. ]

Notes from the 4/02 meeting:

The working group felt that the current wording is clear enough.

478. May a function parameter be an array of an abstract class type?

Section: 8.3.4  dcl.array     Status: NAD     Submitter: Steve Adamczyk     Date: 28 Sep 2004

Consider the following example:

    struct S {
      virtual void v() = 0;

    void f(S sa[10]);     // permitted?

8.3.4  dcl.array paragraph 1 says that a declaration like that of sa is ill-formed:

T is called the array element type; this type shall not be a reference type, the (possibly cv-qualified) type void, a function type or an abstract class type.

On the other hand, 8.3.5  dcl.fct paragraph 3 says that the type of sa is adjusted to S*, which would be permitted:

The type of each parameter is determined from its own decl-specifier-seq and declarator. After determining the type of each parameter, any parameter of type “array of T” or “function returning T” is adjusted to be “pointer to T” or “pointer to function returning T,” respectively.

It is not clear whether the parameter adjustment trumps the prohibition on declaring an array of an abstract class type or not. Implementations differ in this respect: EDG 2.4.2 and MSVC++ 7.1 reject the example, while g++ 3.3.3 and Sun Workshop 8 accept it.

Rationale (April, 2005):

The prohibition in 8.3.4  dcl.array is absolute and does not allow for exceptions. Even though such a type in a parameter declaration would decay to an allowed type, the prohibition applies to the type before the decay.

This interpretation is consistent with the resolution of issue 337, which causes template type deduction to fail if such types are deduced. It was also observed that pointer arithmetic on pointers to abstract classes is very likely to fail, and the fact that the programmer used array notation to declare the pointer type is a strong indication that he/she expected to use subscripting.

18. f(TYPE) where TYPE is void should be allowed

Section: 8.3.5  dcl.fct     Status: NAD     Submitter: unknown     Date: unknown

8.3.5  dcl.fct paragraph 2 says:

If the parameter-declaration-clause is empty, the function takes no arguments. The parameter list (void) is equivalent to the empty parameter list.
Can a typedef to void be used instead of the type void in the parameter list?

Rationale: The IS is already clear that this is not allowed.

66. Visibility of default args vs overloads added after using-declaration

Section: 8.3.6  dcl.fct.default     Status: NAD     Submitter: Mike Miller     Date: 6 Oct 1998

Paragraph 9 of says that extra default arguments added after a using-declaration but before a call are usable in the call, while 7.3.3  namespace.udecl paragraph 9 says that extra function overloads are not. This seems inconsistent, especially given the similarity of default arguments and overloads.

Rationale (10/99): The Standard accurately reflects the intent of the Committee.

434. Unclear suppression of standard conversions while binding reference to lvalue

Section: 8.5.3  dcl.init.ref     Status: NAD     Submitter: Bronek Kozicki     Date: 14 September 2003

In section 8.5.3  dcl.init.ref, paragraph 5, there is following note:

Note: the usual lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) standard conversions are not needed, and therefore are suppressed, when such direct bindings to lvalues are done.

I believe that this note is misleading. There should be either:

The problem:

  1. under current wording it's unclear if following code is legal, or not:
    int main()
      const int ci = 10;
      int * pi = NULL;
      const int * & rpci = pi;
      rpci = &ci;
      *pi = 12; // circumvent constness of "ci"
  2. it is also unclear what behaviour should following program expose:
    int main()
      int * pi = NULL;
      const int * const & rcpci = pi; // 1
      int i = 0;
      pi = &i; // 2
      if (pi == rcpci)
        std::cout << "bound to lvalue" << std::endl;
        std::cout << "bound to temporary rvalue" << std::endl;

There has been discussion on this issue on comp.lang.c++.moderated month ago, see and there seems to be some confusion about it. I understand that note is not normative, but apparently even some compiler writers are misled (try above code snippets on few different compilers, and using different compilation options - notably GCC 3.2.3 with -Wall -pedantic), thus it should be cleared up.

My proposal is to change wording of discussed note to:

Note: result of every standard conversion is never an lvalue, and therefore all standard conversions (clause 4) are suppressed, when such direct bindings to lvalues are done.

Rationale (April, 2005):

As acknowledged in the description of the issue, the referenced text is only a note and has no normative impact. Furthermore, the examples cited do not involve the conversions mentioned in the note, and the normative text is already sufficiently clear that the types in the examples are not reference-compatible.

315. Is call of static member function through null pointer undefined?

Section: 9.4.1  class.static.mfct     Status: NAD     Submitter: Jason Merrill     Date: 7 Oct 2001

Another instance to consider is that of invoking a member function from a null pointer:

  struct A { void f () { } };
  int main ()
    A* ap = 0;
    ap->f ();

Which is explicitly noted as undefined in 9.3.1  class.mfct.nonstatic, even though one could argue that since f() is empty, there is no lvalue->rvalue conversion.

If f is static, however, there seems to be no such rule, and the call is only undefined if the dereference implicit in the -> operator is undefined. IMO it should be.

Incidentally, another thing that ought to be cleaned up is the inconsistent use of "indirection" and "dereference". We should pick one. (This terminology issue has been broken out as issue 342.)

This is related to issue 232

Rationale (October 2003):

We agreed the example should be allowed. p->f() is rewritten as (*p).f() according to 5.2.5  expr.ref. *p is not an error when p is null unless the lvalue is converted to an rvalue (4.1  conv.lval), which it isn't here.

7. Can a class with a private virtual base class be derived from?

Section: 11.2  class.access.base     Status: NAD     Submitter: Jason Merrill     Date: unknown
    class Foo { public: Foo() {}  ~Foo() {} };
    class A : virtual private Foo { public: A() {}  ~A() {} };
    class Bar : public A { public: Bar() {}  ~Bar() {} };
~Bar() calls ~Foo(), which is ill-formed due to access violation, right? (Bar's constructor has the same problem since it needs to call Foo's constructor.) There seems to be some disagreement among compilers. Sun, IBM and g++ reject the testcase, EDG and HP accept it. Perhaps this case should be clarified by a note in the draft.

In short, it looks like a class with a virtual private base can't be derived from.

Rationale: This is what was intended.

17. Footnote 99 should discuss the naming class when describing members that can be accessed from friends

Section: 11.2  class.access.base     Status: NAD     Submitter: unknown     Date: unknown

Footnote 98 says:

As specified previously in clause 11  class.access , private members of a base class remain inaccessible even to derived classes unless friend declarations within the base class declaration are used to grant access explicitly.
This footnote does not fit with the algorithm provided in 11.2  class.access.base paragraph 4 because it does not take into account the naming class concept introduced in this paragraph.

(See also paper J16/99-0002 = WG21 N1179.)

Rationale (10/99): The footnote should be read as referring to immediately-derived classes, and is accurate in that context.

471. Conflicting inherited access specifications

Section: 11.2  class.access.base     Status: NAD     Submitter: Mike Miller     Date: 14 Jun 2004

The Standard does not appear to specify how to handle cases in which conflicting access specifications for a member are inherited from different base classes. For example,

    struct A {
      int i;
    struct B : virtual public A {
      using A::i;
    struct C : virtual public A, public B {
      // "i" is protected from B, public from A

This question affects both the existing wording of 11.2  class.access.base paragraph 4 (“m as a member of N is public ... m as a member of N is private ... m as a member of N is protected”) and the proposed wording for issue 385 (“when a nonstatic data member or nonstatic member function is a protected member of its naming class”).

One possible definition of “is public” would be something like, “if any visible declaration of the entity has public access.” One could also plausibly define the access of m in N to be the minimum of all the visible declarations, or even an error if the visible declarations are inconsistent.

11.2  class.access.base paragraph 1 describes the access of inherited members, so a clarifying statement resolving this issue might plausibly be inserted at the end of that paragraph.

Proposed resolution (October, 2004):

Add the following text as a new paragraph after 11.2  class.access.base paragraph 1:

If a given base class can be reached along more than one path through a derived class's sub-object lattice (10.1  class.mi), a member of that base class could have different accessibility in the derived class along different paths. In such cases, the most permissive access prevails. [Example:

    struct B { static int i; };
    class I: protected B { };
    class D1: public B, public I { };
    class D2: public I, private B { };

i is accessible as a public member of D1 and as a protected member of D2. —end example]

Rationale (03/2005): This question is already covered, in almost identical words, in 11.7  class.paths.

209. Must friend declaration names be accessible?

Section: 11.4  class.friend     Status: NAD     Submitter: Judy Ward     Date: 1 Mar 2000

11.4  class.friend, paragraph 7, says

A name nominated by a friend declaration shall be accessible in the scope of the class containing the friend declaration.

Does that mean the following should be illegal?

    class A { void f(); };
    class B { friend void A::f(); }; // Error: A::f not accessible from B

I discussed this with Bjarne in email, and he thinks it was an editorial error and this was not the committee's intention. The paragraph seems to have been added in the pre-Kona (24 Sept 1996) mailing, and I could not find anything in the previous meeting's (Stockholm) mailings which led me to believe this was intentional. The only compiler vendor which I think currently implements it is the latest release (2.43) of the EDG front end.

Proposed resolution (10/00):

Remove the first sentence of 11.4  class.friend, paragraph 7.

Rationale (04/01):

After the 10/00 vote to accept this issue as a DR with the proposed resolution, it was noted that the first two sentences of 11  class.access paragraph 3 cause the proposed change to have no effect:

Access control is applied uniformly to all names, whether the names are referred to from declarations or expressions. [Note: access control applies to names nominated by friend declarations (11.4  class.friend) and using-declarations (7.3.3  namespace.udecl). ]

In addition to the obvious editing to the text of the note, an exception to the blanket statement in the first sentence would also be required. However, discussion during the 04/01 meeting failed to produce consensus on exactly which names in the friend declaration should be exempted from the requirements of access control.

One possibility would be that only the name nominated as friend should be exempt. However, that approach would make it impossible to name a function as a friend if it used private types in its parameters or return type. Another suggestion was to ignore access for every name used in a friend declaration. That approach raised a question about access within the body of a friend function defined inline in the class body — the body is part of the declaration of a function, but references within the body of a friend function should still be subject to the usual access controls.

Other possibilities were raised, such as allowing the declaration of a friend member function if the declaration were permissible in its containing class, or taking the union of the access within the befriending class and the befriended entity. However, these suggestions would have been complex and difficult to specify correctly.

Ultimately it was decided that the original perceived defect was not sufficiently serious as to warrant the degree of complexity required to resolve it satisfactorily and the issue was consequently declared not to be a defect. It was observed that most of the problems involved with the current state of affairs result from inability to declare a particular member function as a friend; in such cases, an easy workaround is simply to befriend the entire class rather than the specific member function.

445. Wording issue on friend declarations

Section: 11.4  class.friend     Status: NAD     Submitter: Risto Lankinen     Date: 5 Dec 2003

Thus says the section 11.4  class.friend/7 in ISO 14882 C++ standard:

A name nominated by a friend declaration shall be accessible in the scope of the class containing the friend declaration.

The obvious intention of this is to allow a friend declaration to specify a function (or nested class, enum, etc.) that is declared "private" or "protected" in its enclosing class. However, literal interpretation seems to allow a broader access to the befriended function by the whole class that is declaring the friendship.

If the rule were interpreted literally as it is currently written, this would compile (when it, of course, shouldn't be allowed at all):

class C
  static void f();

class D
  friend void C::f();  // A name nominated by friend declaration...

   C::f();  // ... shall be accessible in scope of class declaring friendship

Suggested fix is to reword "in the scope of the class containing the friend declaration" to exclude all other references from the scope of the declaring class, except the friend-declaration itself.

Notes from the March 2004 meeting:

We considered this and concluded that the standard is clear enough.

501. Visibility of friend declarations within the befriending class

Section: 11.4  class.friend     Status: NAD     Submitter: Gabriel Dos Reis     Date: 25 Jan 2005

I just received a query from a user of why line #1 in the following snippet is ill-formed:

    void g(int (*)(int));

    template<class T>
    class A {
       friend int f(int) { return 0; }

       void h() {
           g(f);          // #1

I believe that the special invisibility rule about friends is too complicated and makes life too complicated, especially considering that friends in templates are not templates, nor can they be conveniently rewritten with a “first declare at the namespace scope” rule. I can understand the rules when they make programming easier or prevent some obvious “silly” mistakes; but that does not seem to be the case here.

John Spicer: See two papers that discuss this issue: N0878 by Bill Gibbons, which ultimately gave rise to our current rules, and N0913 by me as an alternative to N0878.

Rationale (April, 2005):

The Standard is clear and consistent; this rule is the result of an explicit decision by the Committee.

19. Clarify protected member access

Section: 11.5  class.protected     Status: NAD     Submitter: unknown     Date: unknown

11.5  class.protected paragraph 1 says:

When a friend or a member function of a derived class references a protected nonstatic member of a base class, an access check applies in addition to ...
Instead of saying "references a protected nonstatic member of a base class", shouldn't this be rewritten to use the concept of naming class as 11.2  class.access.base paragraph 4 does?

Rationale (04/99): This rule is orthogonal to the specification in 11.2  class.access.base paragraph 4.

117. Timing of destruction of temporaries

Section: 12.2  class.temporary     Status: NAD     Submitter: Mike Miller     Date: 14 May 1999

12.2  class.temporary paragraph 4 seems self-contradictory:

the temporary that holds the result of the expression shall persist until the object's initialization is complete... the temporary is destroyed after it has been copied, before or when the initialization completes.
How can it be destroyed "before the initialization completes" if it is required to "persist until the object's initialization is complete?"

Rationale (04/00):

It was suggested that "before the initialization completes" refers to the case in which some part of the initialization terminates by throwing an exception. In that light, the apparent contradiction does not apply.

363. Initialization of class from self

Section: 12.6.1  class.expl.init     Status: NAD     Submitter: Sergey P. Derevyago     Date: 11 July 2002

Is the following code well-formed?

 struct A { /* */ };

 int main()
  A a=a;

Note, that { int a=a; } is pretty legal.

And if so, what is the semantics of the self-initialization of UDT? For example

 #include <stdio.h>

 struct A {
        A()           { printf("A::A() %p\n",            this);     }
        A(const A& a) { printf("A::A(const A&) %p %p\n", this, &a); }
        ~A()          { printf("A::~A() %p\n",           this);     }

 int main()
  A a=a;

can be compiled and prints:

A::A(const A&) 0253FDD8 0253FDD8
A::~A() 0253FDD8

(on some implementations).

Notes from October 2002 meeting:

3.8 paragraph 6 indicates that the references here are valid. It's permitted to take the address of a class object before it is fully initialized, and it's permitted to pass it as an argument to a reference parameter as long as the reference can bind directly. Except for the failure to cast the pointers to void * for the %p in the printfs, these examples are standard-conforming.

307. Initialization of a virtual base class subobject

Section: 12.7  class.cdtor     Status: NAD     Submitter: Andrei Iltchenko     Date: 31 Aug 2001

The second paragraph of section 12.7  class.cdtor contains the following text:

To explicitly or implicitly convert a pointer (an lvalue) referring to an object of class X to a pointer (reference) to a direct or indirect base class B of X, the construction of X and the construction of all of its direct or indirect bases that directly or indirectly derive from B shall have started and the destruction of these classes shall not have completed, otherwise the conversion results in undefined behavior.

Now suppose we have the following piece of code:

    struct  a  {
       a() :  m_a_data(0)  {   }
       a(const a& rhsa) :  m_a_data(rhsa.m_a_data)  {   }
       int   m_a_data;

    struct  b :  virtual a  {
       b() :  m_b_data(0)  {   }
       b(const b& rhsb) :  a(rhsb),  m_b_data(rhsb.m_b_data)  {   }
       int   m_b_data;

    struct  c :  b  {
       c() :  m_c_data(0)  {   }
       c(const c& rhsc) :  a(rhsc),// Undefined behaviour when constru-
                                   // cting an object of type 'c'
                           b(rhsc),  m_c_data(rhsc.m_c_data)  {   }
       int   m_c_data;

    int  main()
    {   c   ac1,   ac2(ac1);   }

The problem with the above snippet is that when the value 'ac2' is being created and its construction gets started, c's copy constructor has first to initialize the virtual base class subobject 'a'. Which requires that the lvalue expression 'rhsc' be converted to the type of the parameter of a's copy constructor, which is 'const a&'. According to the wording quoted above, this can be done without undefined behaviour if and only if b's construction has already started, which is not possible since 'a', being a virtual base class, has to be initialized first by a constructor of the most derived object (12.6.2  class.base.init).

The issue could in some cases be alleviated when 'c' has a user-defined copy constuctor. The constructor could default-initialize its 'a' subobject and then initialize a's members as needed taking advantage of the latitude given in paragraph 2 of 12.6.2  class.base.init.

But if 'c' ends up having the implicitly-defined copy constuctor, there's no way to evade undefined behaviour.

    struct  c :  b  {
       c() :  m_c_data(0)  {   }
       int   m_c_data;

    int  main()
    {   c   ac1,   ac2(ac1);   }

Paragraph 8 of 12.8  class.copy states

The implicitly-defined copy constructor for class X performs a memberwise copy of its subobjects. The order of copying is the same as the order of initialization of bases and members in a user-defined constructor (see 12.6.2  class.base.init). Each subobject is copied in the manner appropriate to its type:

Which effectively means that the implicitly-defined copy constructor for 'c' will have to initialize its 'a' base class subobject first and that must be done with a's copy constructor, which will always require a conversion of an lvalue expression of type 'const c' to an lvalue of type 'const a&'. The situation would be the same if all the three classes shown had implicitly-defined copy constructors.

Suggested resolution:

Prepend to paragraph 2 of 12.7  class.cdtor the following:

Unless the conversion happens in a mem-initializer whose mem-initializer-id designates a virtual base class of X, to explicitly or implicitly convert ...

Notes from the 10/01 meeting:

There is no problem in this example. ac1 is fully initialized before it is used in the initialization of ac2.

26. Copy constructors and default arguments

Section: 12.8  class.copy     Status: NAD     Submitter: Daveed Vandevoorde     Date: 22 Sep 1997

The working paper is quite explicit about

    struct X {
         X(X, X const& = X());
being illegal (because of the chicken & egg problem wrt copying.)

Shouldn't it be as explicit about the following?

    struct Y {
        Y(Y const&, Y = Y());
Rationale: There is no need for additional wording. This example leads to a program which either fails to compile (due to resource limits on recursive inlining) or fails to run (due to unterminated recursion). In either case the implementation may generate an error when the program is compiled.

356. Wording of behavior of generated copy constructor for scalar members

Section: 12.8  class.copy     Status: NAD     Submitter: Steve Clamage     Date: 25 May 2002

Section 12.8  class.copy paragraph 8 says the compiler-generated copy constructor copies scalar elements via the built-in assignment operator. Seems inconsistent. Why not the built-in initialization?

Notes from October 2002 meeting:

The Core Working Group believes this should not be changed. The standard already mentions built-in operators and the assignment operator does clearly define what must be done for scalar types. There is currently no concept of built-in initialization.

444. Overriding and the generated copy assignment operator

Section: 12.8  class.copy     Status: NAD     Submitter: Kerch Holt     Date: 26 Nov 2003

EDG (and g++, for that matter) picks the explicit copy assignment operator, which we think is wrong in this case:

#include <stdio.h>
struct D;   // fwd declaration
struct B {
    D& operator=(D&);
struct D : B {
    D() {}
    D(int ii) { s = ii; }
    using B::operator=;
    int s;
int main() {
    D od, od1(10);
    od = od1; // implicit D::operator=(D&) called, not BASE::operator=(D&)
D& B::operator=(D& d) { 
    printf("B::operator called\n");
    return d;

If you look at 12.8  class.copy paragraph 10 it explicitly states that in such a case the "using B::operator=" will not be considered.

Steve Adamczyk: The fact that the operator= you declared is (D&) and not (const D&) is fooling you. As the standard says, the operator= introduced by the using-declaration does not suppress the generation of the implicit operator=. However, the generated operator= has the (const D&) signature, so it does not hide B::operator=; it overloads it.

Kerch Holt: I'm not sure this is correct. Going by 12.8 P10 first paragraph we think that the other form "operator=(D&)" is generated because the two conditions mentioned were not met in this case. 1) there is no direct base with a "const [volatile] B&" or "B" operator 2) And no member has a operator= either. This implies the implicit operator is "operator=(D&)". So, if that is the case the "hiding" should happen.

Also, in the last paragraph it seems to state that operators brought in from "using", no matter what the parameter is, are always hidden.

Steve Adamczyk: Not really. I think this section is pretty clear about the fact that the implicit copy assignment operator is generated. The question is whether it hides or overloads the one imported by the using-declaration.

Notes from the March 2004 meeting:

(a) Class B does get an implicitly-generated operator=(const B&); (b) the using-declaration brings in two operator= functions from B, the explicitly-declared one and the implicitly-generated one; (c) those two functions overload with the implicitly-generated operator=(const D&) in the derived class, rather than being hidden by the derived-class function, because it does not match either of their signatures; (d) overload resolution picks the explicitly-declared function from the base class because it's the best match in this case. We think the standard wording says this clearly enough.

102. Operator lookup rules do not work well with parts of the library

Section:  over.match.oper     Status: NAD     Submitter: Herb Sutter     Date: 15 Oct 1998

The following example does not work as one might expect:

    namespace N { class C {}; }
    int operator +(int i, N::C) { return i+1; }

    #include <numeric>
    int main() {
        N::C a[10];
        std::accumulate(a, a+10, 0);
According to 3.4.1  basic.lookup.unqual paragraph 6, I would expect that the "+" call inside std::accumulate would find the global operator+. Is this true, or am I missing a rule? Clearly, the operator+ would be found by Koenig lookup if it were in namespace N.

Daveed Vandevoorde: But doesn't unqualified lookup of the operator+ in the definition of std::accumulate proceed in the namespace where the implicit specialization is generated; i.e., in namespace std?

In that case, you may find a non-empty overload set for operator+ in namespace std and the surrounding (global) namespace is no longer considered?

Nathan Myers: Indeed, <string> defines operator+, as do <complex>, <valarray>, and <iterator>. Any of these might hide the global operator.

Herb Sutter: These examples fail for the same reason:

    struct Value { int i; };

    typedef map<int, Value > CMap;
    typedef CMap::value_type CPair;

    ostream & operator<< ( ostream &os, const CPair &cp )
      { return os << cp.first << "/" << cp.second.i; }

    int main() {
      CMap courseMap;
      copy( courseMap.begin(), courseMap.end(),
            ostream_iterator<CPair>( cout, "\n" ) );

    template<class T, class S>
    ostream& operator<< (ostream& out, pair<T,S> pr)
      { return out << pr.first << " : " << pr.second << endl; }

    int main() {
      map <int, string> pl;
      copy( pl.begin(), pl.end(),
            ostream_iterator <places_t::value_type>( cout, "\n" ) );
This technique (copying from a map to another container or stream) should work. If it really cannot be made to work, that would seem broken to me. The reason is does not work is that copy and pair are in namespace std and the name lookup rules do not permit the global operator<< to be found because the other operator<<'s in namespace std hide the global operator. (Aside: FWIW, I think most programmers don't realize that a typedef like CPair is actually in namespace std, and not the global namespace.)

Bill Gibbons: It looks like part of this problem is that the library is referring to names which it requires the client to declare in the global namespace (the operator names) while also declaring those names in namespace std. This would be considered very poor design for plain function names; but the operator names are special.

There is a related case in the lookup of operator conversion functions. The declaration of a conversion function in a derived class does not hide any conversion functions in a base class unless they convert to the same type. Should the same thing be done for the lookup of operator function names, e.g. should an operator name in the global namespace be visible in namespace std unless there is a matching declaration in std?

Because the operator function names are fixed, it it much more likely that a declaration in an inner namespace will accidentally hide a declaration in an outer namespace, and the two declarations are much less likely to interfere with each other if they are both visible.

The lookup rules for operator names (when used implicitly) are already quite different from those for ordinary function names. It might be worthwhile to add one more special case.

Mike Ball : The original SGI proposal said that non-transitive points of instantiation were also considered. Why, when, and by whom was it added?

Rationale (10/99): This appears to be mainly a program design issue. Furthermore, any attempt to address it in the core language would be beyond the scope of what can be done in a Technical Corrigendum.

423. Can a conversion be done on the left operand of a compound assignment?

Section:  over.match.oper     Status: NAD     Submitter: Gennaro Prota     Date: 19 Jun 2003

Is the following well-formed?

  template <typename T>
  class test
    operator T& (){ return m_i; }
    T m_i;
  int main()
    test<int*> t2;
    t2 += 1;  // Allowed?

Is it possible that by "assignment operators" (  over.match.oper paragraph 4) only the built-in candidates for operator= (i.e. excluding +=, *=, etc.) were meant? On one hand the plural ("operators") seems to imply that all the assignment operators are considered. OTOH, there has already been a core DR (221) about a missing distinction between "assignment operator" and "compound assignment operators". Is there a similar defect here?

Steve Adamczyk: The standard is ambiguous. However, I think the ARM was fairly clear about "assignment operators" meaning only "=", and I find that Cfront 3.0.1 accepts the test case (with typename changed to class). I don't know whether that's good or bad, but it's at least a precedent. Given the change of Core Issue 221, if we do nothing further conversions are valid on += and therefore this case is valid.

Note that "t2++;" is unquestionably valid, so one could also argue for the status quo (post-221) on the basis of consistency.

Notes from the October 2003 meeting:

We believe the example is well-formed, and no change other than that in issue 221 is needed.

243. Weighting of conversion functions in direct-initialization

Section:  over.ics.user     Status: NAD     Submitter: Steve Adamczyk     Date: 5 Sep 2000

There is a moderately serious problem with the definition of overload resolution. Consider this example:

    struct B;
    struct A {
    struct B {
        operator A();
    } b;
    int main() {

This is pretty much the definition of "ambiguous," right? You want to convert a B to an A, and there are two equally good ways of doing that: a constructor of A that takes a B, and a conversion function of B that returns an A.

What we discover when we trace this through the standard, unfortunately, is that the constructor is favored over the conversion function. The definition of direct-initialization (the parenthesized form) of a class considers only constructors of that class. In this case, the constructors are the A(B) constructor and the (implicitly-generated) A(const A&) copy constructor. Here's how they are ranked on the argument match:

A(B) exact match (need a B, have a B)
A(const A&) user-defined conversion (B::operator A used to convert B to A)

In other words, the conversion function does get considered, but it's operating with, in effect, a handicap of one user defined conversion. To put that a different way, this problem is a problem of weighting, not a problem that certain conversion paths are not considered.

I believe the reason that the standard's approach doesn't yield the "intuitive" result is that programmers expect copy constructor elision to be done whenever reasonable, so the intuitive cost of using the conversion function in the example above is simply the cost of the conversion function, not the cost of the conversion function plus the cost of the copy constructor (which is what the standard counts).

Suggested resolution:

In a direct-initialization overload resolution case, if the candidate function being called is a copy constructor and its argument (after any implicit conversions) is a temporary that is the return value of a conversion function, and the temporary can be optimized away, the cost of the argument match for the copy constructor should be considered to be the cost of the argument match on the conversion function argument.

Notes from 10/01 meeting:

It turns out that there is existing practice both ways on this issue, so it's not clear that it is "broken". There is some reason to feel that something that looks like a "constructor call" should call a constructor if possible, rather than a conversion function. The CWG decided to leave it alone.

61. Address of static member function "&p->f"

Section: 13.4  over.over     Status: NAD     Submitter: Steve Adamczyk     Date: 13 Oct 1998

Can p->f, where f refers to a set of overloaded functions all of which are static member functions, be used as an expression in an address-of-overloaded-function context? A strict reading of this section suggests "no", because "p->f" is not the name of an overloaded function (it's an expression). I'm happy with that, but the core group should decide and should add an example to document the decision, whichever way it goes.

Rationale (10/99): The "strict reading" correctly reflects the intent of the Committee, for the reason given, and no clarification is required.

247. Pointer-to-member casts and function overload resolution

Section: 13.4  over.over     Status: NAD     Submitter: Martin Sebor     Date: 22 Sep 2000

It is unclear whether the following code is well-formed or not:

    class A { }; 

    struct B : public A
	void foo (); 
	void foo (int);

    int main ()
	void (A::*f)() = (void (A::*)())&B::foo;

13.4  over.over paragraph 1 says,

The function selected is the one whose type matches the target type required in the context. The target can be

This would appear to make the program ill-formed, since the type in the cast is different from that of either interpretation of the address-of-member expression (its class is A, while the class of the address-of-member expression is B)

However, 13.4  over.over paragraph 3 says,

Nonstatic member functions match targets of type "pointer-to-member-function;" the function type of the pointer to member is used to select the member function from the set of overloaded member functions.

The class of which a function is a member is not part of the "function type" (8.3.5  dcl.fct paragraph 4). Paragraph 4 is thus either a misuse of the phrase "function type," a contradiction of paragraph 1, or an explanation of what "matching the target type" means in the context of a pointer-to-member target. By the latter interpretation, the example is well-formed and B::foo() is selected.

Bill Gibbons: I think this is an accident due to vague wording. I think the intent was

The function selected is the one which will make the effect of the cast be that of the identity conversion.

Mike Miller: The "identity conversion" reading seems to me to be overly restrictive. It would lead to the following:

    struct B {
        void f();
        void f(int);
    struct D: B { };
    void (D::* p1)() = &D::f;                  // ill-formed
    void (D::* p2)() = (void (B::*)()) &D::f;  // okay

I would find the need for an explicit cast here surprising, since the downcast is a standard conversion and since the declaration of p1 certainly has enough information to disambiguate the overload set. (See also issue 203.)

Bill Gibbons: There is an interesting situation with using-declarations. If a base class member function is present in the overload set in a derived class due to a using-declaration, it is treated as if it were a derived class member function for purposes of overload resolution in a call (13.3.1  over.match.funcs paragraph 4):

For non-conversion functions introduced by a using-declaration into a derived class, the function is considered to be a member of the derived class for the purpose of defining the type of the implicit object parameter

There is no corresponding rule for casts. Such a rule would be practical, but if the base class member function were selected it would not have the same class as that specified in the cast. Since base-to-derived pointer to member conversions are implicit conversions, it would seem reasonable to perform this conversion implicitly in this case, so that the result of the cast has the right type. The usual ambiguity and access restrictions on the base-to-derived conversion would not apply since they do not apply to calling through the using-declaration either.

On the other hand, if there is no special case for this, we end up with the bizarre case:

    struct A {
        void foo();
    struct B : A {
        using A::foo;
        void foo(int);
    int main() {
        // Works because "B::foo" contains A::foo() in its overload set.
        (void (A::*)())&B::foo;
        // Does not work because "B::foo(int)" does not match the cast.
        (void (A::*)(int))&B::foo;

I think the standard should be clarified by either:

Rationale (10/00): The cited example is well-formed. The function type, ignoring the class specification, is used to select the matching function from the overload set as specified in 13.4  over.over paragraph 3. If the target type is supplied by an explicit cast, as here, the conversion is then performed on the selected pointer-to-member value, with the usual restrictions on what can and cannot be done with the converted value (5.2.9  expr.static.cast paragraph 9, 5.2.10  expr.reinterpret.cast paragraph 9).

27. Overload ambiguities for builtin ?: prototypes

Section: 13.6  over.built     Status: NAD     Submitter: Jason Merrill     Date: 25 Sep 1997

I understand that the lvalue-to-rvalue conversion was removed in London. I generally agree with this, but it means that ?: needs to be fixed:


    bool test;
    Integer a, b;
    test ? a : b;
What builtin do we use? The candidates are
    operator ?:(bool, const Integer &, const Integer &)
    operator ?:(bool, Integer, Integer)
which are both perfect matches.

(Not a problem in FDIS, but misleading.)

Rationale: The description of the conditional operator in 5.16  expr.cond handles the lvalue case before the prototype is considered.

552. Use of typename in the type in a non-type parameter-declaration

Section: 14.2  temp.names     Status: NAD     Submitter: Andre Morozov     Date: 15 December 2005

The discussion in of the use of typename with a qualified-id in a template parameter-declaration in 14.2  temp.names paragraph 2 is confusing:

typename followed by an unqualified-id names a template type parameter. typename followed by a qualified-id denotes the type in a non-type parameter-declaration.

This rule would be clearer if the unqualified-id case were described in terms of resolving the ambiguity of declaring a template parameter name versus referring to a type-name from the enclosing scope, and if the qualified-id case referred to the use of the typename keyword with dependent types in 14.6  temp.res. An example would also be helpful.

Rationale (April, 2006):

The CWG felt that the wording was already clear enough.

114. Virtual overriding by template member function specializations

Section: 14.5.2  temp.mem     Status: NAD     Submitter: Bill Gibbons     Date: 7 May 1999

According to 14.5.2  temp.mem paragraph 4,

A specialization of a member function template does not override a virtual function from a base class.
Bill Gibbons: I think that's sufficiently surprising behavior that it should be ill-formed instead.

As I recall, the main reason why a member function template cannot be virtual is that you can't easily construct reasonable vtables for an infinite set of functions. That doesn't apply to overrides.

Another problem is that you don't know that a specialization overrides until the specialization exists:

    struct A {
        virtual void f(int);
    struct B : A {
        template<class T> void f(T);  // does this override?
But this could be handled by saying: The last case might only involve non-deducible contexts, e.g.
    template<int I> struct X;
    struct A {
        virtual void f(A<5>);
    struct B : A {
        template<int I, int J> void f(A<I+J>);  // does not overrride

    void g(B *b) {
        X<t> x;
        b->f<3,2>(x);  // specialization B::f(A<5>) makes program ill-formed
So I think there are reasonable semantics. But is it useful?

If not, I think the creation of a specialization that would have been an override had it been declared in the class should be an error.

Daveed Vandevoorde: There is real code out there that is written with this rule in mind. Changing the standard on them would not be good form, IMO.

Mike Ball: Also, if you allow template functions to be specialized outside of the class you introduce yet another non-obvious ordering constraint.

Please don't make such a change after the fact.

John Spicer: This is the result of an explicit committee decision. The reason for this rule is that it is too easy to unwittingly override a function from a base class, which was probably not what was intended when the template was written. Overriding should be a conscious decision by the class writer, not something done accidentally by a template.

Rationale (10/99): The Standard correctly reflects the intent of the Committee.

Notes from October 2002 meeting:

This was reopened because of a discussion while reviewing possible extensions.

Notes from April 2003 meeting:

This was discussed again, and the consensus was that we did not want to make a change, and in particular we did not want to make it an error and risk breaking existing code.

47. Template friend issues

Section: 14.5.3  temp.friend     Status: NAD     Submitter: John H. Spicer     Date: 7 Nov 1997

Issue 1

Paragraph 1 says that a friend of a class template can be a template. Paragraph 2 says: A friend template may be declared within a non-template class. A friend function template may be defined within a non-template class.

I'm not sure what this wording implies about friend template definitions within template classes. The rules for class templates and normal classes should be the same: a function template can be declared or defined, but a class template can only be declared in a friend declaration.

Issue 2

Paragraph 4 says: When a function is defined in a friend function declaration in a class template, the function is defined when the class template is first instantiated. I take it that this was intended to mean that a function that is defined in a class template is not defined until the first instantiation. I think this should say that a function that is defined in a class template is defined each time the class is instantiated. This means that a function that is defined in a class template must depend on all of the template parameters of the class template, otherwise multiple definition errors could occur during instantiations. If we don't have a rule like this, compilers would have to compare the definitions of functions to see whether they are the same or not. For example:

    template <class T> struct A {
            friend int f() { return sizeof(T); }
    A<int> ai;
    A<long> ac;
I hope we would all agree that this program is ill-formed, even if long and int have the same size.

From Bill Gibbons:

[1] That sounds right.

[2] Whenever possible, I try to treat instantiated class templates as if they were ordinary classes with funny names. If you write:

    struct A_int {
        friend int f() { return sizeof(int); }
    struct A_long {
        friend int f() { return sizeof(long); }
it is a redefinition (which is not allowed) and an ODR violation. And if you write:
    template <class T, class U> struct A {
                friend int f() { return sizeof(U); }
    A<int,float> ai;
    A<long,float> ac;
the corresponding non-template code would be:
    struct A_int_float {
        friend int f() { return sizeof(float); }
    struct A_long_float {
        friend int f() { return sizeof(float); }
then the two definitions of "f" are identical so there is no ODR violation, but it is still a redefinition. I think this is just an editorial clarification.

Rationale (04/99): The first sub-issue reflects wording that was changed to address the concern before the IS was issued. A close and careful reading of the Standard already leads to the conclusion that the example in the second sub-issue is ill-formed, so no change is needed.

316. Injected-class-name of template used as template template parameter

Section: 14.6.1  temp.local     Status: NAD     Submitter: Jason Merrill     Date: 14 Oct 2001

A gcc hacker recently sent in a patch to make the compiler give an error on code like this:

  template <template <typename> class T> struct A { };

  template <typename U> struct B
    A<B> *p;
presumably because the DR from issue 176 says that we decide whether or not B is to be treated as a template depending on whether a template-argument-list is supplied. I think this is a drafting oversight, and that B should also be treated as a template when passed as a template template parameter. The discussion in the issue list only talks about making the name usable both as a class and as a template.

John Spicer: This case was explicitly discussed and it was agreed that to use the injected name as a template template parameter you need to use the non-injected name.

A (possibly unstated) rule that I've understood about template arguments is that the form of the argument (type/nontype/template) is based only on the argument and not on the kind of template parameter. An example is that "int()" is always "function taking no arguments returning int" and never a convoluted way of saying zero.

In a similar way, we now decide whether or not something is a template based only on the form of the argument.

I think this rule is important for two kinds of cases. The first case involves explicit arguments of function templates:

  template <template <typename> class T> void f(){} // #1
  template <class T> void f(){}  // #2

  template <typename U> struct B {
	void g() {

  int main() {
	B<int> b;

With the current rules, this uses B as a type argument to template #2.

The second case involves the use of a class template for which the template parameter list is unknown at the point where the argument list is scanned:

  template <class T> void f(){}

  template <typename U> struct B {
	void g() {
		f< U::template X<B> >();  // what is B?

  struct Z1 {
	template <class T> struct X {};

  struct Z2 {
	template <template <class> class T> struct X {};

  int main() {
	B<Z1> b1;

	B<Z2> b2;

If B could be used as a template name we would be unable to decide how to treat B at the point that it was scanned in the template argument list.

So, I think it is not an oversight and that it should be left the way it is.

Notes from the 4/02 meeting:

It was agreed that this is Not a Defect.

544. Base class lookup in explicit specialization

Section: 14.6.2  temp.dep     Status: NAD     Submitter: John Spicer     Date: 01 November 2005

There is some question as to whether 14.6.2  temp.dep paragraph 3 applies to the definition of an explicitly-specialized member of a class template:

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.

Consider an example like the following:

    template <class T>
    struct A {
     void foo() {}

    template <class T>
    struct B: A<T> {
     int bar();

    int foo() { return 0; }

    template <>
    int B<int>::bar() { return foo(); }

    int main() {
     return B<int>().bar();

Does foo in the definition of B<int>::bar() refer to ::foo() or to A<int>::foo()?

Rationale (April, 2006):

An explicitly-specialized member of a class template is not, in fact, a member of a class template but a member of a particular specialization of that template. The special treatment of lookup vis-a-vis dependent base classes in 14.6.2  temp.dep thus does not apply, and base class members are found by unqualified name lookup.

334. Is a comma-expression dependent if its first operand is?

Section:  temp.dep.expr     Status: NAD     Submitter: John Spicer     Date: 10 Jan 2002

Is the comma expression in the following dependent?

  template <class T> static void f(T)
  template <class T> void g(T)
    f((T::x, 0));
  struct A {
    static int x;
  void h()

According to the standard, it is, because  temp.dep.expr says that an expression is dependent if any of its sub-expressions is dependent, but there is a question about whether the language should say something different. The type and value of the expression are not really dependent, and similar cases (like casting T::x to int) are not dependent.

Mark Mitchell: If the first operand is dependent, how do we know it does not have an overloaded comma operator?

Rationale (October, 2004):

CWG agreed that such comma expressions are and ought to be dependent, for the reason expressed in Mark Mitchell's comment.

34. Argument dependent lookup and points of instantiation

Section: 14.7.1  temp.inst     Status: NAD     Submitter: Daveed Vandevoorde     Date: 15 Jul 1998

Does Koenig lookup create a point of instantiation for class types? I.e., if I say:

    TT<int> *p;
The namespaces and classes associated with p are those associated with the type pointed to, i.e., TT<int>. However, to determine those I need to know TT<int> bases and its friends, which requires instantiation.

Or should this be special cased for templates?

Rationale: The standard already specifies that this creates a point of instantiation.

489. Must member function templates be instantiated during overload resolution?

Section: 14.7.1  temp.inst     Status: NAD     Submitter: Daveed Vandevoorde     Date: 24 Nov 2004

A related question to that raised in issue 488 is whether member function templates must be instantiated if the compiler can determine that they will not be needed by the function selected by overload resolution. That is explicitly specified for class templates in 14.7.1  temp.inst paragraph 5:

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.

Should the same be true for member function templates? In the example from issue 488,

    struct S {
        template <typename T> S(const T&);
    void f(const S&);
    void f(int);
    void g() {
        enum E { e };
        f(e);    // ill-formed?

a compiler could conceivably determine that f(int) would be selected by overload resolution (because it involves only an integral promotion, while the alternative requires a user-defined conversion) without instantiating the declaration of the S constructor. Should the compiler have that freedom?

Rationale (April, 2005):

In order for this question to come up, there would need to be a “gap” between the the normal rules and the rules for template argument deduction failure. The resolution for issue 488 will close the only such gap of which the CWG is aware. The issue can be reopened if other such cases turn up.

46. Explicit instantiation of member templates

Section: 14.7.2  temp.explicit     Status: NAD     Submitter: John H. Spicer     Date: 28 Jan 1998

Is the second explicit instantiation below well-formed?

    template <class T> struct A {
        template <class T2> void f(T2){}

    template void A<int>::f(char); // okay
    template template void A<int>::f(float); // ?
Since multiple "template<>" clauses are permitted in an explicit specialization, it might follow that multiple "template" keywords should also be permitted in an explicit instantiation. Are multiple "template" keywords not allowed in an explicit instantiation? The grammar permits it, but the grammar permits lots of stuff far weirder than that. My opinion is that, in the absence of explicit wording permitting that kind of usage (as is present for explicit specializations) that such usage is not permitted for explicit instantiations.

Rationale (04/99): The Standard does not describe the meaining of multiple template keywords in this context, so the example should be considered as resulting in undefined behavior according to 1.3.12  defns.undefined .

3. The template compilation model rules render some explicit specialization declarations not visible during instantiation

Section: 14.7.3  temp.expl.spec     Status: NAD     Submitter: Bill Gibbons     Date: unknown

[N1065 issue 1.19] An explicit specialization declaration may not be visible during instantiation under the template compilation model rules, even though its existence must be known to perform the instantiation correctly. For example:

translation unit #1

      template<class T> struct A { };
      export template<class T> void f(T) { A<T> a; }
translation unit #2
      template<class T> struct A { };
      template<> struct A<int> { }; // not visible during instantiation
      template<class T> void f(T);
      void g() { f(1); }
Rationale: This issue was addressed in the FDIS and should have been closed.

88. Specialization of member constant templates

Section: 14.7.3  temp.expl.spec     Status: NAD     Submitter: Jason Merrill     Date: 20 Jan 1999

Is this valid C++? The question is whether a member constant can be specialized. My inclination is to say no.

    template <class T> struct A {
        static const T i = 0;

    template<> const int A<int>::i = 42;
    int main () {
        return A<int>::i;
John Spicer: This is ill-formed because 9.4.2 paragraph 4 prohibits an initializer on a definition of a static data member for which an initializer was provided in the class.

The program would be valid if the initializer were removed from the specialization.

Daveed Vandevoorde: Or at least, the specialized member should not be allowed in constant-expressions.

Bill Gibbons: Alternatively, the use of a member constant within the definition could be treated the same as the use of "sizeof(member class)". For example:

    template <class T> struct A {
        static const T i = 1;
        struct B { char b[100]; };
        char x[sizeof(B)];     // specialization can affect array size
        char y[i];             // specialization can affect array size 

    template<> const int A<int>::i = 42;
    template<> struct A<int>::B { char z[200] };

    int main () {
        A<int> a;
        return sizeof(a.x)   // 200  (unspecialized value is 100)
             + sizeof(a.y);  // 42   (unspecialized value is 1)
For the member template case, the array size "sizeof(B)" cannot be evaluated until the template is instantiated because B might be specialized. Similarly, the array size "i" cannot be evaluated until the template is instantiated.

Rationale (10/99): The Standard is already sufficiently clear on this question.

182. Access checking on explicit specializations

Section: 14.7.3  temp.expl.spec     Status: NAD     Submitter: John Spicer     Date: 8 Nov 1999

John Spicer: Certain access checks are suppressed on explicit instantiations. 14.7.2  temp.explicit paragraph 8 says:

The usual access checking rules do not apply to names used to specify explicit instantiations. [Note: In particular, the template arguments and names used in the function declarator (including parameter types, return types and exception specifications) may be private types or objects which would normally not be accessible and the template may be a member template or member function which would not normally be accessible. ]
I was surprised that similar wording does not exist (that I could find) for explicit specializations. I believe that the two cases should be handled equivalently in the example below (i.e., that the specialization should be permitted).
    template <class T> struct C {
    void f();
    void g();

    template <class T> void C<T>::f(){}
    template <class T> void C<T>::g(){}

    class A {
    class B {};
    void f();

    template void C<A::B>::f();    // okay
    template <> void C<A::B>::g(); // error - A::B inaccessible

    void A::f() {
    C<B> cb;

Mike Miller: According to the note in 14.3  temp.arg paragraph 3,

if the name of a template-argument is accessible at the point where it is used as a template-argument, there is no further access restriction in the resulting instantiation where the corresponding template-parameter name is used.

(Is this specified anywhere in the normative text? Should it be?)

In the absence of text to the contrary, this blanket permission apparently applies to explicitly-specialized templates as well as to implicitly-generated ones (is that right?). If so, I don't see any reason that an explicit instantiation should be treated differently from an explicit specialization, even though the latter involves new program text and the former is just a placement instruction to the implementation.

Proposed Resolution (4/02):

In 14.7.2  temp.explicit delete paragraph 8:

The usual access checking rules do not apply to names used to specify explicit instantiations. [Note: In particular, the template arguments and names used in the function declarator (including parameter types, return types and exception specifications) may be private types or objects which would normally not be accessible and the template may be a member template or member function which would not normally be accessible. ]

In 14.7  temp.spec add the paragraph deleted above as paragraph 7 with the changes highlighted below:

The usual access checking rules do not apply to names used to specify explicit instantiations or explicit specializations. [Note: In particular, tThe template arguments and names used in the function declarator (including parameter types, return types and exception specifications) may be private types or objects which would normally not be accessible and the template may be a member template or member function which would not normally be accessible. ]

Rationale (October 2002):

We reconsidered this and decided that the difference between the two cases (explicit specialization and explicit instantiation) is appropriate. The access rules are sometimes bent when necessary to allow naming something, as in an explicit instantiation, but explicit specialization requires not only naming the entity but also providing a definition somewhere.

285. Identifying a function template being specialized

Section: 14.7.3  temp.expl.spec     Status: NAD     Submitter: Erwin Unruh     Date: 01 May 2001

The Standard does not describe how to handle an example like the following:

    template <class T> int f(T, int);
    template <class T> int f(int, T);

    template<> int f<int>(int, int) { /*...*/ }

It is impossible to determine which of the function templates is being specialized. This problem is related to the discussion of issue 229, in which one of the objections raised against partial specialization of function templates is that it is not possible to determine which template is being specialized.

Notes from 10/01 meeting:

It was decided that while this is true, it's not a problem. You can't call such functions anyway; the call would be ambiguous.

99. Partial ordering, references and cv-qualifiers

Section:     Status: NAD     Submitter: Mike Miller     Date: 5 Mar 1999


    template <class T> void f(T&);
    template <class T> void f(const T&);
    void m() {
        const int p = 0;
Some compilers treat this as ambiguous; others prefer f(const T&). The question turns out to revolve around whether paragraph 2 says what it ought to regarding the removal of cv-qualifiers and reference modifiers from template function parameters in doing type deduction.

John Spicer: The partial ordering rules as originally proposed specified that, for purposes of comparing parameter types, you remove a top level reference, and after having done that you remove top level qualifiers. This is not what is actually in the IS however. The IS says that you remove top level qualifiers and then top level references.

The original rules were intended to prefer f(A<T>) over f(const T&).

Rationale (10/99): The Standard correctly reflects the intent of the Committee.

(October 2002) This is resolved by issue 214.

211. Constructors should not be allowed to return normally after an exception

Section: 15  except     Status: NAD     Submitter: Bruce Mellows     Date: 8 Mar 2000

The grammar should be changed so that constructor function-try-blocks must end with a throw-expression.

Rationale (04/00):

No change is needed. It is already the case that flowing off the end of a function-try-block of a constructor rethrows the exception, and return statements are prohibited in constructor function-try-blocks (15.3  except.handle paragraphs 15-16.

104. Destroying the exception temp when no handler is found

Section: 15.1  except.throw     Status: NAD     Submitter: Jonathan Schilling     Date: 21 Mar 1999

Questions regarding when a throw-expression temporary object is destroyed.

Section 15.1  except.throw paragraph 4 describes when the temporary is destroyed when a handler is found. But what if no handler is found:

    struct A {
        A() { printf ("A() \n"); }
        A(const A&) { printf ("A(const A&)\n"); }
        ~A() { printf ("~A() \n"); }

    void t() { exit(0); }

    int main() {
        throw A();
Does A::~A() ever execute here? (Or, in case two constructions are done, are there two destructions done?) Is it implementation-defined, analogously to whether the stack is unwound before terminate() is called (15.3  except.handle paragraph 9)?

Or what if an exception specification is violated? There are several different scenarios here:

    int glob = 0; // or 1 or 2 or 3

    struct A {
        A() { printf ("A() \n"); }
        A(const A&) { printf ("A(const A&)\n"); }
        ~A() { printf ("~A() \n"); }

    void u() {
        switch (glob) {
        case 0:  exit(0);
        case 1:  throw "ok";
        case 2:  throw 17;
        default: throw;

    void foo() throw(const char*, std::bad_exception) {
        throw A();

    int main() {
        try {
        catch (const char*) { printf("in handler 1\n"); }
        catch (std::bad_exception) { printf("in handler 2\n"); }
The case where u() exits is presumably similar to the terminate() case. But in the cases where the program goes on, A::~A() should be called for the thrown object at some point. But where does this happen? The standard doesn't really say. Since an exception is defined to be "finished" when the unexpected() function exits, it seems to me that is where A::~A() should be called — in this case, as the throws of new (or what will become new) exceptions are made out of u(). Does this make sense?

Rationale (10/99): Neither std::exit(int) nor std::abort() destroys temporary objects, so the exception temporary is not destroyed when no handler is found. The original exception object is destroyed when it is replaced by an unexpected() handler. The Standard is sufficiently clear on these points.

308. Catching exceptions with ambiguous base classes

Section: 15.3  except.handle     Status: NAD     Submitter: Sergey P. Derevyago     Date: 04 Sep 2001

15.3  except.handle paragraph 3 contains the following text:

A handler is a match for a throw-expression with an object of type E if

I propose to alter this text to allow to catch exceptions with ambiguous public base classes by some of the public subobjects. I'm really sure that if someone writes:

    try {
        // ...
    catch (Matherr& m) {
        // ...
he really wants to catch all Matherrs rather than to allow some of the Matherrs to escape:
    class SomeMatherr : public Matherr { /* */ };
    struct TrickyBase1 : public SomeMatherr { /* */ };
    struct TrickyBase2 : public SomeMatherr { /* */ };
    struct TrickyMatherr : public TrickyBase1, TrickyBase2 { /* */ };

According to the standard TrickyMatherr will leak through the catch (Matherr& m) clause. For example:

    #include <stdio.h>

    struct B {};
    struct B1 : B {};
    struct B2 : B {};
    struct D : B1, B2 {};  // D() has two B() subobjects

    void f() { throw D(); }

    int main()
     try { f(); }
     catch (B& b) { puts("B&"); }  // passed
     catch (D& d) { puts("D&"); }  // really works _after_ B&!!!

Also I see one more possible solution: to forbid objects with ambiguous base classes to be "exceptional objects" (for example Borland C++ goes this way) but it seems to be unnecessary restrictive.

Notes from the 10/01 meeting:

The Core Working Group did not feel this was a significant problem. Catching either of the ambiguous base classes would be surprising, and giving an error on throwing an object that has an ambiguous base class would break existing code.

346. Typo in 15.4

Section: 15.4  except.spec     Status: NAD     Submitter: Lois Goldthwaite     Date: 18 March 2002

15.4  except.spec paragraph 13 contains the following text. I believe 'implicitLY' marked below should be replaced with 'implicit.'

An implicitly declared special member function (clause 12  special) shall have an exception-specification. If f is an implicitly declared default constructor, copy constructor, destructor, or copy 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 implicitly 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. [Example:

    struct A {
        A(const A&) throw();
        ~A() throw(X);
    struct B {
        B() throw();
        B(const B&) throw();
        ~B() throw(Y);
    struct D : public A, public B {
            //  Implicit declaration of  D::D();
            //  Implicit declaration of  D::D(const   D&)   throw();
            //  Implicit declaration of  D::~D()   throw   (X,Y);

Furthermore, if A::~A() or B::~B() were virtual, D::~D() would not be as restrictive as that of A::~A, and the program would be ill-formed since a function that overrides a virtual function from a base class shall have an exception-specification at least as restrictive as that in the base class. ]

The example code shows structs whose destructors have exception specifications which throw certain types. There is no defect here, but it doesn't sit well with our general advice elsewhere that destructors should not throw. I wish I could think of some other way to illustrate this section.

Notes from October 2002 meeting:

This was previously resolved by an editorial change.

37. When is uncaught_exception() true?

Section: 15.5.3  except.uncaught     Status: NAD     Submitter: Daveed Vandevoorde     Date: 10 Aug 1998

The term "throw exception" seems to sometimes refer to an expression of the form "throw expr" and sometimes just to the "expr" portion thereof.

As a result it is not quite clear to me whether when "uncaught_exception()" becomes true: before or after the temporary copy of the value of "expr".

Is there a definite consensus about that?

Rationale: The standard is sufficiently clear; the phrase "to be thrown" indicates that the throw itself (which includes the copy to the temporary object) has not yet begun. The footnote in 15.5.1  except.terminate paragraph 1 reinforces this ordering.

See also issue 475.

266. No grammar sentence symbol

Section: gram     Status: NAD     Submitter: Hans Aberg     Date: 2 Dec 2000

The grammar in Appendix A does not indicate a grammar sentence symbol and is therefore formally not a grammar.

Rationale (04/01):

Appendix A does not claim to be a formal grammar. The specification is clear enough in its current formulation.

81. Null pointers and C compatability

Section: diff     Status: NAD     Submitter: Steve Clamage     Date: 27 Oct 1998

Annex C lists C compatibility issues. One item not in the annex came up in a discussion in comp.std.c++.

Consider this C and C++ code:

    const int j = 0;
    char* p = (char*)j;

Rationale (10/99): Because j is not a constant expression in C, this code fragment has implementation-defined behavior in C. There is no incompatibility with C resulting from the fact that C++ defines this behavior.

167. Deprecating static functions

Section: D.2  depr.static     Status: NAD     Submitter: Darin Adler     Date: 8 Sep 1999

D.2  depr.static says that declaring namespace-scope objects as static is deprecated. Declaring namespace-scope functions as static should also be deprecated.

Proposed resolution (10/99): In both  namespace.unnamed paragraph 2 and D.2  depr.static paragraph 1, replace

when declaring objects in a namespace scope
when declaring entities in a namespace scope
In addition, there are a number of locations in the Standard where use of or reference to static should be reconsidered. These include:

Rationale (04/00):

This issue, along with issue 174, has been subsumed by issue 223. Until the committee determines the meaning of deprecation, it does not make sense either to extend or reduce the number of features to which it is applied.

174. Undeprecating global static

Section: D.2  depr.static     Status: NAD     Submitter: Lawrence Crowl     Date: 25 Oct 1999

The decision to deprecate global static should be reversed.

  1. We cannot deprecate static because it is an important part of C and to abandon it would make C++ unnecessarily incompatible with C.
  2. Because templates may be instantiated on members of unnamed namespaces, some compilation systems may place such symbols in the global linker space, which could place a significant burden on the linker. Without static, programmers have no mechanism to avoid the burden.

Rationale (04/00):

This issue, along with issue 167, has been subsumed by issue 223. Until the committee determines the meaning of deprecation, it does not make sense either to extend or reduce the number of features to which it is applied.

Issues with "Dup" Status

82. Definition of "using" a constant expression

Section: 3.2  basic.def.odr     Status: dup     Submitter: Bill Gibbons     Date: 31 Dec 1998

The wording in 3.2  basic.def.odr paragraph 2 about "potentially evaluated" is incomplete. It does not distinguish between expressions which are used as "integral constant expressions" and those which are not; nor does it distinguish between uses in which an objects address is taken and those in which it is not. (A suitable definition of "address taken" could be written without actually saying "address".)

Currently the definition of "use" has two parts (part (a) and (d) below); but in practice there are two more kinds of "use" as in (b) and (c):

  1. Use in "sizeof" or a non-polymorphic "typeid". Neither the value nor the address is really used. No definition is needed at all.
  2. Use as an integral constant expression. Only the value is used. A static data member with its initializer given in the class need not have a namespace-scope definition.
  3. Use which requires the value, which is known at compile time because the object is const, of integral or enum type, and initialized with an integral constant expression. Only the value need be used, but an implementation is not required to use the value from the initializer; it might access the object. So in the original example, the namespace-scope definition is required even though most compilers will not require it.
  4. All other uses require that the object actually exist because its address will be taken implicitly or explicitly.
We discussed (b) and decided that the namespace-scope definition was not needed, but the wording did not make it into the standard.

I don't think we discussed (c).

Rationale (04/99): The substantive part of this issue is covered by Core issue 48

12. Default arguments on different declarations for the same function and the Koenig lookup

Section: 3.4.2  basic.lookup.argdep     Status: dup     Submitter: Daveed Vandevoorde     Date: unknown

Given the following test case:

    enum E { e1, e2, e3 };
    void f(int, E e = e1);
    void f(E, E e = e1);
    void g() {
        void f(long, E e = e2);
        f(1); // calls ::f(int, E)
        f(e1); // ?

First note that Koenig lookup breaks the concept of hiding functions through local extern declarations as illustrated by the call `f(1)'. Should the WP show this as an example?

Second, it appears the WP is silent as to what happens with the call `f(e1)': do the different default arguments create an ambiguity? is the local choice preferred? or the global?

Tentative Resolution (10/98) In 3.4.2  basic.lookup.argdep paragraph 2, change

If the ordinary unqualified lookup of the name finds the declaration of a class member function, the associated namespaces and classes are not considered.
If the ordinary unqualified lookup of the name finds the declaration of a class member function or the declaration of a function at block scope, the associated namespaces and classes are not considered.

Rationale (04/99): The proposal would also apply to local using-declarations (per Mike Ball) and was therefore deemed undesirable. The ambiguity issue is dealt with in Core issue 1

313. Class with single conversion function to integral as array size in new

Section: 5.3.4     Status: dup     Submitter: Bill Gibbons     Date: 22 Oct 2001

Should it be allowed to use an object of a class type having a single conversion function to an integral type as an array size in the first bound of the type in an array new?

  struct A {
    operator int();
  } a;
  int main () {
    new int[a];

There are similar accommodations for the expression in a delete (5.3.5  expr.delete paragraph 1) and in a switch (6.4.2  stmt.switch paragraph 2). There is also widespread existing practice on this (g++, EDG, MSVC++, and Sun accept it, and even cfront 3.0.2).

Rationale (October, 2004):

Duplicate of issue 299.

548. qualified-ids in declarations

Section: 8.3  dcl.meaning     Status: dup     Submitter: Martin Sebor     Date: 18 November 2005

According to 8.3  dcl.meaning paragraph 1, the declarator in the definition or explicit instantiation of a namespace member can only be qualified if the definition or explicit instantiation appears outside the member's namespace:

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.

There is no similar restriction on a qualified-id in a class definition (9  class paragraph 5):

If a class-head contains a nested-name-specifier, the class-specifier shall refer to a class that was previously declared directly in the class or namespace to which the nested-name-specifier refers (i.e., neither inherited nor introduced by a using-declaration), and the class-specifier shall appear in a namespace enclosing the previous declaration.

An elaborated-type-specifier in an explicit instatiation containing a qualified-id is also not prohibited from appearing in the namespace nominated by its nested-name-specifier (14.7.2  temp.explicit paragraph 2):

An explicit instantiation shall appear in an enclosing namespace of its template. If the name declared in the explicit instantiation is an unqualified name, the explicit instantiation shall appear in the namespace where its template is declared.

(This asymmetry is due to the removal of inappropriate mention of classes in 8.3  dcl.meaning by issue 40 and a failure to insert the intended restrictions elsewhere.)

An example of this inconsistency is:

    namespace N {
      template <class T> struct S { };
      template <class T> void foo () { }

      template struct N::S<int>;   // OK
      template void N::foo<int>(); // ill-formed

It is not clear that any purpose is served by the “outside of its namespace” restriction on declarators in definitions and explicit instantiations; if possible, it would be desirable to reconcile the treatment of declarators and class names by removing the restriction on declarators (which appears to be widespread implementation practice, anyway).

Rationale (April, 2006):

This is the same as issue 482.

15. Default arguments for parameters of function templates

Section: 8.3.6  dcl.fct.default     Status: dup     Submitter: unknown     Date: unknown

8.3.6  dcl.fct.default paragraph 4 says:

For non-template functions, default arguments can be added in later declarations of a functions in the same scope.
Why say for non-template functions? Why couldn't the following allowed?
    template <class T> struct B {
        template <class U> inline void f(U);
    template <class T> template <class U>
    inline void B<T>::f(U = int) {} // adds default arguments
                                       // is this well-formed?
    void g()
        B<int> b;
If this is ill-formed, chapter 14 should mention this.

Rationale: This is sufficiently clear in the standard. Allowing additional default arguments would be an extension.

Notes from October 2002 meeting:

The example here is flawed. It's not clear what is being requested. One possibility is the extension introduced by issue 226. Other meanings don't seem to be useful.

72. Linkage and storage class specifiers for templates

Section: 14  temp     Status: dup     Submitter: Mike Ball     Date: 19 Oct 1998

John Spicer: The standard does say that a namespace scope template has external linkage unless it is a function template declared "static". It doesn't explicitly say that the linkage of the template is also the linkage of the instantiations, but I believe that is the intent. For example, a storage class is prohibited on an explicit specialization to ensure that a specialization cannot be given a different storage class than the template on which it is based.

Mike Ball: This makes sense, but I couldn't find much support in the document. Sounds like yet another interpretation to add to the list.

John Spicer: The standard does not talk about the linkage of instantiations, because only "names" are considered to have linkage, and instances are not really names. So, from an implementation point of view, instances have linkage, but from a language point of view, only the template from which the instances are generated has linkage.

Mike Ball: Which is why I think it would be cleaner to eliminate storage class specifiers entirely and rely on the unnamed namespace. There is a statement that specializations go into the namespace of the template. No big deal, it's not something it says, so we live with what's there.

John Spicer: That would mean prohibiting static function templates. I doubt those are common, but I don't really see much motivation for getting rid of them at this point.

"export" is an additional attribute that is separate from linkage, but that can only be applied to templates with external linkage.

Mike Ball: I can't find that restriction in the standard, though there is one that templates in an unnamed namespace can't be exported. I'm pretty sure that we intended it, though.

John Spicer: I can't find it either. The "inline" case seems to be addressed, but not static. Surely this is an error as, by definition, a static template can't be used from elsewhere.

Rationale: Duplicate of Core issue 69.

200. Partial ordering and explicit arguments

Section:  temp.func.order     Status: dup     Submitter: Martin Sebor     Date: 28 Jan 2000

The description of how the partial ordering of template functions is determined in  temp.func.order paragraphs 3-5 does not make any provision for nondeduced template parameters. For example, the function call in the following code is ambiguous, even though one template is "obviously" more specialized than the other:

    template <class T> T f(int);
    template <class T, class U> T f(U);
    void g() {
The reason is that neither function parameter list allows template parameter T to be deduced; both deductions fail, so neither template is considered more specialized than the other and the function call is ambiguous.

One possibility of addressing this situation would be to incorporate explicit template arguments from the call in the argument deduction using the transformed function parameter lists. In this case, that would result in finding the first template to be more specialized than the second.

Rationale (04/00):

This issue is covered in a more general context in issue 214.

375. Confusing example on lookup with typename

Section: 14.6  temp.res     Status: dup     Submitter: Manish Pagey     Date: 23 August 2002

Section 14.6  temp.res paragraph 4 uses the following example to show that qualified name lookup described in Section 3.4.3  basic.lookup.qual applies even in the presence of "typename":

  struct A {
    struct X { } ;
    int X ;
  } ;
  template<class T> void f(T t) {
    typename T::X x ; // ill-formed: finds the data member X
                      // not the member type X

This example is confusing because the definition of the template function itself is not ill formed unless it is instantiated with "A" as the template parameter. In other words, the example should be modified to something like:

  struct A {
    struct X { } ;
    int X ; 
  } ; 
  struct B {
    struct X { } ;
  } ;
  template<class T> void f(T t) {
    typename T::X x ; 
  void foo() {
    A a ; 
    B b ;
    f(b) ; // OK -- finds member type B::X.
    f(a) ; // ill-formed: finds the data member A::X not 
           // the member type A::X.

Notes from October 2002 meeting:

This is a duplicate of Core Issue 345.

133. Exception specifications and checking

Section: 15.4  except.spec     Status: dup     Submitter: Daveed Vandevoorde     Date: 25 June 1999

15.4  except.spec paragraph 1 says,

An exception-specification shall appear only on a function declarator in a function, pointer, reference or pointer to member declaration or definition.
This wording forbids exception specifications in declarations where they might plausibly occur (e.g., an array of function pointers). This restriction seems arbitrary. It's also unclear whether this wording allows declarations such as
    void (*f())() throw(int);  // returns a pointer to a function
                               // that might throw "int"

At the same time, other cases are allowed by the wording in paragraph 1 (e.g., a pointer to a pointer to a function), but no checking for such cases is specified in paragraph 3. For example, the following appears to be allowed:

    void (*p)() throw(int);
    void (**pp)() throw() = &p;

Rationale (10/99): Duplicate of issues 87 and 92.

79. Alignment and placement new

Section:     Status: dup     Submitter: Herb Sutter     Date: 15 Dec 1998

The example in reads:

[Example: This can be useful for constructing an object at a known address:
    char place[sizeof(Something)];
    Something* p = new (place) Something();
end example]
This example has potential alignment problems. One way to correct it would be to change the definition of place to read:
    char* place = new char[sizeof(Something)];

Rationale (10/99): This is an issue for the Library Working Group.

Issues with "Extension" Status

220. All deallocation functions should be required not to throw

Section:     Status: extension     Submitter: Herb Sutter     Date: 31 Mar 2000

[Picked up by evolution group at October 2002 meeting.]

The default global operators delete are specified to not throw, but there is no requirement that replacement global, or class-specific, operators delete must not throw. That ought to be required.

In particular:

We already require that all versions of an allocator's deallocate() must not throw, so that part is okay.

Rationale (04/00):

  1. Replacement deallocation functions are already required not to throw an exception (cf  lib.res.on.functions paragraph 2, as applied to paragraph 12 and paragraph 11).
  2. Section  lib.replacement.functions is describing the signatures of the functions to be replaced; exception specfications are not part of the signature.
  3. There does not appear to be any pressing need to require that class member deallocation functions not throw.

294. Can static_cast drop exception specifications?

Section: 5.2.9  expr.static.cast     Status: extension     Submitter: Steve Adamczyk     Date: 27 Jun 2001

[Picked up by evolution group at October 2002 meeting.]

Is it okay for a static_cast to drop exception specifications?

    void f() throw(int);
    int main () {
      static_cast<void (*)() throw()>(f);  // Okay?
      void (*p)() throw() = f;  // Error

The fact that a static_cast is defined, more or less, as an initialization suggests that a check ought to be made.

One tricky point: this is another case where the general rule that the reverse of an implicit cast is allowed as a static_cast bites you -- the reverse conversion doesn't drop exception specifications, and so is okay. Perhaps this should be treated like casting away constness.

Mike Miller comments : I don't think that case can arise. According to 15.4  except.spec,

An exception-specification shall appear only on a function declarator in a function, pointer, reference, or pointer to member declaration or definition.

We strengthened that in issue 87 (voted to DR status in Copenhagen) to

An exception-specification shall appear only on a function declarator for a function type, pointer to function type, reference to function type, or pointer to member function type that is the top-level type of a declaration or definition, or on such a type appearing as a parameter or return type in a function declarator.

As I read that, you can't put an exception-specification on the type-id in a static_cast, which means that a static_cast can only weaken, not strengthen, the exception specification.

The core WG discussed this at the 10/01 meeting and agreed.

256. Overflow in size calculations

Section: 5.3.4     Status: extension     Submitter: James Kanze     Date: 15 Oct 2000

[Picked up by evolution group at October 2002 meeting.]

(See also issue 476.)

The size requested by an array allocation is computed by multiplying the number of elements requested by the size of each element and adding an implementation-specific amount for overhead. It is possible for this calculation to overflow. Is an implementation required to detect this situation and, for instance, throw std::bad_alloc?

On one hand, the maximum allocation size is one of the implementation limits specifically mentioned in Annex B  limits, and, according to 1.4  intro.compliance paragraph 2, an implementation is only required to "accept and correctly execute" programs that do not violate its resource limits.

On the other hand, it is difficult or impossible for user code to detect such overflows in a portable fashion, especially given that the array allocation overhead is not fixed, and it would be a service to the user to handle this situation gracefully.

Rationale (04/01):

Each implementation is required to document the maximum size of an object (Annex B  limits). It is not difficult for a program to check array allocations to ensure that they are smaller than this quantity. Implementations can provide a mechanism in which users concerned with this problem can request extra checking before array allocations, just as some implementations provide checking for array index and pointer validity. However, it would not be appropriate to require this overhead for every array allocation in every program.

13. extern "C" for Parameters of Function Templates

Section: 7.5     Status: extension     Submitter: John Spicer     Date: unknown

[Picked up by evolution group at October 2002 meeting.]

How can we write a function template, or member function of a class template that takes a C linkage function as a parameter when the function type depends on one of the template parameter types?

    extern "C" void f(int);
    void g(char);

    template <class T> struct A {
        A(void (*fp)(T));

    A<char> a1(g);  // okay
    A<int> a2(f);   // error
Another variant of the same problem is:
    extern "C" void f(int);
    void g(char);

    template <class T> void h( void (*fp)(T) );
    int main() {
        h(g);  // okay
        h(f);  // error

Suggested resolution: (John Spicer)

Somehow permit a language linkage to be specified as part of a function parameter declaration. i.e.

    template <class T> struct A {
        A( extern "C" void (*fp)(T) );

    template <class T> void h( extern "C" void (*fp)(T) );
Suggested resolution: (Bill Gibbons)

The whole area of linkage needs revisiting. Declaring calling convention as a storage class was incorrect to begin with; it should be a function qualifier, as in:

    void f( void (*pf)(int) c_linkage );
instead of the suggested:
    void f( extern "C" void (*pf)(int) );
I would like to keep calling convention on the "next round" issues list, including the alternative of using function qualifiers.

And to that end, I suggest that the use of linkage specifiers to specify calling convention be deprecated - which would make any use of linkage specifiers in a parameter declaration deprecated.

Martin Sebor: 7.5, paragraph 4 says that "A linkage-specification shall occur only in namespace scope..." I'm wondering why this restriction is necessary since it prevents, among other things, the use of the functions defined <cmath> in generic code that involves function objects. For example, the program below is ill-formed since std::pointer_to_binary_function<> takes a pointer to a function with extern "C++" linkage which is incompatible with the type of the double overload of std::pow.

Relaxing the restriction to allow linkage specification in declarations of typedefs in class scope would allow std::pointer_to_binary_function<> ctor to be overloaded on both types (i.e., extern "C" and extern "C++"). An alternative would be to allow for the linkage specification to be deduced along with the type.

    #include <cmath>
    #include <functional>
    #include <numeric>

    int main () {
      double a[] = { 1, 2, 3 };
      return std::accumulate (a, a + 3, 2.0,
        std::pointer_to_binary_function<double, double, double>(std::pow));

107. Linkage of operator functions

Section: 7.5     Status: extension     Submitter: Stephen Clamage     Date: 21 Apr 1999

[Picked up by evolution group at October 2002 meeting.]

Steve Clamage: I can't find anything in the standard that prohibits a language linkage on an operator function. For example:

extern "C" int operator+(MyInt, MyInt) { ... }

Clearly it is a bad idea, you could have only one operator+ with "C" linkage in the entire program, and you can't call the function from C code.

Mike Miller: Well, you can't name an operator function in C code, but if the arguments are compatible (e.g., not references), you can call it from C code via a pointer. In fact, because the language linkage is part of the function type, you couldn't pass the address of an operator function into C code unless you could declare the function to be extern "C".

Fergus Henderson: In the general case, for linkage to languages other than C, this could well make perfect sense.

Steve Clamage:

But is it disallowed (as opposed to being stupid), and if so, where in the standard does it say so?

Mike Miller: I don't believe there's a restriction. Whether that is because of the (rather feeble) justification of being able to call an operator from C code via a pointer, or whether it was simply overlooked, I don't know.

Fergus Henderson: I don't think it is disallowed. I also don't think there is any need to explicitly disallow it.

Steve Clamage: I don't think the standard is clear enough on this point. I'd like to see a clarification.

I think either of these two clarifications would be appropriate:

  1. A linkage specification on an operator function is ill-formed.
  2. A linkage specification on an operator function is well-formed, but the semantics (e.g. name mangling) are implementation-defined. In addition, the rule about multiple functions with the same name having "C" linkage applies.
  3.     extern "C" T operator+(T,T); // ok
        extern "C" T operator-(T,T); // ok
        extern "C" U operator-(U);   // error, two extern "C" operator- 

Mike Miller: I think the point here is that something like

    extern "xyzzy" bool operator<(S&,S&)
could well make sense, if language xyzzy is sufficiently compatible with C++, and the one-function rule only applies to extern "C", not to other language linkages. Given that it might make sense to have general language linkages for operators, is it worthwhile to make an exception to the general rule by saying that you can have any language linkage on an operator function except "C" linkage? I don't like exceptions to general rules unless they're very well motivated, and I don't see sufficient motivation to make one here.

Certainly this capability isn't very useful. There are lots of things in C++ that aren't very useful but just weren't worth special-casing out of the language. I think this falls into the same category.

Mike Ball: I DON'T want to forbid operator functions within an extern "C". Rather I want to add operator functions to that sentence in paragraph 4 of 7.5 which reads

A C language linkage is ignored for the names of class members and the member function type of class member functions.
My reason is simple: C linkage makes a total hash of scope. Any "C" functions declared with the same name in any namespace scope are the same function. In other words, namespaces are totally ignored.

This provision was added in toward the end of the standardization process, and was, I thought, primarily to make it possible to put the C library in namespace std. Otherwise, it seems an unwarrented attack on the very concept of scope. We (wisely) didn't force this on static member functions, since it would essentially promote them to the global scope.

Now I think that programmers think of operator functions as essentially part of a class. At least for one very common design pattern they are treated as part of the class interface. This pattern is the reason we invented Koenig lookup for operator functions.

What happens when such a class definition is included, deliberately or not, in an extern "C" declaration? The member operators continue to work, but the non-member operators can suddenly get strange and hard to understand messages. Quite possibly, they get the messages only when combined with other classes in other compilation units. You can argue that the programmer shouldn't put the class header in a linkage delaration in the first place, but I can still find books that recommend putting `extern "C"' around entire header files, so it's going to happen.

I think that including operator functions in the general exclusion from extern "C" doesn't remove a capability, rather it ensurs a capability that programmers already think they have.

Rationale (10/00):

The benefits of creating an exception for operator functions were outweighed by the complexity of adding another special case to the rules.

168. C linkage for static member functions

Section: 7.5     Status: extension     Submitter: Darin Adler     Date: 9 Sep 1999

[Picked up by evolution group at October 2002 meeting.]

7.5 paragraph 4 says,

A C language linkage is ignored for the names of class members and the member function types of class member functions.
This makes good sense, since C linkage names typically aren't compatible with the naming used for member functions at link time, nor is C language linkage function type necessarily compatible with the calling convention for passing this to a non-static member function.

But C language linkage type (not name) for a static member function is invaluable for a common programming idiom. When calling a C function that takes a pointer to a function, it's common to use a private static member function as a "trampoline" which retrieves an object reference (perhaps by casting) and then calls a non-static private member function. If a static member function can't have a type with C language linkage, then a global or friend function must be used instead. These alternatives expose more of a class's implementation than a static member function; either the friend function itself is visible at namespace scope alongside the class definition or the private member function must be made public so it can be called by a non-friend function.

Suggested Resolution: Change the sentence cited above to:

A C language linkage is ignored for the names of class members and the member function types of non-static class member functions.
The example need not be changed because it doesn't involve a static member function.

The following workaround accomplishes the goal of not exposing the class's implementation, but at the cost of significant superstructure and obfuscation:

    // foo.h
    extern "C" typedef int c_func(int);
    typedef int cpp_func(int);
    class foo
      c_func* GetCallback();
      static int Callback(int);

    // foo.cpp
    #include "foo.h"

    // A local pointer to the static member that will handle the callback.
    static cpp_func* cpp_callback=0;

    // The C function that will actually get registered.
    extern "C" int CFunk(int i)
      return cpp_callback(i);

    c_func* foo::GetCallback()
      cpp_callback = &Callback;    // Only needs to be done once.
      return &CFunk;

Rationale (10/99): The Standard correctly reflects the intent of the Committee.

359. Type definition in anonymous union

Section: 9.5  class.union     Status: extension     Submitter: Al Grant     Date: 31 May 2002

Is this legal? Should it be?

  struct E {
    union {
      struct { int x; } s;
    } v;

One compiler faults a type definition (i.e. of the anonymous struct) since it is in an anonymous union [9.5  class.union paragraph 2: "The member-specification of an anonymous union shall only define non-static data members."].

I would suggest that compiler B is correctly interpreting the standard but that this is a defect in the standard. There is no reason to disallow definition of anonymous structs.

Furthermore, is it really necessary to disallow definition of named types in anonymous unions in general, as long as the types do not need fully qualified names for external linkage? Why should this be illegal?

  struct E {
    union {
      typedef int Int;
      struct X { X *next; Int n; } list;
    } v;

Notes from October 2002 meeting:

There was agreement that the standard says such declarations are invalid; therefore this must be considered as an extension. There was general feeling that this extension would not be too useful, though Jason Merrill was sympathetic to the argument. It was also agreed that if this were to be changed it would require careful wording so as not to allow too many cases.

6. Should the optimization that allows a class object to alias another object also allow the case of a parameter in an inline function to alias its argument?

Section: 12.8  class.copy     Status: extension     Submitter: unknown     Date: unknown

[Picked up by evolution group at October 2002 meeting.]

(See also paper J16/99-0005 = WG21 N1182.)

At the London meeting, 12.8  class.copy paragraph 15 was changed to limit the optimization described to only the following cases:

One other case was deemed desirable as well: However, there are cases when this aliasing was deemed undesirable and, at the London meeting, the committee was not able to clearly delimit which cases should be allowed and which ones should be prohibited.

Can we find an appropriate description for the desired cases?

Rationale (04/99): The absence of this optimization does not constitute a defect in the Standard, although the proposed resolution in the paper should be considered when the Standard is revised.

150. Template template parameters and default arguments

Section: 14.3.3  temp.arg.template     Status: extension     Submitter: Mike Miller     Date: 3 Aug 1999

[Picked up by evolution group at October 2002 meeting.]

How are default template arguments handled with respect to template template parameters? Two separate questions have been raised:

  1. Do default template arguments allow a template argument to match a template parameter with fewer template parameters, and can the template template parameter be specialized using the smaller number of template arguments? For example,
        template <class T, class U = int>
        class ARG { };
        template <class X, template <class Y> class PARM>
        void f(PARM<X>) { }    // specialization permitted?
        void g() {
            ARG<int> x;        // actually ARG<int, int>
            f(x);              // does ARG (2 parms, 1 with default)
                               // match PARM (1 parm)?
    Template template parameters are deducible (  temp.deduct.type paragraph 9), but 14.3.3  temp.arg.template does not specify how matching is done.

    Jack Rouse: I implemented template template parameters assuming template signature matching is analogous to function type matching. This seems like the minimum reasonable implementation. The code in the example would not be accepted by this compiler. However, template default arguments are compile time entities so it seems reasonable to relax the matching rules to allow cases like the one in the example. But I would consider this to be an extension to the language.

    Herb Sutter: An open issue in the LWG is that the standard doesn't explicitly permit or forbid implementations' adding additional template-parameters to those specified by the standard, and the LWG may be leaning toward explicitly permitting this. [Under this interpretation,] if the standard is ever modified to allow additional template-parameters, then writing "a template that takes a standard library template as a template template parameter" won't be just ugly because you have to mention the defaulted parameters; it would not be (portably) possible at all except possibly by defining entire families of overloaded templates to account for all the possible numbers of parameters vector<> (or anything else) might actually have. That seems unfortunate.

  2. Are default arguments permitted in the template parameter list of a template template parameter? For example,
        template <template <class T, class U = int> class PARM>
        class C {
            PARM<int> pi;

    Jack Rouse: I decided they could not in the compiler I support. This continues the analogy with function type matching. Also, I did not see a strong need to allow default arguments in this context.

    A class template used as a template template argument can have default template arguments from its declarations. How are the two sources of default arguments to be reconciled? The default arguments from the template template formal could override. But it could be cofusing if a template-id using the argument template, ARG<int>, behaves differently from a template-id using the template formal name, FORMAL<int>.

Rationale (10/99): Template template parameters are intended to be handled analogously to function function parameters. Thus the number of parameters in a template template argument must match the number of parameters in a template template parameter, regardless of whether any of those paramaters have default arguments or not. Default arguments are allowed for the parameters of a template template parameter, and those default arguments alone will be considered in a specialization of the template template parameter within a template definition; any default arguments for the parameters of a template template argument are ignored.

Note (Mark Mitchell, February, 2006):

Perhaps it is already obvious to all, but it seems worth noting that this extension would change the meaning of conforming programs:

    struct Dense { static const unsigned int dim = 1; };

    template <template <typename> class View,
              typename Block>
    void operator+(float, View<Block> const&);

    template <typename Block,
              unsigned int Dim = Block::dim>
    struct Lvalue_proxy { operator float() const; };

    void test_1d (void) {
        Lvalue_proxy<Dense> p;
        float b;
        b + p;

If Lvalue_proxy is allowed to bind to View, then the template operator+ will be used to perform addition; otherwise, Lvalue_proxy's implicit conversion to float, followed by the built-in addition on floats will be used.

229. Partial specialization of function templates

Section: 14.5.4  temp.class.spec     Status: extension     Submitter: Dave Abrahams     Date: 1 Apr 2000

Library issue 225 poses the following questions:

  1. How can a 3rd party library implementor (lib1) write a version of a standard algorithm which is specialized to work with his own class template?
  2. How can another library implementor (lib2) write a generic algorithm which will take advantage of the specialized algorithm in lib1?

For example, a programmer might want to provide a version of std::swap that would be used for any specialization of a particular class template. It is possible to do that for specific types, but not for all specializations of a template.

The problem is due to the fact that programmers are forbidden to add overloads to namespace std, although specializations are permitted. One suggested solution would be to allow partial specialization of function templates, analogous to partial specialization of class templates.

Library issue 225 contains a detailed proposal for adding partial specialization of function templates (not reproduced here in the interest of space and avoiding multiple-copy problems). This Core issue is being opened to provide for discussion of the proposal within the core language working group.

Notes from 10/00 meeting:

A major concern over the idea of partial specialization of function templates is that function templates can be overloaded, unlike class templates. Simply naming the function template in the specialization, as is done for class specialization, is not adequate to identify the template being specialized.

In view of this problem, the library working group is exploring the other alternative, permitting overloads to be added to functions in namespace std, as long as certain restrictions (to be determined) are satisfied.

(See also documents N1295 and N1296 and issue 285.)

Notes from 10/01 meeting:

The Core Working Group decided to ask the Library Working Group for guidance on whether this feature is still needed to resolve a library issue. The answer at present is "we don't know."

Rationale (October, 2004):

The Core Working Group decided that the Evolution Working Group is the appropriate forum in which to explore the desirability and form of this feature.