| Document number: | J16/01-0055 = WG21 N1341 |
| Date: | 9 November, 2001 |
| Project: | Programming Language C++ |
| Reference: | ISO/IEC IS 14882:1998(E) |
| Reply to: | J. Stephen Adamczyk |
| jsa@edg.com |
This document contains the C++ core language issues on which the Committee (J16 + WG21) has not yet acted, that is, issues issues with status "Ready," "Review," "Drafting," and "Open."
This document is part of a group of related documents that together describe the issues that have been raised regarding the C++ Standard. The other documents in the group are:
The purpose of these documents is to record the disposition of issues which have come before the Core Language Working Group of the ANSI (J16) and ISO (WG21) C++ Standard Committee.
Issues represent potential defects in the ISO/IEC IS 14882:1998(E) document; they are not necessarily formal ISO Defect Reports (DRs). While some issues will eventually be elevated to DR status, others will be disposed of in other ways. (See Issue Status below.)
This set of documents exists in two slightly different versions. The Committee Version (this version) is the master version and should be made available only to committee members. The Public Version is extracted from the Committee Version by removal of sensitive and/or unnecessary information, such as drafting assignments, reflector message numbers, and the like.
Committee members with the HTML version of the IS may wish to place the issues list documents in the same directory; references to the IS in the issues can then be explored by hyperlink.
The most current public version of this document can be found at http://www.dkuug.dk/jtc1/sc22/wg21. Requests for further information about these documents should include the document number, reference ISO/IEC 14882:1998(E), and be submitted to the Information Technology Information Council (ITI), 1250 Eye Street NW, Washington, DC 20005, USA.
Information regarding how to obtain a copy of the C++ Standard, join the Standard Committee, or submit an issue can be found in the C++ FAQ at http://www.research.att.com/~austern/csc/faq.html . Public discussion of the C++ Standard and related issues occurs on newsgroup comp.std.c++.
Issues progress through various statuses as the Core Language Working Group and, ultimately, the full J16 and WG21 committees deliberate and act. For ease of reference, issues are grouped in these documents by their status. Issues have one of the following statuses:
Open: The issue is new or the working group has not yet formed an opinion on the issue. If a Suggested Resolution is given, it reflects the opinion of the issue's submitter, not necessarily that of the working group or the Committee as a whole.
Drafting: Informal consensus has been reached in the working group and is described in rough terms in a Tentative Resolution, although precise wording for the change is not yet available.
Review: Exact wording of a Proposed Resolution is now available for an issue on which the working group previously reached informal consensus.
Ready: The working group has reached consensus that the issue is a defect in the Standard, the Proposed Resolution is correct, and the issue is ready to forward to the full Committee for ratification as a proposed defect report.
DR: The full J16 Committee has approved the item as a proposed defect report. The Proposed Resolution in an issue with this status reflects the best judgment of the Committee at this time regarding the action that will be taken to remedy the defect; however, the current wording of the Standard remains in effect until such time as a Technical Corrigendum or a revision of the Standard is issued by ISO.
Dup: The issue is identical to or a subset of another issue, identified in a Rationale statement.
NAD: The working group has reached consensus that the issue is not a defect in the Standard. A Rationale statement describes the working group's reasoning.
Extension: The working group has reached consensus that the issue is not a defect in the Standard but is a request for an extension to the language. Under ISO rules, extensions cannot be considered for at least five years from the approval of the Standard, at which time the Standard will be open for review. The working group expresses no opinion on the merits of an issue with this status; however, the issue will be maintained on the list for possible future consideration when extension proposals will be in order.
From reflector message 8228.
Paragraphs 1 and 2 of 3.4.2 basic.lookup.koenig say, in part,
When an unqualified name is used as the postfix-expression in a function call (5.2.2 expr.call )... namespace-scope friend function declarations (11.4 class.friend ) not otherwise visible may be found... the set of declarations found by the lookup of the function name [includes] the set of declarations found in the... classes associated with the argument types.The most straightforward reading of this wording is that if a function of namespace scope (as opposed to a class member function) is declared as a friend in a class, and that class is an associated class in a function call, the friend function will be part of the overload set, even if it is not visible to normal lookup.
Consider the following example:
namespace A {
class S;
};
namespace B {
void f(A::S);
};
namespace A {
class S {
int i;
friend void B::f(S);
};
}
void g() {
A::S s;
f(s); // should find B::f(A::S)
}
This example would seem to satisfy the criteria from
3.4.2
basic.lookup.koenig
:
A::S is an associated class of the argument, and
A::S has a
friend declaration of the namespace-scope function
B::f(A::S),
so Koenig lookup should include B::f(A::S) as part of the
overload set in the call.
Another interpretation is that, instead of finding the friend declarations in associated classes, one only looks for namespace-scope functions, visible or invisible, in the namespaces of which the the associated classes are members; the only use of the friend declarations in the associated classes is to validate whether an invisible function declaration came from an associated class or not and thus whether it should be included in the overload set or not. By this interpretation, the call f(s) in the example will fail, because B::f(A::S) is not a member of namespace A and thus is not found by the lookup.
Notes from 10/99 meeting: The second interpretation is correct. The wording should be revised to make clear that Koenig lookup works by finding "invisible" declarations in namespace scope and not by finding friend declarations in associated classes.
Proposed resolution (04/01): The "associated classes" are handled adequately under this interpretation by 3.4.2 basic.lookup.koenig paragraph 3, which describes the lookup in the associated namespaces as including the friend declarations from the associated classes. Other mentions of the associated classes should be removed or qualified to avoid the impression that there is a lookup in those classes:
In 3.4.2 basic.lookup.koenig, change
When an unqualified name is used as the postfix-expression in a function call (5.2.2 expr.call), other namespaces not considered during the usual unqualified lookup (3.4.1 basic.lookup.unqual) may be searched, and namespace-scope friend function declarations (11.4 class.friend) not otherwise visible may be found.
to
When an unqualified name is used as the postfix-expression in a function call (5.2.2 expr.call), other namespaces not considered during the usual unqualified lookup (3.4.1 basic.lookup.unqual) may be searched, and in those namespaces, namespace-scope friend function declarations (11.4 class.friend) not otherwise visible may be found.
In 3.4.2 basic.lookup.koenig paragraph 2, delete the words and classes in the following two sentences:
If the ordinary unqualified lookup of the name finds the declaration of a class member function, the associated namespacesand classesare not considered. Otherwise the set of declarations found by the lookup of the function name is the union of the set of declarations found using ordinary unqualified lookup and the set of declarations found in the namespacesand classesassociated with the argument types.
(See also issues 95, 136, 138, 139, 165, 166, and 218.)
The Standard does not appear to address how the rules for order of initialization apply to static data members of class templates.
Suggested resolution: Add the following verbiage to either 3.6.2 basic.start.init or 9.4.2 class.static.data:
Initialization of static data members of class templates shall be performed during the initialization of static data members for the first translation unit to have static initialization performed for which the template member has been instantiated. This requirement shall apply to both the static and dynamic phases of initialization.
Notes from 04/01 meeting:
Enforcing an order of initialization on static data members of class templates will result in substantial overhead on access to such variables. The problem is that the initialization be required as the result of instantiation in a function used in the initialization of a variable in another translation unit. In current systems, the order of initialization of static data data members of class templates is not predictable. The proposed resolution is to state that the order of initialization is undefined.
Proposed resolution (04/01, updated slightly 10/01):
Replace the following sentence in 3.6.2 basic.start.init paragraph 1:
Objects with static storage duration defined in namespace scope in the same translation unit and dynamically initialized shall be initialized in the order in which their definition appears in the translation unit.
with
Dynamic initialization of an object is either ordered or unordered. Explicit specializations and definitions of class template static data members have ordered initialization. Other class template static data member instances have unordered initialization. Other objects defined in namespace scope have ordered initialization. Objects defined within a single translation unit and with ordered initialization shall be initialized in the order of their definitions in the translation unit. The order of initialization is unspecified for objects with unordered initialization and for objects defined in different translation units.
Note (07/01):
Brian McNamara argues against the proposed resolution. The following excerpt captures the central point of a long message on comp.std.c++:
(The full text is in core message 9263.)
I have a class for representing linked lists which looks something liketemplate <class T> class List { ... static List<T>* sentinel; ... }; template <class T> List<T>* List<T>::sentinel( new List<T> ); // static member definitionThe sentinel list node is used to represent "nil" (the null pointer cannot be used with my implementation, for reasons which are immaterial to this discussion). All of the List's non-static member functions and constructors depend upon the value of the sentinel. Under the proposed resolution for issue #270, Lists cannot be safely instantiated before main() begins, as the sentinel's initialization is "unordered".
(Some readers may propose that I should use the "singleton pattern" in the List class. This is undesirable, for reasons I shall describe at the end of this post at the location marked "[*]". For the moment, indulge me by assuming that "singleton" is not an adequate solution.)
Though this is a particular example from my own experience, I believe it is representative of a general class of examples. It is common to use static data members of a class to represent the "distinguished values" which are important to instances of that class. It is imperative that these values be initialized before any instances of the class are created, as the instances depend on the values.
In a comp.std.c++ posting on 28 Jul 2001, Brian McNamara proposes the following alternative resolution:
Replace the following sentence in 3.6.2 basic.start.init paragraph 1:
Objects with static storage duration defined in namespace scope in the same translation unit and dynamically initialized shall be initialized in the order in which their definition appears in the translation unit.with
Objects with static storage duration defined in namespace scope shall be initialized in the order described below.and then after paragraph 1, add this text:
Dynamic initialization is either ordered or quasi-ordered. Explicit specializations of class template static data members have ordered initialization. Other class template static data member instances have quasi-ordered initialization. All other objects defined in namespace scope have ordered initialization. The order of initialization is specified as follows:along with a non-normative note along the lines of
- Objects that are defined within a single translation unit and that have ordered initialization shall be initialized in the order of their definitions in the translation unit.
- Objects that are defined only within a single translation unit and that have quasi-ordered initialization shall also be initialized in the order of their definitions in the translation unit -- that is, as though these objects had ordered initialization.
- Objects that are defined within multiple translation units (which, therefore, must have quasi-ordered initialization) shall be initialized as follows: in exactly one translation unit (which one is unspecified), the object shall be treated as though it has ordered initialization; in the other translation units which define the object, the object will be initialized before all other objects that have ordered initialization in those translation units.
- For any two objects, "X" and "Y", with static storage duration and defined in namespace scope, if the previous bullets do not imply a relationship for the initialization ordering between "X" and "Y", then the relative initialization order of these objects is unspecified.
[ Note: The intention is that translation units can each be compiled separately with no knowledge of what objects may be re-defined in other translation units. Each translation unit can contain a method which initializes all objects (both quasi-ordered and ordered) as though they were ordered. When these translation units are linked together to create an executable program, all of these objects can be initialized by simply calling the initialization methods (one from each translation unit) in any order. Quasi-ordered objects require some kind of guard to ensure that they are not initialized more than once (the first attempt to initialize such an object should succeed; any subsequent attempts should simply be ignored). ]
Erwin Unruh replies: There is a point which is not mentioned with this posting. It is the cost for implementing the scheme. It requires that each static template variable is instantiated in ALL translation units where it is used. There has to be a flag for each of these variables and this flag has to be checked in each TU where the instantiation took place.
I would reject this idea and stand with the proposed resolution of issue 270.
There just is no portable way to ensure the "right" ordering of construction.
Notes from 10/01 meeting:
The Core Working Group reaffirmed its previous decision.
From reflector messages 8072-8073.
Jack Rouse: 3.8 basic.life paragraph 1 includes:
The lifetime of an object is a runtime property of the object. The lifetime of an object of type T begins when:Consider the code:
- storage with the proper alignment and size for type T is obtained, and
- if T is a class type with a non-trivial constructor (12.1 class.ctor ), the constructor call has completed.
struct B {
B( int = 0 );
~B();
};
struct S {
B b1;
};
int main()
{
S s = { 1 };
return 0;
}
In the code above, class S does have a non-trivial constructor, the
default constructor generated by the compiler. According the text
above, the lifetime of the auto s would never begin because a
constructor for S is never called. I think the second case in the
text needs to include aggregate initialization.
Mike Miller: I see a couple of ways of fixing the problem. One way would be to change "the constructor call has completed" to "the object's initialization is complete."
Another would be to add following "a class type with a non-trivial constructor" the phrase "that is not initialized with the brace notation (8.5.1 dcl.init.aggr )."
The first formulation treats aggregate initialization like a constructor call; even POD-type members of an aggregate could not be accessed before the aggregate initialization completed. The second is less restrictive; the POD-type members of the aggregate would be usable before the initialization, and the members with non-trivial constructors (the only way an aggregate can acquire a non-trivial constructor) would be protected by recursive application of the lifetime rule.
Proposed resolution (04/01):
In 3.8 basic.life paragraph 1, change
If T is a class type with a non-trivial constructor (12.1 class.ctor), the constructor call has completed.
to
If T is a class type with a non-trivial constructor (12.1 class.ctor), the initialization is complete. [Note: the initialization can be performed by a constructor call or, in the case of an aggregate with an implicitly-declared non-trivial default constructor, an aggregate initialization (8.5.1 dcl.init.aggr).]
3.10 basic.lval paragraph 15 lists the types via which an lvalue can be used to access the stored value of an object; using an lvalue type that is not listed results in undefined behavior. It is permitted to add cv-qualification to the actual type of the object in this access, but only at the top level of the type ("a cv-qualified version of the dynamic type of the object").
However, 4.4 conv.qual paragraph 4 permits a "conversion [to] add cv-qualifiers at levels other than the first in multi-level pointers." The combination of these two rules allows creation of pointers that cannot be dereferenced without causing undefined behavior. For instance:
int* jp;
const int * const * p1 = &jp;
*p1; // undefined behavior!
The reason that *p1 results in undefined behavior is that the type of the lvalue is const int * const", which is not "a cv-qualified version of" int*.
Since the conversion is permitted, we must give it defined semantics, hence we need to fix the wording in 3.10 basic.lval to include all possible conversions of the type via 4.4 conv.qual.
Proposed resolution (04/01):
Add a new bullet to 3.10 basic.lval paragraph 15, following "a cv-qualified version of the dynamic type of the object:"
Is the temporary created during copy-initialization of a class object treated as an lvalue or an rvalue? That is, is the following example well-formed or not?
struct B { };
struct A {
A(A&); // not const
A(const B&);
};
B b;
A a = b;
According to 8.5 dcl.init paragraph 14, the initialization of a is performed in two steps. First, a temporary of type A is created using A::A(const B&). Second, the resulting temporary is used to direct-initialize a using A::A(A&).
The second step requires binding a reference to non-const to the temporary resulting from the first step. However, 8.5.3 dcl.init.ref paragraph 5 requires that such a reference be bound only to lvalues.
It is not clear from 3.10 basic.lval whether the temporary created in the process of copy-initialization should be treated as an lvalue or an rvalue. If it is an lvalue, the example is well-formed, otherwise it is ill-formed.
Proposed resolution (04/01):
In 8.5 dcl.init paragraph 14, insert the following after "the call initializes a temporary of the destination type":
The temporary is an rvalue.
In 15.1 except.throw paragraph 3, replace
The temporary is used to initialize the variable...
with
The temporary is an lvalue and is used to initialize the variable...
(See also issue 84.)
The definition of "friend" in 11.4 class.friend says:
A friend of a class is a function or class that is not a member of the class but is permitted to use the private and protected member names from the class. ...A nested class, i.e. INNER in the example below, is a member of class OUTER. The sentence above states that it cannot be a friend. I think this is a mistake.
class OUTER {
class INNER;
friend class INNER;
class INNER {};
};
Proposed resolution (04/01):
Change the first sentence of 11.4 class.friend as follows:
A friend of a class is a function or class that isnot a member of the class but is allowedgiven permission to use the private and protected member names from the class.The name of a friend is not in the scope of the class, and the friend is not called with the member access operators (5.2.5 expr.ref) unless it is a member of another class.A class specifies its friends, if any, by way of friend declarations. Such declarations give special access rights to the friends, but they do not make the nominated friends members of the befriending class.
From messages 9152-3.
The example in 14.5.4 temp.class.spec paragraph 6 is incorrect. It reads,
template<class T> struct A {
class C {
template<class T2> struct B { };
};
};
// partial specialization of A<T>::C::B<T2>
template<class T> template<class T2>
struct A<T>::C::B<T2*> { };
A<short>::C::B<int*> absip; // uses partial specialization
Because C is a class rather than a struct, the use of the name B is inaccessible.
Proposed Resolution (10/01):
Change class C to struct C in the example in 14.5.4 temp.class.spec paragraph 6. The example becomes
template<class T> struct A {
struct C {
template<class T2> struct B { };
};
};
// partial specialization of A<T>::C::B<T2>
template<class T> template<class T2>
struct A<T>::C::B<T2*> { };
A<short>::C::B<int*> absip; // uses partial specialization
From reflector messages 8377-8380.
Mike Miller: A question about typename came up in the discussion of issue 68 that is somewhat relevant to the idea of omitting typename in contexts where it is clear that a type is required: consider something like
template <class T>
class X {
friend class T::nested;
};
Is typename required here? If so, where would it go? (The
grammar doesn't seem to allow it anywhere in an
elaborated-type-specifier that has a class-key.)
Bill Gibbons: The class applies to the last identifier in the qualified name, since all the previous names must be classes or namespaces. Since the name is specified to be a class it does not need typename. [However,] it looks like 14.6 temp.res paragraph 3 requires typename and the following paragraphs do not exempt this case. This is not what we agreed on.
Proposed resolution (04/01):
In 14.6 temp.res paragraph 5, change
The keyword typename is not permitted in a base-specifier or in a mem-initializer; in these contexts a qualified-name that depends on a template-parameter (14.6.2 temp.dep) is implicitly assumed to be a type name.
to
A qualified name used as the name in a mem-initializer-id, a base-specifier, or an elaborated-type-specifier (in the class-key and enum forms) is implicitly assumed to name a type, without the use of the typename keyword. [Note: the typename keyword is not permitted by the syntax of these constructs.]
(The expected resolution for issue 254 will remove the typename forms from the grammar for elaborated-type-specifier. If that resolution is adopted, the parenthetical phrase "(in the class-key and enum forms)" in the preceding wording should be removed because those will be the only forms of elaborated-type-specifier.)
From messages 8956, 8959, and 8975.
According to 14.7 temp.spec paragraph 5,
No program shall explicitly instantiate any template more than once, both explicitly instantiate and explicitly specialize a template, or specialize a template more than once for a given set of template-arguments.
This rule has an impact on library issue 120. Library authors would like to have the freedom to specialize (or not) various library functions without having to document their choices, while users need the flexibility to explicitly instantiate library functions in certain translation units.
If this rule could be slightly weakened, it would reduce the need for constraining either the library author or the programmer. For instance, the rule might be recast to say that if a specialization is followed by an explicit instantiation in the same translation unit, the explicit instantiation is ignored. A specialization and an explicit instantiation of the same template in two different translation units would still be an error, no diagnostic required.
Proposed resolution (04/01):
Replace the first sentence of 14.7 temp.spec paragraph 5,
No program shall explicitly instantiate any template more than once, both explicitly instantiate and explicitly specialize a template, or specialize a template more than once for a given set of template-arguments.
by
For a given template and a given set of template-arguments,
- an explicit instantiation shall appear at most once in a program,
- an explicit specialization shall be defined at most once according to 3.2 basic.def.odr in a program, and
- both an explicit instantiation and a declaration of an explicit specialization shall not appear in a program unless the explicit instantiation follows a declaration of the explicit specialization.
Replace 14.7.2 temp.explicit paragraph 4,
The definition of a non-exported function template, a non-exported member function template, or a non-exported member function or static data member of a class template shall be present in every translation unit in which it is explicitly instantiated.
by
For a given set of template parameters, if an explicit instantiation of a template appears after a declaration of an explicit specialization for that template, the explicit instantiation has no effect. Otherwise, the definition of a non-exported function template, a non-exported member function template, or a non-exported member function or static data member of a class template shall be present in every translation unit in which it is explicitly instantiated.
(From messages 9067-8, 9070-91.)
Consider this example:
namespace N {
template <class T> void f(T){}
template <class T> void g(T){}
template <> void f(int);
template <> void f(char);
template <> void g(char);
}
using namespace N;
namespace M {
template <> void N::f(char){} // prohibited by standard
template <class T> void g(T){}
template <> void g(char){} // specialization of M::g or ambiguous?
template void f(long); // instantiation of N::f?
}
template <class T> void g(T){}
template <> void N::f(char){} // okay
template <> void f(int){} // is this a valid specialization of N::f?
template void g(int); // instantiation of ::g(int) or ambiguous?
The question here is whether unqualified names made visible by a using-directive can be used as the declarator in an explicit instantiation or explicit specialization.
Note that this question is already answered for qualified names in 8.3 dcl.meaning paragraph 1. In a qualified name such as N::f, f must be a member of class or namespace N, not a name made visible in N by a using-directive (or a using-declaration, for that matter).
The standard does not, as far as I can tell, specify the behavior of these cases one way or another.
My opinion is that names from using-directives should not be considered when looking up the name in an unqualified declarator in an explicit specialization or explicit instantiation. In such cases, it is reasonable to insist that the programmer know exactly which template is being specialized or instantiated, and that a qualified name must be used if the template is a member of a namespace.
As the example illustrates, allowing names from using-directives to be used would also have the affect of making ambiguous otherwise valid instantiation and specialization directives.
Furthermore, permitting names from using-directives would require an additional rule to prohibit the explicit instantiation of an entity in one namespace from being done in another (non-enclosing) namespace (as in the instantiation of f in namespace M in the example).
Mike Miller: I believe the explicit specialization case is already covered by 7.3.1.2 namespace.memdef paragraph 2, which requires using a qualified name to define a namespace member outside its namespace.
John Spicer: 7.3.1.2 namespace.memdef deals with namespace members. An explicit specialization directive deals with something that is a specialization of a namespace member. I don't think the rules in 7.3.1.2 namespace.memdef could be taken to apply to specializations unless the standard said so explicitly.
Proposed resolution (suggested 04/01, proposed 10/01):
(The first change below will need to be revised in accordance with the resolution of issue 284 to add a cross-reference to the text dealing with class names.)
Add in 14.7.2 temp.explicit paragraph 2 before the example:
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. [Note: Regarding qualified names in declarators, see 8.3 dcl.meaning.]
Change the first sentence of 7.3.1.2 namespace.memdef paragraph 1 from
Members of a namespace can be defined within that namespace.
to
Members (including explicit specializations of templates (14.7.3 temp.expl.spec)) of a namespace can be defined within that namespace.
Change the first sentence of 7.3.1.2 namespace.memdef paragraph 2 from
Members of a named namespace can also be defined...
to
Members (including explicit specializations of templates (14.7.3 temp.expl.spec)) of a named namespace can also be defined...
Change the last sentence of 14.7.3 temp.expl.spec paragraph 2 from
If the declaration is not a definition, the specialization may be defined later in the namespace in which the explicit specialization was declared, or in a namespace that encloses the one in which the explicit specialization was declared.
to
If the declaration is not a definition, the specialization may be defined later (7.3.1.2 namespace.memdef).
From message 8976.
3.2 basic.def.odr paragraph 2 says that a deallocation function is "used" by a new-expression or delete-expression appearing in a potentially-evaluated expression. 3.2 basic.def.odr paragraph 3 requires only that "used" functions be defined.
This wording runs afoul of the typical implementation technique for polymorphic delete-expressions in which the deallocation function is invoked from the virtual destructor of the most-derived class. The problem is that the destructor must be defined, because it's virtual, and if it contains an implicit reference to the deallocation function, the deallocation function must also be defined, even if there are no relevant new-expressions or delete-expressions in the program.
For example:
struct B { virtual ~B() { } };
struct D: B {
void operator delete(void*);
~D() { }
};
Is it required that D::operator delete(void*) be defined, even if no B or D objects are ever created or deleted?
Suggested resolution: Add the words "or if it is found by the lookup at the point of definition of a virtual destructor (12.4 class.dtor)" to the specification in 3.2 basic.def.odr paragraph 2.
Notes from 04/01 meeting:
The consensus was in favor of requiring that any declared non-placement operator delete member function be defined if the destructor for the class is defined (whether virtual or not), and similarly for a non-placement operator new if a constructor is defined.
Proposed resolution (10/01):
In 3.2 basic.def.odr paragraph 2, add the indicated text:
An allocation or deallocation function for a class is used by a new expression appearing in a potentially-evaluated expression as specified in 5.3.4 expr.new and 12.5 class.free. A deallocation function for a class is used by a delete expression appearing in a potentially-evaluated expression as specified in 5.3.5 expr.delete and 12.5 class.free. A non-placement allocation or deallocation function for a class is used by the definition of a constructor of that class. A non-placement deallocation function for a class is used by the definition of the destructor of that class, or by being selected by the lookup at the point of definition of a virtual destructor (12.4 class.dtor). [Footnote: An implementation is not required to call allocation and deallocation functions from constructors or destructors; however, this is a permissible implementation technique.]
3.2 basic.def.odr paragraph 4 has a note listing the contexts that require a class type to be complete. It does not list use as a base class as being one of those contexts.
Proposed resolution (10/01):
In 3.2 basic.def.odr paragraph 4 add a new bullet at the end of the note as the next-to-last bullet:
Can a typedef T to a cv-qualified class type be used in a qualified name T::x?
struct A { static int i; };
typedef const A CA;
int main () {
CA::i = 0; // Okay?
}
Suggested answer: Yes. All the compilers I tried accept the test case.
Proposed resolution (10/01):
In 3.4.3.1 class.qual paragraph 1 add the indicated text:
If the nested-name-specifier of a qualified-id nominates a class, the name specified after the nested-name-specifier is looked up in the scope of the class (10.2 class.member.lookup), except for the cases listed below. The name shall represent one or more members of that class or of one of its base classes (clause 10 class.derived). If the class-or-namespace-name of the nested-name-specifier names a cv-qualified class type, it nominates the underlying class (the cv-qualifiers are ignored).
The wording in 3.8 basic.life paragraph 6 allows an lvalue designating an out-of-lifetime object to be used as the operand of a static_cast only if the conversion is "ultimately to "char&" or unsigned char&." This description excludes the possibility of using a cv-qualified version of these types for no apparent reason.
Notes on 04/01 meeting:
The wording should be changed to allow cv-qualified char types.
Proposed resolution (04/01):
In 3.8 basic.life paragraph 6 change the third bullet:
(From message 9134.)
There is currently no restriction on the use of the inline specifier in friend declarations. That would mean that the following usage is permitted:
struct A {
void f();
};
struct B {
friend inline void A::f();
};
void A::f(){}
I believe this should be disallowed because a friend declaration in one class should not be able to change attributes of a member function of another class.
More generally, I think that the inline attribute should only be permitted in friend declarations that are definitions.
Notes from the 04/01 meeting:
The consensus agreed with the suggested resolution. This outcome would be similar to the resolution of issue 136.
Proposed resolution (10/01):
Add to the end of 7.1.2 dcl.fct.spec paragraph 3:
If the inline specifier is used in a friend declaration, that declaration shall be a definition or the function shall have previously been declared inline.
From reflector messages 8332-4, 8337.
According to 7.2 dcl.enum paragraph 5, the underlying type of an enum is an unspecified integral type, which could potentially be unsigned int. The promotion rules in 4.5 conv.prom paragraph 2 say that such an enumeration value used in an expression will be promoted to unsigned int. This means that a conforming implementation could give the value false for the following code:
enum { zero };
-1 < zero; // might be false
This is counterintuitive. Perhaps the description of the underlying
type of an enumeration should say that an unsigned underlying type
can be used only if the values of the enumerators cannot be
represented in the corresponding signed type. This approach would
be consistent with the treatment of integral promotion of
bitfields (4.5
conv.prom
paragraph 3).
On a related note, 7.2 dcl.enum paragraph 5 says,
the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int.
This specification does not allow for an enumeration like
enum { a = -1, b = UINT_MAX };
Since each enumerator can fit in an int or unsigned int, the underlying type is required to be no larger than int, even though there is no such type that can represent all the enumerators.
Proposed resolution (04/01):
Change 7.2 dcl.enum paragraph 5 as follows:
It is implementation-defined which integral type is used as the underlying type for an enumeration except that the underlying type shall not be larger than int unlessthe value of an enumerator cannot fit in an int or unsigned intneither int nor unsigned int can represent all the enumerator values. Furthermore, the underlying type shall not be an unsigned type if the corresponding signed type can represent all the enumerator values.
See also issue 58.
Notes from 04/01 meeting:
It was noted that 4.5 conv.prom promotes unsigned types smaller than int to (signed) int. The signedness chosen by an implementation for small underlying types is therefore unobservable, so the last sentence of the proposed resolution above should apply only to int and larger types. This observation also prompted discussion of an alternative approach to resolving the issue, in which the bmin and bmax of the enumeration would determine the promoted type rather than the underlying type. Steve Adamczyk is investigating this alternative.
Proposed resolution (10/01):
Change 4.5 conv.prom paragraph 2 from
An rvalue of type wchar_t (3.9.1 basic.fundamental) or an enumeration type (7.2 dcl.enum) can be converted to an rvalue of the first of the following types that can represent all the values of its underlying type: int, unsigned int, long, or unsigned long.to
An rvalue of type wchar_t (3.9.1 basic.fundamental) can be converted to an rvalue of the first of the following types that can represent all the values of its underlying type: int, unsigned int, long, or unsigned long. An rvalue of an enumeration type (7.2 dcl.enum) can be converted to an rvalue of the first of the following types that can represent all the values of the enumeration (i.e., the values in the range bmin to bmax as described in 7.2 dcl.enum): int, unsigned int, long, or unsigned long.
From messages 8946 and 8948.
According to 7.3.3 namespace.udecl paragraph 12,
When a using-declaration brings names from a base class into a derived class scope, member functions in the derived class override and/or hide member functions with the same name and parameter types in a base class (rather than conflicting).
Note that this description says nothing about the cv-qualification of the hiding and hidden member functions. This means, for instance, that a non-const member function in the derived class hides a const member function with the same name and parameter types instead of overloading it in the derived class scope. For example,
struct A {
virtual int f() const;
virtual int f();
};
struct B: A {
B();
int f();
using A::f;
};
const B cb;
int i = cb.f(); // ill-formed: A::f() const hidden in B
The same terminology is used in 10.3 class.virtual paragraph 2:
If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name and same parameter list as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf.
Notes on the 04/01 meeting:
The hiding and overriding should be on the basis of the function signature, which includes any cv-qualification on the function.
Proposed resolution (04/01):
In 7.3.3 namespace.udecl paragraph 12 change:
When a using-declaration brings names from a base class into a derived class scope, member functions in the derived class override and/or hide member functions with the same name and parameter types in a base class (rather than conflicting).to read:
When a using-declaration brings names from a base class into a derived class scope, member functions and member function templates in the derived class override and/or hide member functions and member function templates with the same name and signature (1.3.1 defns.argument) in a base class (rather than conflicting).
In 10.3 class.virtual paragraph 2 change:
If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name and same parameter list as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf.to read:
If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name and signature (1.3.10 defns.signature) as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf.
Note: Should this use the term "parameter-type-list" introduced in issue 140 instead of "signature"?
This concerns the inconsistent treatment of cv qualifiers on reference types and function types. The problem originated with GCC bug report c++/2810. The bug report is available at http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view&pr=2810&database=gcc
8.3.2 dcl.ref describes references. Of interest is the statement (my emphasis)
Cv-qualified references are ill-formed except when the cv-qualifiers are introduced through the use of a typedef or of a template type argument, in which case the cv-qualifiers are ignored.
Though it is strange to ignore 'volatile' here, that is not the point of this defect report. 8.3.5 dcl.fct describes function types. Paragraph 4 states,
In fact, if at any time in the determination of a type a cv-qualified function type is formed, the program is ill-formed.
No allowance for typedefs or template type parameters is made here, which is inconsistent with the equivalent reference case.
The GCC bug report was template code which attempted to do,
template <typename T> void foo (T const &);
void baz ();
...
foo (baz);
in the instantiation of foo, T is `void ()' and an attempt is made to const qualify that, which is ill-formed. This is a surprise.
Suggested resolution:
Replace the quoted sentence from paragraph 4 in 8.3.5 dcl.fct with
cv-qualified functions are ill-formed, except when the cv-qualifiers are introduced through the use of a typedef or of a template type argument, in which case the cv-qualifiers are ignored.
Adjust the example following to reflect this.
Proposed resolution (10/01):
In 8.3.5 dcl.fct paragraph 4, replace
The effect of a cv-qualifier-seq in a function declarator is
not the same as adding cv-qualification on top of the function type,
i.e., it does not create a cv-qualified function type. In fact, if at
any time in the determination of a type a cv-qualified function type
is formed, the program is ill-formed. [Example:
typedef void F();
struct S {
const F f; // ill-formed
};
-- end example]
by
The effect of a cv-qualifier-seq in a function declarator is
not the same as adding cv-qualification on top of the function type.
In the latter case, the cv-qualifiers are ignored. [Example:
typedef void F();
struct S {
const F f; // ok; equivalent to void f();
};
-- end example]
Strike the last bulleted item in 14.8.2 temp.deduct paragraph 2, which reads
Attempting to create a cv-qualified function type.
From message 9258.
We've been looking at implementing value-initialization. At one point, some years back, I remember Bjarne saying that something like X() in an expression should produce an X object with the same value one would get if one created a static X object, i.e., the uninitialized members would be zero-initialized because the whole object is initialized at program startup, before the constructor is called.
The formulation for default-initialization that made it into TC1 (in 8.5 dcl.init) is written a little differently (see issue 178), but I had always assumed that it would still be a valid implementation to zero the whole object and then call the default constructor for the troublesome "non-POD but no user-written constructor" cases.
That almost works correctly, but I found a problem case:
struct A {
A();
~A();
};
struct B {
// B is a non-POD with no user-written constructor.
// It has a nontrivial generated constructor.
const int i;
A a;
};
int main () {
// Value-initializing a "B" doesn't call the default constructor for
// "B"; it value-initializes the members of B. Therefore it shouldn't
// cause an error on generation of the default constructor for the
// following:
new B();
}
If the definition of the B::B() constructor is generated, an error is issued because the const member "i" is not initialized. But the definition of value-initialization doesn't require calling the constructor, and therefore it doesn't require generating it, and therefore the error shouldn't be detected.
So this is a case where zero-initializing and then calling the constructor is not equivalent to value-initializing, because one case generates an error and the other doesn't.
This is sort of unfortunate, because one doesn't want to generate all the required initializations at the point where the "()" initialization occurs. One would like those initializations to be packaged in a function, and the default constructor is pretty much the function one wants.
I see several implementation choices:
Personally, I find option 1 the least objectionable.
Proposed resolution (10/01):
Add the indicated wording to the third-to-last sentence of 3.2 basic.def.odr pararaph 2:
A default constructor for a class is used by default initialization or value initialization as specified in 8.5 dcl.init.
Add a footnote to the indicated bullet in 8.5 dcl.init paragraph 5:
Add the indicated wording to the first sentence of 12.1 class.ctor paragraph 7:
An implicitly-declared default constructor for a class is implicitly defined when it is used (in the sense defined in 3.2 basic.def.odr) to create an object of its class type (1.8 intro.object).
From message 9264.
Another glitch in the TC1/core issue 178 definition of value-initialization: it's no longer an error to value-initialize a reference. That makes an example like
typedef struct { int &r; } S;
int main() {
S(); // Error in C++98, okay in TC1!
}
valid, which has got to be wrong. See 8.5
dcl.init paragraph
5, where there is wording that forbids default-initialization of a
reference, but not value-initialization thereof. As noted in
issue 302, if
the default constructor were required to be generated when a
value-initialization is done, that would force an error.
Proposed resolution (10/01):
Add the indicated wording to the indicated sentence in 8.5 dcl.init paragraph 5:
A program that calls for default-initialization or value-initialization of an entity of reference type is ill-formed.
In message 9323, Andrew Koenig said that he has made this proposed change as a quasi-editorial change in TC1. He has since confirmed that the change has been made already.
I think that the definition of a POD class in the current version of the Standard is overly permissive in that it allows for POD classes for which a user-defined operator function operator& may be defined. Given that the idea behind POD classes was to achieve compatibility with C structs and unions, this makes 'Plain old' structs and unions behave not quite as one would expect them to.
In the C language, if x and y are variables of struct or union type S that has a member m, the following expression are allowed: &x, x.m, x = y. While the C++ standard guarantees that if x and y are objects of a POD class type S, the expressions x.m, x = y will have the same effect as they would in C, it is still possible for the expression &x to be interpreted differently, subject to the programmer supplying an appropriate version of a user-defined operator function operator& either as a member function or as a non-member function.
This may result in surprising effects. Consider:
// POD_bomb is a POD-struct. It has no non-static non-public data members,
// no virtual functions, no base classes, no constructors, no user-defined
// destructor, no user-defined copy assignment operator, no non-static data
// members of type pointer to member, reference, non-POD-struct, or
// non-POD-union.
struct POD_bomb {
int m_value1;
int m_value2;
int operator&()
{ return m_value1++; }
int operator&() const
{ return m_value1 + m_value2; }
};
3.9 basic.types paragraph 2 states:
For any complete POD object type T, whether or not the object
holds a valid value of type T, the underlying bytes
(1.7
intro.memory) making up the object can be copied into
an array of char or unsigned char [footnote:
By using, for example, the library functions (17.4.1.2
lib.headers) memcpy or memmove]. If the
content of the array of char or unsigned char is
copied back into the object, the object shall subsequently hold its
original value. [Example:
#define N sizeof(T)
char buf[N];
T obj; // obj initialized to its original value
memcpy(buf, &obj, N);
// between these two calls to memcpy,
// obj might be modified
memcpy(&obj, buf, N);
// at this point, each subobject of obj of scalar type
// holds its original value
—end example]
Now, supposing that the complete POD object type T in the example above is POD_bomb, and we cannot any more count on the assertions made in the comments to the example. Given a standard conforming implementation, the code will not even compile. And I see no legal way of copying the contents of an object of a complete object type POD_bomb into an array of char or unsigned char with memcpy or memmove without making use of the unary & operator. Except, of course, by means of an ugly construct like:
struct POD_without_ampersand {
POD_bomb a_bomb;
} obj;
#define N sizeof(POD_bomb)
char buf[N];
memcpy(buf, &obj, N);
memcpy(&obj, buf, N);
The fact that the definition of a POD class allows for POD classes for which a user-defined operator& is defined, may also present major obstacles to implementers of the offsetof macro from <cstddef>
18.1 lib.support.types paragraph 5 says:
The macro offsetof accepts a restricted set of type arguments in this International Standard. type shall be a POD structure or a POD union (clause 9 class). The result of applying the offsetof macro to a field that is a static data member or a function is undefined."
Consider a well-formed C++ program below:
#include <cstddef>
#include <iostream>
struct POD_bomb {
int m_value1;
int m_value2;
int operator&()
{ return m_value1++; }
int operator&() const
{ return m_value1 + m_value2; }
};
// POD_struct is a yet another example of a POD-struct.
struct POD_struct {
POD_bomb m_nonstatic_bomb1;
POD_bomb m_nonstatic_bomb2;
};
int main()
{
std::cout << "offset of m_nonstatic_bomb2: " << offsetof(POD_struct,
m_nonstatic_bomb2) << '\n';
return 0;
}
See Jens Maurer's paper 01-0038=N1324 for an analysis of this issue.
Notes from 10/01 meeting:
A consensus was forming around the idea of disallowing operator& in POD classes when it was noticed that it is permitted to declare global-scope operator& functions, which cause the same problems. After more discussion, it was decided that such functions should not be prohibited in POD classes, and implementors should simply be required to "get the right answer" in constructs such as offsetof and va_start that are conventionally implemented using macros that use the "&" operator. It was noted that one can cast the original operand to char & to de-type it, after which one can use the built-in "&" safely.
Proposed resolution:
[Footnote: Note that offsetof is required to work as specified even if unary operator& is overloaded for any of the types involved.]
[Footnote: Note that va_start is required to work as specified even if unary operator& is overloaded for parmN.]
Although 8.3 dcl.meaning requires that a declaration of a qualified-id refer to a member of the specified namespace or class and that the member not have been introduced by a using-declaration, it applies only to names declared in a declarator. It is not clear whether there is existing wording enforcing the same restriction for qualified-ids in class-specifiers and elaborated-type-specifiers or whether additional wording is required. Once such wording is found/created, the proposed resolution of issue 275 must be modified accordingly.
Notes from 10/01 meeting:
The sentiment was that this should be required on class definitions, but not on elaborated type specifiers in general (which are references, not declarations). We should also make sure we consider explicit instantiations, explicit specializations, and friend declarations.
Proposed resolution (10/01):
Add after the end of 9.1 class.name paragraph 3:
When a nested-name-specifier is specified in a class-head or in an elaborated-type-specifier, the resulting qualified name shall refer to a previously declared member of the class or namespace to which the nested-name-specifier refers, and the member shall not have been introduced by a using-declaration in the scope of the class or namespace nominated by the nested-name-specifier.
From reflector messages 8501-2, 8507.
9.8 class.local paragraph 1 says,
Declarations in a local class can use only type names, static variables, extern variables and functions, and enumerators from the enclosing scope.The definition of when an object or function is "used" is found in 3.2 basic.def.odr paragraph 2 and essentially says that the operands of sizeof and non-polymorphic typeid operators are not used. (The resolution for issue 48 will add contexts in which integral constant expressions are required to the list of non-uses.)
This definition of "use" would presumably allow code like
void foo() {
int i;
struct S {
int a[sizeof(i)];
};
};
which is required for C compatibility.
However, the restrictions on nested classes in 9.7 class.nest paragraph 1 are very similar to those for local classes, and the example there explicitly states that a reference in a sizeof expression is a forbidden use (abbreviated for exposition):
class enclose {
public:
int x;
class inner {
void f(int i)
{
int a = sizeof(x); // error: refers to enclose::x
}
};
};
[As a personal note, I have seen real-world code that was exactly like this; it was hard to persuade the author that the required writearound, sizeof(((enclose*) 0)->x), was an improvement over sizeof(x). —wmm]
Similarly, 9.2 class.mem paragraph 9 would appear to prohibit examples like the following:
struct B {
char x[10];
};
struct D: B {
char y[sizeof(x)];
};
Suggested resolution: Add cross-references to 3.2 basic.def.odr following the word "use" in both 9.7 class.nest and 9.8 class.local , and change the example in 9.7 class.nest to indicate that a reference in a sizeof expression is permitted. In 9.2 class.mem paragraph 9, "referred to" should be changed to "used" with a cross_reference to 3.2 basic.def.odr.
Notes from 10/01 meeting:
It was noted that the suggested resolution did not make the sizeof() example in 9.7 class.nest valid. Although the reference to the argument of sizeof() is not regarded as a use, the right syntax must be used nonetheless to reference a non-static member from the enclosing class. The use of the member name by itself is not valid. The consensus within the core working group was that nothing should be done about this case. It was later discovered that 9.4 class.static paragraph 3 states that
The definition of a static member shall not use directly the names of the nonstatic members of its class or of a base class of its class (including as operands of the sizeof operator). The definition of a static member may only refer to these members to form pointer to members (5.3.1 expr.unary.op) or with the class member access syntax (5.2.5 expr.ref).
This seems to reinforce the decision of the working group.
The use of "use" should still be cross-referenced. The statements in 9.7 class.nest and 9.8 class.local should also be rewritten to state the requirement positively rather than negatively as the list of "can't"s is already missing some cases such as template parameters.
Proposed resolution:
In 9.2 class.mem paragraph 9, replace
Except when used to form a pointer to member (5.3.1 expr.unary.op), when used in the body of a nonstatic member function of its class or of a class derived from its class (9.3.1 class.mfct.nonstatic), or when used in a mem-initializer for a constructor for its class or for a class derived from its class (12.6.2 class.base.init), a nonstatic data or function member of a class shall only be referred to with the class member access syntax (5.2.5 expr.ref).
with the following paragraph
Each use (3.2 basic.def.odr) of a nonstatic data member or nonstatic member function of a class shall be expressed as a class member access (5.2.5 expr.ref), except when it appears in the formation a pointer to member (5.3.1 expr.unary.op, when it appears in the body of a nonstatic member function of its class or of a class derived from its class (9.3.1 class.mfct.nonstatic), or when it appears in a mem-initializer for a constructor for its class or for a class derived from its class (12.6.2 class.base.init).
In 9.7 class.nest paragraph 1, replace the last sentence,
Except by using explicit pointers, references, and object names, declarations in a nested class can use only type names, static members, and enumerators from the enclosing class.
with the following
Except by using explicit pointers, references, and object names, declarations in a nested class shall not use (including as operands of the sizeof operator) nonstatic data members or nonstatic member functions from the enclosing class.
In the example following 9.7 class.nest paragraph 1, change the comment on the first statement of function f to emphasize that sizeof(x) is an error. The example reads in full:
int x;
int y;
class enclose {
public:
int x;
static int s;
class inner {
void f(int i)
{
int a = sizeof(x); // error: direct use of enclose::x even in sizeof
x = i; // error: assign to enclose::x
s = i; // OK: assign to enclose::s
::x = i; // OK: assign to global x
y = i; // OK: assign to global y
}
void g(enclose* p, int i)
{
p->x = i; // OK: assign to enclose::x
}
};
};
inner* p = 0; // error: inner not in scope
--- end example]
In 9.8 class.local paragraph 1, replace the last sentence,
Declarations in a local class can use only type names, static variables, extern variables and functions, and enumerators from the enclosing scope.
with the following
Declarations in a local class shall not use (3.2 basic.def.odr) objects with automatic storage duration from the enclosing scope.
From reflector message core-7816.
The ambiguity text in 10.2 class.member.lookup may not say what we intended. It makes the following example ill-formed:
struct A {
int x(int);
};
struct B: A {
using A::x;
float x(float);
};
int f(B* b) {
b->x(3); // ambiguous
}
This is a name lookup ambiguity because of 10.2
class.member.lookup
paragraph 2:
... Each of these declarations that was introduced by a using-declaration is considered to be from each sub-object of C that is of the type containing the declaration designated by the using-declaration. If the resulting set of declarations are not all from sub-objects of the same type, or the set has a nonstatic member and includes members from distinct sub-objects, there is an ambiguity and the program is ill-formed.This contradicts the text and example in paragraph 12 of 7.3.3 namespace.udecl .
Proposed Resolution (10/00):
Replace the two cited sentences from 10.2 class.member.lookup paragraph 2 with the following:
The resulting set of declarations shall all be from sub-objects of the same type, or there shall be a set of declarations from sub-objects of a single type that contains using-declarations for the declarations found in all other sub-object types. Furthermore, for nonstatic members, the resulting set of declarations shall all be from a single sub-object, or there shall be a set of declarations from a single sub-object that contains using-declarations for the declarations found in all other sub-objects. Otherwise, there is an ambiguity and the program is ill-formed.
Replace the examples in 10.2 class.member.lookup paragraph 3 with the following:
struct A {
int x(int);
static int y(int);
};
struct V {
int z(int);
};
struct B: A, virtual V {
using A::x;
float x(float);
using A::y;
static float y(float);
using V::z;
float z(float);
};
struct C: B, A, virtual V {
};
void f(C* c) {
c->x(3); // ambiguous -- more than one sub-object A
c->y(3); // not ambiguous
c->z(3); // not ambiguous
}
Notes from 04/01 meeting:
The following example should be accepted but is rejected by the wording above:
struct A { static void f(); };
struct B1: virtual A {
using A::f;
};
struct B2: virtual A {
using A::f;
};
struct C: B1, B2 { };
void g() {
C::f(); // OK, calls A::f()
}
From reflector messages 8289 and 8305.
13.3.1.1 over.match.call paragraph 3 says that when a call of the form
(&C::f)()is written, the set of overloaded functions named by C::f must not contain any nonstatic member functions. A footnote gives the rationale: if a member of C::f is a nonstatic member function, &C::f is a pointer to member constant, and therefore the call is invalid.
This is clear, it's implementable, and it doesn't directly contradict anything else in the standard. However, I'm not sure it's consistent with some similar cases.
In 13.4 over.over paragraph 5, second example, it is made amply clear that when &C::f is used as the address of a function, e.g.,
int (*pf)(int) = &C::f;the overload set can contain both static and nonstatic member functions. The function with the matching signature is selected, and if it is nonstatic &C::f is a pointer to member function, and otherwise &C::f is a normal pointer to function.
Similarly, 13.3.1.1.1 over.call.func paragraph 3 makes it clear that
C::f();is a valid call even if the overload set contains both static and nonstatic member functions. Overload resolution is done, and if a nonstatic member function is selected, an implicit this-> is added, if that is possible.
Those paragraphs seem to suggest the general rule that you do overload resolution first and then you interpret the construct you have according to the function selected. The fact that there are static and nonstatic functions in the overload set is irrelevant; it's only necessary that the chosen function be static or nonstatic to match the context.
Given that, I think it would be more consistent if the (&C::f)() case would also do overload resolution first. If a nonstatic member is chosen, the program would be ill-formed.
Proposed resolution (04/01):
Change the indicated text in 13.3.1.1 over.match.call paragraph 3:
The fourth case arises from a postfix-expression of the form &F, where F names a set of overloaded functions. In the context of a function call,the set of functions named by F shall contain only non-member functions and static member functions. [Footnote: If F names a non-static member function, &F is a pointer-to-member, which cannot be used with the function call syntax.] And in this context using &F behaves the same as using&F is treated the same as the name F by itself. Thus, (&F)(expression-listopt) is simply (F)(expression-listopt), which is discussed in 13.3.1.1.1 over.call.func. If the function selected by overload resolution according to 13.3.1.1.1 over.call.func is a nonstatic member function, the program is ill-formed. [Footnote: When F is a nonstatic member function, a reference of the form &A::F is a pointer-to-member, which cannot be used with the function-call syntax, and a reference of the form &F is an invalid use of the "&" operator on a nonstatic member function.] (The resolution of &F in other contexts is described in 13.4 over.over.)
According to 13.3.1.1.2 over.call.object paragraph 2, when the primary-expression E in the function call syntax evaluates to a class object of type "cv T", a surrogate call function corresponding to an appropriate conversion function declared in a direct or indirect base class B of T is included or not included in the set of candidate functions based on class B being accessible.
For instance in the following code sample, as per the paragraph in question, the expression c(3) calls f2, instead of the construct being ill-formed due to the conversion function A::operator fp1 being inaccessible and its corresponding surrogate call function providing a better match than the surrogate call function corresponding to C::operator fp2:
void f1(int) { }
void f2(float) { }
typedef void (* fp1)(int);
typedef void (* fp2)(float);
struct A {
operator fp1()
{ return f1; }
};
struct B : private A { };
struct C : B {
operator fp2()
{ return f2; }
};
int main()
{
C c;
c(3); // f2 is called, instead of the construct being ill-formed.
return 0;
}
The fact that the accessibility of a base class influences the overload resolution process contradicts the fundamental language rule (3.4 basic.lookup paragraph 1, and 13.3 over.match paragraph 2) that access checks are applied only once name lookup and function overload resolution (if applicable) have succeeded.
Proposed resolution (10/01, same as suggested resolution):
In 13.3.1.1.2 over.call.object paragraph 2, replace the last sentence
Similarly, surrogate call functions are added to the set of candidate functions for each conversion function declared in an accessible base class provided the function is not hidden within T by another intervening declaration.
with
Similarly, surrogate call functions are added to the set of candidate functions for each conversion function declared in a base class of T provided the function is not hidden within T by another intervening declaration. If such a surrogate function is eventually selected as the best viable function and its corresponding conversion function is from an ambiguous (10.2 class.member.lookup) base class of T, the program is ill-formed.
Does dropping a cv-qualifier on a reference binding prevent the binding as far as overload resolution is concerned? Paragraph 4 says "Other restrictions on binding a reference to a particular argument do not affect the formation of a conversion sequence." This was intended to refer to things like access checking, but some readers have taken that to mean that any aspects of reference binding not mentioned in this section do not preclude the binding.
Proposed resolution (10/01):
In 13.3.3.1.4 over.ics.ref paragraph 4 add the indicated text:
Other restrictions on binding a reference to a particular argument that are not based on the types of the reference and the argument do not affect the formation of a standard conversion sequence, however.
(Issue reopened in messages 9099-9101.)
template <class T> void f(T);
template <class T> void g(T);
template <class T> void g(T,T);
int main()
{
(&f<int>);
(&g<int>);
}
The question is whether &f<int> identifies a unique function.
&g<int> is clearly ambiguous.
13.4 over.over paragraph 1 says that a function template name is considered to name a set of overloaded functions. I believe it should be expanded to say that a function template name with an explicit template argument list is also considered to name a set of overloaded functions.
In the general case, you need to have a destination type in order to identify a unique function. While it is possible to permit this, I don't think it is a good idea because such code depends on there only being one template of that name that is visible.
The EDG front end issues an error on this use of "f". egcs 1.1.1 allows it, but the most current snapshot of egcs that I have also issues an error on it.
It has been pointed out that when dealing with nontemplates, the rules for taking the address of a single function differ from the rules for an overload set, but this asymmetry is needed for C compatibility. This need does not exist for the template case.
My feeling is that a general rule is better than a general rule plus an exception. The general rule is that you need a destination type to be sure that the operation will succeed. The exception is when there is only one template in the set and only then when you provide values for all of the template arguments.
It is true that in some cases you can provide a shorthand, but only if you encourage a fragile coding style (that will cause programs to break when additional templates are added).
I think the standard needs to specify one way or the other how this case should be handled. My recommendation would be that it is ill-formed.
Nico Josuttis: Consider the following example:
template <int VAL>
int add (int elem)
{
return elem + VAL;
}
std::transform(coll.begin(), coll.end()
coll.begin(),
add<10>);
If John's recommendation is adopted, this code will become ill-formed. I bet there will be a lot of explanation for users necessary why this fails and that they have to change add<10> to something like (int (*)(int))add<10>.
This example code is probably common practice because this use of the STL is typical and is accepted in many current implementations. I strongly urge that this issue be resolved in favor of keeping this code valid.
Bill Gibbons: I find this rather surprising. Shouldn't a template-id which specifies all of the template arguments be treated like a declaration-only explicit instantiation, producing a set of ordinary function declarations? And when that set happens to contain only one function, shouldn't the example code work?
(See also issue 250.)
Notes from 04/01 meeting:
The consensus of the group was that the add example should not be an error.
Proposed resolution (04/01):
In 13.4 over.over paragraph 1 change:
A function template name is considered to name a set of overloaded functions in such contexts.to:
A function template name is considered to name a set of overloaded functions in such contexts, unless it is followed by an explicit template argument specification (14.8.1 temp.arg.explicit) that identifies a single function template specialization.
Add just before the first example of 14.8.1 temp.arg.explicit paragraph 2:
If a template argument list is specified and it, along with any default template arguments, identifies a single function template specialization, then the resulting template-id is an lvalue of function type (which is subject to function-to-pointer conversion (4.3 conv.func)).
Change the first example of 14.8.1 temp.arg.explicit paragraph 2:
template<class X, class Y> X f(Y);
void g()
{
int i = f<int>(5.6); // Y is deduced to be double
int j = f(5.6); // ill-formed: X cannot be deduced
}
to read:
template<class X, class Y> X f(Y);
void g()
{
int i = f<int>(5.6); // Y is deduced to be double
int j = f(5.6); // ill-formed: X cannot be deduced
f<void>(f<int, bool>); // Y for outer f deduced to be
// int (*)(bool)
f<void>(f<int>); // ill-formed: f<int> does not denote an
// lvalue but an overload set
}
Note: this interacts with the resolution of issue 226.
The prohibition of default template arguments for function templates is a misbegotten remnant of the time where freestanding functions were treated as second class citizens and required all template arguments to be deduced from the function arguments rather than specified.
The restriction seriously cramps programming style by unnecessarily making freestanding functions different from member functions, thus making it harder to write STL-style code.
Suggested resolution:
Replace
A default template-argument shall not be specified in a function template declaration or a function template definition, nor in the template-parameter-list of the definition of a member of a class template.
by
A default template-argument shall not be specified in the template-parameter-list of the definition of a member of a class template.
The actual rules are as stated for arguments to class templates.
Notes from 10/00 meeting:
The core language working group was amenable to this change. Questions arose, however, over the interaction between default template arguments and template argument deduction: should it be allowed or forbidden to specify a default argument for a deduced parameter? If it is allowed, what is the meaning: should one or the other have priority, or is it an error if the default and deduced arguments are different?
Notes from the 10/01 meeting:
It was decided that default arguments should be allowed on friend declarations only when the declaration is a definition. It was also noted that it is not necessary to insist that if there is a default argument for a given parameter all following parameters have default arguments, because (unlike in the class case) arguments can be deduced if they are not specified.
Note that there is an interaction with issue 115.
Proposed resolution (10/01):
In 14.1 temp.param paragraph 9, replace
A default template-argument may be specified in a class template declaration or a class template definition. A default template-argument shall not be specified in a function template declaration or a function template definition, nor in the template-parameter-list of the definition of a member of a class template.
with
A default template-argument may be specified in a template declaration. A default template-argument shall not be specified in the template-parameter-lists of the definition of a member of a class template that appears outside of the member's class.
In 14.8 temp.fct.spec paragraph 9, replace
A default template-argument shall not be specified in a friend template declaration.
with
A default template-argument shall not be specified in a friend class template declaration. If a friend function template declaration specifies a default template-argument, that declaration must be a definition and shall be the only declaration of the function template in the translation unit.
In 14.8 temp.fct.spec paragraph 11, replace
If a template-parameter has a default template-argument, all subsequent template-parameters shall have a default template-argument supplied.
with
If a template-parameter of a class template has a default template-argument, all subsequent template-parameters shall have a default template-argument supplied.
In 14.8 temp.fct.spec paragraph 1, replace
Template arguments can either be explicitly specified when naming the function template specialization or be deduced (14.8.2 temp.deduct) from the context, e.g. from the function arguments in a call to the function template specialization.
with
Template arguments can be explicitly specified when naming the function template specialization, deduced from the context (14.8.2 temp.deduct), e.g., deduced from the function arguments in a call to the function template specialization), or obtained from default template arguments.
In 14.8.1 temp.arg.explicit paragraph 2, replace
Trailing template arguments that can be deduced (14.8.2 temp.deduct) may be omitted from the list of explicit template-arguments.
with
Trailing template arguments that can be deduced (14.8.2 temp.deduct) or obtained from default template-arguments may be omitted from the list of explicit template-arguments.
In 14.8.2 temp.deduct paragraph 1, replace
The values can be either explicitly specified or, in some cases, deduced from the use.
with
The values can be explicitly s