| Document number: | J16/99-0027 = WG21 D1203 |
| Date: | 24 May 1999 |
| Project: | Programming Language C++ |
| Reference: | ISO/IEC IS 14882:1998(E) |
| Reply to: | William M. Miller |
| wmm@fastdial.net |
| Index by IS Reference | Index by Issue Number |
The purpose of this document 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.)
The most current public version of this document can be found at http://www.dkuug.dk/jtc1/sc22/wg21. Requests for further information about this document should include the document number above, reference ISO/IEC 14882:1998(E), and be submitted to the Information Technology Information Council (ITI), 1250 Eye Street NW, Washington, DC 20005, USA.
Public information as to how to obtain a copy of the C++ Standard,
join the Standard Committee, submit an issues, or comment on an issue
can be found in the C++ FAQ at
http://reality.sgi.com/austern_mti/std-c++/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 this document 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.
The text of 3.8 basic.life paragraph 2 currently reads,
The phrase "an object of type" is obviously incorrect. I believe it should read "an object of POD type." Does anyone disgree?
Proposed Resolution (04/99):
As suggested.
5.3.4 expr.new paragraph 6 says:
The expression in a direct-new-declarator shall have integral type (3.9.1 basic.fundamental ) with a non-negative value.I assume the intent was to also allow enumeral types, as we do in 5.2.1 expr.sub ?
Proposed Resolution (04/99):
Replace "integral type" by "enumeration or integral type" in
5.3.4 expr.new
paragraph 6.
Can a typedef redeclaration be done within a class?
class X {
typedef int I;
typedef int I;
};
See also
9.2 class.mem
,
Core issue 36
,
and
Core issue 85
.
Proposed Resolution (04/99):
Change 7.1.3 dcl.typedef
paragraph 2
from "In a given scope" to "In a given non-class scope."
The following code does not compile with the EDG compiler:
volatile const int a = 5;
int b[a];
The standard,
7.1.5.1 dcl.type.cv
, says:
A variable of const-qualified integral or enumeration type initialized by an integral constant expression can be used in integral constant expressions.This doesn't say it can't be const volatile-qualified, although I think that was what was intended.
Proposed Resolution: Change 7.1.5.1 dcl.type.cv to read:
Consider the following:
extern "C" void f();
namespace N {
extern "C" void f();
}
using N::f;
According to
7.3.3 namespace.udecl
paragraph 11, the using-declaration is an error:
If a function declaration in namespace scope or block scope has the same name and the same parameter types as a function introduced by a using-declaration, the program is ill-formed.Based on the context (7.3.3 namespace.udecl paragraph 10 simply reiterates the requirements of 3.3 basic.scope ), one might wonder if the failure to exempt extern "C" functions was intentional or an oversight. After all, there is only one function f() involved, because it's extern "C", so ambiguity is not a reason to prohibit the using-declaration.
This also breaks the relatively strong parallel between extern "C" functions and typedefs established in our discussion of Core issue 14 in Santa Cruz. There the question was for using-directives:
typedef unsigned int size_t;
extern "C" int f();
namespace N {
typedef unsigned int size_t;
extern "C" int f();
}
using namespace N;
int i = f(); // ambiguous "f"?
size_t x; // ambiguous "size_t"?
We decided for both that there was no ambiguity because each
pair of declarations declares the same entity. (According to
3 basic
paragraph 3, a typedef name is not an entity, but a type is; thus the
declarations of size_t declare the same entity "unsigned int".)
In the context of using-declarations, there is no explicit extension of the restrictions in 3.3 basic.scope paragraph 4 except as noted above for function declarations; thus the parallel scenario for a typedef is not ill-formed:
typedef unsigned int size_t;
namespace N {
typedef unsigned int size_t;
};
using N::size_t; // okay, both declarations
// refer to the same entity
I think
7.3.3 namespace.udecl
paragraph 11 ought to be rewritten as:
If a function declaration in namespace scope or block scope has the same name and the same parameter types as a function introduced by a using-declaration, and the declarations do not declare the same function, the program is ill-formed.
Proposed Resolution (04/99):
As suggested.
Section 7.3.4 namespace.udir paragraph 4 uses the term extended-namespace-definition three times:
If a namespace is extended by an extended-namespace-definition after a using-directive for that namespace is given, the additional members of the extended namespace and the members of namespaces nominated by using-directives in the extended-namespace-definition can be used after the extended-namespace-definition.I think the intent is clear, but unfortunately I cannot find any other mention (or definition) of this term.
Mike Miller: True enough; in Section 7.3.1 namespace.def [the grammar] it's called an extension-namespace-definition.
Proposed Resolution (04/99):
Systematically replace "extended-namespace-definition" by "extension-namespace-definition".
(From J16/99-0005 = WG21 N1182, "Proposed Resolutions for Core Language Issues 6, 14, 20, 40, and 89")
There are two sub-issues. The first concerns the statement in 8.3 dcl.meaning paragraph 1,
The id-expression of a declarator-id shall be a simple identifier except for the declaration of some special functions (12.3 class.conv , 12.4 class.dtor , 13.5 over.oper ) and for the declaration of template specializations or partial specializations (14.7 temp.spec ).The second sub-issue is regarding another statement in the same paragraph:
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 ) or nested class (9.7 class.nest ) outside of its class, the definition or explicit instantiation of a function, variable or class member of a namespace outside of its namespace, or...Analysis
The problem in the first sub-issue is that the wrong syntactic non-terminal is mentioned. The relevant portions of the grammar are:
If an unqualified-id is used as the id-expression of a declarator-id, it shall be a simple identifier except...However, it does not appear that this restriction has any meaning; all of the possible cases of unqualified-ids are represented in the list of exceptions! Rather than recasting the sentence into a correct but useless form, it would be better to remove it altogether.
The second sub-issue deals with the conditions under which a qualified-id can be used in a declarator, including "the definition of a...nested class" and "the definition or explicit instantiation of a...class member of a namespace." However, the name in a class definition is not part of a declarator; these constructs do not belong in a list of declarator contexts.
Proposed Resolution (04/99):
The suggested resolution for the first sub-issue overlooked the fact that the existing wording has the additional effect of prohibiting the use of the non-identifier syntax for declaring other than the listed entities. Thus the proposed wording (adopted in full committee 04/99) for the first sub-issue is:
Change 8.3 dcl.meaning paragraph 1 from:
The id-expression of a declarator-id shall be a simple identifier except...to:
An unqualified-id occurring in a declarator-id shall be a simple identifier except...
The proposed change for the second sub-issue is as suggested (removal
of "nested class" and "class member" from the list), but
the full committee has not yet acted on it.
The standard says, in 9.2 class.mem paragraph 4:
A member-declarator can contain a constant-initializer only if it declares a static member (9.4 class.static ) of integral or enumeration type, see 9.4.2 class.static.data .But later, in the section on static class data member initialization, 9.4.2 class.static.data paragraph 4, it says:
If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression (5.19 expr.const ). In that case, the member can appear in integral constant expressions within its scope.The first paragraph should be modified to make it clear that it is not possible to initialize a static data member in-line with a constant-initializer if that data member is of integral (or enumeration) type, and yet not const.
Proposed Resolution (04/99): Change the sentence in 9.2 class.mem paragraph 4 to read:
A member-declarator can contain a constant-initializer only if it declares a static member (9.4 class.static ) of const integral or const enumeration type, see 9.4.2 class.static.data .
Paragraph 2 says that "the object-expression is always evaluated" when the class member syntax is used to refer to a static member. This presumably should say that the object expression is evaluated if the member access is performed, i.e., not if the overall expression is the operand of sizeof or the unevaluated branch of ?:, ||, or &&.
Proposed Resolution (04/99):
Replace "is always evaluated" by "is evaluated" in
9.4 class.static
paragraph 2.
Issue 1
12.8 class.copy (From J16/99-0005 = WG21 N1182, "Proposed Resolutions for Core Language Issues 6, 14, 20, 40, and 89")
There are three related sub-issues in this issue, all dealing with the elision of copy constructors as described in 12.8 class.copy paragraph 15:
After discussion in Santa Cruz, the core group decided that sub-issue #1 required no change; the necessity of an accessible and unambiguous copy constructor is made clear in 12.2 class.temporary paragraph 1 and need not be repeated in this text. The remaining two sub-issues appear to be valid criticisms and should be addressed.
Proposed Resolution (04/99): The paragraph in question should be rewritten as follows. In addition, references to this section should be added to the index under "temporary, elimination of," "elimination of temporary," and "copy, constructor elision."
class Thing {
public:
Thing();
~Thing();
Thing(const Thing&);
};
Thing f() {
Thing t;
return t;
}
Thing t2 = f();
Here the criteria for elision can be combined to eliminate two calls to
the copy constructor of class Thing: the copying of the local
automatic object t into the temporary object for the return value
of function f() and the copying of that temporary object into
object t2. Effectively, the construction of the local object t
can be viewed as directly initializing the global object t2, and
that object's destruction will occur at program exit.
—end example]
The explanation in 14.3.2 temp.arg.nontype paragraph 2 of why a string literal cannot be used as a template argument leaves something to be desired:
...because a string literal is an object with internal linkage.I can't find anything that says that a string literal has internal linkage. In fact, I'd be pretty surprised if I did, since linkage is defined (in 3.5 basic.link ) strictly in terms of names, and a string literal doesn't have a name. Actually, I think that it's the namelessness of a string literal that prevents it from being a template argument; only the third and fourth bullets of 14.3.2 temp.arg.nontype paragraph 1 could conceivably apply, and both of those require that the entity have a name (i.e., that they be given as an id-expression).
Proposed Resolution (04/99):
Replace "is an object with internal linkage" by "has no name" in 14.3.2 temp.arg.nontype
paragraph 2.
Section 7.3.3 namespace.udecl paragraph 8 says:
A using-declaration is a declaration and can therefore be used repeatedly where (and only where) multiple declarations are allowed.It contains the following example:
Indeed, if the double using-declaration for A::i is prohibited in f, why should it be allowed in namespace A1?
Proposed Resolution (04/99): Change the comment "// error: double declaration" to "// OK: double declaration". (This should be reviewed against existing practice.)
See also
Core issue 56
and
Core issue 85
.
Problem Description: At least five of the examples in 14.7.3 temp.expl.spec have errors.
Proposed Resolution:
1. Change the example in paragraph 8 from:
[Example::
// file #1
#include <vector>
// Primary class template vector
export template<class T> void f(t) {
vector<T> vec; // should match the specialization
/* ... */
}
// file #2
#include <vector>
class B { };
// Explicit specialization of vector for vector<B>
template<class T> class vector<B> { /* ... */ }
template<class T> void f(T);
void g(B b) {
f(b); // ill formed:
// f<B> should refer to vector<B>, but the
// specialization was not declared with the
// definition of f in file #1
}
—end example]
to:
[Example:
// file #1
#include <vector>
// Primary class template vector
export template<class T> void f(T) {
using std::vector;
vector<T> vec; // should match the specialization
/* ... */
};
// file #2
#include <vector>
class B { };
// Explicit specialization of vector for vector<B>
namespace std {
template<> class vector<B> { /* ... */ };
}
template<class T> void f(T);
void g(B b) {
f(b); // ill formed:
// f<B> should refer to vector<B>, but the
// specialization was not declared with the
// definition of f in file #1
}
—end example]
2. Change the example in paragraph 9 from:
[Example:
namespace N {
template<class T> class X { /* ... */ };
template<class T> class Y { /* ... */ };
template<> class X<int> { /* ... */ }; // OK: specialization
// in same namespace
template<> class Y<double>; // forward declare intent to
// specialize for double
}
template<> class N::Y<double> { /* ... */ }; // OK: specialization
// in same namespace
—end example]
to:
[Example:
namespace N {
template<class T> class X { /* ... */ };
template<class T> class Y { /* ... */ };
template<> class X<int> { /* ... */ }; // OK: specialization
// in same namespace
template<> class Y<double>; // forward declare intent to
// specialize for double
}
namespace N {
template<> class Y<double> { /* ... */ }; // OK: specialization
// in same namespace
}
—end example]
3. The example in paragraph 16 as it appears in the IS:
[Example:
template<class T> struct A {
void f(T);
template<class X> void g(T, X);
void h(T) { }
};
// specialization
template<> void A<int>::f(int);
// out of class member template definition
template<class T> template<class X> void A<T>::g(T,X) { }
// member template partial specialization
template<> template<class X> void A<int>::g(int, X);
// member template specialization
template<> template<>
void A<int>::g(int, char); // X deduced as char
template<> template<>
void A<int>::g<char>(int, char); // X specified as char
// member specialization even if defined in class definition
template<> void A<int>::h(int) { }
—end example]
The word 'partial' in the third comment in the example
should be removed because this example does not illustrate partial
specialization. Also, the two specializations of template<>
template<> void A<int>::g(int, char); violate
14.7 temp.spec
, paragraph 5, which reads:
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. An implementation is not required to diagnose a violation of this rule.Proposed resolution:
[Example:
template<class T> struct A {
void f(T);
template<class X1> void g1(T, X1);
template<class X2> void g2(T, X2);
void h(T) { }
};
// specialization
template<> void A<int>::f(int);
// out of class member template definition
template<class T> template<class X> void A<T>::g(T,X) { }
// member template specialization
template<> template<class X> void A<int>::g(int, X);
// member template specialization
template<> template<>
void A<int>::g1(int, char); // X1 deduced as char
template<> template<>
void A<int>::g2<char>(int, char); // X2 specified as char
// member specialization even if defined in class definition
template<> void A<int>::h(int) { }
—end example]
4. Remove the spurious semicolon (or the curly brackets) from the end of the last line in the example in paragraph 17. This is the example as it appears in the IS:
[Example:
template<class T1> class A {
template<class T2> class B {
void mf();
};
};
template<> template<> A<int>::B<double> { };
template<> template<> void A<char>::B<char>::mf() {};
—end example]
5. Remove spurious semicolons (or curly brackets) from the specializations of mf1 and mf2 in the example in paragraph 18. This is the text of the example as it appears in the IS:
[Example:
template<class T1> class A {
template<class T2> class B {
template<class T3> void mf1(T3);
void mf2();
};
};
template<> template<class X>
class A<int>::B { };
template<> template<> template<class T>
void A<int>::B<double>::mf1(T t) { };
template<class Y> template<>
void A<Y>::B<double>::mf2() { }; // ill-formed; B<double> is specialized but
// its enclosing class template A is not
—end example]
Section 3.4.2 basic.lookup.koenig includes the following:
struct A {
union U {};
friend void f(U);
};
struct B {
struct S {};
friend void f(S);
};
int main() {
A::U u;
f(u); // okay: A is an associated class
A::S s;
f(s); // error: no matching f(), B is not an associated class
}
Certainly the enclosing class should also be an associated class for nested
class types, shouldn't it?
Proposed Resolution (04/99):
As suggested.
The C++ standard has inherited the definition of the 'exit' function more or less unchanged from ISO C.
However, when the 'exit' function is called, objects of static extent which have been initialised, will be destructed if their types posses a destructor.
In addition, the C++ standard has inherited the definition of the 'signal' function and its handlers from ISO C, also pretty much unchanged.
The C standard says that the only standard library functions that may be called while a signal handler is executing, are the functions 'abort', 'signal' and 'exit'.
This introduces a bit of a nasty turn, as it is not at all unusual for the destruction of static objects to have fairly complex destruction semantics, often associated with resource release. These quite commonly involve apparently simple actions such as calling 'fclose' for a FILE handle.
Having observed some very strange behaviour in a program recently which in handling a SIGTERM signal, called the 'exit' function as indicated by the C standard.
But unknown to the programmer, a library static object performed some complicated resource deallocation activities, and the program crashed.
The C++ standard says nothing about the interaction between signals, exit and static objects. My observations, was that in effect, because the destructor called a standard library function other than 'abort', 'exit' or 'signal', while transitively in the execution context of the signal handler, it was in fact non-compliant, and the behaviour was undefined anyway.
This is I believe a plausible judgement, but given the prevalence of this common programming technique, it seems to me that we need to say something a lot more positive about this interaction.
Curiously enough, the C standard fails to say anything about the analogous interaction with functions registered with 'atexit' ;-)
Proposed Resolution (10/98):
The current Committee Draft of the next version of the ISO C standard specifies that the only standard library function that may be called while a signal handler is executing is 'abort'. This would solve the above problem.
[This issue should remain open until it has been decided that the next
version of the C++ standard will use the next version of the C standard
as the basis for the behavior of 'signal'.]
From J16/98-0026 = WG21 N1169, "Proposed Defect Reports on ISO/IEC
14882, Programming Languages - C++":
A reference is rebindable. This is surprising and unnatural. This can also cause subtle optimizer bugs.Example:
struct T { int& ri; T (int& r) : ri (r) { } }; void bar (T*); void foo () { int i; T x (i); x.ri = 3; // the optimizer understands that this is really i = 3 bar (&x); x.ri = 4; // optimizer assumes that this writes to i, but this is incorrect } int gi; void bar (T* p) { p->~T (); new (p) T (gi); }If we replace T& with T* const in the example then undefined behavior result and the optimizer is correct.Proposal: make T& equivalent to T* const by extending the scope of 3.8 basic.life paragraph 9 to references.
(See also J16/99-0005 = WG21 N1182, "Proposed Resolutions for Core Language Issues 6, 14, 20, 40, and 89")
Proposed Resolution
Add a new bullet to the list of restrictions in 3.8 basic.life paragraph 7, following the second bullet ("the new object is of the same type..."):
- the type of the objects, if a class type, does not contain any non-static data member whose type is const-qualified or a reference type, and
52. Non-static members, member selection and access checking
Section: 5.2.5 expr.ref Status: drafting Submitter: Steve Adamczyk Date: 13 Oct 1998
5.2.5 expr.ref paragraph 4 should make it clear that when a nonstatic member is referenced in a member selection operation, the type of the left operand is implicitly cast to the naming class of the member. This allows for the detection of access and ambiguity errors on that implicit cast.
Proposed Resolution (04/99): A non-normative note in 11.2 class.access.base paragraph 4 already indicates this. Hence, the relevant part of that note should be made normative.
53. Lvalue-to-rvalue conversion before certain static_casts
Section: 5.2.9 expr.static.cast Status: drafting Submitter: Steve Adamczyk Date: 13 Oct 1998
Section 5.2.9 expr.static.cast paragraph 6 should make it clear that when any of the "inverse of any standard conversion sequence" static_casts are done, the operand undergoes the lvalue-to-rvalue conversions first.
Proposed Resolution (04/99): As suggested.
73. Pointer equality
Section: 5.10 expr.eq Status: drafting Submitter: Nathan Myers Date: 13 Nov 1998
Nathan Myers: In 5.10 expr.eq , we have:
Pointers to objects or functions of the same type (after pointer conversions) can be compared for equality. Two pointers of the same type compare equal if and only if they are both null, both point to the same object or function, or both point one past the end of the same array.What does this say, when we haveint i[1]; int j[1];about the expression (i+1 == j) ? It seems to require padding between i[0] and j[0] so that the comparison will come out false.
I think this may be a defect, in that the quoted paragraph extends operator=='s domain too far beyond operator<'s. It should permit (but not require) an off-the-end pointer to compare equal to another object, but not to any element of the same array.Mike Miller: I think this is reading more into the statement in 5.10 expr.eq paragraph 1 than is actually there. What does it mean for a pointer to "point to" an object? I can't find anything that definitively says that i+1 cannot "point to" j[0] (although it's obviously not required to do so). If i+1 is allowed to "point to" j[0], then i+1==j is allowed to be true, and there's no defect. There are places where aliasing is forbidden, but the N+1th element of an array doesn't appear to be one of them.
To put it another way, "points to" is undefined in the Standard. The only definition I can think of that encompasses the possible ways in which a pointer can get its value (e.g., the implementation-defined conversion of an arbitrary integer value to a pointer) is that it means "having the same value representation as would be produced by applying the (builtin) & operator to an lvalue expression designating that object". In other words, if the bits are right, it doesn't matter how you produced the value, as long as you didn't perform any operations that have undefined results. The expression i+1 is not undefined, so if the bits of i+1 are the same as those of &j[0], then i+1 "points to" j[0] and i+i==j is allowed to be true.
Tom MacDonald: C9X contains the following words for the "==" operator:
Two pointers compare equal if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.Matt Austern: I don't think there's anything wrong with saying that the result ofint x[1]; int y[1]; std::cout << (y == x + 1) << std::endl;is implementation defined, or even that it's undefined.Mike Miller: A similar question could be raised about different objects that (sequentially) share the same storage. Consider the following:
struct B { virtual void f(); }; struct D1: B { }; struct D2: B { }; void g() { B* bp1 = new D1; B* bp2 = new (bp1) D2; bp1 == bp2; // ??? }Section 3.8 basic.life paragraph 5 does not list this kind of comparison among the pointer operations that cause undefined behavior, so presumably the comparison is allowed. However, 5.10 expr.eq paragraph 1 describes pointer comparison in terms of "[pointing] to the same object," which bp1 and bp2 clearly do not do. How should we describe the result of this comparison?Proposed Resolution (04/99): An implementation should not be forced to provide padding at the end of arrays; however coming up with the correct words is tricky. In particular, the C9X wording does not seem satisfactory.
94. Inconsistencies in the descriptions of constant expressions
Section: 5.19 expr.const Status: drafting Submitter: Mike Miller Date: 8 Feb 1999
- According to 9.4.2 class.static.data paragraph 4, a static const integral or const enumeration data member initialized with an integral constant expression "can appear in integral constant expressions within its scope" [emphasis mine]. This means that the following is not permitted:
struct S { static const int c = 5; }; int a[S::c]; // error: S::c not in scopeIs this restriction intentional? If so, what was the rationale for the restriction?Bjarne Stroustrup: I think that once you have said S::, c is in scope so that
int a[S::c];is ok.Mike Miller: I'd like to think that's what it meant, but I don't believe that's what it said. According to 3.3 basic.scope paragraph 1, the scope of a name is the region "in which that name may be used as an unqualified name." You can, indeed, use a qualified name to refer to a name that is not in scope, but that only goes to reinforce my point that "S::c" is not in scope at the point where the expression containing it is used. I think the phrase "within its scope" is at best misleading and should be removed. (Unless there's a reason I'm missing for restricting the use of static member constants to their scope.)
- According to 5.19 expr.const paragraph 1, integral constant expressions can "involve...const variables or static data members of integral or enumeration types initialized with constant expressions." However, in 5.19 expr.const paragraph 3, arithmetic constant expressions cannot include them. This seems a rather gratuitous distinction and one likely to bite programmers trained always to use const variables instead of preprocessor definitions. Again, is there a rationale for the difference?
As far as I can tell from 5.19 expr.const paragraph 2, "arithmetic constant expressions" (as distinct from "integral constant expressions") are used only in static initializers to distinguish between static and dynamic initialization. They include floating point types and exclude non-type template parameters, as well as the const variables and static data members.
- There is a minor error in 5.19 expr.const paragraph 2. The first sentence says, "Other expressions are considered constant expressions only for the purpose of non-local static object initialization." However, 6.7 stmt.dcl paragraph 4 appears to rely on the same definition dealing with the initialization of local static objects. I think that the words "non-local" should be dropped and a cross reference to 6.7 stmt.dcl added.
- 5.19 expr.const paragraph 4 says, "An expression that designates the address of a member or base class of a non-POD class object (clause 9) is not an address constant expression (12.7 class.cdtor )."
I'm guessing that should be "non-static member," like the similar prohibition in 12.7 class.cdtor regarding out-of-lifetime access to members of non-POD class objects.
Proposed Resolutions (04/99):
- Remove the phrase "within its scope" in 9.4.2 class.static.data paragraph 4; this sub-issue is now "ready".
- As suggested.
- NAD; the proposed change would be a cleanup, but since the change is unobservable it is not worth applying.
- Change the sentence in 5.19 expr.const paragraph 4 to "An expression that designates the address of a subobject of a non-POD class object is not an address constant expression." This sub-issue is now "ready".
69. Storage class specifiers on template declarations
Section: 7.1.1 dcl.stc Status: drafting Submitter: Mike Ball Date: 17 Oct 1998
I cannot find anything in the standard that tells me the meaning of a storage-class-specifier on a function template declaration. In particular, there is no indication what effect, if any, it has on the storage class of the instantiations.
There is an explicit prohibition of storage-class-specifiers on explicit specializations.
For example, if we have
template<class T> static int foo(T) { return sizeof(T); }does this generate static functions for all instantiations? By 7.1.1 dcl.stc the storage class applies to the name declared in the declarator, which is the template foo, not an instantiation of foo, which is named with a template-id. There is a statement in clause 14 that template names have linkage, which supports the contention that "static" applies to the template, not to instantiations.So what does the specifier mean? Lacking a direct statement in the standard, I see the following posibilities, in my preference order.
Of course, if anybody can find some concrete statement, that would settle it.
- storage-class-specifiers have no meaning on template declarations, their use being subsumed by "export" (for the template name) and the unnamed namespace (for instantiations)
- storage-class-specifiers have no effect on the template name, but do affect the linkage of the instantiations, though this now applies linkage to template-ids, which I can find no support for. I suspect this is what was intended, though I don't remember
From 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: This makes sense, but I couldn't find much support in the document. Sounds like yet another interpretation to add to the list.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.John: Agreed.
Mike: 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."export" is an additional attribute that is separate from linkage, but that can only be applied to templates with external linkage.John: 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.
Mike: 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: 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.
Proposed Resolution (04/99): Disallow storage class specifiers on template declarations, as is stated in 7.1.1 dcl.stc paragraph 4. However, other places suggest otherwise and will need to be revised accordingly.
68. Grammar does not allow "friend class A<int>;"
Section: 7.1.5.3 dcl.type.elab Status: drafting Submitter: Mike Ball Date: 17 Oct 1998
I can't find the answer to the following in the standard. Does anybody have a reference?
The syntax for elaborated type specifier is
elaborated-type-specifier:
Which does not allow the productionclass-key ::opt nested-name-specifieropt identifier
enum ::opt nested-name-specifieropt identifier
typename ::opt nested-name-specifier identifier
typename ::opt nested-name-specifier templateopt template-id
If an elaborated-type-specifier is the sole constituent of a declaration, the declaration is ill-formed unless it is an explicit specialization (14.7.3 temp.expl.spec ), an explicit instantiation (14.7.2 temp.explicit ) or it has one of the following forms:
class-key identifier:
friend class-key identifier ;
friend class-key :: identifier ;
friend class-key nested-name-specifier identifier ;class foo<int> // foo is a templateOn the other hand, a friend declaration seems to require this production,An elaborated-type-specifier shall be used in a friend declaration for a class.*And in 14.5.3 temp.friend we find the example[Footnote: The class-key of the elaborated-type-specifier is required. —end footnote]
[Example:Is there some special dispensation somewhere to allow the syntax in this context? Is there something I've missed about elaborated-type-specifier? Is it just another bug in the standard?template<class T> class task; template<class T> task<T>* preempt(task<T>*); template<class T> class task { // ... friend void next_time(); friend void process(task<T>*); friend task<T>* preempt<T>(task<T>*); template<class C> friend int func(C); friend class task<int>; template<class P> friend class frd; // ... };Proposed Resolution (04/99): This is a bug. Among others, 7.1.5.3 dcl.type.elab should be updated (perhaps using the grammatical construct class-name).
4. Does extern "C" affect the linkage of function names with internal linkage?
Section: 7.5 dcl.link Status: drafting Submitter: Mike Anderson Date: unknown
(Previously numbered 864.)
7.5 dcl.link paragraph 6 says the following:
At most one of a set of overloaded functions with a particular name can have C linkage.
Does this apply to static functions as well? For example, is the following well-formed?extern "C" { static void f(int) {} static void f(float) {} };Can a function with internal linkage "have C linkage" at all (assuming that phrase means "has extern "C" linkage"), for how can a function be extern "C" if it's not extern? The function type can have extern "C" linkage -- but I think that's independent of the linkage of the function name. It should be perfectly reasonable to say, in the example above, that extern "C" applies only to the types of f(int) and f(float), not to the function names, and that the rule in 7.5 dcl.link paragraph 6 doesn't apply.Suggested resolution: The extern "C" linkage specification applies only to the type of functions with internal linkage, and therefore some of the rules that have to do with name overloading don't apply.
Proposed Resolution:
The intent is to distingush implicit linkage from explicit linkage for both name linkage and language (function type) linkage. (It might be more clear to use the terms name linkage and type linkage to distinguish these concepts. A function can have a name with one kind of linkage and a type with a different kind of linkage. The function itself has no linkage: it has no name, only the declaration has a name. This becomes more obvious when you consider function pointers.)
The tentatively agreed proposal is to apply implicit linkage to names declared in brace-enclosed linkage specifications and to non-top-level names declared in simple linkage specifications; and to apply explicit linkage to top-level names declared in simple linkage specifications.
The language linkage of any function type formed through a function declarator is that of the nearest enclosing linkage-specification. For purposes of determining whether the declaration of a namespace-scope name matches a previous declaration, the language linkage portion of the type of a function declaration (that is, the language linkage of the function itself, not its parameters, return type or exception specification) is ignored.
For a linkage-specification using braces, i.e.
extern string-literal { declaration-seqopt }the linkage of any declaration of a namespace-scope name (including local externs) which is not contained in a nested linkage-specification, is not declared to have no linkage (static), and does not match a previous declaration is given the linkage specified in the string-literal. The language linkage of the type of any function declaration of a namespace-scope name (including local externs) which is not contained in a nested linkage-specification and which is declared with function declarator syntax is the same as that of a matching previous declaration, if any, else is specified by string-literal.For a linkage-specification without braces, i.e.
extern string-literal declarationthe linkage of the names declared in the top-level declarators of declaration is specified by string-literal; if this conflicts with the linkage of any matching previous declarations, the program is ill-formed. The language linkage of the type of any top-level function declarator is specified by string-literal; if this conflicts with the language linkage of the type of any matching previous function declarations, the program is ill-formed. The effect of the linkage-specification on other (non top-level) names declared in declaration is the same as that of the brace-enclosed form.[This needs additional drafting work.]
29. Linkage of locally declared functions
Section: 7.5 dcl.link Status: drafting Submitter: Mike Ball Date: 19 Mar 1998
Consider the following:
extern "C" void foo() { extern void bar(); bar(); }Does "bar()" have "C" language linkage?The ARM is explicit and says
A linkage-specification for a function also applies to functions and objects declared within it.The DIS saysIn a linkage-specification, the specified language linkage applies to the function types of all function declarators, function names, and variable names introduced by the declaration(s).Is the body of a function definition part of the declaration?From Mike Miller:
Yes: from 7 dcl.dcl paragraph 1,
declaration:
and 8.4 dcl.fct.def paragraph 1:function-definition
function-definition:
At least that's how I'd read it.decl-specifier-seqopt declarator ctor-initializeropt function-body
From Dag Brück:
Consider the following where extern "C" has been moved to a separate declaration:
extern "C" void foo(); void foo() { extern void bar(); bar(); }I think the ARM wording could possibly be interpreted such that bar() has "C" linkage in my example, but not the DIS wording.As a side note, I have always wanted to think that placing extern "C" on a function definition or a separate declaration would produce identical programs.
Proposed Resolution:
See the proposed resolution for Core issue 4 , which covers this case.
The ODR should also be checked to see whether it addresses name and type linkage.
1. What if two using-declarations refer to the same function but the declarations introduce different default-arguments?
Section: 8.3.6 dcl.fct.default Status: drafting Submitter: Bill Gibbons Date: unknown
3.3 basic.scope paragraph 4 says:
Given a set of declarations in a single declarative region, each of which specifies the same unqualified name,8.3.6 dcl.fct.default paragraph 9 says:
- they shall all refer to the same entity, or all refer to functions ...
When a declaration of a function is introduced by way of a using-declaration (7.3.3 namespace.udecl , any default argument information associated with the declaration is imported as well.This is not really clear regarding what happens in the following case:namespace A { extern "C" void f(int = 5); } namespace B { extern "C" void f(int = 7); } using A::f; using B::f; f(); // ???Proposed Resolution:Add the following at the end of 13.3.3 over.match.best :
If the best viable function resolves to a function for which multiple declarations were found, and if at least two of these declarations specify a default argument that made the function viable, the program is ill-formed. [Example:namespace A { extern "C" void f(int = 5); } namespace B { extern "C" void f(int = 5); } using A::f; using B::f; void use() { f(3); // OK, default argument was not used for viability f(); // Error: found default argument twice! }—end example]This wording needs some work to "see through" using-declarations.
5. CV-qualifiers and type conversions
Section: 8.5 dcl.init Status: drafting Submitter: Josee Lajoie Date: unknown
The description of copy-initialization in 8.5 dcl.init paragraph 14 says:
Should "destination type" in this last bullet refer to "cv-unqualified destination type" to make it clear that the destination type excludes any cv-qualifiers? This would make it clearer that the following example is well-formed:
- If the destination type is a (possibly cv-qualified) class type:
...- Otherwise (i.e. for the remaining copy-initialization cases), user-defined conversion sequences that can convert from the source type to the destination type or (when a conversion function is used) to a derived class thereof are enumerated ... if the function is a constructor, the call initializes a temporary of the destination type. ...
struct A { A(A&); }; struct B : A { }; struct C { operator B&(); }; C c; const A a = c; // allowed?The temporary created with the conversion function is an lvalue of type B. If the temporary must have the cv-qualifiers of the destination type (i.e. const) then the copy-constructor for A cannot be called to create the object of type A from the lvalue of type const B. If the temporary has the cv-qualifiers of the result type of the conversion function, then the copy-constructor for A can be called to create the object of type A from the lvalue of type const B. This last outcome seems more appropriate.Proposed Resolution:
As above.
39. Conflicting amgibuity rules
Section: 10.2 class.member.lookup Status: drafting Submitter: Neal M Gafter Date: 20 Aug 1998
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:
The above example should be well-formed.
9. Clarification of access to base class members
Section: 11.2 class.access.base Status: drafting Submitter: unknown Date: unknown
11.2 class.access.base paragraph 4 says:
A base class is said to be accessible if an invented public member of the base class is accessible. If a base class is accessible, one can implicitly convert a pointer to a derived class to a pointer to that base class.Given the above, is the following well-formed?class D; class B { protected: int b1; friend void foo( D* pd ); }; class D : protected B { }; void foo( D* pd ) { if ( pd->b1 > 0 ); // Is 'b1' accessible? }Can you access the protected member b1 of B in foo? Can you convert a D* to a B* in foo?1st interpretation:
A public member of B is accessible within foo (since foo is a friend), therefore foo can refer to b1 and convert a D* to a B*.
2nd interpretation:
B is a protected base class of D, and a public member of B is a protected member of D and can only be accessed within members of D and friends of D. Therefore foo cannot refer to b1 and cannot convert a D* to a B*.
(See also J16/99-0002 = WG21 N1179.)
Proposed Resolution (04/99): The fourth bullet of 11.2 class.access.base paragraph 4 creates an infinite recursion by not excluding the class of which m is a member from consideration as the base class B. The algorithm should be revised to remove the infinite recursion; with that change, it is clear that the example is ill-formed.
16. Access to members of indirect private base classes
Section: 11.2 class.access.base Status: drafting Submitter: unknown Date: unknown
The text in 11.2 class.access.base paragraph 4 does not seem to handle the following cases:
class D; class B { private: int i; friend class D; }; class C : private B { }; class D : private C { void f() { B::i; //1: well-formed? i; //2: well-formed? } };The member i is not a member of D and cannot be accessed in the scope of D. What is the naming class of the member i on line //1 and line //2?(See also paper J16/99-0002 = WG21 N1179.)
Proposed Resolution (04/99): As described for Core issue 9 . With that change, it is clear that the example is ill-formed.
45. Access to nested classes
Section: 11.8 class.access.nest Status: drafting Submitter: Daveed Vandevoorde Date: 29 Sep 1998
Example:
#include <iostream.h> class C { // entire body is private struct Parent { Parent() { cout << "C::Parent::Parent()\n"; } }; struct Derived : Parent { Derived() { cout << "C::Derived::Derived()\n"; } }; Derived d; }; int main() { C c; // Prints message from both nested classes return 0; }How legal/illegal is this? Paragraphs that seem to apply here are:11 class.access paragraph 1:
A member of a class can beand 11.8 class.access.nest paragraph 1:
- private; that is, its name can be used only by members and friends of the class in which it is declared. [...]
The members of a nested class have no special access to members of an enclosing class, nor to classes or functions that have granted friendship to an enclosing class; the usual access rules (clause 11 class.access ) shall be obeyed. [...]This makes me think that the ': Parent' part is OK by itself, but that the implicit call of 'Parent::Parent()' by 'Derived::Derived()' is not.From Mike Miller:
I think it is completely legal, by the reasoning given in the (non-normative) 11.8 class.access.nest paragraph 2. The use of a private nested class as a base of another nested class is explicitly declared to be acceptable there. I think the rationale in the comments in the example ("// OK because of injection of name A in A") presupposes that public members of the base class will be public members in a (publicly-derived) derived class, regardless of the access of the base class, so the constructor invocation should be okay as well.
I can't find anything normative that explicitly says that, though.
Proposed Resolution:
A member class should have access to the members of the enclosing class in the same manner as if it had been declared a friend of the enclosing class. See paper J16/99-0009 = WG21 N1186.
38. Explicit template arguments and operator functions
Section: 14.2 temp.names Status: drafting Submitter: John Wiegley Date: 17 Aug 1998
It appears from the grammar that explicit template arguments cannot be specified for overloaded operator names. Does this mean that template operators can never be friends?
But assuming that I read things wrong, then I should be able to specify a global template 'operator +' by writing:
friend A::B operator + <>(char&);From John Spicer:You should be able to have explicit template arguments on operator function, but the grammar does seem to prohibit it (unless I'm reading it incorrectly). This is an error in the grammar, they should be permitted.
Tentative Resolution:
As suggested.
Issues with "Open" Status
95. Elaborated type specifiers referencing names declared in friend decls
Section: 7.3.1.2 namespace.memdef Status: open 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 7.3.1.2 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,
friend declarations (11.4 class.friend ) may introduce a (possibly not visible) name into an enclosing name-space
while 3.3.1 basic.scope.pdecl paragraph 6 says exactly the opposite:friend declarations refer to functions or classes that are members of the nearest enclosing namespace, but they do not introduce new names into that namespace (7.3.1.2 namespace.memdef ).
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 7.1.5.3 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 7.1.5.3 dcl.type.elab paragraph 3 says:
The class-key or enum keyword present in the elaborated-type-specifier shall agree in kind with the declaration to which the name in the elaborated-type-specifier refers. This rule also applies to the form of elaborated-type-specifier that declares a class-name or friend class since it can be construed as referring to the definition of the class. Thus, in any elaborated-type-specifier, the enum keyword shall be used to refer to an enumeration (7.2 dcl.enum ), the union class-key shall be used to refer to a union (clause 9 class ), and either the class or struct class-key shall be used to refer to a class (clause 9 class ) declared using the class or struct class-key.
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 thatclass B; union B {};andunion 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:
While it was generally agreed that it is poor practice to take advantage of an external declaration once it had gone out of scope, some argued that a translator had to remember the declaration for checking anyway, so why not acknowledge this? The compromise adopted was to decree essentially that block scope rules apply, but that a conforming implementation need not diagnose a failure to redeclare an external identifier that had gone out of scope (undefined behavior).
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.
107. Linkage of operator functions
Section: 7.5 dcl.link Status: open Submitter: Stephen Clamage Date: 21 Apr 1999
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.
66. Visibility of default args vs overloads added after using-declaration
Section: 8.3.6 dcl.fct.default Status: open 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 paragraph9 says that extra function overloads are not. This seems inconsistent, especially given the similarity of default arguments and overloads.
78. Section 8.5 paragraph 9 should state it only applies to non-static objects
Section: 8.5 dcl.init Status: open Submitter: Judy Ward Date: 15 Dec 1998Paragraph 9 of 8.5 dcl.init says:
If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD class type (or array thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlying class type shall have a user-declared default constructor. Otherwise, if no initializer is specified for an object, the object and its subobjects, if any, have an indeterminate initial value; if the object or any of its subobjects are of const-qualified type, the program is ill-formed.It should be made clear that this paragraph does not apply to static objects.Tentative Resolution: Put the word "non-static" before "object".
85. Redeclaration of member class
Section: 9.1 class.name Status: open Submitter: Steve Adamczyk Date: 25 Jan 1999
In 9.1 class.name paragraph 2, there is the example
void g() { struct s; // hide global struct s // with a local declaration s* p; // refer to local struct s struct s { char* p; }; // define local struct s struct s; // redeclaration, has no effect }The final redeclaration is invalid according to 9.2 class.mem paragraph 1 last sentence.See also Core issue 36 and Core issue 56 .
80. Class members with same name as class
Section: 9.2 class.mem Status: open Submitter: Jason Merrill Date: 5 Dec 1998
Between the May '96 and September '96 working papers, the text in 9.2 class.mem paragraph 13:
If T is the name of a class, then each of the following shall have a name different from T:was changed by removing the word 'static'. Looking over the meeting minutes from Stockholm, none of the proposals seem to include this change, which breaks C compatibility and is not mentioned in the compatibility annex. Was this change actually voted in by the committee?
- every static data member of class T;
Specifically, this breaks /usr/include/netinet/in.h under Linux, in which "struct ip_opts" shares its name with one of its members.
57. Empty unions
Section: 9.5 class.union Status: open Submitter: Steve Adamczyk Date: 13 Oct 1998
There doesn't seem to be a prohibition in 9.5 class.union against a declaration like
union { int : 0; } x;Should that be valid? If so, 8.5 dcl.init paragraph 5 third bullet, which deals with default-initialization of unions, should say that no initialization is done if there are no data members.What about:
union { } x; static union { };If the first example is well-formed, should either or both of these cases be well-formed as well?
58. Signedness of bit fields of enum type
Section: 9.6 class.bit Status: open Submitter: Steve Adamczyk Date: 13 Oct 1998
Section 9.6 class.bit paragraph 4 needs to be more specific about the signedness of bit fields of enum type. How much leeway does an implementation have in choosing the signedness of a bit field? In particular, does the phrase "large enough to hold all the values of the enumeration" mean "the implementation decides on the signedness, and then we see whether all the values will fit in the bit field", or does it require the implementation to make the bit field signed or unsigned if that's what it takes to make it "large enough"?
8. Access to template arguments used in a function return type and in the nested name specifier
Section: 11 class.access Status: open Submitter: Mike Ball Date: unknown
Consider the following example:
class A { class A1{}; static void func(A1, int); static void func(float, int); static const int garbconst = 3; public: template < class T, int i, void (*f)(T, int) > class int_temp {}; template<> class int_temp<A1, 5, func> { void func1() }; friend int_temp<A1, 5, func>::func1(); int_temp<A1, 5, func>* func2(); }; A::int_temp<A::A1, A::garbconst + 2, &A::func>* A::func2() {...}ISSUE 1:In 11 class.access paragraph 5 we have:
All access controls in clause 11 affect the ability to access a class member name from a particular scope... In particular, access controls apply as usual to member names accessed as part of a function return type, even though it is not possible to determine the access privileges of that use without first parsing the rest of the function declarator.
This means, if we take the loosest possible definition of "access from a particular scope", that we have to save and check later the following namesA::int_temp A::A1 A::garbconst (part of an expression) A::func (after overloading is done)I suspect that member templates were not really considered when this was written, and that it might have been written rather differently if they had been. Note that access to the template arguments is only legal because the class has been declared a friend, which is probably not what most programmers would expect.Rationale:
Not a defect. This behavior is as intended.
ISSUE 2:
Now consider void A::int_temp<A::A1, A::garbconst + 2, &A::func>::func1() {...} By my reading of 11.8 class.access.nest , the references to A::A1, A::garbconst and A::func are now illegal, and there is no way to define this function outside of the class. Is there any need to do anything about either of these Issues?
This issue needs work.
17. Footnote 99 should discuss the naming class when describing members that can be accessed from friends
Section: 11.2 class.access.base Status: open 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.)
77. The definition of friend does not allow nested classes to be friends
Section: 11.4 class.friend Status: open Submitter: Judy Ward Date: 15 Dec 1998The 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 {}; };
10. Can a nested class access its own class name as a qualified name if it is a private member of the enclosing class?
Section: 11.8 class.access.nest Status: open Submitter: Josee Lajoie Date: unknown
Paragraph 1 says: "The members of a nested class have no special access to members of an enclosing class..."
This prevents a member of a nested class from being defined outside of its class definition. i.e. Should the following be well-formed?
class D { class E { static E* m; }; }; D::E* D::E::m = 1; // ill-formedThis is because the nested class does not have access to the member E in D. 11 class.access paragraph 5 says that access to D::E is checked with member access to class E, but unfortunately that doesn't give access to D::E. 11 class.access paragraph 6 covers the access for D::E::m, but it doesn't affect the D::E access. Are there any implementations that are standard compliant that support this?Here is another example:
class C { class B { C::B *t; //2 error, C::B is inaccessible }; };This causes trouble for member functions declared outside of the class member list. For example:class C { class B { B& operator= (const B&); }; }; C::B& C::B::operator= (const B&) { } //3If the return type (i.e. C::B) is access checked in the scope of class B (as implied by 11 class.access paragraph 5) as a qualified name, then the return type is an error just like referring to C::B in the member list of class B above (i.e. //2) is ill-formed.This issue depends on the outcome of Core issue 45 .
86. Lifetime of temporaries in query expressions
Section: 12.2 class.temporary Status: open Submitter: Steve Adamczyk Date: Jan 1999In 12.2 class.temporary paragraph 5, should binding a reference to the result of a "?" operation, each of whose branches is a temporary, extend both temporaries?
Here's an example:
const SFileName &C = noDir ? SFileName("abc") : SFileName("bcd");Do the temporaries created by the SFileName conversions survive the end of the full expression?
102. Operator lookup rules do not work well with parts of the library
Section: 13.3.1.2 over.match.oper Status: open 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?
59. Clarification of overloading and UDC to reference type
Section: 13.3.1.4 over.match.copy Status: open Submitter: Steve Adamczyk Date: 13 Oct 1998
Sections 13.3.1.4 over.match.copy and 13.3.1.5 over.match.conv should be clarified regarding the treatment of conversion functions which return reference types.
Suggested resolution:
In 13.3.1.4 over.match.copy paragraph 1, change
Conversion functions that return "reference to T" return lvalues of type T and are therefore considered to yield T for this process of selecting candidate functions.toConversion functions that return "reference to X" return lvalues of type X and are therefore considered to yield X for this process of selecting candidate functions.In 13.3.1.5 over.match.conv paragraph 1, changeConversion functions that return "reference to T" return lvalues of type T and are therefore considered to yield T for this process of selecting candidate functions.toConversion functions that return "reference to cv2 X" return lvalues of type "cv2 X" and are therefore considered to yield X for this process of selecting candidate functions.
51. Overloading and user-defined conversions
Section: 13.3.3 over.match.best Status: open Submitter: Steve Adamczyk Date: 13 Oct 1998
In 13.3.3 over.match.best paragraph 1, bullet 4 of the second set of bullets, there is a cross-reference to 8.5 dcl.init and 13.3.1.5 over.match.conv . I believe it should also reference 13.3.1.6 over.match.ref . I think the phrase "initialization by user-defined conversion" was intended to refer to all initializations using user-defined conversions, and not just the case in 13.3.1.5 over.match.conv . Referring to only 13.3.1.5 over.match.conv suggests a narrower meaning of the phrase.
13.3.1.4 over.match.copy , although it does deal with initialization by user-defined conversion, does not need to be referenced because it deals with class --> class cases, and therefore there are no standard conversions involved that could be compared.
84. Overloading and conversion loophole used by auto_ptr
Section: 13.3.3.1 over.best.ics Status: open Submitter: Steve Adamczyk Date: 10 Dec 1998
By the letter of the standard, the conversions required to make auto_ptr work should be accepted.
However, there's good reason to wonder if there isn't a bug in the standard here. Here's the issue: line 16 in the example below comes down to
copy-initialize an auto_ptr<Base> from an auto_ptr<Derived> rvalueTo do that, we first look to see whether we can convert an auto_ptr<Derived> to an auto_ptr<Base>, by enumerating the constructors of auto_ptr<Base> and the conversion functions of auto_ptr<Derived>. There's a single possible way to do the conversion, namely the conversion functionauto_ptr<Derived>::operator auto_ptr<Base>()(generated from the template). (The constructor auto_ptr<Base>(auto_ptr_ref<Base>) doesn't work because it requires a user-defined conversion on the argument.)So far, so good. Now, we do the copy step:
direct-initialize an auto_ptr<Base> from an auto_ptr<Base> rvalueThis, as we've gone to great lengths to set up, is done by calling the conversion functionauto_ptr<Base>::operator auto_ptr_ref<Base>()(generated from the template), and then the constructorauto_ptr<Base>(auto_ptr_ref<Base>)(generated from the template).The problem with this interpretation is that it violates the long-standing common-law rule that only a single user-defined conversion will be called to do an implicit conversion. I find that pretty disturbing. (In fact, the full operation involves two conversion functions and two constructors, but "copy" constructors are generally considered not to be conversions.)
The direct-initialization second step of a copy-initialization was intended to be a simple copy -- you've made a temporary, and now you use a copy constructor to copy it. Because it is defined in terms of direct initialization, however, it can exploit the loophole that auto_ptr is based on.
To switch to personal opinion for a second, I think it's bad enough that auto_ptr has to exploit a really arcane loophole of overload resolution, but in this case it seems like it's exploiting a loophole on a loophole.
struct Base { // 2 static void sink(auto_ptr<Base>); // 3 }; // 4 struct Derived : Base { // 5 static void sink(auto_ptr<Derived>); // 6 }; // 7 auto_ptr<Derived> source() { // 8 auto_ptr<Derived> p(source()); // 9 auto_ptr<Derived> pp(p); // 10 Derived::sink(source()); // 11 p = pp; // 12 p = source(); // 13 auto_ptr<Base> q(source()); // 14 auto_ptr<Base> qp(p); // 15 Base::sink(source()); // 16 q = pp; // 17 q = source(); // 18 return p; // 19 return source(); }
60. Reference binding and valid conversion sequences
Section: 13.3.3.1.4 over.ics.ref Status: open Submitter: Steve Adamczyk Date: 13 Oct 1998
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.
83. Overloading and deprecated conversion of string literal
Section: 13.3.3.2 over.ics.rank Status: open Submitter: Steve Adamczyk Date: 24 Jan 1999In 13.3.3.2 over.ics.rank , we have
This does not work right with respect to the deprecated conversion from string literal to "char *". Consider
- S1 and S2 differ only in their qualification conversion and yield similar types T1 and T2 (4.4 conv.qual ), respectively, and the cv-qualification signature of type T1 is a proper subset of the cv-qualification signature of type T2, [Example:
int f(const int *); int f(int *); int i; int j = f(&i); // Calls f(int *)—end example] or, if not that,void f(char *); void f(const char *); f("abc");The two conversion sequences differ only in their qualification conversions, and the destination types are similar. The cv-qualification signature of "char *", is a proper subset of the cv-qualification signature of "const char *", so f(char *) is chosen, which is wrong. The rule should be like the one for conversion to bool -- the deprecated conversion should be worse than another exact match that is not the deprecated conversion.
61. Address of static member function "&p->f"
Section: 13.6 over.built Status: open 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.
72. Linkage and storage class specifiers for templates
Section: 14 temp Status: open 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.
105. Meaning of "template function"
Section: 14 temp Status: open Submitter: Daveed Vandevoorde Date: 16 Apr 1999
The phrase "template function" is sometimes used to refer to a template (e.g., in 14 temp paragraph 8) and sometimes to refer to a function generated from a template (e.g., 13.4 over.over paragraph 4).
Suggested Resolution:
The phrase should mean "a function generated from a template" (or might perhaps include explicit specializations).
96. Syntactic disambiguation using the template keyword
Section: 14.2 temp.names Status: open Submitter: John Spicer Date: 16 Feb 1999The following is the wording from 14.2 temp.names paragraphs 4 and 5 that discusses the use of the "template" keyword following . or -> and in qualified names.
When the name of a member template specialization appears after . or -> in a postfix-expression, or after nested-name-specifier in a qualified-id, and the postfix-expression or qualified-id explicitly depends on a template-parameter (14.6.2 temp.dep ), the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template. [Example:
class X { public: template<size_t> X* alloc(); template<size_t> static X* adjust(); }; template<class T> void f(T* p) { T* p1 = p->alloc<200>(); // ill-formed: < means less than T* p2 = p->template alloc<200>(); // OK: < starts template argument list T::adjust<100>(); // ill-formed: < means less than T::template adjust<100>(); // OK: < starts explicit qualification }—end example]If a name prefixed by the keyword template is not the name of a member template, the program is ill-formed. [Note: the keyword template may not be applied to non-template members of class templates. ] The whole point of this feature is to say that the "template" keyword is needed to indicate that a "<" begins a template parameter list in certain contexts. The constraints in paragraph 5 leave open to debate certain cases.
First, I think it should be made more clear that the template name must be followed by a template argument list when the "template" keyword is used in these contexts. If we don't make this clear, we would have to add several semantic clarifications instead. For example, if you say "p->template f()", and "f" is an overload set containing both templates and nontemplates: a) is this valid? b) are the nontemplates in the overload set ignored? If the user is forced to write "p->template f<>()" it is clear that this is valid, and it is equally clear that nontemplates in the overload set are ignored. As this feature was added purely to provide syntactic guidance, I think it is important that it otherwise have no semantic implications.
I propose that paragraph 5 be modified to:
If a name prefixed by the keyword template is not the name of a member template, or an overload set containing one or more member templates, the program is ill-formed. If the name prefixed by the template keyword is not followed by a template-argument-list, the program is ill-formed.
62. Unnamed members of classes used as type parameters
Section: 14.3.1 temp.arg.type Status: open Submitter: Steve Adamczyk Date: 13 Oct 1998
Section 14.3.1 temp.arg.type paragraph 2 says
A local type, a type with no linkage, an unnamed type or a type compounded from any of these types shall not be used as a template-argument for a template type-parameter.It probably wasn't intended that classes with unnamed members should be included in this list, but they are arguably compounded from unnamed types.
23. Some questions regarding partial ordering of function templates
Section: 14.5.5.2 temp.func.order Status: open Submitter: unknown Date: unknown
Issue 1:
14.5.5.2 temp.func.order paragraph 2 says:
Given two overloaded function templates, whether one is more specialized than another can be determined by transforming each template in turn and using argument deduction (14.8.2 temp.deduct ) to compare it to the other.14.8.2 temp.deduct now has 4 subsections describing argument deduction in different situations. I think this paragraph should point to a subsection of 14.8.2 temp.deduct .Rationale:
This is not a defect; it is not necessary to pinpoint cross-references to this level of detail.
Issue 2:
14.5.5.2 temp.func.order paragraph 4 says:
Using the transformed function parameter list, perform argument deduction against the other function template. The transformed template is at least as specialized as the other if, and only if, the deduction succeeds and the deduced parameter types are an exact match (so the deduction does not rely on implicit conversions).In "the deduced parameter types are an exact match", the terms exact match do not make it clear what happens when a type T is compared to the reference type T&. Is that an exact match?Issue 3:
14.5.5.2 temp.func.order paragraph 5 says:
A template is more specialized than another if, and only if, it is at least as specialized as the other template and that template is not at least as specialized as the first.What happens in this case:template<class T> void f(T,int); template<class T> void f(T, T); void f(1,1);For the first function template, there is no type deduction for the second parameter. So the rules in this clause seem to imply that the second function template will be chosen.Rationale:
This is not a defect; the standard unambiguously makes the above example ill-formed due to ambiguity.
108. Are classes nested in templates dependent?
Section: 14.6.2.1 temp.dep.type Status: open Submitter: Mark Mitchell Date: 14 Apr 1999
Mark Mitchell (via John Spicer): Given:
template <class T> struct S { struct I1 { typedef int X; }; struct I2 : public I1 { X x; }; };Is this legal? The question really boils down to asking whether or not I1 is a dependent type. On the one hand, it doesn't seem to fit any of the qualifications in 14.6.2.1 temp.dep.type . On the other, 14.7.3 temp.expl.spec allows explicit specialization of a member class of a class template, so something like:
template <> struct S<double>::I1 { int X; }; is apparently legal. But, then, `X' no longer refers to a type name. So, it seems like `I1' should be classified as dependent. What am I missing?
Erwin Unruh: I wrote that particular piece of text and I just missed the problem above. It is intended to be a dependent type. The reasoning is that I1 is just a shorthand for S
::I1 which clearly is dependent.Suggested Resolution: (Erwin Unruh)
I think the list of what is a dependent type should be extended to cover "a type declared and used within the same template" modulo of phrasing.
2. How can dependent names be used in member declarations that appear outside of the class template definition?
Section: 14.6.4 temp.dep.res Status: open Submitter: unknown Date: unknown
template <class T> class Foo { public: typedef int Bar; Bar f(); }; template <class T> typename Foo<T>::Bar Foo<T>::f() { return 1;} --------------------In the class template definition, the declaration of the member function is interpreted as:int Foo<T>::f();In the definition of the member function that appears outside of the class template, the return type is not known until the member function is instantiated. Must the return type of the member function be known when this out-of-line definition is seen (in which case the definition above is ill-formed)? Or is it OK to wait until the member function is instantiated to see if the type of the return type matches the return type in the class template definition (in which case the definition above is well-formed)?Suggested resolution: (John Spicer)
My opinion (which I think matches several posted on the reflector recently) is that the out-of-class definition must match the declaration in the template. In your example they do match, so it is well formed.
I've added some additional cases that illustrate cases that I think either are allowed or should be allowed, and some cases that I don't think are allowed.
template <class T> class A { typedef int X; }; template <class T> class Foo { public: typedef int Bar; typedef typename A<T>::X X; Bar f(); Bar g1(); int g2(); X h(); X i(); int j(); }; // Declarations that are okay template <class T> typename Foo<T>::Bar Foo<T>::f() { return 1;} template <class T> typename Foo<T>::Bar Foo<T>::g1() { return 1;} template <class T> int Foo<T>::g2() { return 1;} template <class T> typename Foo<T>::X Foo<T>::h() { return 1;} // Declarations that are not okay template <class T> int Foo<T>::i() { return 1;} template <class T> typename Foo<T>::X Foo<T>::j() { return 1;}In general, if you can match the declarations up using only information from the template, then the declaration is valid.Declarations like Foo::i and Foo::j are invalid because for a given instance of A<T>, A<T>::X may not actually be int if the class is specialized.
This is not a problem for Foo::g1 and Foo::g2 because for any instance of Foo<T> that is generated from the template you know that Bar will always be int. If an instance of Foo is specialized, the template member definitions are not used so it doesn't matter whether a specialization defines Bar as int or not.
21. Can a default argument for a template argument appear in a friend declaration?
Section: 14.6.4 temp.dep.res Status: open Submitter: unknown Date: unknown
14.1 temp.param paragraph 10 says:
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 as default function arguments are (8.3.6 dcl.fct.default )."Can a default argument for a template argument appear in a friend declaration? If so, when is this default argument considered for template instantiations?For example,
template<class T1, class T2 = int> class A; class B { friend<class T1 = int, class T2> class A; };Is this well-formed? If it is, should the IS say when the default argument for T1 is considered for instantiations of class A?
63. Class instantiation from pointer conversion to void*, null and self
Section: 14.7.1 temp.inst Status: open Submitter: Steve Adamczyk Date: 13 Oct 1998
A template is implicitly instantiated because of a "pointer conversion" on an argument. This was intended to include related-class conversions, but it also inadvertently includes conversions to void*, null pointer conversions, cv-qualification conversions and the identity conversion.
It is not clear whether a reinterpret_cast of a pointer should cause implicit instantiation.
44. Member specializations
Section: 14.7.3 temp.expl.spec Status: open Submitter: Nathan Myers Date: 19 Sep 1998
Some compilers reject the following:
struct A { template <int I> void f(); template <> void f<0>(); };on the basis of 14.7.3 temp.expl.spec paragraph 2:An explicit specialization shall be declared in the namespace of which the template is a member, or, for member templates, in the namespace of which the enclosing class or enclosing class template is a member. An explicit specialization of a member function, member class or static data member of a class template shall be declared in the namespace of which the class template is a member. ...claiming that the specialization above is not "in the namespace of which the enclosing class ... is a member". Elsewhere, declarations are sometimes required to be "at" or "in" "namespace scope", which is not what it says here. Paragraph 17 says:A member or a member template may be nested within many enclosing class templates. If the declaration of an explicit specialization for such a member appears in namespace scope, the member declaration shall be preceded by a template<> for each enclosing class template that is explicitly specialized.The qualification "if the declaration ... appears in namespace scope", implies that it might appear elsewhere. The only other place I can think of for a member specialization is in class scope.Was it the intent of the committee to forbid the construction above? (Note that A itself is not a template.) If so, why?
64. Partial ordering to disambiguate explicit specialization
Section: 14.7.3 temp.expl.spec Status: open Submitter: Steve Adamczyk Date: 13 Oct 1998
Paragraph 12 should address partial ordering. It wasn't updated when that change was made and conflicts with 14.5.5.2 temp.func.order paragraph 1.
88. Specialization of member constant templates
Section: 14.7.3 temp.expl.spec Status: open Submitter: Jason Merrill Date: 20 Jan 1999Is 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 class.static.data 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.
99. Partial ordering, references and cv-qualifiers
Section: 14.8.2.1 temp.deduct.call Status: open Submitter: Mike Miller Date: 5 Mar 1999Consider:
template <class T> void f(T&); template <class T> void f(const T&); void m() { const int p = 0; f(p); }Some compilers treat this as ambiguous; others prefer f(const T&). The question turns out to revolve around whether 14.8.2.1 temp.deduct.call 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&).
70. Is an array bound a nondeduced context?
Section: 14.8.2.4 temp.deduct.type Status: open Submitter: Jack Rouse Date: 29 Sep 1998
Paragraph 4 lists contexts in which template formals are not deduced. Were template formals in an expression in the array bound of an array type specification intentionally left out of this list? Or was the intent that such formals always be explicitly specified? Otherwise I believe the following should be valid:
template <int I> class IntArr {}; template <int I, int J> void concat( int (&d)[I+J], const IntArr<I>& a, const IntArr<J>& b ) {} int testing() { IntArr<2> a; IntArr<3> b; int d[5]; concat( d, a, b ); }Can anybody shed some light on this?From John Spicer:
Expressions involving nontype template parameters are nondeduced contexts, even though they are omitted from the list in 14.8.2.4 temp.deduct.type paragraph 4. See 14.8.2.4 temp.deduct.type paragraphs 12-14:
- A template type argument cannot be deduced from the type of a non-type template-argument.
...
- If, in the declaration of a function template with a non-type template-parameter, the non-type template-parameter is used in an expression in the function parameter-list, the corresponding template-argument must always be explicitly specified or deduced elsewhere because type deduction would otherwise always fail for such a template-argument.
98. Branching into try block via switch label
Section: 15 except Status: open Submitter: Jack Rouse Date: 23 Feb 1999At the top of clause 15, in paragraph 2, it says:
A goto, break, return, or continue statement can be used to transfer control out of a try block or handler, but not into one.What about switch statements?switch ( f() ) { case 1: try { g(); case 2: h(); } catch (...) { // handler } break; }Suggested Resolution (John Spicer ): Our interpretation has been that all transfers into try blocks and handlers are prohibited.
104. Destroying the exception temp when no handler is found
Section: 15.1 except.throw Status: open 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() { std::set_terminate(t); 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() { std::set_unexpected(u); try { foo(); } 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?
87. Exception specifications on function parameters
Section: 15.4 except.spec Status: open Submitter: Steve Adamczyk Date: 25 Jan 1999In 15.4 except.spec paragraph 2:
An exception-specification shall appear only on a function declarator in a function, pointer, reference or pointer to member declaration or definition.Does that mean in the top-level function declarator, or one at any level? Can one, for example, specify an exception specification on a pointer-to-function parameter of a function?void f(int (*pf)(float) throw(A))Suggested answer: no. The exception specifications are valid only on the top-level function declarators.However, if exception specifications are made part of a function's type as has been tentatively agreed, they would have to be allowed on any function declaration.
92. Should exception specifications be part of the type system?
Section: 15.4 except.spec Status: open Submitter: Jonathan Schilling Date: 2 Feb 1999It was tentatively agreed at the Santa Cruz meeting that exception specifications should fully participate in the type system.
This is such a major change that it deserves to be a separate issue.
See also Core issue 25 and Core issue 87 .
79. Alignment and placement new
Section: 18.4.1.3 lib.new.delete.placement Status: open Submitter: Herb Sutter Date: 15 Dec 1998
The example in 18.4.1.3 lib.new.delete.placement reads:
[Example: This can be useful for constructing an object at a known address:This example has potential alignment problems. One way to correct it would be to change the definition of place to read:char place[sizeof(Something)]; Something* p = new (place) Something();—end example]char* place = new char[sizeof(Something)];
81. Null pointers and C compatability
Section: D depr Status: open Submitter: Steve Clamage Date: 27 Oct 1998
Annex D 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;
106. Creating references to references during template deduction/instantiation
Section: unknown Status: open Submitter: Bjarne Stroustrup Date: unknown
The main defect is in the library, where the binder template can easily lead to reference-to-reference situations. See also paper J16/99-0011 = WG21 N1188.
Andrew Koenig illustrates the problem with the following example:
int& f(); template<typename T> void poof(T (*)(), T&); int main() { int n; poof(&f, n); // Second parameter of type int & &? }Suggested Resolution: As suggested: a reference-to-reference-to-T should be equivalent to a reference-to-T. However, such multi-level references would be allowed only in types constructed via typedef and template argument substitution, not directly. In addition to being analogous to the treatment of cv-qualifiers, this restriction avoids the lexical surprise of && being treated as a logical-and rather than as a reference-to-reference (per Daveed Vandevoorde).
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):
We discussed (b) and decided that the namespace-scope definition was not needed, but the wording did not make it into the standard.
- Use in "sizeof" or a non-polymorphic "typeid". Neither the value nor the address is really used. No definition is needed at all.
- 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.
- 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.
- All other uses require that the object actually exist because its address will be taken implicitly or explicitly.
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.koenig 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.koenig 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.toIf 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
Issues with "NAD" Status
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.
91. A union's associated types should include the union itself
Section: 3.4.2 basic.lookup.koenig 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.
71. Incorrect cross reference
Section: 5 expr Status: NAD Submitter: Neal Gafter Date: 15 Oct 1998An 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 13.3.1.2 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.
54. Static_cast from private base to derived class
Section: 5.2.9 expr.static.cast Status: NAD Submitter: Steve Adamczyk Date: 13 Oct 1998
Is it okay to use a static_cast to cast from a private base class to a derived class? That depends on what the words "valid standard conversion" in paragraph 8 mean -- do they mean the conversion exists, or that it would not get an error if it were done? I think the former was intended -- and therefore a static_cast from a private base to a derived class would be allowed.
Rationale (04/99): A static_cast from a private base to a derived class is not allowed outside a member from the derived class, because 4.10 conv.ptr paragraph 3 implies that the conversion is not valid. (Classic style casts work.)
31. Looking up new/delete
Section: 5.3.4 expr.new Status: NAD Submitter: Daveed Vandevoorde Date: 23 Jun 1998
Section 12.5 class.free 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 expr.new 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 class.free ), 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 ).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".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. ]
Rationale:
Paragraphs 16 through 18 are sufficiently correct and unambiguous as written.
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.
97. Use of bool constants in integral constant expressions
Section: 5.19 expr.const Status: NAD Submitter: Andy Koenig Date: 18 Feb 1999
Consider:
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
{literals, enumerators, const variables, or static members} of integral of integral or enumeration types...
Or does it meanliterals (of any type), enumerators, const variables, or {static members of integral or enumeration types}
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.
14. extern "C" functions and declarations in different namespaces
Section: 7.5 dcl.link Status: NAD Submitter: Erwin Unruh Date: unknown
Issue 1
7.5 dcl.link paragraph 6 says the following:
extern "C" functions in multiple namespaces refer to the same function.
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-formedFor 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.
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.
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.
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.
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.
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:
Given:
bool test; Integer a, b; test ? a : b;What builtin do we use? The candidates areoperator ?:(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.
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.
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; f(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.
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 #2template<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.
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.
Issues with "Extension" Status
11. How do the keywords typename/template interact with using-declarations?
Section: 7.3.3 namespace.udecl Status: extension Submitter: Bill Gibbons Date: unknown
Issue 1:
The working paper is not clear about how the typename/template keywords interact with using-declarations:
templateWhen the rules for typename and the similar use of template were decided, we chose to require that they be used at every reference. The way to avoid typename at every use is to declare a typedef; then the typedef name itself is known to be a type. For using-declarations, we decided that they do not introduce new declarations but rather are aliases for existing declarations, like symbolic links. This makes it unclear whether the declaration "X c;" above should be well-formed, because there is no new name declared so there is no declaration with a "this is a type" attribute. (The same problem would occur with the template keyword when a member template of a dependent class is used). I think these are the main options:struct A { typedef int X; }; template void f() { typename A ::X a; // OK using typename A ::X; // OK typename X b; // ill-formed; X must be qualified X c; // is this OK? } Suggested Resolution:
- Continue to allow typename in using-declarations, and template (for member templates) too. Attach the "is a type" or "is a template" attribute to the placeholder name which the using-declaration "declares"
- Disallow typename and template in using-declarations (just as class-keys are disallowed now). Allow typename and template before unqualified names which refer to dependent qualified names through using-declarations.
- Document that this is broken.
The core WG already resolved this issue according to (1), but the wording does not seem to have been added to the standard. New wording needs to be drafted.
Issue 2:
Either way, one more point needs clarification. If the first option is adopted:
template<class T> struct A { struct X { }; }; template<class T> void g() { using typename A<T>::X; X c; // if this is OK, then X by itself is a type int X; // is this OK? }When "g" is instantiated, the two declarations of X are compatible (7.3.3 namespace.udecl paragraph 10). But there is no way to know this when the definition of "g" is compiled. I think this case should be ill-formed under the first option. (It cannot happen under the second option.) If the second option is adopted:template<class T> struct A { struct X { }; }; template<class T> void g() { using A<T>::X; int X; // is this OK? }Again, the instantiation would work but there is no way to know that in the template definition. I think this case should be ill-formed under the second option. (It would already be ill-formed under the first option.)From John Spicer:
The "not a new declaration" decision is more of a guiding principle than a hard and fast rule. For example, a name introduced in a using-declaration can have different access than the original declaration.Tentative Resolution:Like symbolic links, a using-declaration can be viewed as a declaration that declares an alias to another name, much like a typedef.
In my opinion, "X c;" is already well-formed. Why would we permit typename to be used in a using-declaration if not to permit this precise usage?
In my opinion, all that needs to be done is to clarify that the "typeness" or "templateness" attribute of the name referenced in the using-declaration is attached to the alias created by the using-declaration. This is solution #1.
The rules for multiple declarations with the same name in the same scope should treat a using-declaration which names a type as a typedef, just as a typedef of a class name is treated as a class declaration. This needs drafting work. Also see Core issue 36 .
Rationale (04/99): Any semantics associated with the typename keyword in using-declarations should be considered an extension.
109. Allowing ::template in using-declarations
Section: 7.3.3 namespace.udecl Status: extension 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:
using typenameopt ::opt nested-name-specifier unqualified-id ;
and nested-name-specifier never ends in "template".
Is that intentional?
Bill Gibbons :
It certainly appears to be, since we have:
qualified-id:
so it would be easier to specify using-declaration as:::opt nested-name-specifier templateopt unqualified-id
using typenameopt qualified-id ;
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.
13. extern "C" for Parameters of Function Templates
Section: 7.5 dcl.link Status: extension Submitter: John Spicer Date: unknown
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); // errorAnother 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.
15. Default arguments for parameters of function templates
Section: 8.3.6 dcl.fct.default Status: extension 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; b.f(); }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.
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(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:
- the source is a temporary object
- the return value optimization
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.
- aliasing a parameter in an inline function call to the function call argument.
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.
Issues with "DR" Status
41. Clarification of lookup of names after declarator-id
Section: 3.4.1 basic.lookup.unqual Status: DR Submitter: Mike Miller Date: 1 Sep 1998
Footnotes 26 and 29 both use the phrase "following the function declarator" incorrectly: the function declarator includes the parameter list, but the footnotes make clear that they intend what's said to apply to names inside the parameter list. Presumably the phrase should be "following the function declarator-id."
Proposed Resolution (04/99): Change the text in 3.4.1 basic.lookup.unqual paragraph 6 from:
A name used in the definition of a function [footnote: This refers to unqualified names following the function declarator; such a name may be used as a type or as a default argument name in the parameter-declaration-clause, or may be used in the function body. end footnote] that is ...to:A name used in the definition of a function following the function's declarator-id [footnote: This refers to unqualified names that occur, for instance, in a type or default argument expression in the parameter-declaration-clause or used in the function body. end footnote] that is ...Change the text in 3.4.1 basic.lookup.unqual ] paragraph 8 from:A name used in the definition of a function that is a member function (9.3 class.mfct ) [footnote: That is, an unqualified name following the function declarator; such a name may be used as a type or as a default argument name in the parameter-declaration-clause, or may be used in the function body, or, if the function is a constructor, may be used in the expression of a mem-initializer. end footnote] of class X shall be ...to:A name used in the definition of a member function (9.3 class.mfct ) of class X following the function's declarator-id [footnote: That is, an unqualified name that occurs, for instance, in a type or default argument expression in the parameter-declaration-clause, in the function body, or in an expression of a mem-initializer in a constructor definition. end footnote] shall be ...
33. Argument dependent lookup and overloaded functions
Section: 3.4.2 basic.lookup.koenig Status: DR Submitter: Jason Merrill Date: 15 Jul 1998
If an argument used for lookup is the address of a group of overloaded functions, are there any associated namespaces or classes? What if it's the address of a function template?
My inclination is to say no to both.
From Mike Miller:
We discussed this on the reflector a few weeks ago. I'll leave the template case for the Core III experts, but I'd find it surprising if the overload case weren't handled as the obvious generalization of the single-function case. For a single function, the associated namespaces are those of the types used in the parameters and return type; I would expect that using an overloaded function name would simply be the union of the namespaces from the members of the overload set. That would be the simplest and most intuitive, IMHO -- is there an argument for doing it differently?
Proposed Resolution (04/99): In 3.4.2 basic.lookup.koenig paragraph 2, add following the last bullet in the list of associated classes and namespaces for various argument types (not a bullet itself because overload sets and templates do not have a type):
In addition, if the argument is the name or address of a set of overloaded functions and/or function templates, its associated classes and namespaces are the union of those associated with each of the members of the set: the namespace in which the function or function templates is defined and the classes and namespaces associated with its (non-dependent) parameter types and return type.
43. Copying base classes (PODs) using memcpy
Section: 3.9 basic.types Status: DR Submitter: Nathan Myers Date: 15 Sep 1998
Can you use memcpy on non-member POD subobjects of non-POD objects?
In 3.9 basic.types paragraphs 2 and 3 we have:
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*. 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 elided]Paragraph 3 doesn't repeat the restriction of paragraph 2. Should it be assumed? Otherwise only complete POD types are copyable to an array of char and back, but scribbling over subobjects is OK. (Or perhaps a "distinct T object" is a complete object...)*[Footnote: By using, for example, the library functions (17.4.1.2 lib.headers ) memcpy or memmove. end footnote]For any POD type T, if two pointers to T point to distinct T 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.Proposed Resolution (04/99): Change the text in 3.9 basic.types paragraph 2 from:
For any complete POD object type T, ...to:For any object (other than a base class subobject) of POD type T, ...Change the text in 3.9 basic.types paragraph 3 from:For any POD type T, if two pointers to T point to distinct T objects obj1 and obj2,to:For any POD type T, if two pointers to T point to distinct T objects obj1 and obj2, where neither obj1 nor obj2 is a base class subobject, ...
65. Typo in default argument example
Section: 8.3.6 dcl.fct.default Status: DR Submitter: Mike Miller Date: 6 Oct 1998
Proposed Resolution (04/99): Change the text in the example of section 8.3.6 dcl.fct.default paragraph 5 from:
... g will be called with the value f(1).to:... g will be called with the value f(2).
35. Definition of default-initialization
Section: 8.5 dcl.init Status: DR Submitter: Andrew Koenig Date: 29 Jul 1998
Given:
struct S1 { int x; }; struct S2 { int x; double y; }; struct S3 { int x; double y; string s; };Once upon a time, we went through a fairly protracted discussion to ensure that S1().x would be guaranteed to be 0. Note that if we declarevoid f() { S1 s1; // ... }there is no guarantee of the value of s1.x, and that is intentional. But S1().x is different, because S1() is an rvalue, and unless all of its members are defined, the effect of copying it is undefined.Similarly, S2().x and S2().y are also defined to be equal to zero, and here it really matters for many implementations, because if S2().y is just a bunch of random bits, it is entirely possible that trying to copy S2().y will yield a floating-point trap.
However, rather to my surprise, the standard does not define the value of S3().x or S3().y, because S3 is not a POD. It does define S3().s (by running the string constructor), but once a structure is no longer a POD, the values of uninitialized members are no longer guaranteed in expressions of the form T().
In my opinion, this definition is a mistake, and the committee's intention was to zero-initialize all members that do not have an explicitly defined constructor, whether or not the class is a POD. See also paper J16/99-0014 = WG21 N1191.
Proposed Resolution (04/99): Add the following text to the end of section 8.5 dcl.init paragraph 5:
To value-initialize an object of type T means:
- if T is a class type (9 class ) with a user-declared constructor (12.1 class.ctor ), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
- if T is a non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized;
- if T is an array type, then each element is value-initialized;
- otherwise, the storage for the object is zero-initialized.
Change "default-initialization" to "value-initialization" in 5.2.3 expr.type.conv paragraph 2 and in 8.5.1 dcl.init.aggr paragraph 7.
48. Definitions of unused static members
Section: 9.4.2 class.static.data Status: DR Submitter: Bill Gibbons Date: 23 Nov 1997
Also see section: 3.2 basic.def.odr .
Originally, all static data members still had to be defined outside the class whether they were used or not.
But that restriction was supposed to be lifted so that static data members need not be defined outside the class unless they are used in a manner which requires their definition, in the same manner as namespace-scope variables. In particular, if an integral/enum const static data member is initialized within the class, and its address is never taken, we agreed that no namespace-scope definition was required.
For example:
struct A { static const int size = 10; int array[size]; }; int main() { A a; return 0; }However, 9.4.2 class.static.data paragraph 4 says:The member shall still be defined in a namespace scope if it is used in the program and the namespace scope definition shall not contain an initializer.A narrow interpreration of "used" in this rule would make the example ill-formed because there is no namespace-scope definition of "size". A better wording for this rule would be:The member shall still be defined in a namespace scope if it is used in the program in the manner described in 3.2 basic.def.odr . The namespace scope definition shall not contain an initializer.Also, the wording in 3.2 basic.def.odr paragraph 2:An expression is potentially evaluated unless either it is the operand of the sizeof operator (5.3.3 expr.sizeof ), or it is the operand of the typeid operator and does not designate an lvalue of polymorphic class type (5.2.8 expr.typeid ).is incomplete because it does not mention the use of a compile-time constant as an array bound or template argument. It should say something like:An expression is potentially evaluated unless it is the operand of the sizeof operator (5.3.3 expr.sizeof ), the operand of the typeid operator, an integral constant-expression used as an array bound or an integral constant-expression used as a template-argument for a non-reference template-parameter; and the expression does not designate an lvalue of polymorphic class type (5.2.8 expr.typeid ).Proposed Resolution (04/99): Change the first sentence of 3.2 basic.def.odr paragraph 2 from:
An expression is potentially evaluated unless either it is the operand of the sizeof operator (5.3.3 expr.sizeof ), or it is the operand of the typeid operator and does not designate an lvalue of polymorphic class type (5.2.8 expr.typeid ).to:An expression is potentially evaluated unless it appears where an integral constant expression is required (see 5.19 expr.const ), is the operand of the sizeof operator (5.3.3 expr.sizeof ), or is the operand of the typeid operator and the expression does not designate an lvalue of polymorphic class type (5.2.8 expr.typeid ).
32. Clarification of explicit instantiation of non-exported templates
Section: 14 temp Status: DR Submitter: Daveed Vandevoorde Date: 10 Jul 1998
Section 14 temp paragraph 8 says:
A non-exported template that is neither explicitly specialized nor explicitly instantiated must be defined in every translation unit in which it is implicitly instantiated (14.7.1 temp.inst ) or explicitly instantiated (14.7.2 temp.explicit ); no diagnostic is required.Shouldn't the first underlined phrase be omitted to avoid conflict with the second underlined phrase?From John Spicer:
The first "explicitly instantiated" is intended to mean "explicitly instantiated in some other translation unit".
Proposed Resolution (04/99): Change the text in 14 temp paragraph 8 from:
A non-exported template that is neither explicitly specialized nor explicitly instantiated must be defined in every translation unit in which it is implicitly instantiated (14.7.1 temp.inst ) or explicitly instantiated (14.7.2 temp.explicit ); no diagnostic is required.to:A non-exported template must be defined in every translation unit in which it is implicitly instantiated (14.7.1 temp.inst ), unless the corresponding specialization is explicitly instantiated (14.7.2 temp.explicit ) in some translation unit; no diagnostic is required. [Note: See also 14.7.2 temp.explicit ]
49. Restriction on non-type, non-value template arguments
Section: 14.1 temp.param Status: DR Submitter: Mike Miller Date: 16 Oct 1998
The example in 14.6.4 temp.dep.res paragraph 8 is:
template<int* a> struct R { /*...*/ }; int* p; R<p> w;There was a French comment was that this is an error, and there was general agreement with that.I've been looking for the verbiage that specifies that this is an error and haven't found it. In particular, nothing in 14.1 temp.param ("Template parameters") nor 14.3.2 temp.arg.nontype ("Template non-type arguments") appears to rule out this case. (14.3.2 temp.arg.nontype paragraph 1 allows an argument to be "the name of an object or function with external linkage," with no limitation on the kinds of parameters such a name can match; "p" is, in fact, such a name.)
Should the resolution of the French comment include beefing up one or both of these sections to cover the applicable rules explicitly?
Proposed Resolution (04/99): Change the example in 14.1 temp.param paragraph 8 from:
template<int *a> struct R { /* ... */ }; template<int b[5]> struct S { /* ... */ }; int *p; R<p> w; // OK S<p> x; // OK due to parameter adjustment int v[5]; R<v> y; // OK due to implicit argument conversion S<v> z; // OK due to both adjustment and conversionto:template<int *a> struct R { /* ... */ }; template<int b[5]> struct S { /* ... */ }; int p; R<&p> w; // OK S<&p> x; // OK due to parameter adjustment int v[5]; R<v> y; // OK due to implicit argument conversion S<v> z; // OK due to both adjustment and conversionFurthermore, in 14.3.2 temp.arg.nontype paragraph 1:
- the fourth bullet item should be changed from "...where the & is optional if the name refers to a function or array;" to "...where the & is optional if the name refers to a function or array, or if the corresponding template-parameter is a reference;"
- the third bullet item should be removed.
30. Valid uses of '::template'
Section: 14.2 temp.names Status: DR Submitter: Daveed Vandevoorde Date: 28 May 1998
I have a request for clarification regarding a issue similar to John Wiegley's, but wrt. the ::template syntax. More precisely, where is
X::template Yallowed? (It is required for dependent X where Y is a template-id, I believe, but it doesn't seem to be disallowed elsewhere.)The question also holds for '.template' and '->template'.
Proposed Resolution (04/99): Append to 14.2 temp.names paragraph 5:
Furthermore, names of member templates shall not be prefixed by the keyword template if the postfix-expression or qualified-id does not appear in the scope of a template. [Note: just as is the case with the typename prefix, the template prefix is allowed in cases where it is not strictly necessary; i.e., when the expression on the left of the -> or ., or the nested-name-specifier is not dependent on a template-parameter. ]
22. Template parameter with a default argument that refers to itself
Section: 14.6.4 temp.dep.res Status: DR Submitter: unknown Date: unknown
14.1 temp.param paragraph 13 says:
The scope of a template-parameter extends from its point of declaration until the end of its template. In particular, a template-parameter can be used in the declaration of subsequent template-parameters and their default arguments.Is the following well-formed?template<class U = U> class X { ... };Proposed Resolution (04/99): Change 14.1 temp.param paragraph 14 from:
A template-parameter cannot be used in preceding template-parameters or their default arguments.to:A template-parameter cannot be used in preceding template-parameters, in their default arguments, or in its own default argument.
25. Exception specifications and pointers to members
Section: 15.4 except.spec Status: DR Submitter: unknown Date: unknown
15.4 except.spec paragraph 3 should say what happens when two pointers to members with different exception specifications are assigned to each other, initialized with one another, etc.
Proposed Resolution (04/99): Change the text in 15.4 except.spec paragraph 3 from:
Similarly, any function or pointer to function assigned to, or initializing, a pointer to function shall only allow exceptions that are allowed by the pointer or function being assigned to or initialized.to:A similar restriction applies to assignment to and initialization of pointers to functions, pointers to member functions, and references to functions: the target entity shall allow at least the exceptions allowed by the source value in the assignment or initialization.Index by IS Reference
Section Issue # Status Title 3.2 50 NAD Converting pointer to incomplete type to same type 3.2 82 dup Definition of "using" a constant expression 3.3.6 42 NAD Redefining names from base classes 3.4.1 41 DR Clarification of lookup of names after declarator-id 3.4.2 12 dup Default arguments on different declarations for the same function and the Koenig lookup 3.4.2 33 DR Argument dependent lookup and overloaded functions 3.4.2 90 drafting Should the enclosing class be an "associated class" too? 3.4.2 91 NAD A union's associated types should include the union itself 3.6.3 28 drafting 'exit', 'signal' and static object destruction 3.8 89 drafting Object lifetime does not account for reference rebinding 3.8 93 ready Missing word in 3.2 basic.life paragraph 2 3.9 43 DR Copying base classes (PODs) using memcpy 5 71 NAD Incorrect cross reference 5.2.5 52 drafting Non-static members, member selection and access checking 5.2.9 53 drafting Lvalue-to-rvalue conversion before certain static_casts 5.2.9 54 NAD Static_cast from private base to derived class 5.3.4 31 NAD Looking up new/delete 5.3.4 74 ready Enumeration value in direct-new-declarator 5.7 55 NAD Adding/subtracting pointer and enumeration value 5.10 73 drafting Pointer equality 5.19 94 drafting Inconsistencies in the descriptions of constant expressions 5.19 97 NAD Use of bool constants in integral constant expressions 7.1.1 69 drafting Storage class specifiers on template declarations 7.1.3 56 ready Redeclaring typedefs within classes 7.1.5.1 76 ready Are const volatile variables considered "constant expressions"? 7.1.5.3 68 drafting Grammar does not allow "friend class A<int>;" 7.3.1.2 95 open Elaborated type specifiers referencing names declared in friend decls 7.3.3 11 extension How do the keywords typename/template interact with using-declarations? 7.3.3 36 review Using-declarations in multiple-declaration contexts 7.3.3 101 ready Redeclaration of extern "C" names via using-declarations 7.3.3 109 extension Allowing ::template in using-declarations 7.3.4 103 ready Is it extended-namespace-definition or extension-namespace-definition ? 7.5 4 drafting Does extern "C" affect the linkage of function names with internal linkage? 7.5 13 extension extern "C" for Parameters of Function Templates 7.5 14 NAD extern "C" functions and declarations in different namespaces 7.5 29 drafting Linkage of locally declared functions 7.5 107 open Linkage of operator functions 8.3 40 ready Syntax of declarator-id 8.3.5 18 NAD f(TYPE) where TYPE is void should be allowed 8.3.6 1 drafting What if two using-declarations refer to the same function but the declarations introduce different default-arguments? 8.3.6 15 extension Default arguments for parameters of function templates 8.3.6 65 DR Typo in default argument example 8.3.6 66 open Visibility of default args vs overloads added after using-declaration 8.5 5 drafting CV-qualifiers and type conversions 8.5 35 DR Definition of default-initialization 8.5 78 open Section 8.5 paragraph 9 should state it only applies to non-static objects 9.1 85 open Redeclaration of member class 9.2 75 ready In-class initialized members must be const 9.2 80 open Class members with same name as class 9.4 67 ready Evaluation of left side of object-expression 9.4.2 48 DR Definitions of unused static members 9.5 57 open Empty unions 9.6 58 open Signedness of bit fields of enum type 10.2 39 drafting Conflicting amgibuity rules 11 8 open Access to template arguments used in a function return type and in the nested name specifier 11.2 7 NAD Can a class with a private virtual base class be derived from? 11.2 9 drafting Clarification of access to base class members 11.2 16 drafting Access to members of indirect private base classes 11.2 17 open Footnote 99 should discuss the naming class when describing members that can be accessed from friends 11.4 77 open The definition of friend does not allow nested classes to be friends 11.5 19 NAD Clarify protected member access 11.8 10 open Can a nested class access its own class name as a qualified name if it is a private member of the enclosing class? 11.8 45 drafting Access to nested classes 12.2 86 open Lifetime of temporaries in query expressions 12.8 6 extension 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? 12.8 20 ready Some clarifications needed for 12.8 para 15 12.8 26 NAD Copy constructors and default arguments 13.3.1.2 102 open Operator lookup rules do not work well with parts of the library 13.3.1.4 59 open Clarification of overloading and UDC to reference type 13.3.3 51 open Overloading and user-defined conversions 13.3.3.1 84 open Overloading and conversion loophole used by auto_ptr 13.3.3.1.4 60 open Reference binding and valid conversion sequences 13.3.3.2 83 open Overloading and deprecated conversion of string literal 13.6 27 NAD Overload ambiguities for builtin ?: prototypes 13.6 61 open Address of static member function "&p->f" 14 32 DR Clarification of explicit instantiation of non-exported templates 14 72 open Linkage and storage class specifiers for templates 14 105 open Meaning of "template function" 14.1 49 DR Restriction on non-type, non-value template arguments 14.2 30 DR Valid uses of '::template' 14.2 38 drafting Explicit template arguments and operator functions 14.2 96 open Syntactic disambiguation using the template keyword 14.3.1 62 open Unnamed members of classes used as type parameters 14.3.2 100 ready Clarify why string literals are not allowed as template arguments 14.5.3 47 NAD Template friend issues 14.5.5.2 23 open Some questions regarding partial ordering of function templates 14.6.2.1 108 open Are classes nested in templates dependent? 14.6.4 2 open How can dependent names be used in member declarations that appear outside of the class template definition? 14.6.4 21 open Can a default argument for a template argument appear in a friend declaration? 14.6.4 22 DR Template parameter with a default argument that refers to itself 14.7.1 34 NAD Argument dependent lookup and points of instantiation 14.7.1 63 open Class instantiation from pointer conversion to void*, null and self 14.7.2 46 NAD Explicit instantiation of member templates 14.7.3 3 NAD The template compilation model rules render some explicit specialization declarations not visible during instantiation 14.7.3 24 review Errors in examples in 14.7.3 14.7.3 44 open Member specializations 14.7.3 64 open Partial ordering to disambiguate explicit specialization 14.7.3 88 open Specialization of member constant templates 14.8.2.1 99 open Partial ordering, references and cv-qualifiers 14.8.2.4 70 open Is an array bound a nondeduced context? 15 98 open Branching into try block via switch label 15.1 104 open Destroying the exception temp when no handler is found 15.4 25 DR Exception specifications and pointers to members 15.4 87 open Exception specifications on function parameters 15.4 92 open Should exception specifications be part of the type system? 15.5.3 37 NAD When is uncaught_exception() true? 18.4.1.3 79 open Alignment and placement new D 81 open Null pointers and C compatability unknown 106 open Creating references to references during template deduction/instantiation Index by Issue Number
Issue # Section Status Title 1 8.3.6 drafting What if two using-declarations refer to the same function but the declarations introduce different default-arguments? 2 14.6.4 open How can dependent names be used in member declarations that appear outside of the class template definition? 3 14.7.3 NAD The template compilation model rules render some explicit specialization declarations not visible during instantiation 4 7.5 drafting Does extern "C" affect the linkage of function names with internal linkage? 5 8.5 drafting CV-qualifiers and type conversions 6 12.8 extension 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? 7 11.2 NAD Can a class with a private virtual base class be derived from? 8 11 open Access to template arguments used in a function return type and in the nested name specifier 9 11.2 drafting Clarification of access to base class members 10 11.8 open Can a nested class access its own class name as a qualified name if it is a private member of the enclosing class? 11 7.3.3 extension How do the keywords typename/template interact with using-declarations? 12 3.4.2 dup Default arguments on different declarations for the same function and the Koenig lookup 13 7.5 extension extern "C" for Parameters of Function Templates 14 7.5 NAD extern "C" functions and declarations in different namespaces 15 8.3.6 extension Default arguments for parameters of function templates 16 11.2 drafting Access to members of indirect private base classes 17 11.2 open Footnote 99 should discuss the naming class when describing members that can be accessed from friends 18 8.3.5 NAD f(TYPE) where TYPE is void should be allowed 19 11.5 NAD Clarify protected member access 20 12.8 ready Some clarifications needed for 12.8 para 15 21 14.6.4 open Can a default argument for a template argument appear in a friend declaration? 22 14.6.4 DR Template parameter with a default argument that refers to itself 23 14.5.5.2 open Some questions regarding partial ordering of function templates 24 14.7.3 review Errors in examples in 14.7.3 25 15.4 DR Exception specifications and pointers to members 26 12.8 NAD Copy constructors and default arguments 27 13.6 NAD Overload ambiguities for builtin ?: prototypes 28 3.6.3 drafting 'exit', 'signal' and static object destruction 29 7.5 drafting Linkage of locally declared functions 30 14.2 DR Valid uses of '::template' 31 5.3.4 NAD Looking up new/delete 32 14 DR Clarification of explicit instantiation of non-exported templates 33 3.4.2 DR Argument dependent lookup and overloaded functions 34 14.7.1 NAD Argument dependent lookup and points of instantiation 35 8.5 DR Definition of default-initialization 36 7.3.3 review Using-declarations in multiple-declaration contexts 37 15.5.3 NAD When is uncaught_exception() true? 38 14.2 drafting Explicit template arguments and operator functions 39 10.2 drafting Conflicting amgibuity rules 40 8.3 ready Syntax of declarator-id 41 3.4.1 DR Clarification of lookup of names after declarator-id 42 3.3.6 NAD Redefining names from base classes 43 3.9 DR Copying base classes (PODs) using memcpy 44 14.7.3 open Member specializations 45 11.8 drafting Access to nested classes 46 14.7.2 NAD Explicit instantiation of member templates 47 14.5.3 NAD Template friend issues 48 9.4.2 DR Definitions of unused static members 49 14.1 DR Restriction on non-type, non-value template arguments 50 3.2 NAD Converting pointer to incomplete type to same type 51 13.3.3 open Overloading and user-defined conversions 52 5.2.5 drafting Non-static members, member selection and access checking 53 5.2.9 drafting Lvalue-to-rvalue conversion before certain static_casts 54 5.2.9 NAD Static_cast from private base to derived class 55 5.7 NAD Adding/subtracting pointer and enumeration value 56 7.1.3 ready Redeclaring typedefs within classes 57 9.5 open Empty unions 58 9.6 open Signedness of bit fields of enum type 59 13.3.1.4 open Clarification of overloading and UDC to reference type 60 13.3.3.1.4 open Reference binding and valid conversion sequences 61 13.6 open Address of static member function "&p->f" 62 14.3.1 open Unnamed members of classes used as type parameters 63 14.7.1 open Class instantiation from pointer conversion to void*, null and self 64 14.7.3 open Partial ordering to disambiguate explicit specialization 65 8.3.6 DR Typo in default argument example 66 8.3.6 open Visibility of default args vs overloads added after using-declaration 67 9.4 ready Evaluation of left side of object-expression 68 7.1.5.3 drafting Grammar does not allow "friend class A<int>;" 69 7.1.1 drafting Storage class specifiers on template declarations 70 14.8.2.4 open Is an array bound a nondeduced context? 71 5 NAD Incorrect cross reference 72 14 open Linkage and storage class specifiers for templates 73 5.10 drafting Pointer equality 74 5.3.4 ready Enumeration value in direct-new-declarator 75 9.2 ready In-class initialized members must be const 76 7.1.5.1 ready Are const volatile variables considered "constant expressions"? 77 11.4 open The definition of friend does not allow nested classes to be friends 78 8.5 open Section 8.5 paragraph 9 should state it only applies to non-static objects 79 18.4.1.3 open Alignment and placement new 80 9.2 open Class members with same name as class 81 D open Null pointers and C compatability 82 3.2 dup Definition of "using" a constant expression 83 13.3.3.2 open Overloading and deprecated conversion of string literal 84 13.3.3.1 open Overloading and conversion loophole used by auto_ptr 85 9.1 open Redeclaration of member class 86 12.2 open Lifetime of temporaries in query expressions 87 15.4 open Exception specifications on function parameters 88 14.7.3 open Specialization of member constant templates 89 3.8 drafting Object lifetime does not account for reference rebinding 90 3.4.2 drafting Should the enclosing class be an "associated class" too? 91 3.4.2 NAD A union's associated types should include the union itself 92 15.4 open Should exception specifications be part of the type system? 93 3.8 ready Missing word in 3.2 basic.life paragraph 2 94 5.19 drafting Inconsistencies in the descriptions of constant expressions 95 7.3.1.2 open Elaborated type specifiers referencing names declared in friend decls 96 14.2 open Syntactic disambiguation using the template keyword 97 5.19 NAD Use of bool constants in integral constant expressions 98 15 open Branching into try block via switch label 99 14.8.2.1 open Partial ordering, references and cv-qualifiers 100 14.3.2 ready Clarify why string literals are not allowed as template arguments 101 7.3.3 ready Redeclaration of extern "C" names via using-declarations 102 13.3.1.2 open Operator lookup rules do not work well with parts of the library 103 7.3.4 ready Is it extended-namespace-definition or extension-namespace-definition ? 104 15.1 open Destroying the exception temp when no handler is found 105 14 open Meaning of "template function" 106 unknown open Creating references to references during template deduction/instantiation 107 7.5 open Linkage of operator functions 108 14.6.2.1 open Are classes nested in templates dependent? 109 7.3.3 extension Allowing ::template in using-declarations