| Document number: | PL22.16/11-0081 = WG21 N3311 |
| Date: | 2011-09-06 |
| Project: | Programming Language C++ |
| Reference: | ISO/IEC IS 14882:2003 |
| Reply to: | William M. Miller |
| Edison Design Group, Inc. | |
| wmm@edg.com |
This document contains the C++ core language issues for which the Committee (J16 + WG21) has decided that no action is required, that is, issues with status "NAD" ("Not A Defect"), "dup" (duplicate), "concepts," and "extension."
This document is part of a group of related documents that together describe the issues that have been raised regarding the C++ Standard. The other documents in the group are:
For more information, including a description of the meaning of the issue status codes and instructions on reporting new issues, please see the Active Issues List.
Section references in this document reflect the section numbering of document PL22.16/10-0215 = WG21 N3290.
The intent appears to be that the following example is well-formed, even though D::f(int) hides B2::f():
struct B1 { void f(); };
struct B2 { void f(); };
struct[[base_check]] D: B1, B2 {
using B1::f;
void f(int);
};
However, this is not reflected in the current wording.
Rationale (November, 2010):
The consensus of the CWG was that the using-declaration does, indeed, hide B2::f() and thus D should be ill-formed.
_N3225_.D.2 [depr.static] says that declaring namespace-scope objects as static is deprecated. Declaring namespace-scope functions as static should also be deprecated.
Proposed resolution (10/99): In both 7.3.1.1 [namespace.unnamed] paragraph 2 and _N3225_.D.2 [depr.static] paragraph 1, replace
when declaring objects in a namespace scopewith
when declaring entities in a namespace scopeIn addition, there are a number of locations in the Standard where use of or reference to static should be reconsidered. These include:
Rationale (04/00):
This issue, along with issue 174, has been subsumed by issue 223. Until the committee determines the meaning of deprecation, it does not make sense either to extend or reduce the number of features to which it is applied.
The decision to deprecate global static should be reversed.
Rationale (04/00):
This issue, along with issue 167, has been subsumed by issue 223. Until the committee determines the meaning of deprecation, it does not make sense either to extend or reduce the number of features to which it is applied.
The standard is inconsistent in its use of a hyphen on the following: nontype vs. non-type, non-dependent vs. nondependent, non-deduced vs. nondeduced, and non-template vs. nontemplate. We should pick a preferred form.
Notes from the March 2004 meeting:
If this isn't a purely editorial issue, nothing is. We're referring this to the editor. We prefer the hyphenated forms.
This issue is for tracking various concerns that are raised in paper N3057.
Rationale (August, 2010):
The paper was voted in in Pittsburgh.
2.14.8 [lex.ext] paragraphs 3-4 state in notes that the arguments to a literal operator template “can only contain characters from the basic source character set.” This restriction does not appear to occur anywhere in normative text, however.
Rationale (July, 2009):
The characters in the template arguments are the characters comprising n, the integer literal, or f, the floating literal. As such, they are constrained by the grammar to be members of the basic character set, and no further normative restriction is needed.
User-defined literals should not be part of C++0x unless they have implementation experience.
Rationale (March, 2011):
The feature has been implemented.
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.
decltype applied to a function call expression requires a complete type (5.2.2 [expr.call] paragraph 3 and 3.2 [basic.def.odr] paragraph 4), even though decltype's result might be used in a way that does not actually require a complete type. This might cause undesired and excessive template instantiations. Immediately applying decltype should not require a complete type, for example, for the return type of a function call.
Additional note (October, 2010):
Another potential consideration in this question is the use of the return type in template argument deduction. If the return type is a specialization of a class template, one would want an error occurring in the instantiation of that specialization to cause a deduction failure, which would argue in favor of requiring the type to be complete. (However, that might also be covered by “when the completeness of the class type affects the semantics of the program” in 14.7.1 [temp.inst] paragraph 1.)
Rationale (November, 2010):
The CWG was persuaded by the SFINAE consideration.
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 3.3.7 [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.
The wording of 3.4.1 [basic.lookup.unqual] paragraph 2 is misleading. It says:
The declarations from the namespace nominated by a using-directive become visible in a namespace enclosing the using-directive; see 7.3.4 [namespace.udir].
According to 7.3.4 [namespace.udir] paragraph 1, that namespace is
the nearest enclosing namespace which contains both the using-directive and the nominated namespace.
That would seem to imply the following:
namespace outer {
namespace inner {
int i;
}
void f() {
using namespace inner;
}
int j = i; // inner::i is "visible" in namespace outer
}
Suggested resolution: Change the first sentence of 3.4.1 [basic.lookup.unqual] paragraph 2 to read:
The declarations from the namespace nominated by a using-directive become visible in the scope in which the using-directive appears after the using-directive.
Notes from the 4/02 meeting:
After a lot of discussion of possible wording changes, we decided the wording should be left alone. 3.4.1 [basic.lookup.unqual] paragraph 2 is not intended to be a full specification; that's in 7.3.4 [namespace.udir] paragraph 1. See also 3.3.6 [basic.scope.namespace] paragraph 1.
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.
In discussing issue 197, the question arose as to whether the handling of fundamental types in argument-dependent lookup is actually what is desired. This question needs further discussion.
Rationale (March, 2011):
There does not seem to be sufficient motivation at this point, with an additional eleven years' experience, to make a change.
I believe the following code example should unambiguously call the member operator+. Am I right?
//--- some library header ---
//
namespace N1 {
template<class T> struct Base { };
template<class T> struct X {
struct Y : public Base<T> { // here's a member operator+
Y operator+( int _Off ) const { return Y(); }
};
Y f( unsigned i ) { return Y() + i; } // the "+" in question
};
}
//--- some user code ---
//
namespace N2 {
struct Z { };
template<typename T> // here's another operator+
int* operator+( T , unsigned ) { static int i ; return &i ; }
}
int main() {
N1::X< N2::Z > v;
v.f( 0 );
}
My expectation is that 3.4.2 [basic.lookup.argdep] would govern, specifically:
If the ordinary unqualified lookup of the name finds the declaration of a class member function, the associated namespaces and classes are not considered.So I think the member should hide the otherwise-better-matching one in the associated namespace. Here's what compilers do:
Agree with me and call the member operator+: Borland 5.5, Comeau 4.3.0.1, EDG 3.0.1, Metrowerks 8.0, MSVC 6.0
Disagree with me and try to call N2::operator+: gcc 2.95.3, 3.1.1, and 3.2; MSVC 7.0
Simple so far, but someone tells me that 13.3.1.2 [over.match.oper] muddies the waters. There, paragraph 10 summarizes that subclause:
[Note: the lookup rules for operators in expressions are different than the lookup rules for operator function names in a function call, ...In particular, consider the above call to "Y() + unsigned" and please help me step through 13.3.1.2 [over.match.oper] paragraph 3:
... for a binary operator @ with a left operand of a type whose cv-unqualified version is T1 and a right operand of a type whose cv-unqualified version is T2,OK so far, here @ is +, and T1 is N1::X::Y.
three sets of candidate functions, designated member candidates, non-member candidates and built-in candidates, are constructed as follows:[and later are union'd together to get the candidate list]
If T1 is a class type, the set of member candidates is the result of the qualified lookup of T1::operator@ (over.call.func); otherwise, the set of member candidates is empty.So there is one member candidate, N1::X::Y::operator+.
The set of non-member candidates is the result of the unqualified lookup of operator@ in the context of the expression according to the usual rules for name lookup in unqualified function calls (basic.lookup.argdep) except that all member functions are ignored.
*** This is the question: What does that last phrase mean? Does it mean:
a) first apply the usual ADL rules to generate a candidate list, then ignore any member functions in that list (this is what I believe and hope it means, and in particular it means that the presence of a member will suppress names that ADL would otherwise find in the associated namespaces); or
b) something else?
In short, does N2::operator+ make it into the candidate list? I think it shouldn't. Am I right?
John Spicer: I believe that the answer is sort-of "a" above. More specifically, the unqualified lookup consists of a "normal" unqualified lookup and ADL. ADL always deals with only namespace members, so the "ignore members functions" part must affect the normal lookup, which should ignore class members when searching for an operator.
I suspect that the difference between compilers may have to do with details of argument-dependent lookup. In the example given, the argument types are "N1::X<N2::Z>::Y" and "unsigned int". In order for N2::operator+ to be a candidate, N2 must be an associated namespace.
N1::X<N2::Z>::Y is a class type, so 3.4.2 [basic.lookup.argdep] says that its associated classes are its direct and indirect base classes, and its namespaces are the namespaces of those classes. So, its associated namespace is just N1.
3.4.2 [basic.lookup.argdep] also says:
If T is a template-id, its associated namespaces and classes are the namespace in which the template is defined; for member templates, the member template's class; the namespaces and classes associated with the types of the template arguments provided for template type parameters (excluding template template parameters); the namespaces in which any template template arguments are defined; and the classes in which any member templates used as template template arguments are defined. [Note: non-type template arguments do not contribute to the set of associated namespaces. ]First of all, there is a problem with the term "is a template-id". template-id is a syntactic constuct and you can't really talk about a type being a template-id. Presumably, this is intended to mean "If T is the type of a class template specialization ...". But does this apply to N1::X<N2::Z>::Y? Y is a class nested within a class template specialization. In addition, its base class is a class template specialization.
I think this raises two issues:
Notes from the April 2003 meeting:
The ADL rules in the standard sort of look at if they are fully recursive, but in fact they are not; in some cases, enclosing classes and base classes are considered, and in others they are not. Microsoft and g++ did fully-recursive implementations, and EDG and IBM did it the other way. Jon Caves reports that Microsoft saw no noticeable difference (e.g., no complaints from customers internal or external) when they made this change, so we believe that even if the rules are imperfect the way they are in the standard, they are clear and the imperfections are small enough that programmers will not notice them. Given that, it seemed prudent to make no changes and just close this issue.
The template-id issue is spun off as issue 403.
There is a discrepancy between the syntaxes allowed for defining a constructor and a destructor of a class template. For example:
template <class> struct S { S(); ~S (); };
template <class T> S<T>::S<T>() { } // error
template <class T> S<T>::~S<T>() { } // okay
The reason for this is that 3.4.3.1 [class.qual] paragraph 2 says that S::S is “considered to name the constructor,” which is not a template and thus cannot accept a template argument list. On the other hand, the second S in S::~S finds the injected-class-name, which “can be used with or without a template-argument-list” (14.6.1 [temp.local] paragraph 1) and thus satisfies the requirement to name the destructor's class (12.4 [class.dtor] paragraph 1).
Would it make sense to allow the template-argument-list in the constructor declaration and thus make the language just a little easier to use?
Rationale (July, 2007):
The CWG noted that the suggested change would be confusing in the case where the class template had both template and non-template constructors.
3.5 [basic.link] paragraph 8 says,
A name with no linkage (notably, the name of a class or enumeration declared in a local scope (3.3.3 [basic.scope.local] )) shall not be used to declare an entity with linkage.This wording does not, but should, prohibit use of an unnamed local type in the declaration of an entity with linkage. For example,
void f() {
extern struct { } x; // currently allowed
}
Proposed resolution: Change the text in 3.5 [basic.link] paragraph 8 from:
A name with no linkage (notably, the name of a class or enumeration declared in a local scope (3.3.3 [basic.scope.local])) shall not be used to declare an entity with linkage.to:
A name with no linkage (notably, the name of a class or enumeration declared in a local scope (3.3.3 [basic.scope.local])) or an unnamed type shall not be used to declare an entity with linkage.In section 3.5 [basic.link] paragraph 8, add to the example, before the closing brace of function f:
extern struct {} x; // ill-formed
Rationale (10/00): The proposed change would have introduced an incompatibility with the C language. For example, the global declaration
static enum { A, B, C } abc;
represents an idiom that is used in C but would be prohibited under this resolution.
According to 3.2 [basic.def.odr] paragraph 5, it is possible for a static data member of a class template to be defined more than once in a given program provided that each such definition occurs in a different translation unit and the ODR is met.
Now consider the following example:
src1.cpp:
#include <iostream>
int initializer()
{
static int counter;
return counter++;
}
int g_data1 = initializer();
template<class T>
struct exp {
static int m_data;
};
template<class T>
int exp<T>::m_data = initializer();
int g_data2 = initializer();
extern int g_data3;
int main()
{
std::cout << exp<char>::m_data << ", " << g_data1 << ", "
<< g_data2 << ", " << g_data3 << std::endl;
return 0;
}
src2.cpp:
extern int initializer();
int g_data3 = initializer();
template<class T>
struct exp {
static int m_data;
};
template<class T>
int exp<T>::m_data = initializer();
void func()
{
exp<char>::m_data++;
}
The specialization exp<char>::m_data is implicitly instaniated in both translation units, hence (14.7.1 [temp.inst] paragraph 1) its initialization occurs. And for both definitions of exp<T>::m_data the ODR is met. According to 3.6.2 [basic.start.init] paragraph 1:
Objects with static storage duration defined in namespace scope in the same translation unit and dynamically initialized shall be initialized in the order in which their definition appears in the translation unit.
But for exp<T>::m_data we have two definitions. Does it mean that both g_data1 and g_data3 are guaranteed to be dynamically initialized before exp<char>::m_data?
Suggested Resolution: Insert the following sentence before the last two sentences of 3.2 [basic.def.odr] paragraph 5:
In the case of D being a static data member of a class template the following shall also hold:
- for a given (not explicit) specialization of D initialized dynamically (3.6.2 [basic.start.init]), the accumulated set of objects initialized dynamically in namespace scope before the specialization of D shall be the same in every translation unit that contains the definition for this specialization.
Notes from 10/01 meeting:
It was decided that this issue is not linked to issue 270 and that there is no problem, because there is only one instantiation (see 2.2 [lex.phases] paragraph 8).
The subject line pretty much says it all. It's a possibility that hadn't ever occurred to me. I don't see any prohibition in the standard, and I also don't think the possibility introduces any logical inconsistencies. The proper behavior, presumably, would be to go through the list of already-constructed objects (not including the current one, since its constructor wouldn't have finished executing) and destroy them in reverse order. Not fundamentally hard, and I'm sure lots of existing implementations already do that.
I'm just not sure whether the standard was intended to support this, or whether it's just that nobody else thought of it either. If the former, then a non-normative note somewhere in 3.6.2 [basic.start.init] might be nice.
Rationale (October 2004):
There is nothing in the Standard to indicate that this usage is prohibited, so it must be presumed to be permitted.
3.8 [basic.life] and 12.4 [class.dtor] discuss explicit management of object lifetime. It seems clear that most object lifetime issues apply to sub-objects (array elements, and data members) as well. The standard supports
struct X { T t } x;
T* pt = &x.t;
pt->~T();
new(pt) T;
and this kind of behavior is useful in allocators.
However the standard does not seem to prohibit the same operations on base sub-objects.
struct D: B{ ... } d;
B* pb = &d;
pb->~B();
new(pb) B;
However if B and/or D have virtual member functions or virtual bases, it is unlikely that this code will result in a well-formed D object in current implementations (note that the various lines may be in different functions).
Suggested resolution: 12.4 [class.dtor] should be modified so that explicit destruction of base-class sub-objects be made illegal, or legal only under some restrictive conditions.
Rationale (04/01):
Reallocation of a base class subobject is already disallowed by 3.8 [basic.life] paragraph 7.
3.8 [basic.life] was never adjusted for threads. In particular, it describes what may be done with objects in various intervals. In general when the Standard uses words like “during,” it is referring to intervals defined by “sequenced before” ordering. In this context, however, all the specifications need to use the “happens before” ordering.
Suggested resolution:
Add the following at the beginning of 3.8 [basic.life]:
All statements about the ordering of evaluations in this section, using words like “before,” “after,” and “during,” refer to the “happens before” order defined in 1.10 [intro.multithread]. [Note: We ignore situations in which evaluations are unordered by “happens before,” since these require a data race (1.10 [intro.multithread]), which already results in undefined behavior. —end note]
Rationale (August, 1020):
The text is already in the FCD.
Following the definition in 9 [class] paragraph 4 the following is a valid POD (actually a POD-struct):
struct test
{
const int i;
};
The legality of PODs with const members is also implied by the text of 5.3.4 [expr.new] paragraph 15 bullet 1, sub-bullet 2 and 12.6.2 [class.base.init] paragraph 4 bullet 2.
3.9 [basic.types] paragraph 3 states that
For any POD type T, if two pointers to T point to distinct objects obj1 and obj2, if the value of obj1 is copied into obj2, using the memcpy library function, obj2 shall subsequently hold the same value as obj1.
[Note: this text was changed by TC1, but the essential point stays the same.]
This implies that the following is required to work:
test obj1 = { 1 };
test obj2 = { 2 };
memcpy( &obj2, &obj1, sizeof(test) );
The memcpy of course changes the value of the const member, surely something that shouldn't be allowed.
Suggested resolution:
It is recommended that 3.9 [basic.types] paragraph 3 be reworded to exclude PODs which contain (directly or indirectly) members of const-qualified type.
Rationale (October, 2004):
7.1.6.1 [dcl.type.cv] paragraph 4 already forbids modifying a const member of a POD struct. The prohibition need not be repeated in 3.9 [basic.types].
3.9 [basic.types] paragraph 11 requires that a class type have a trivial copy constructor in order to be classified as a literal type. This seems overly restrictive; presumably having a constexpr copy constructor would suffice. (Note that a trivial copy constructor is a constexpr constructor according to 7.1.5 [dcl.constexpr] paragraph 4.)
Rationale (June, 2008):
A copy constructor takes a reference as its first parameter, thus no user-declared copy constructor can be constexpr.
3.9.1 [basic.fundamental] paragraph 6 states,
As described below, bool values behave as integral types.
This sentence looks definitely out of order: how can a value behave as a type?
Suggested resolution:
Remove the sentence entirely, as it doesn't supply anything that isn't already stated in the following paragraphs and in the referenced section about integral promotion.
Rationale (July, 2007):
This is, at most, an editorial issue with no substantive impact. The suggestion has been forwarded to the project editor for consideration.
Rationale (August, 2011)
This is an editorial issue that has been transmitted to the project editor.
The C++ standard says in 3.10 [basic.lval], in paragraph 15:
an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union),
Note that it is a literal copy from the C standard, but this is of course not the problem.
In C, union is not defined as an aggregate type. Therefore it is appropriate to say “aggregate or union.” But things changed in C++: aggregate type includes union type now (though not all unions are aggregates), and it becomes clear that the “union” in “aggregate or union” is redundant and should be deleted.
The above cited paragraph could be changed to:
an aggregate type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate)
Rationale (October, 2006):
As noted in the issue, not all unions are aggregates, but those that are not aggregates still allow aliasing. That part of the specification would be lost with the suggested change.
Historically, based on C's treatment, cv-qualification of non-class rvalues has been ignored in C++. With the advent of rvalue references, it's not quite as clear that this is desirable. For example, some implementations are reported to print const rvalue for the following program:
const int bar() {
return 5;
}
void pass_int(int&& i) {
printf("rvalue\n");
}
void pass_int(const int&& i) {
printf("const rvalue\n");
}
int main() {
pass_int(bar());
}
Rationale (August, 2010):
The current specification is as intended.
Paragraph 3 of section 4.5 [conv.prom] contains a statement saying that if a bit-field is larger than int or unsigned int, no integral promotions apply to it. This phrase needs further clarification, as it is hardly possible to fugure out what it means. See below.
Assuming a machine with a size of general-purpose register equal 32 bits (where a byte takes up 8 bits) and a C++ implementation where an int is 32 bits and a long is 64 bits. And the following snippet of code:
struct ExternalInterface {
long field1:36, field2:28;
};
int main() {
ExternalInterface myinstance = { 0x100000001L, 0x12,};
if(myinstance.field1 < 0x100000002L) { //do something }
}
Does the standard prohibit the implementation from promoting field1's value into two general purpose registers? And imposes a burden of using shift machine instructions to work with the field's value? What else could that phrase mean?
Either alternative is implementation specific, so I don't understand why the phrase "If the bit-field is larger yet, no integral promotions apply to it" made it to the standard.
Notes from 10/01 meeting:
The standard of course does not dictate what an implementation might do with regard to use of registers or shift instructions in the generated code. The phrase cited means only that a larger bit-field does not undergo integral promotions, and therefore it retains the type with which it was declared (long in the above example). The Core Working Group judged that this was sufficiently clear in the standard.
Note that 9.6 [class.bit] paragraph 1 indicates that any bits in excess of the size of the underlying type are padding bits and do not participate in the value representation. Therefore the field1 bit field in the above example is not capable of holding the indicated values, which require more than 32 bits.
Section 4.9 [conv.fpint] paragraph 1 states:
An rvalue of a floating point type can be converted to an rvalue of an integer type. The conversion truncates; that is, the fractional part is discarded.
Here, the concepts of “truncation” and “fractional part” seem to be used without precise definitions. When -3.14 is converted into an integer, is the truncation toward zero or away from zero? Is the fractional part -0.14 or 0.86? The standard seem to give no clear answer to these.
Suggested resolution:
Replace “truncates” with “truncates toward zero.”
Replace “the fractional part” with “the fractional part (where that of x is defined as x-floor(x) for nonnegative x and x-ceiling(x) for negative x);” there should be a better wording for this, or the entire statement “that is, the fractional part is discarded” can be removed, once the meaning of “truncation” becomes unambiguous as above.
Rationale (October, 2006):
The specification is clear enough: “fractional part” refers to the digits following the decimal point, so that -3.14 converted to int becomes -3.
In the following code, I expect both "null" and "FALSE" to be null pointer constants -- and that the code should compile and output the string "int*" twice to cout:
#include <iostream>
using namespace std;
void foo(int* p)
{
cout << "int*" << endl;
}
int main(void)
{
const int null = 0;
foo(null);
const bool FALSE = false;
foo(FALSE);
}
ISO/IEC 14882-1998 4.10 [conv.ptr] states:
An integral constant expression rvalue of integer type that evaluates to zero (called a /null pointer constant/) can be converted to a pointer type.
Stroustrup appears to agree with me -- he states (3rd edition page 88):
In C, it has been popular to define a macro NULL to represent the zero pointer. Because of C++`s tighter type checking, the use of plain 0, rather than any suggested NULL macro, leads to fewer problems. If you feel you must define NULL, use:const int NULL = 0;
However gcc 3.3.1 rejects this code with the errors:
bug.cc:17: error: invalid conversion from `int' to `int*'
bug.cc:19: error: cannot convert `const bool' to `int*' for argument `1' to `
void foo(int*)'
I have reported this as a bug (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13867), but the gcc team states that 4.10 requires that a null pointer constant must be an rvalue -- and no implicit conversion from an lvalue to an rvalue is required (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=396):
a null pointer constant is an integral constant expression rvalue that evaluates to zero [4.10/1] in this case `null' is an lvalue. The standard does not specify that lvalue->rvalue decay happens here, so `null' is not a null pointer constant.
I disagree with the gcc teams interpretation -- I don't see why 3.10 [basic.lval] doesn't apply:
Whenever an lvalue appears in a context where an rvalue is expected, the lvalue is converted to an rvalue;
The insertion of the word rvalue appears to have occurred during standardization -- it is not present in either Stroustrup 2nd edition or the 3rd edition. Does the committee deliberately intend to exclude an lvalue as a null pointer constant by adding the word rvalue? If so, it leads to the rather bizarre fact that "null" is not a null pointer constant, but "null + 0" is!
Notes from the March 2004 meeting:
We think this is just a bug in gcc. The const variable does get converted to an rvalue in this context. This case is not really any different than cases like
const int null = 0; int i = null;or
const int i = 1; int a[i];(which are accepted by gcc). No one would argue that the second lines of those examples are invalid because the variables are lvalues, and yet the conversions to rvalue happen implicitly for the same reason cited above -- the contexts require an rvalue.
An operator expression can, according to 5 [expr] paragraph 2, require transformation into function call syntax. The reference in that paragraph is to 13.5 [over.oper] , but it should be to 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.
5.1.2 [expr.prim.lambda] paragraph 2 says,
A closure object behaves as a function object (20.8 [function.objects])...
This linkage to <functional> increases the dependency of the language upon the library and is inconsistent with the definition of “freestanding” in 17.6.1.3 [compliance].
Rationale (July, 2009):
The reference to 20.8 [function.objects] appears in a note, not in normative text, and is intended only to clarify the meaning of the term “function object.” The CWG does not believe that this reference creates any dependency on any library facility.
The following case is ill-formed:
int f (int&);
void* f (const int&);
int main()
{
int i;
[=] ()-> decltype(f(i)) { return f(i); };
}
The decltype(f(i)) is not of the form decltype((x)), and also not within the body of the lambda, so the special rewriting rule doesn't apply. So, the call in the decltype selects the first overload, and the call in the body selects the second overload, and there's no conversion from void* to int, so the return-statement is ill-formed.
This pattern is likely to arise frequently because of the retrictions on deducing the return type from the body of the lambda.
Daveed Vandevoorde: The pattern may be common, but it probably doesn't matter much in practice. It's most likely that overloaded functions that differ only in the cv-qualification of their parameters will have related return types.
Rationale (October, 2009):
The consensus of the CWG was that this is not a sufficiently important problem to warrant changing the existing specification.
According to 5.1.2 [expr.prim.lambda] paragraph 21,
When the lambda-expression is evaluated, the entities that are captured by copy are used to direct-initialize each corresponding non-static data member of the resulting closure object.
This apparently means that if the capture-default is to copy, entities captured by default, implicitly, are copied even in cases where the copy constructors of such entities are explicit. It should be required that such entities be captured explicitly instead.
See also issue 1020.
Rationale (August, 2010):
The behavior is according to the original design and is similar to what would happen if the constructor of the closure object initialized the members for the captured entities using mem-initializers. CWG did not see sufficient motivation to change the design.
[Picked up by evolution group at October 2002 meeting.]
Is it okay for a static_cast to drop exception specifications?
void f() throw(int);
int main () {
static_cast<void (*)() throw()>(f); // Okay?
void (*p)() throw() = f; // Error
}
The fact that a static_cast is defined, more or less, as an initialization suggests that a check ought to be made.
One tricky point: this is another case where the general rule that the reverse of an implicit cast is allowed as a static_cast bites you -- the reverse conversion doesn't drop exception specifications, and so is okay. Perhaps this should be treated like casting away constness.
Mike Miller comments : I don't think that case can arise. According to 15.4 [except.spec],
An exception-specification shall appear only on a function declarator in a function, pointer, reference, or pointer to member declaration or definition.
We strengthened that in issue 87 (voted to DR status in Copenhagen) to
An exception-specification shall appear only on a function declarator for a function type, pointer to function type, reference to function type, or pointer to member function type that is the top-level type of a declaration or definition, or on such a type appearing as a parameter or return type in a function declarator.
As I read that, you can't put an exception-specification on the type-id in a static_cast, which means that a static_cast can only weaken, not strengthen, the exception specification.
The core WG discussed this at the 10/01 meeting and agreed.
Note (March, 2008):
The Evolution Working Group recommended closing this issue with no further consideration. See paper J16/07-0033 = WG21 N2173.
It is not specified under what conditions an object pointer created by converting a function pointer, as described in 5.2.10 [expr.reinterpret.cast] paragraph 8, will be safely-derived, particularly in light of the conditionally-supported, implementation-defined nature of such conversions.
Notes from the March, 2009 meeting:
If this is to be addressed, the result should not be as suggested, i.e., a requirement for implementation documentation appearing only in a note. At the least, such a requirement must be in normative text.
Rationale (July, 2009):
The definition of “safely-derived pointer” is clearly and exclusively formulated in terms of pointers to objects. So no implementation is required to maintain safe pointer derivation through conversion to and from a function-pointer type.
On the other hand, any garbage-collecting implementation is free to treat function pointers the same as object pointers for purposes of collection. This would provide the effect of safe pointer derivation through function-pointer types. An implementation is even free to document this behavior, if it so chooses.
However, converting a pointer to a dynamically-allocated object into a function pointer would be a very strange and almost always pointless and unsafe thing to do. There is no need for the standard to encourage this sort of behavior, even to the extent of adding a note mentioning the possibility.
In 5.3.1 [expr.unary.op], part of paragraph 7 describes how to compute the negative of an unsigned quantity:
The negative of an unsigned quantity is computed by subtracting its value from 2n, where n is the number of bits in the promoted operand. The type of the result is the type of the promoted operand.
According to this method, -0U will get the value 2n - 0 = 2n, where n is the number of bits in an unsigned int. However, 2n is obviously out of the range of values representable by an unsigned int and thus not the actual value of -0U. To get the result, a truncating conversion must be applied.
Rationale (April, 2007):
As noted in the issue description, a “truncating conversion” is needed. This conversion is supplied without need of an explicit mention, however, by the nature of unsigned arithmetic given in 3.9.1 [basic.fundamental] paragraph 4:
Unsigned integers, declared unsigned, shall obey the laws of arithmetic modulo 2n where n is the number of bits in the value representation of that particular size of integer.
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.
Clause 5 [expr] paragraph 4 appears to grant an implementation the right to generate code for a function call like
f(new T1, new T2)
in the order
Suggested resolution: either forbid the ordering above or expand the requirement for reclaiming storage to include exceptions thrown in all operations between the allocation and the completion of the constructor.
Rationale (10/99): Even in the "traditional" ordering of the calls to allocation functions and constructors, memory can still leak. For instance, if T1 were successfully constructed and then the construction of T2 were terminated by an exception, the memory for T1 would be lost. Programmers concerned about memory leaks will avoid this kind of construct, so it seems unnecessary to provide special treatment for it to avoid the memory leaks associated with one particular implementation strategy.
According to 5.4 [expr.cast] paragraph 4, one possible interpretation of an old-style cast is as a static_cast followed by a const_cast. One would therefore expect that the expressions marked #1 and #2 in the following example would have the same validity and meaning:
struct S {
operator const int* ();
};
void f(S& s) {
const_cast<int*>(static_cast<const int*>(s)); // #1
(int*) s; // #2
}
However, a number of implementations issue an error on #2.
Is the intent that (T*)x should be interpreted as something like
const_cast<T*>(static_cast<const volatile T*>(x))
Rationale (July, 2009):
According to the straightforward interpretation of the wording, the example should work. This appears to be just a compiler bug.
According to 5.5 [expr.mptr.oper] paragraph 6,
The result of a .* expression whose second operand is a pointer to a data member is of the same value category (3.10 [basic.lval]) as its first operand.
This is incorrect if the member has a reference type, in which case the result is an lvalue.
Rationale (September, 2010):
There are no pointers to member of reference type; see 8.3.3 [dcl.mptr] paragraph 3.
An expression of the form pointer + enum (see paragraph 5) is not given meaning, and ought to be, given that paragraph 2 of this section makes it valid. Presumably, the enum value should be converted to an integral value, and the rest of the processing done on that basis. Perhaps we want to invoke the integral promotions here.
[Should this apply to (pointer - enum) too?]
Rationale (04/99): Paragraph 1 invokes "the usual arithmetic conversions" for operands of enumeration type.
(It was later pointed out that the builtin operator T* operator+(T*, ptrdiff_t) (13.6 [over.built] paragraph 13) is selected by overload resolution. Consequently, according to 13.3.1.2 [over.match.oper] paragraph 7, the operand of enumeration type is converted to ptrdiff_t before being interpreted according to the rules in 5.7 [expr.add] .)
Code that was portable in C90 and C++98 is no longer portable with the introduction of data types longer than long; code that could previously cast size_t and ptrdiff_t to long without loss of precision (because long was the largest type) can no longer rely on that idiom.
The CWG discussed this during the Berlin (April, 2006) meeting. The general consensus was that this was unavoidable: there are valid reasons for implementations to keep long at a size less than that required for address arithmetic.
See paper J16/06-0053 = WG21 N1983, which also suggests the possibility of required diagnostics for problematic cases as an alternative to restricting the size of size_t and ptrdiff_t.
Rationale (October, 2006):
This is not an area in which the Standard should override the decisions of implementors who wish to maintain the size of long for backward compatibility but need a larger size_t to deal with expanded address spaces. Also, diagnostics of the sort described are better treated as quality of implementation issues rather than topics for standardization.
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 2.14.2 [lex.icon] : No mention of true or false as an integer literal.
From 2.14.6 [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
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.
Does an explicit temporary of an integral type qualify as an integral constant expression? For instance,
void* p = int(); // well-formed?
It would appear to be, since int() is an explicit type conversion according to 5.2.3 [expr.type.conv] (at least, it's described in a section entitled "Explicit type conversion") and type conversions to integral types are permitted in integral constant expressions (5.19 [expr.const]). However, this reasoning is somewhat tenuous, and some at least have argued otherwise.
Note (March, 2008):
This issue should be closed as NAD as a result of the rewrite of 5.19 [expr.const] in conjunction with the constexpr proposal.
Rationale (August, 2011):
As given in the preceding note.
According to 5.19 [expr.const] paragraph 1,
In particular, except in sizeof expressions, functions, class objects, pointers, or references shall not be used, and assignment, increment, decrement, function-call, or comma operators shall not be used.
Given a case like
enum E { e };
int operator+(int, E);
int i[4 + e];
does this mean that the overloaded operator+ is not considered (because it can't be called), or is it selected by overload resolution, thus rendering the program ill-formed?
Rationale (April, 2005):
All expressions, including constant expressions, are subject to overload resolution. The example is ill-formed.
typeid expressions can never be constant, whether or not the operand is a polymorphic class type. The result of the expression is a reference, and the typeinfo class that the reference refers to is polymorphic, with a virtual destructor - it can never be a literal type.
Rationale (July, 2009):
The intent of this specification was that the address of such a typeinfo object could be treated as an address constant and thus usable in constant initialization (contrary to the statement in the comment, the result of typeid is an lvalue, not a reference).
When jumping past initialization of a local static variable the value of the static becomes indeterminate. Seems like this behavior should be illegal just as it is for local variables with automatic linkage.
Here is an example:
struct X {
X(int i) : x(i) {}
int x;
};
int f(int c) {
if (c)
goto ly; // error here for jumping past next stmt.
static X a = 1;
ly:
return a.x; // either 1 or 0 depending on implementation.
}
6.7 [stmt.dcl] P3 should be changed to:
A program that jumps from a point where a local variable with automatic or static storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has POD type (3.9) and is declared without an initializer (8.5).This would imply "static X a = 1;" should be flagged as an error. Note that this behavior a may be a "quality of implementation issue" which may be covered in 6.7 P4. Paragraph 4 seems to make the choice of static/dynamic initialization indeterminate. Making this an error and thus determinate seems the correct thing to do since that is what is already required of automatic variables.
Steve Adamczyk: Some version of this may be appropriate, but it's common to have code that is executed only the first time it is reached, and to have an initialization of a static variable inside such a piece of code. In such a case, on executions after the first there is indeed a jump over the declaration, but the static variable is correctly initialized -- it was initialized the first time the routine was called.
void f() {
static bool first_time = true;
if (!first_time) goto after_init;
static int i = g();
first_time = false;
after_init:
...
}
Rationale (October, 2004):
The CWG sees no reason to change this specification. Local static variables are different from automatic variables: automatic variables, if not explicitly initialized, can have indeterminate (“garbage”) values, including trap representations, while local static variables are subject to zero initialization and thus cannot have garbage values.
The latitude granted to implementations regarding performing dynamic initialization of local static objects as if it were static initialization is exactly parallel to namespace scope objects (3.6.2 [basic.start.init]), as are the restrictions on programmer assumptions.
Because a definition is also a declaration, it might make sense to change uses of "declaration or definition" to simply "declaration".
Notes from the March 2004 meeting:
Jens Maurer prepared drafting for this issue, but we find ourselves reluctant to actually make the changes. Though correct, they seemed more likely to be misread than the existing wording.
Proposed resolution:
Remove in 1.3 [intro.defs] “parameter” the indicated words:
an object or reference declared as part of a function declaration or definition, or in the catch clause of an exception handler, that acquires a value on entry to the function or handler; ...
Remove in 14.1 [temp.param] paragraph 10 the indicated words:
The set of default template-arguments available for use with a template declaration or definition is obtained by merging the default arguments from the definition (if in scope) and all declarations in scope in the same way default function arguments are (...).
Remove in 14.6 [temp.res] paragraph 2 the indicated words:
A name used in a template declaration or definition and that is dependent on a template-parameter is assumed not to name a type unless the applicable name lookup finds a type name or the name is qualified by the keyword typename.
Remove in 14.6.4.1 [temp.point] paragraph 1 the indicated words:
Otherwise, the point of instantiation for such a specialization immediately follows the namespace scope declaration or definition that refers to the specialization.
Remove in 14.6.4.1 [temp.point] paragraph 3 the indicated words:
Otherwise, the point of instantiation for such a specialization immediately precedes the namespace scope declaration or definition that refers to the specialization.
Remove in 14.7.3 [temp.expl.spec] paragraph 21 the indicated words:
Default function arguments shall not be specified in a declaration or a definition for one of the following explicit specializations:[Note: default function arguments may be specified in the declaration or definition of a member function of a class template specialization that is explicitly specialized. ]
- ...
Remove in 14.8.2.5 [temp.deduct.type] paragraph 18 the indicated words:
[Note: a default template-argument cannot be specified in a function template declaration or definition; ...]
Remove in 17.6.2.2 [using.headers] paragraph 3 the indicated words:
A translation unit shall include a header only outside of any external declaration or definition, and shall include the header lexically before the first reference to any of the entities it declares or first defines in that translation unit.
Rationale (October, 2004):
CWG felt that readers might misunderstand “declaration” as meaning “non-definition declaration.”
9.5 [class.union] paragraph 3 implies that anonymous unions in unnamed namespaces need not be declared static (it only places that restriction on anonymous unions "declared in a named namespace or in the global namespace").
However, 7.1.1 [dcl.stc] paragraph 1 says that "global anonymous unions... shall be declared static." This could be read as prohibiting anonymous unions in unnamed namespaces, which are the preferred alternative to the deprecated use of static.
Rationale (10/99): An anonymous union in an unnamed namespace is not "a global anonymous union," i.e., it is not a member of the global namespace.
In clause 7.1.2 [dcl.fct.spec], para. 3, the following sentence
A function defined within a class definition is an inline function.
should, if I am not mistaken, instead be:
A function defined within a class declaration is an inline function."
Notes from October 2002 meeting:
This is not a defect. Though there is a long history, going back to the ARM, of use of the term "class declaration" to mean the definition of the class, we believe "class definition" is clearer. We have opened issue 379 to deal with changing all other uses of "class declaration" to "class definition" where appropriate.
A customer reports that when he attempts to replace ::operator new with a user-defined function, the standard library calls the default function by preference if the user-defined function is inline. I believe that our compiler is correct, and that such a replacement function isn't allowed to be inline, but I'm not sure there's sufficiently explicit language in the standard.
In general, of course, the definition of an inline function must be present in every translation unit where the function is called. (7.1.2 [dcl.fct.spec], par 4) It could be argued that this requirement doesn't quite address replacement functions: what we're dealing with is the odd case where we've already got one definition and the user is supplying a different one. I'd like to see something specifically addressing the case of a replacement function.
So what do we have? I see discussion of requirement for a replacement ::operator new in three places: 17.6.4.6 [replacement.functions], 18.6.1.1 [new.delete.single] par 2, and 3.7.4 [basic.stc.dynamic] par 2-3. I don't see anything explicitly saying that the replacement function may not be inline. The closest I can find is 18.6.1.1 [new.delete.single] par 2, which says that "a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library". One might argue that "with this function signature" rules out inline, but that strikes me as a slight stretch.
Have I missed anything?
Andrew Koenig: I think you've turned up a problem in 7.1.2 [dcl.fct.spec] paragraph 4. Consider:
// Translation unit 1
#include <iostream>
extern void foo(void (*)());
inline void bar() {
std::cout << "Hello, world!" << std::endl;
}
int main() {
foo(bar);
}
// Translation unit 2
void foo(void (*f)()) { (*f)(); }
Are you really trying to tell me that this program is ill-formed because the definition of bar is not available in translation unit 2?
I think not. The actual words in 7.1.2 [dcl.fct.spec] par 4 are
An inline function shall be defined in every translation unit in which it is used...and I think at in this context, ``used'' should be interpreted to mean that foo is used only in translation unit 1, where it is converted to a value of type void(*)().
Notes from October 2003 meeting:
We don't think Andy Koenig's comment requires any action; "used" is already defined appropriately.
We agree that this replacement should not be allowed, but we think it's a library issue (in the rules for allowed replacements). Forwarded to library group; it's issue 404 on the library issues list.
Is the following valid?
template <class T> void f(T) {
typedef int x;
typedef T x;
}
int main() {
f(1);
}
There is an instantiation where the function is valid. Is an implementation allowed to issue an error on the template declaration because the types on the typedef are not the same (7.1.3 [dcl.typedef])?
How about
typedef T x; typedef T2 x;?
It can be argued that these cases should be allowed because they aren't necessarily wrong, but it can also be argued that there's no reason to write things like the first case above, and if such a case appears it's more likely to be a mistake than some kind of intentional test that int and T are the same type.
Notes from the October 2003 meeting:
We believe that all these cases should be allowed, and that errors should be required only when an instance of the template is generated. The current standard wording does not seem to disallow such cases, so no change is required.
Rationale (August, 2011):
The requirements apply to definitions, not declarations.
9.2 [class.mem] paragraph 2 says,
A class is considered a completely-defined object type (3.9 [basic.types]) (or complete type) at the closing } of the class-specifier. Within the class member-specification, the class is regarded as complete within function bodies, default arguments, and exception-specifications (including such things in nested classes). Otherwise it is regarded as incomplete within its own class member-specification.
In particular, the return type of a member function is not listed as a context in which the class type is considered complete; instead, that case is handled as an exception to the general rule in 8.3.5 [dcl.fct] paragraph 6 requiring a complete type in the definition of a function:
The type of a parameter or the return type for a function definition shall not be an incomplete class type (possibly cv-qualified) unless the function definition is nested within the member-specification for that class (including definitions in nested classes defined within the class).
These rules have implications for the use of decltype. (The following examples use the not-yet-accepted syntax for specifying the return type of a function after its declarator, but the questions apply to the current syntax as well.) Consider:
struct deduced {
int test() { return 0; }
auto eval( deduced& d )->decltype( d.test() ) {
return d.test();
}
};
5.2.5 [expr.ref] paragraph 1 requires that the class type of the object or pointer expression in a class member access expression be complete, so this usage is ill-formed.
A related issue is the use of this in a decltype specifier:
struct last_one {
int test() { return 0; }
auto eval()->decltype( this->test() ) {
return test();
}
};
9.3.2 [class.this] paragraph 1 allows use of this only in the body of a non-static member function, and the return type is not part of the function-body.
Do we want to change the rules to allow these kinds of decltype expressions?
Rationale (February, 2008):
In the other cases where a class type is considered complete within the definition of the class, it is possible to defer handling the construct until the end of the definition. That is not possible for types, as the type may be needed immediately in subsequent declarations.
It was also noted that the primary utility of decltype is in generic contexts; within a single class definition, other mechanisms are possible (e.g., use of a member typedef in both the declaration of the operand of the decltype and to replace the decltype itself).
The first bullet of 7.1.6.2 [dcl.type.simple] paragraph 4 says,
There are two clarifications to this specification that would assist the reader. First, it would be useful to have a note highlighting the point that a parenthesized expression is neither an id-expression nor a member access expression.
Second, the phrase “the type of the entity named by e” is unclear as to whether cv-qualification in the object or pointer expression is or is not part of that type. Rephrasing this to read, “the declared type of the entity,” or adding “(ignoring any cv-qualification in the object expression or pointer expression),” would clarify the intent.
Rationale (February, 2008):
The text is clear enough. In particular, both of these points are illustrated in the last two lines of the example contrasting decltype(a->x) and decltype((a->x)): in the former, the expression has no parentheses, thus satisfying the requirements of the first bullet and yielding the declared type of A::x, while the second has parentheses, falling into the third bullet and picking up the const from the object expression in the member access.
Because type deduction for the auto specifier is described in 7.1.6.4 [dcl.spec.auto] paragraph 6 as equivalent to the deduction that occurs in a call to a function template, the adjustment of the argument type from A to A& specified in 14.8.2.1 [temp.deduct.call] paragraph 3 is performed when the initializer is an lvalue. As a result, in the following example, ra has the type A& and not, as might be expected, A&&:
class A { };
void f() {
A a;
auto&& ra = a;
}
It is unclear whether this is surprising enough, and potentially widely-enough used, to warrant making an exception to the current rules to handle this case differently.
Rationale (September, 2008):
It is important that the deduction rules be the same in the function and auto cases. The result of this example might be surprising, but maintaining a consistent model for deduction is more important.
Although in most contexts “= expression” can be replaced by “{ expression }”, enumerator-definitions accept only the “=” form. This could be surprising.
Additional note (October, 2009):
The Committee may wish to consider default arguments in this light as well.
Rationale (August, 2010):
This suggestion was considered and rejected by EWG.
The text of 7.2 [dcl.enum] paragraph 2 explicitly forbids unnamed scoped enumerations:
The optional identifier shall not be omitted in the declaration of a scoped enumeration.
There does not appear to be a good rationale for this restriction since a typedef name can be used to name the enumerators. It is also inconsistent with similar constructs. For example,
typedef enum class { e } E;
E x = E::e;
is ill-formed, but
typedef struct { enum { s }; } S;
int y = S::s;
is well-formed.
Rationale (August, 2011):
The use of typedef names for linkage purposes is intended for C compatibility and should not be extended to features that are not part of the C subset of C++.
I received an inquiry/complaint that you cannot re-open a namespace using a qualified name. For example, the following program is ok, but if you uncomment the commented lines you get an error:
namespace A {
namespace N {
int a;
}
int b;
namespace M {
int c;
}
}
//namespace A::N {
// int d;
//}
namespace A {
namespace M {
int e;
}
}
int main()
{
A::N::a = 1;
A::b = 2;
A::M::c = 3;
// A::N::d = 4;
A::M::e = 5;
}
Andrew Koenig: There's a name lookup issue lurking here. For example:
int x;
namespace A {
int x;
namespace N {
int y;
};
}
namespace A::N {
int* y = &x; // which x?
}
Jonathan Caves: I would assume that any rule would state that:
namespace A::B {
would be equivalent to:
namespace A {
namespace B {
so in your example 'x' would resolve to A::x
BTW: we have received lots of bug reports about this "oversight".
Lawrence Crowl: Even worse is
int x;
namespace A {
int x;
}
namespace B {
int x;
namespace ::A {
int* y = &x;
}
}
I really don't think that the benefits of qualified names here is worth
the cost.
Notes from April 2003 meeting:
We're closing this because it's on the Evolution working group list.
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 11.3 [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,
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 7.1.6.3 [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 7.1.6.3 [dcl.type.elab] paragraph 3 says:
class B;
union B {};
and
union B {};
class B;
are both invalid. I think this paragraph is intended to say that. I'm
not so sure it actually does say that, though.
Mike Miller: Regarding I think the intent was to model this after the existing rule for local declarations of functions (which dates back to C)
Actually, that's not the C (1989) rule. To quote the Rationale from X3.159-1989:
Regarding Getting this right requires being somewhat more rigorous
Yes, I think if this is to be made illegal, it would have to be done with the ODR; the name-lookup-based current rules clearly (IMHO) don't apply. (Although to be fair, the [non-normative] note in 3.3 [basic.scope] paragraph 4 sounds as if it expects friend invisible injection to trigger the multiple-declaration provisions of that paragraph; it's just that there's no normative text implementing that expectation.)
Bill Gibbons: Nor does the ODR currently disallow:
translation unit #1 struct A;
translation unit #2 union A;
since it only refers to class definitions, not declarations.
But the obvious form of the missing rule (all declarations of a class within a program must have compatible struct/class/union keys) would also answer the original question.
The declarations need not be visible. For example:
translation unit #1 int f() { return 0; }
translation unit #2: void g() {
extern long f();
}
is ill-formed even though the second "f" is not a visible declaration.
Rationale (10/99): The main issue (differing behavior of standalone and embedded elaborated-type-specifiers) is as the Committee intended. The remaining questions mentioned in the discussion may be addressed in dealing with related issues.
(See also issues 136, 138, 139, 143, 165, and 166.)
7.3.1.2 [namespace.memdef] paragraph 2 says,
Members of a named namespace can also be defined outside that namespace by explicit qualification (3.4.3.2 [namespace.qual] ) of the name being defined, provided that the entity being defined was already declared in the namespace...It is not clear whether block-scope extern declarations and friend declarations are sufficient to permit the named entities to be defined outside their namespace. For example,
namespace NS {
struct A { friend struct B; };
void foo() { extern void bar(); }
}
struct NS::B { }; // 1) legal?
void NS::bar() { } // 2) legal?
Rationale (10/99): Entities whose names are "invisibly injected" into a namespace as a result of friend declarations are not "declared" in that namespace until an explicit declaration of the entity appears at namespace scope. Consequently, the definitions in the example are ill-formed.
(See also issues 95, 136, 138, 139, 143, and 166.)
Consider the following example:
class C {
public: enum E {};
friend void* operator new(size_t, E);
friend void operator delete(void*, E);
};
void foo() {
C::E e;
C* ptr = new(e) C();
}
This code, which is valid in global scope, becomes ill-formed when the class definition is moved into a namespace, and there is no way to make it valid:
namespace N {
class C {
public: enum E {};
friend void* operator new(size_t, E);
friend void operator delete(void*, E);
};
}
void foo() {
N::C::E e;
N::C* ptr = new(e) N::C();
}
The reason for this is that non-member allocation and deallocation functions are required to be members of the global scope (3.7.4.1 [basic.stc.dynamic.allocation] paragraph 1, 3.7.4.2 [basic.stc.dynamic.deallocation] paragraph 1), unqualified friend declarations declare names in the innermost enclosing namespace (7.3.1.2 [namespace.memdef] paragraph 3), and these functions cannot be declared in global scope at a point where the friend declarations could refer to them using qualified-ids because their second parameter is a member of the class and thus can't be named before the class containing the friend declarations is defined.
Possible solutions for this conundrum include invention of some mechanism to allow a friend declaration to designate a namespace scope other than the innermost enclosing namespace in which the friend class or function is to be declared or to relax the innermost enclosing namespace lookup restriction in 7.3.1.2 [namespace.memdef] paragraph 3 for friend declarations that nominate allocation and deallocation functions.
Rationale (April, 2006):
The CWG acknowledged that it is not always possible to move code from the global scope into a namespace but felt that this problem was not severe enough to warrant changing the language to accommodate it. Possible solutions include moving the enumeration outside the class or defining member allocation and deallocation functions.
7.3.1.2 [namespace.memdef] paragraph 3 is intended to prevent injection of names from friend declarations into the containing namespace scope:
If a friend declaration in a non-local class first declares a class or function the friend class or function is a member of the innermost enclosing namespace. The name of the friend is not found by unqualified lookup (3.4.1 [basic.lookup.unqual]) or by qualified lookup (3.4.3 [basic.lookup.qual]) until a matching declaration is provided in that namespace scope (either before or after the class definition granting friendship).
However, this does not address names declared by elaborated-type-specifiers that are part of the friend declaration. Are these names intended to be visibly injected? For example, is the following well-formed?
class A {
friend class B* f();
};
B* bp; // Is B visible here?
Implementations differ in their treatment of this example: EDG and MSVC++ 8.0 accept it, while g++ 4.1.1 rejects it.
Rationale (July, 2009):
The current specification does not restrict injection of names in elaborated-type-specifiers, and the consensus of the CWG was that no change is needed on this point.
Daveed Vandevoorde : While reading Core issue 11 I thought it implied the following possibility:
template<typename T>
struct B {
template<int> void f(int);
};
template<typename T>
struct D: B<T> {
using B<T>::template f;
void g() { this->f<1>(0); } // OK, f is a template
};
However, the grammar for a using-declaration reads:
and nested-name-specifier never ends in "template".
Is that intentional?
Bill Gibbons :
It certainly appears to be, since we have:
Rationale (04/99): Any semantics associated with the template keyword in using-declarations should be considered an extension.
Notes from the April 2003 meeting:
We decided to make no change and to close this issue as not-a-defect. This is not needed functionality; the example above, for example, can be written with ->template. This issue has been on the issues list for years as an extension, and there has been no clamor for it.
It was also noted that knowing that something is a template is not enough; there's still the issue of knowing whether it is a class or function template.
Additional note (February, 2011):
This issue is being reopened for further consideration after
additional discussion
using T::template X; // ill-formed
for a class template member X of base class T, one could write
template<U> using X = typename T::template X<U>;
Rationale (March, 2011):
There was insufficient motivation for a change at this point.
7.3.3 [namespace.udecl] paragraph says,
A using-declaration shall not name a template-id.It is not clear whether this prohibition applies to the entity for which the using-declaration is a synonym or to any name that appears in the using-declaration. For example, is the following code well-formed?
template <typename T>
struct base {
void bar ();
};
struct der : base<int>
{
using base<int>::bar; // ill-formed ?
};
Rationale (10/99): 7.3.3 [namespace.udecl] paragraph 1 says, "A using-declaration introduces a name..." It is the name that is thus introduced that cannot be a template-id.
Now that the concept of "conditionally-supported" is available (see N1564), perhaps asm should not be required of every implementation.
Rationale (October, 2004):
This is covered in paper N1627. We would like to keep asm as a keyword for all implementations, however, to enhance portability by preventing programmers from inadvertently using it as an identifier.
Issue 1
7.5 [dcl.link] paragraph 6 says the following:
extern "C" int f(void);
namespace A {
extern "C" int f(void);
};
using namespace A;
int i = f(); // Ok because only one function f() or
// ill-formed
For name lookup, both declarations of f are visible and overloading cannot
distinguish between them. Has the compiler to check that these functions
are really the same function or is the program in error?
Rationale: These are the same function for all purposes.
Issue 2
A similar question may arise with typedefs:
// vendor A
typedef unsigned int size_t;
// vendor B
namespace std {
typedef unsigned int size_t;
}
using namespace std;
size_t something(); // error?
Is this valid because the typedef size_t refers to the same type in both
namespaces?
Rationale (04/99): In 7.3.4 [namespace.udir] paragraph 4:
If name lookup finds a declaration for a name in two different namespaces, and the declarations do not declare the same entity and do not declare functions, the use of the name is ill-formed.The term entity applied to typedefs refers to the underlying type or class (3 [basic] , paragraph 3); therefore both declarations of size_t declare the same entity and the above example is well-formed.
[Picked up by evolution group at October 2002 meeting.]
Steve Clamage: I can't find anything in the standard that prohibits a language linkage on an operator function. For example:
extern "C" int operator+(MyInt, MyInt) { ... }
Clearly it is a bad idea, you could have only one operator+ with "C" linkage in the entire program, and you can't call the function from C code.
Mike Miller: Well, you can't name an operator function in C code, but if the arguments are compatible (e.g., not references), you can call it from C code via a pointer. In fact, because the language linkage is part of the function type, you couldn't pass the address of an operator function into C code unless you could declare the function to be extern "C".
Fergus Henderson: In the general case, for linkage to languages other than C, this could well make perfect sense.
Steve Clamage:
But is it disallowed (as opposed to being stupid), and if so, where in the standard does it say so?
Mike Miller: I don't believe there's a restriction. Whether that is because of the (rather feeble) justification of being able to call an operator from C code via a pointer, or whether it was simply overlooked, I don't know.
Fergus Henderson: I don't think it is disallowed. I also don't think there is any need to explicitly disallow it.
Steve Clamage: I don't think the standard is clear enough on this point. I'd like to see a clarification.
I think either of these two clarifications would be appropriate:
extern "C" T operator+(T,T); // ok
extern "C" T operator-(T,T); // ok
extern "C" U operator-(U); // error, two extern "C" operator-
Mike Miller: I think the point here is that something like
extern "xyzzy" bool operator<(S&,S&)
could well make sense, if language xyzzy is sufficiently compatible with
C++, and the one-function rule only applies to extern "C", not to other
language linkages. Given that it might make sense to have general
language linkages for operators, is it worthwhile to make an exception
to the general rule by saying that you can have any language linkage
on an operator function except "C" linkage? I don't like exceptions to
general rules unless they're very well motivated, and I don't see
sufficient motivation to make one here.
Certainly this capability isn't very useful. There are lots of things in C++ that aren't very useful but just weren't worth special-casing out of the language. I think this falls into the same category.
Mike Ball: I DON'T want to forbid operator functions within an extern "C". Rather I want to add operator functions to that sentence in paragraph 4 of 7.5 [dcl.link] which reads
A C language linkage is ignored for the names of class members and the member function type of class member functions.My reason is simple: C linkage makes a total hash of scope. Any "C" functions declared with the same name in any namespace scope are the same function. In other words, namespaces are totally ignored.
This provision was added in toward the end of the standardization process, and was, I thought, primarily to make it possible to put the C library in namespace std. Otherwise, it seems an unwarrented attack on the very concept of scope. We (wisely) didn't force this on static member functions, since it would essentially promote them to the global scope.
Now I think that programmers think of operator functions as essentially part of a class. At least for one very common design pattern they are treated as part of the class interface. This pattern is the reason we invented Koenig lookup for operator functions.
What happens when such a class definition is included, deliberately or not, in an extern "C" declaration? The member operators continue to work, but the non-member operators can suddenly get strange and hard to understand messages. Quite possibly, they get the messages only when combined with other classes in other compilation units. You can argue that the programmer shouldn't put the class header in a linkage delaration in the first place, but I can still find books that recommend putting `extern "C"' around entire header files, so it's going to happen.
I think that including operator functions in the general exclusion from extern "C" doesn't remove a capability, rather it ensurs a capability that programmers already think they have.
Rationale (10/00):
The benefits of creating an exception for operator functions were outweighed by the complexity of adding another special case to the rules.
Note (March, 2008):
The Evolution Working Group recommended closing this issue with no further consideration. See paper J16/07-0033 = WG21 N2173.
[Picked up by evolution group at October 2002 meeting.]
7.5 [dcl.link] paragraph 4 says,
A C language linkage is ignored for the names of class members and the member function types of class member functions.This makes good sense, since C linkage names typically aren't compatible with the naming used for member functions at link time, nor is C language linkage function type necessarily compatible with the calling convention for passing this to a non-static member function.
But C language linkage type (not name) for a static member function is invaluable for a common programming idiom. When calling a C function that takes a pointer to a function, it's common to use a private static member function as a "trampoline" which retrieves an object reference (perhaps by casting) and then calls a non-static private member function. If a static member function can't have a type with C language linkage, then a global or friend function must be used instead. These alternatives expose more of a class's implementation than a static member function; either the friend function itself is visible at namespace scope alongside the class definition or the private member function must be made public so it can be called by a non-friend function.
Suggested Resolution: Change the sentence cited above to:
A C language linkage is ignored for the names of class members and the member function types of non-static class member functions.The example need not be changed because it doesn't involve a static member function.
The following workaround accomplishes the goal of not exposing the class's implementation, but at the cost of significant superstructure and obfuscation:
// foo.h
extern "C" typedef int c_func(int);
typedef int cpp_func(int);
class foo
{
private:
c_func* GetCallback();
static int Callback(int);
};
// foo.cpp
#include "foo.h"
// A local pointer to the static member that will handle the callback.
static cpp_func* cpp_callback=0;
// The C function that will actually get registered.
extern "C" int CFunk(int i)
{
return cpp_callback(i);
}
c_func* foo::GetCallback()
{
cpp_callback = &Callback; // Only needs to be done once.
return &CFunk;
}
Rationale (10/99): The Standard correctly reflects the intent of the Committee.
Note (March, 2008):
The Evolution Working Group recommended closing this issue with no further consideration. See paper J16/07-0033 = WG21 N2173.
Is this code valid:
extern "C" void f();
namespace N
{
int var;
extern "C" void f(){ var = 10; }
}
The two declarations of f refer to the same external function, but is this a valid way to declare and define f?
And is the definition of f considered to be in namespace N or in the global namespace?
Notes from October 2002 meeting:
Yes, this example is valid. See 7.5 [dcl.link] paragraph 6, which contains a similar example with the definition in the global namespace instead. There is only one f, so the question of whether the definition is in the global namespace or the namespace N is not meaningful. The same function is found by name lookup whether it is found from the declaration in namespace N or the declaration in the global namespace, or both (7.3.4 [namespace.udir] paragraph 4).
The grammar for alignment-specifier in 7.6.1 [dcl.attr.grammar] paragraph 1 is:
There is no such nonterminal as alignment-expression; it should be assignment-expression instead.
Rationale (August, 2011)
This is an editorial issue that has been transmitted to the project editor.
The [[noreturn]] attribute, as specified in 7.6.3 [dcl.attr.noreturn], applies to function declarations and is not integrated with the type system. This is incompatible with existing practice (as in gcc) and should be reconsidered.
Rationale (July, 2009):
The CWG reaffirmed the previous decisions not to have attributes apply to types and did not believe that the benefits were sufficient for this case to make an exception to the general rule.
C has rejected the notion of attributes, and introduced the noreturn facility as a keyword. To continue writing clean, portable code we should replace the [[noreturn]] attribute with a noreturn keyword, following the usual convention that while C obfuscates new keywords with _Capital and adds a macro to map to the comfortable spelling, C++ simply adopts the all-lowercase spelling.
Rationale (August, 2010):
CWG felt that an attribute was a more appropriate representation for this feature.
Do we really need the & ref-qualifier? We could get the same behavior without it if we relaxed the restriction on ref-qualified and non-ref-qualified overloads in the same set:
| with the & ref-qualifier | without the & ref-qualifier |
struct S {
void f();
};
|
struct S {
void f();
};
|
struct S {
void f() &;
};
|
struct S {
void f();
void f() && = delete;
};
|
struct S {
void f() &&;
};
|
struct S {
void f() &&;
};
|
struct S {
void f() &;
void f() &&;
};
|
struct S {
void f();
void f() &&;
};
|
The main objection I can see to this change is that we would lose the notational convenience of the & ref-qualifier, which would need to be replaced by a pair of declarations. We might overcome this by still allowing a single & on a function (although it would not be a ref-qualifier) as a synonym to a non-ref-qualified declaration plus a deleted ref-qualified declaration.
The biggest asymmetry between the implicit object parameter and regular parameters is not in reference binding but in type deduction. Consider:
template <class R, class C, class A> void f(R (C::*p)(A));
With these members:
struct S {
void mv(std::string);
void mr(std::string&);
void ml(std::string&&);
};
then
f(&S::mv); // deduces A = string
f(&S::mr); // deduces A = string&
f(&S::ml); // deduces A = string&&
On the other hand, with these members:
struct S {
void mv(std::string);
void mr(std::string) &;
void ml(std::string) &&
};
then
f(&S::mv); // deduces C = S f(&S::mr); // illegal f(&S::ml); // illegal
To make template f work with any pointer to member function, I need three overloads of f. Add cv-qualifiers and it's twelve overloads!
And then there is the interaction with concepts. Consider this type:
struct Value {
Value& operator=(const Value&) &;
};
Is it, say, Regular? If so, will the following compile, and what is the outcome?
template <Regular T> void f() {
T() = T();
}
void g() {
f<Value>();
}
If Value is not Regular, that is a good motivation to avoid ever using & ref-qualifiers on operator= (and probably on any member functions).
If Value is Regular, then either f<Value>() doesn't compile, violating one of the principal motivations for concepts, or it calls Value::operator= on an rvalue, which was explicitly prohibited.
Rationale, March, 2009:
The CWG did not feel that the suggested change was a signficant improvement over the existing specification.
In deciding whether a construct is an object declaration or a function declaration, 8.2 [dcl.ambig.res] contains the following gem: "In that context, the choice is between a function declaration [...] and an object declaration [...] Just as for the ambiguities mentioned in 6.8 [stmt.ambig], the resolution is to consider any construct that could possibly be a declaration a declaration."
To what declaration do the last two "declarations" refer? Object, function, or (following from the syntax) possibly parameter declarations?
Notes from the 4/02 meeting:
This is not a defect. Section 8.2 [dcl.ambig.res] reads:
The ambiguity arising from the similarity between a function-style cast and a declaration mentioned in 6.8 [stmt.ambig] can also occur in the context of a declaration. In that context, the choice is between a function declaration with a redundant set of parentheses around a parameter name and an object declaration with a function-style cast as the initializer. Just as for the ambiguities mentioned in 6.8 [stmt.ambig], the resolution is to consider any construct that could possibly be a declaration a declaration.
The wording "any construct" in the last sentence is not limited to top-level constructs. In particular, the function declaration encloses a parameter declaration, whereas the object declaration encloses an expression. Therefore, in case of ambiguity between these two cases, the declaration is parsed as a function declaration.
Consider the following program:
struct Point
{
Point(int){}
};
struct Lattice
{
Lattice(Point, Point, int){}
};
int main(void)
{
int a, b;
Lattice latt(Point(a), Point(b), 3); /* Line X */
}
The problem concerns the line marked /* Line X */, which is an ambiguous declarations for either an object or a function. The clause that governs this ambiguity is 8.2 [dcl.ambig.res] paragraph 1, and reads:
The ambiguity arising from the similarity between a function-style cast and a declaration mentioned in 6.8 [stmt.ambig] can also occur in the context of a declaration. In that context, the choice is between a function declaration with a redundant set of parentheses around a parameter name and an object declaration with a function-style cast as the initializer. Just as for the ambiguities mentioned in 6.8 [stmt.ambig], the resolution is to consider any construct that could possibly be a declaration a declaration. [Note: a declaration can be explicitly disambiguated by a nonfunction-style cast, by a = to indicate initialization or by removing the redundant parentheses around the parameter name. ]
Based on this clause there are two possible interpretations of the declaration in line X:
Note that the last sentence before the "[Note:" is not much help, because both options are declarations.
Steve Adamczyk: a number of people replied to this posting on comp.std.c++ saying that they did not see a problem. The original poster replied:
I can't do anything but agree with your argumentation. So there is only one correct interpretation of clause 8.2 [dcl.ambig.res] paragraph 1, but I have to say that with some rewording, the clause can be made a lot clearer, like stating explicitly that the entire declaration must be taken into account and that function declarations are preferred over object declarations.
I would like to suggest the following as replacement for the current clause 8.2 [dcl.ambig.res] paragraph 1:
The ambiguity arising from the similarity between a functionstyle cast and a declaration mentioned in 6.8 [stmt.ambig] can also occur in the context of a declaration. In that context, the choice is between a function declaration with a redundant set of parentheses around a parameter name and an object declaration with a function-style cast as the initializer. The resolution is to consider any construct that could possibly be a function declaration a function declaration. [Note: To disambiguate, the whole declaration might have to be examined to determine if it is an object or a function declaration.] [Note: a declaration can be explicitly disambiguated by a nonfunction-style cast, by a = to indicate initialization or by removing the redundant parentheses around the parameter name. ]
Notes from the 4/02 meeting:
The working group felt that the current wording is clear enough.
Consider the following example:
struct S {
virtual void v() = 0;
};
void f(S sa[10]); // permitted?
8.3.4 [dcl.array] paragraph 1 says that a declaration like that of sa is ill-formed:
T is called the array element type; this type shall not be a reference type, the (possibly cv-qualified) type void, a function type or an abstract class type.
On the other hand, 8.3.5 [dcl.fct] paragraph 3 says that the type of sa is adjusted to S*, which would be permitted:
The type of each parameter is determined from its own decl-specifier-seq and declarator. After determining the type of each parameter, any parameter of type “array of T” or “function returning T” is adjusted to be “pointer to T” or “pointer to function returning T,” respectively.
It is not clear whether the parameter adjustment trumps the prohibition on declaring an array of an abstract class type or not. Implementations differ in this respect: EDG 2.4.2 and MSVC++ 7.1 reject the example, while g++ 3.3.3 and Sun Workshop 8 accept it.
Rationale (April, 2005):
The prohibition in 8.3.4 [dcl.array] is absolute and does not allow for exceptions. Even though such a type in a parameter declaration would decay to an allowed type, the prohibition applies to the type before the decay.
This interpretation is consistent with the resolution of issue 337, which causes template type deduction to fail if such types are deduced. It was also observed that pointer arithmetic on pointers to abstract classes is very likely to fail, and the fact that the programmer used array notation to declare the pointer type is a strong indication that he/she expected to use subscripting.
According to 8.3.4 [dcl.array] paragraph 1,
In a declaration T D where D has the form
D1 [ constant-expressionopt ] attribute-specifieropt
and the type of the identifier in the declaration T D1 is “derived-declarator-type-list T”, then the type of the identifier of D is an array type; if the type of the identifier of D contains the auto type-specifier, the program is ill-formed.
This has the effect of prohibiting a declaration like
int v[1];
auto (*p)[1] = &v;
This restriction is unnecessary and presumably unintentional.
Note also that the statement that “the type of the identifier of D is an array type” is incorrect when the nested declarator is not simply a declarator-id. A similar problem exists in the wording of 8.5.3 [dcl.init.ref] paragraph 3 for function types.
Rationale (March, 2011):
The functionality of the auto specifier was intentionally restricted to simple cases; supporting complex declarators like this was explicitly discussed and rejected when the feature was adopted.
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.
According to 8.3.5 [dcl.fct] paragraph 5, top-level cv-qualifiers on parameter types are deleted when determining the function type. It is not clear how or whether this adjustment should be applied to parameters of function templates when the parameter has a dependent type, however. For example:
template<class T> struct A {
typedef T arr[3];
};
template<class T> void f(const typename A<T>::arr) { } // #1
template void f<int>(const A<int>::arr);
template <class T> struct B {
void g(T);
};
template <class T> void B<T>::g(const T) { } // #2
If the const in #1 is dropped, f<int> has a parameter type of A* rather than the const A* specified in the explicit instantiation. If the const in #2 is not dropped, we fail to match the definition of B::g to its declaration.
Rationale (November, 2010):
The CWG agreed that this behavior is intrinsic to the different ways cv-qualification applies to array types and non-array types.
Paragraph 9 of says that extra default arguments added after a using-declaration but before a call are usable in the call, while 7.3.3 [namespace.udecl] paragraph 9 says that extra function overloads are not. This seems inconsistent, especially given the similarity of default arguments and overloads.
Rationale (10/99): The Standard accurately reflects the intent of the Committee.
It would seem intuitively that a deleted function cannot throw an exception, but 8.4.3 [dcl.fct.def.delete] does not mention that. This could conceivably be useful in SFINAE contexts.
Rationale (November, 2010):
Any reference to a deleted function is ill-formed, so it doesn't really matter whether they are noexcept or not.
The current wording of the WP allows aggregate initialization of parameters in function calls. For example, 13.3.3.1.5 [over.ics.list] paragraph 4 reads:
Otherwise, if the parameter has an aggregate type which can be initialized from the initializer list according to the rules for aggregate initialization (8.5.1 [dcl.init.aggr]), the implicit conversion sequence is a user-defined conversion sequence. [Example:
struct A { int m1; double m2; }; void f(A); f( {'a', 'b'} ); // OK: f(A(int,double)) user-defined conversion f( {1.0} ); // error: narrowing—end example]
The rules for aggregate initialization in 8.5.1 [dcl.init.aggr] paragraph 11 allow braces to be elided in the initializer
In a declaration of the form
T x = { a };
It is not clear whether this phrasing should be interpreted as allowing brace elision only in a simple-declaration and thus not in a function argument or whether this restriction is inadvertent and should be removed.
Rationale (November, 2010):
The restriction is intentional. Support for aggregate initialization is principally intended for legacy code and C compatibility, not for code written using the new facilities of the language.
In section 8.5.3 [dcl.init.ref], paragraph 5, there is following note:
Note: the usual lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) standard conversions are not needed, and therefore are suppressed, when such direct bindings to lvalues are done.
I believe that this note is misleading. There should be either:
The problem:
int main()
{
const int ci = 10;
int * pi = NULL;
const int * & rpci = pi;
rpci = &ci;
*pi = 12; // circumvent constness of "ci"
}
int main()
{
int * pi = NULL;
const int * const & rcpci = pi; // 1
int i = 0;
pi = &i; // 2
if (pi == rcpci)
std::cout << "bound to lvalue" << std::endl;
else
std::cout << "bound to temporary rvalue" << std::endl;
}
There has been discussion on this issue on comp.lang.c++.moderated month ago, see http://groups.google.pl/groups?threadm=9bed99bb.0308041153.1c79e882%40posting.google.com and there seems to be some confusion about it. I understand that note is not normative, but apparently even some compiler writers are misled (try above code snippets on few different compilers, and using different compilation options - notably GCC 3.2.3 with -Wall -pedantic), thus it should be cleared up.
My proposal is to change wording of discussed note to:
Note: result of every standard conversion is never an lvalue, and therefore all standard conversions (clause 4) are suppressed, when such direct bindings to lvalues are done.
Rationale (April, 2005):
As acknowledged in the description of the issue, the referenced text is only a note and has no normative impact. Furthermore, the examples cited do not involve the conversions mentioned in the note, and the normative text is already sufficiently clear that the types in the examples are not reference-compatible.
According to the logic in 8.5.3 [dcl.init.ref] paragraph 5, the following example should create a temporary array and bind the reference to that temporary:
const char (&p)[10] = "123";
That is presumably not intended (issue 450 calls a similar outcome for rvalue arrays “implausible”). Current implementations reject this example.
Rationale (August, 2010):
The Standard does not describe initialization of array temporaries, so a program that requires such is ill-formed.
Note (October, 2010):
Although in general an object of array type cannot be initialized from another object of array type, there is special provision in 8.5.2 [dcl.init.string] for doing so when the source object is a string literal, as in this example. The issue is thus being reopened for further consideration in this light.
Notes from the November, 2010 meeting:
The CWG agreed that the current wording appears to permit this example but still felt that array temporaries are undesirable. Wording should be added to disallow this usage.
Proposed resolution (November, 2010):
Change 8.5.3 [dcl.init.ref] paragraph 5 as follows:
...
If the initializer expression is a string literal (2.14.5 [lex.string]), the program is ill-formed.
Otherwise, a temporary of type...
(See also issue 1232, which argues in favor of allowing array temporaries.)
Rationale (March, 2011):
In consideration of the arguments made in issue 1232, CWG agreed to allow array temporaries and there is thus no reason to prohibit them in this case.
The exposition of list initialization using an array in 8.5.4 [dcl.init.list] paragraph 4 raises the question of whether an empty initializer list is permitted, as declaration of an array with a zero bound is ill-formed.
Rationale (October, 2009):
The description is intended as an aid to understanding the concepts, not as a literal transformation that is performed. An implementation is permitted to allocate a zero-length array, even if such as array cannot be decclared (e.g., via a new-expression).
Consider the example,
struct A { char c; };
void f (char d) {
A a = { d + 1 };
}
This code is now ill-formed because of the narrowing conversion from the int result type of the addition, not because of any real narrowing. This seems like an embarrassment for C++0x. It would be better not to get an error about any arithmetic involving non-constant operands just because it might overflow with some values.
Rationale (November, 2010):
The CWG agreed that this behavior is unfortunate but felt that it would be too difficult to formulate a satisfactory set of rules for handling complex expressions correctly for a small gain in utility (the user can simply add a cast in order to avoid the error).
Non-static data member initializers should not be part of C++0x unless they have implementation experience.
Notes from the August, 2010 meeting:
The C++/CLI dialect has a very similar feature that has been implemented.
Rationale (March, 2011):
The full Committee voted not to remove this feature.
It's not clear how lookup of a non-dependent qualified name should be handled in a non-static member function of a class template. For example,
struct A {
int f(int);
static int f(double);
};
struct B {};
template<typename T> struct C : T {
void g() {
A::f(0);
}
};
The call to A::f inside C::g() appears non-dependent, so one might expect that it would be bound at template definition time to A::f(double). However, the resolution for issue 515 changed 9.3.1 [class.mfct.non-static] paragraph 3 to transform an id-expression to a member access expression using (*this). if lookup resolves the name to a non-static member of any class, making the reference dependent. The result is that if C is instantiated with A, A::f(int) is called; if C is instantiated with B, the call is ill-formed (the call is transformed to (*this).A::f(0), and there is no A subobject in C<B>). Both these results seem unintuitive.
(See also issue 1017.)
Notes from the November, 2010 meeting:
The CWG agreed that the resolution of issue 515 was ill-advised and should be reversed.Rationale (March, 2011):
The analysis is incorrect; whether the reference is dependent or not, overload resolution chooses A::f(int) because of the rules in 13.3.1.1.1 [over.call.func] paragraph 3 dealing with contrived objects for static member functions.
Move semantics for *this should not be part of C++0x unless they have implementation experience.
Rationale (March, 2011):
The full Committee voted not to remove this feature.
Another instance to consider is that of invoking a member function from a null pointer:
struct A { void f () { } };
int main ()
{
A* ap = 0;
ap->f ();
}
Which is explicitly noted as undefined in 9.3.1 [class.mfct.non-static], even though one could argue that since f() is empty, there is no lvalue->rvalue conversion.
If f is static, however, there seems to be no such rule, and the call is only undefined if the dereference implicit in the -> operator is undefined. IMO it should be.
Incidentally, another thing that ought to be cleaned up is the inconsistent use of "indirection" and "dereference". We should pick one. (This terminology issue has been broken out as issue 342.)
This is related to issue 232.
Rationale (October 2003):
We agreed the example should be allowed. p->f() is rewritten as (*p).f() according to 5.2.5 [expr.ref]. *p is not an error when p is null unless the lvalue is converted to an rvalue (4.1 [conv.lval]), which it isn't here.
The current wording of 9.4.2 [class.static.data] only allows a static data member to be initialized within the class definition if it is const. This restriction should be removed.
Rationale (July, 2009):
The consensus of the CWG was that there is insufficient motivation for such a change. A non-const static data member must still be defined outside the class if it is used, and the value of such a member cannot be used in a constant expression, unlike a constant static data member, so there is no real advantage to putting the initializer inside the class definition instead of in the definition of the static data member. The apparent parallel with non-static data member initialization is also not compelling; for example, the initializer for a non-static data member can contain forward references to members declared later in the class, while the same is not true of static data member initializers.
Is this legal? Should it be?
struct E {
union {
struct { int x; } s;
} v;
};
One compiler faults a type definition (i.e. of the anonymous struct) since it is in an anonymous union [9.5 [class.union] paragraph 2: "The member-specification of an anonymous union shall only define non-static data members."].
I would suggest that compiler B is correctly interpreting the standard but that this is a defect in the standard. There is no reason to disallow definition of anonymous structs.
Furthermore, is it really necessary to disallow definition of named types in anonymous unions in general, as long as the types do not need fully qualified names for external linkage? Why should this be illegal?
struct E {
union {
typedef int Int;
struct X { X *next; Int n; } list;
} v;
};
Notes from October 2002 meeting:
There was agreement that the standard says such declarations are invalid; therefore this must be considered as an extension. There was general feeling that this extension would not be too useful, though Jason Merrill was sympathetic to the argument. It was also agreed that if this were to be changed it would require careful wording so as not to allow too many cases.
Note (March, 2008):
The Evolution Working Group recommended closing this issue with no further consideration. See paper J16/07-0033 = WG21 N2173.
Can a member of a union be of a class that has a user-declared non-default constructor? The restrictions on union membership in 9.5 [class.union] paragraph 1 only mention default and copy constructors:
An object of a class with a non-trivial default constructor (12.1 [class.ctor]), a non-trivial copy constructor (12.8 [class.copy]), a non-trivial destructor (12.4 [class.dtor]), or a non-trivial copy assignment operator (13.5.3 [over.ass], 12.8 [class.copy]) cannot be a member of a union...
(12.1 [class.ctor] paragraph 11 does say, “a non-trivial constructor,” but it's not clear whether that was intended to refer only to default and copy constructors or to any user-declared constructor. For example, 12.2 [class.temporary] paragraph 3 also speaks of a “non-trivial constructor,” but the cross-references there make it clear that only default and copy constructors are in view.)
Note (March, 2008):
This issue was resolved by the adoption of paper J16/08-0054 = WG21 N2544 (“Unrestricted Unions”) at the Bellevue meeting.
Rationale (August, 2011):
As given in the preceding note.
9.3 [class.mfct] paragraph 5 says this about member functions defined lexically outside the class:
the member function name shall be qualified by its class name using the :: operator
9.4.2 [class.static.data] paragraph 2 says this about static data members:
In the definition at namespace scope, the name of the static data member shall be qualified by its class name using the :: operator
I would have expected similar wording in 9.7 [class.nest] paragraph 3 for nested classes. Without such wording, the following seems to be legal (and is allowed by all the compilers I have):
struct base {
struct nested;
};
struct derived : base {};
struct derived::nested {};
Is this just an oversight, or is there some rationale for this behavior?
Rationale (July, 2008):
The wording in 9 [class] paragraph 10 (added by the resolution of issue 284, which was approved after this issue was raised) makes the example ill-formed:
If a class-head contains a nested-name-specifier, the class-specifier shall refer to a class that was previously declared directly in the class or namespace to which the nested-name-specifier refers (i.e., neither inherited nor introduced by a using-declaration), and the class-specifier shall appear in a namespace enclosing the previous declaration.
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.
Footnote 98 says:
As specified previously in clause 11 [class.access] , private members of a base class remain inaccessible even to derived classes unless friend declarations within the base class declaration are used to grant access explicitly.This footnote does not fit with the algorithm provided in 11.2 [class.access.base] paragraph 4 because it does not take into account the naming class concept introduced in this paragraph.
(See also paper J16/99-0002 = WG21 N1179.)
Rationale (10/99): The footnote should be read as referring to immediately-derived classes, and is accurate in that context.
The Standard does not appear to specify how to handle cases in which conflicting access specifications for a member are inherited from different base classes. For example,
struct A {
public:
int i;
};
struct B : virtual public A {
protected:
using A::i;
};
struct C : virtual public A, public B {
// "i" is protected from B, public from A
};
This question affects both the existing wording of 11.2 [class.access.base] paragraph 4 (“m as a member of N is public ... m as a member of N is private ... m as a member of N is protected”) and the proposed wording for issue 385 (“when a nonstatic data member or nonstatic member function is a protected member of its naming class”).
One possible definition of “is public” would be something like, “if any visible declaration of the entity has public access.” One could also plausibly define the access of m in N to be the minimum of all the visible declarations, or even an error if the visible declarations are inconsistent.
11.2 [class.access.base] paragraph 1 describes the access of inherited members, so a clarifying statement resolving this issue might plausibly be inserted at the end of that paragraph.
Proposed resolution (October, 2004):
Add the following text as a new paragraph after 11.2 [class.access.base] paragraph 1:
If a given base class can be reached along more than one path through a derived class's sub-object lattice (10.1 [class.mi]), a member of that base class could have different accessibility in the derived class along different paths. In such cases, the most permissive access prevails. [Example:
struct B { static int i; }; class I: protected B { }; class D1: public B, public I { }; class D2: public I, private B { };i is accessible as a public member of D1 and as a protected member of D2. —end example]
Rationale (03/2005): This question is already covered, in almost identical words, in 11.6 [class.paths].
11.3 [class.friend], paragraph 7, says
A name nominated by a friend declaration shall be accessible in the scope of the class containing the friend declaration.
Does that mean the following should be illegal?
class A { void f(); };
class B { friend void A::f(); }; // Error: A::f not accessible from B
I discussed this with Bjarne in email, and he thinks it was an editorial error and this was not the committee's intention. The paragraph seems to have been added in the pre-Kona (24 Sept 1996) mailing, and I could not find anything in the previous meeting's (Stockholm) mailings which led me to believe this was intentional. The only compiler vendor which I think currently implements it is the latest release (2.43) of the EDG front end.
Proposed resolution (10/00):
Remove the first sentence of 11.3 [class.friend], paragraph 7.
Rationale (04/01):
After the 10/00 vote to accept this issue as a DR with the proposed resolution, it was noted that the first two sentences of 11 [class.access] paragraph 3 cause the proposed change to have no effect:
Access control is applied uniformly to all names, whether the names are referred to from declarations or expressions. [Note: access control applies to names nominated by friend declarations (11.3 [class.friend]) and using-declarations (7.3.3 [namespace.udecl]). ]
In addition to the obvious editing to the text of the note, an exception to the blanket statement in the first sentence would also be required. However, discussion during the 04/01 meeting failed to produce consensus on exactly which names in the friend declaration should be exempted from the requirements of access control.
One possibility would be that only the name nominated as friend should be exempt. However, that approach would make it impossible to name a function as a friend if it used private types in its parameters or return type. Another suggestion was to ignore access for every name used in a friend declaration. That approach raised a question about access within the body of a friend function defined inline in the class body — the body is part of the declaration of a function, but references within the body of a friend function should still be subject to the usual access controls.
Other possibilities were raised, such as allowing the declaration of a friend member function if the declaration were permissible in its containing class, or taking the union of the access within the befriending class and the befriended entity. However, these suggestions would have been complex and difficult to specify correctly.
Ultimately it was decided that the original perceived defect was not sufficiently serious as to warrant the degree of complexity required to resolve it satisfactorily and the issue was consequently declared not to be a defect. It was observed that most of the problems involved with the current state of affairs result from inability to declare a particular member function as a friend; in such cases, an easy workaround is simply to befriend the entire class rather than the specific member function.
Thus says the section 11.3 [class.friend]/7 in ISO 14882 C++ standard:
A name nominated by a friend declaration shall be accessible in the scope of the class containing the friend declaration.
The obvious intention of this is to allow a friend declaration to specify a function (or nested class, enum, etc.) that is declared "private" or "protected" in its enclosing class. However, literal interpretation seems to allow a broader access to the befriended function by the whole class that is declaring the friendship.
If the rule were interpreted literally as it is currently written, this would compile (when it, of course, shouldn't be allowed at all):
class C
{
private:
static void f();
};
class D
{
friend void C::f(); // A name nominated by friend declaration...
D()
{
C::f(); // ... shall be accessible in scope of class declaring friendship
}
};
Suggested fix is to reword "in the scope of the class containing the friend declaration" to exclude all other references from the scope of the declaring class, except the friend-declaration itself.
Notes from the March 2004 meeting:
We considered this and concluded that the standard is clear enough.
I just received a query from a user of why line #1 in the following snippet is ill-formed:
void g(int (*)(int));
template<class T>
class A {
friend int f(int) { return 0; }
void h() {
g(f); // #1
}
};
I believe that the special invisibility rule about friends is too complicated and makes life too complicated, especially considering that friends in templates are not templates, nor can they be conveniently rewritten with a “first declare at the namespace scope” rule. I can understand the rules when they make programming easier or prevent some obvious “silly” mistakes; but that does not seem to be the case here.
John Spicer: See two papers that discuss this issue: N0878 by Bill Gibbons, which ultimately gave rise to our current rules, and N0913 by me as an alternative to N0878.
Rationale (April, 2005):
The Standard is clear and consistent; this rule is the result of an explicit decision by the Committee.
After the adoption of the wording for extended friend declarations, we now have this new paragraph in 11.3 [class.friend]:
A friend declaration that does not declare a function shall have one of the following forms:
friend elaborated-type-specifier ;
friend simple-type-specifier ;
friend typename-specifier ;
But what about friend class templates? Should the following examples compile in C++0x?
template< template <class> class T >
struct A{ friend T; };
template< class > struct C;
struct B{ friend C; };
Proposed resolution (June, 2008):
Change 11.3 [class.friend] paragraph 3 as follows:
A friend declaration that does not declare a function shall have one of the following forms:
friend elaborated-type-specifier ;
friend simple-type-specifier ;
friend typename-specifier ;
friend ::opt nested-name-specifieropt template-name ;
friend identifier ;
In the last alternative, the identifier shall name a template template-parameter. [Note: a friend declaration may be the declaration in a template-declaration (clause 14 [temp], 14.5.4 [temp.friend]). —end note] If the type specifier in a friend declaration designates a (possibly cv-qualified) class type or a class template, that class or template is declared as a friend; otherwise, the friend declaration is ignored. [Example:...
Rationale (September, 2008):
The proposed extension is not needed. The template case can be handled simply by providing a template header:
template <typename T> friend X<T>;
11.4 [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.
The restrictions on protected access in 11.4 [class.protected] apply only to forming pointers to members and to member access expressions. It should be considered whether to extend these restrictions to pointer-to-member expressions as well. For example,
struct base {
protected: int x;
};
struct derived : base {
void foo(base* b) {
b->x = 123; // not ok
(b->*(&derived::x)) = 123; // ok?!
}
};
Rationale (August, 2010):
Access applies to use of names, so the check must be done at the point at which the pointer-to-member is formed. It is not possible to tell from the pointer to member at runtime what the access was.
An implicitly-declared special member function is defined as deleted (12.1 [class.ctor] paragraph 5, 12.4 [class.dtor] paragraph 3, 12.8 [class.copy] paragraphs 5 and 10) if any of the corresponding functions it would call from base classes is inaccessible. This is inconsistent with the treatment of access control in overload resolution and template argument deduction, where accessibility is ignored (but may result in an ill-formed program). This should be made consistent.
Rationale (July, 2009):
The current treatment is sufficiently useful to warrant the inconsistency with the other handling of access control. In particular, it enables such cases to be detected by SFINAE.
12.2 [class.temporary] paragraph 4 seems self-contradictory:
the temporary that holds the result of the expression shall persist until the object's initialization is complete... the temporary is destroyed after it has been copied, before or when the initialization completes.How can it be destroyed "before the initialization completes" if it is required to "persist until the object's initialization is complete?"
Rationale (04/00):
It was suggested that "before the initialization completes" refers to the case in which some part of the initialization terminates by throwing an exception. In that light, the apparent contradiction does not apply.
Temporaries created in the expression of the range-based for statement are not given special treatment, so they only persist to the end of the expression. This can lead to undefined behavior as __range and the iterators are used in the expansion of the statement. Such temporaries should have their lifetimes extended until the end of the statement.
Rationale (October, 2009):
In the expansion, expression is used to initialize a reference. If expression is a temporary, its lifetime is thus extended to that of the reference, which is the entire for statement.
A posting in comp.lang.c++.moderated prompted me to try the following code:
struct S {
template<typename T, int N> (&operator T())[N];
};
The goal is to have a (deducible) conversion operator template to a reference-to-array type.
This is accepted by several front ends (g++, EDG), but I now believe that 12.3.2 [class.conv.fct] paragraph 1 actually prohibits this. The issue here is that we do in fact specify (part of) a return type.
OTOH, I think it is legitimate to expect that this is expressible in the language (preferably not using the syntax above ;-). Maybe we should extend the syntax to allow the following alternative?
struct S {
template<typename T, int N> operator (T(&)[N])();
};
Eric Niebler: If the syntax is extended to support this, similar constructs should also be considered. For instance, I can't for the life of me figure out how to write a conversion member function template to return a member function pointer. It could be useful if you were defining a null_t type. This is probably due to my own ignorance, but getting the syntax right is tricky.
Eg.
struct null_t {
// null object pointer. works.
template<typename T> operator T*() const { return 0; }
// null member pointer. works.
template<typename T,typename U> operator T U::*() const { return 0; }
// null member fn ptr. doesn't work (with Comeau online). my error?
template<typename T,typename U> operator T (U::*)()() const { return 0; }
};
Martin Sebor: Intriguing question. I have no idea how to do it in a single declaration but splitting it up into two steps seems to work:
struct null_t {
template <class T, class U>
struct ptr_mem_fun_t {
typedef T (U::*type)();
};
template <class T, class U>
operator typename ptr_mem_fun_t<T, U>::type () const {
return 0;
}
};
Note: In the April 2003 meeting, the core working group noticed that the above doesn't actually work.
Note (June, 2010):
It has been suggested that template aliases effectively address this issue. In particular, an identity alias like
template<typename T> using id = T;
provides the necessary syntactic sugar to be able to specify types with trailing declarator elements as a conversion-type-id. For example, the two cases discussed above could be written as:
struct S {
template<typename T, int N>
operator id<T[N]>&();
template<typename T, typename U>
operator id<T (U::*)()>() const;
};
This issue should thus be closed as now NAD.
Rationale (August, 2011):
As given in the preceding note.
Is the following code well-formed?
struct A { /* */ };
int main()
{
A a=a;
}
Note, that { int a=a; } is pretty legal.
And if so, what is the semantics of the self-initialization of UDT? For example
#include <stdio.h>
struct A {
A() { printf("A::A() %p\n", this); }
A(const A& a) { printf("A::A(const A&) %p %p\n", this, &a); }
~A() { printf("A::~A() %p\n", this); }
};
int main()
{
A a=a;
}
can be compiled and prints:
A::A(const A&) 0253FDD8 0253FDD8 A::~A() 0253FDD8
(on some implementations).
Notes from October 2002 meeting:
3.8 [basic.life] paragraph 6 indicates that the references here are valid. It's permitted to take the address of a class object before it is fully initialized, and it's permitted to pass it as an argument to a reference parameter as long as the reference can bind directly. Except for the failure to cast the pointers to void * for the %p in the printfs, these examples are standard-conforming.
The second paragraph of section 12.7 [class.cdtor] contains the following text:
To explicitly or implicitly convert a pointer (an lvalue) referring to an object of class X to a pointer (reference) to a direct or indirect base class B of X, the construction of X and the construction of all of its direct or indirect bases that directly or indirectly derive from B shall have started and the destruction of these classes shall not have completed, otherwise the conversion results in undefined behavior.
Now suppose we have the following piece of code:
struct a {
a() : m_a_data(0) { }
a(const a& rhsa) : m_a_data(rhsa.m_a_data) { }
int m_a_data;
};
struct b : virtual a {
b() : m_b_data(0) { }
b(const b& rhsb) : a(rhsb), m_b_data(rhsb.m_b_data) { }
int m_b_data;
};
struct c : b {
c() : m_c_data(0) { }
c(const c& rhsc) : a(rhsc),// Undefined behaviour when constru-
// cting an object of type 'c'
b(rhsc), m_c_data(rhsc.m_c_data) { }
int m_c_data;
};
int main()
{ c ac1, ac2(ac1); }
The problem with the above snippet is that when the value 'ac2' is being created and its construction gets started, c's copy constructor has first to initialize the virtual base class subobject 'a'. Which requires that the lvalue expression 'rhsc' be converted to the type of the parameter of a's copy constructor, which is 'const a&'. According to the wording quoted above, this can be done without undefined behaviour if and only if b's construction has already started, which is not possible since 'a', being a virtual base class, has to be initialized first by a constructor of the most derived object (12.6.2 [class.base.init]).
The issue could in some cases be alleviated when 'c' has a user-defined copy constuctor. The constructor could default-initialize its 'a' subobject and then initialize a's members as needed taking advantage of the latitude given in paragraph 2 of 12.6.2 [class.base.init].
But if 'c' ends up having the implicitly-defined copy constuctor, there's no way to evade undefined behaviour.
struct c : b {
c() : m_c_data(0) { }
int m_c_data;
};
int main()
{ c ac1, ac2(ac1); }
Paragraph 8 of 12.8 [class.copy] states
The implicitly-defined copy constructor for class X performs a memberwise copy of its subobjects. The order of copying is the same as the order of initialization of bases and members in a user-defined constructor (see 12.6.2 [class.base.init]). Each subobject is copied in the manner appropriate to its type:
- if the subobject is of class type, the copy constructor for the class is used;
Which effectively means that the implicitly-defined copy constructor for 'c' will have to initialize its 'a' base class subobject first and that must be done with a's copy constructor, which will always require a conversion of an lvalue expression of type 'const c' to an lvalue of type 'const a&'. The situation would be the same if all the three classes shown had implicitly-defined copy constructors.
Suggested resolution:
Prepend to paragraph 2 of 12.7 [class.cdtor] the following:
Unless the conversion happens in a mem-initializer whose mem-initializer-id designates a virtual base class of X, to explicitly or implicitly convert ...
Notes from the 10/01 meeting:
There is no problem in this example. ac1 is fully initialized before it is used in the initialization of ac2.
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.
Jack Rouse: In 12.8 [class.copy] paragraph 8, the standard includes the following about the copying of class subobjects in such a constructor:
Mike Miller: I'm more concerned about 12.8 [class.copy] paragraph 7, which lists the situations in which an implicitly-defined copy constructor can render a program ill-formed. Inaccessible and ambiguous copy constructors are listed, but not a copy constructor with a cv-qualification mismatch. These two paragraphs taken together could be read as requiring the calling of a copy constructor with a non-const reference parameter for a const data member.
Proposed Resolution (November, 2006):
This issue is resolved by the proposed resolution for issue 535.
Rationale (August, 2011):
These concerns have been addressed by other changes.
Section 12.8 [class.copy] paragraph 8 says the compiler-generated copy constructor copies scalar elements via the built-in assignment operator. Seems inconsistent. Why not the built-in initialization?
Notes from October 2002 meeting:
The Core Working Group believes this should not be changed. The standard already mentions built-in operators and the assignment operator does clearly define what must be done for scalar types. There is currently no concept of built-in initialization.
EDG (and g++, for that matter) picks the explicit copy assignment operator, which we think is wrong in this case:
#include <stdio.h>
struct D; // fwd declaration
struct B {
D& operator=(D&);
};
struct D : B {
D() {}
D(int ii) { s = ii; }
using B::operator=;
int s;
};
int main() {
D od, od1(10);
od = od1; // implicit D::operator=(D&) called, not BASE::operator=(D&)
}
D& B::operator=(D& d) {
printf("B::operator called\n");
return d;
}
If you look at 12.8 [class.copy] paragraph 10 it explicitly states that in such a case the "using B::operator=" will not be considered.
Steve Adamczyk: The fact that the operator= you declared is (D&) and not (const D&) is fooling you. As the standard says, the operator= introduced by the using-declaration does not suppress the generation of the implicit operator=. However, the generated operator= has the (const D&) signature, so it does not hide B::operator=; it overloads it.
Kerch Holt: I'm not sure this is correct. Going by 12.8 P10 first paragraph we think that the other form "operator=(D&)" is generated because the two conditions mentioned were not met in this case. 1) there is no direct base with a "const [volatile] B&" or "B" operator 2) And no member has a operator= either. This implies the implicit operator is "operator=(D&)". So, if that is the case the "hiding" should happen.
Also, in the last paragraph it seems to state that operators brought in from "using", no matter what the parameter is, are always hidden.
Steve Adamczyk: Not really. I think this section is pretty clear about the fact that the implicit copy assignment operator is generated. The question is whether it hides or overloads the one imported by the using-declaration.
Notes from the March 2004 meeting:
(a) Class B does get an implicitly-generated operator=(const B&); (b) the using-declaration brings in two operator= functions from B, the explicitly-declared one and the implicitly-generated one; (c) those two functions overload with the implicitly-generated operator=(const D&) in the derived class, rather than being hidden by the derived-class function, because it does not match either of their signatures; (d) overload resolution picks the explicitly-declared function from the base class because it's the best match in this case. We think the standard wording says this clearly enough.
Is the following a “copy assignment operator?”
struct A {
const A& operator=(const A&) volatile;
};
12.8 [class.copy] paragraph 9 doesn't say one way or the other whether cv-qualifiers on the function are allowed. (A similar question applies to the const case, but I avoided that example because it seems so wrong one tends to jump to a conclusion before seeing what the standard says.)
Since the point of the definition of “copy assignment operator” is to control whether the compiler generates a default version if the user doesn’t, I suspect the correct answer is that neither const nor volatile cv-qualification on operator= should be allowed for a “copy assignment operator.” A user can write an operator= like that, but it doesn't affect whether the compiler generates the default one.
Proposed Resolution (November, 2006):
Change 12.8 [class.copy] paragraph 9 as follows:
A user-declared copy assignment operator X::operator= is a non-static non-template non-volatile non-const member function of class X with exactly one parameter of type X, X&, const X&, volatile X& or const volatile X&.
[Drafting note: If a user-declared volatile operator= prevented the implicit declaration of the copy assignment operator, all assignments for objects of the given class (even to non-volatile objects) would pay the penalty for volatile write accesses in the user-declared operator=, despite not needing it.]
Additional note (December, 2008):
The proposed resolution addresses only cv-qualified assignment operators and is silent on ref-qualified versions. However, it would seem that the spirit of the resolution would indicate that a ref-qualified assignment operator would not be considered a copy assignment operator.
There appears to be an emerging idiom that relies on the idea that providing an lvalue-only assignment operator would prevent assignment to rvalues:
struct A {
A& operator=(const A&) &; // disable assignemt to rvalue
};
The resolution should also be reconsidered in light of the use of a const-qualified assignment operator as part of the implementation of a proxy class, where the proxy object itself is constant and should not be changed, but the copy assignment operator would apply to the object to which the proxy object refers.
Rationale (March, 2009):
It was decided that cv-qualified and ref-qualified assignment operators should be considered copy assignment operators if they have the required parameter type.
For increased regularity between built-in types and class types, the copy assignment operator can be qualified with &, preventing assignment to an rvalue. The LWG is making that change in the Standard Library. It would seem a good idea to make a similar change, where possible, in the specification of implicitly-declared assignment operators. This would be the case when all subobjects of class type have a non-deleted copy assignment operator that is &-qualified.
Rationale (July, 2009):
The LWG decided not to add reference qualifiers in the library, which reduces the motivation for making this change to implicit assignment operators.
Paper N2987 suggests that an implicitly-declared copy or move constructor should be explicit if the corresponding constructor of any of its subobjects is explicit. During the discussion at the October, 2009 meeting, the CWG deemed this a separable question from the major emphasis of that paper, and this issue was opened as a placeholder for that discussion.
See also issue 1051.
Rationale (November, 2010):
The CWG did not see a correlation between the explicitness of a base class constructor and that of an implicitly-declared derived class constructor.
12.8 [class.copy] paragraph 12 says that a defaulted move constructor is defined as deleted if the class has a non-static data member or direct or virtual base class with a type that does not have a move constructor and is not trivially copyable. This seems more strict than is necessary; the subobject need not be trivially copyable, it should be enough for the selected constructor not to throw. In any case, the wording should be phrased in terms of the function selected by overload resolution rather than properties of the subobject type, and similarly for move assignment.
Rationale (November, 2010):
The CWG felt that the current specification was consistent and not overly problematic; users can add their own move constructor if needed.
In 12.8 [class.copy] paragraph 25, a move assignment operator is defined as deleted if it has any direct or indirect virtual base class. This could be relaxed to apply only if a virtual base is derived from more than once in the DAG.
Rationale (August, 2010):
CWG felt that there was insufficient motivation to change this at this time.
Inheriting constructors should not be part of C++0x unless they have implementation experience.
Rationale (March, 2011):
The full Committee voted not to remove this feature.
The following example does not work as one might expect:
namespace N { class C {}; }
int operator +(int i, N::C) { return i+1; }
#include <numeric>
int main() {
N::C a[10];
std::accumulate(a, a+10, 0);
}
According to
3.4.1 [basic.lookup.unqual]
paragraph 6, I would expect that the "+" call inside
std::accumulate would find the global operator+.
Is this true, or am I
missing a rule? Clearly, the operator+ would be found by Koenig lookup
if it were in namespace N.
Daveed Vandevoorde: But doesn't unqualified lookup of the operator+ in the definition of std::accumulate proceed in the namespace where the implicit specialization is generated; i.e., in namespace std?
In that case, you may find a non-empty overload set for operator+ in namespace std and the surrounding (global) namespace is no longer considered?
Nathan Myers: Indeed, <string> defines operator+, as do <complex>, <valarray>, and <iterator>. Any of these might hide the global operator.
Herb Sutter: These examples fail for the same reason:
struct Value { int i; };
typedef map<int, Value > CMap;
typedef CMap::value_type CPair;
ostream & operator<< ( ostream &os, const CPair &cp )
{ return os << cp.first << "/" << cp.second.i; }
int main() {
CMap courseMap;
copy( courseMap.begin(), courseMap.end(),
ostream_iterator<CPair>( cout, "\n" ) );
}
template<class T, class S>
ostream& operator<< (ostream& out, pair<T,S> pr)
{ return out << pr.first << " : " << pr.second << endl; }
int main() {
map <int, string> pl;
copy( pl.begin(), pl.end(),
ostream_iterator <places_t::value_type>( cout, "\n" ) );
}
This technique (copying from a map to another container or stream)
should work. If it really cannot be made to work, that would seem broken
to me. The reason is does not work is that copy and pair are in
namespace std and the name lookup rules do not permit the global
operator<< to be found because the other operator<<'s
in namespace std
hide the global operator. (Aside: FWIW, I think most programmers don't
realize that a typedef like CPair is actually in namespace std, and not
the global namespace.)
Bill Gibbons: It looks like part of this problem is that the library is referring to names which it requires the client to declare in the global namespace (the operator names) while also declaring those names in namespace std. This would be considered very poor design for plain function names; but the operator names are special.
There is a related case in the lookup of operator conversion functions. The declaration of a conversion function in a derived class does not hide any conversion functions in a base class unless they convert to the same type. Should the same thing be done for the lookup of operator function names, e.g. should an operator name in the global namespace be visible in namespace std unless there is a matching declaration in std?
Because the operator function names are fixed, it it much more likely that a declaration in an inner namespace will accidentally hide a declaration in an outer namespace, and the two declarations are much less likely to interfere with each other if they are both visible.
The lookup rules for operator names (when used implicitly) are already quite different from those for ordinary function names. It might be worthwhile to add one more special case.
Mike Ball : The original SGI proposal said that non-transitive points of instantiation were also considered. Why, when, and by whom was it added?
Rationale (10/99): This appears to be mainly a program design issue. Furthermore, any attempt to address it in the core language would be beyond the scope of what can be done in a Technical Corrigendum.
Is the following well-formed?
template <typename T>
class test
{
public:
operator T& (){ return m_i; }
private:
T m_i;
};
int main()
{
test<int*> t2;
t2 += 1; // Allowed?
}
Is it possible that by "assignment operators" (13.3.1.2 [over.match.oper] paragraph 4) only the built-in candidates for operator= (i.e. excluding +=, *=, etc.) were meant? On one hand the plural ("operators") seems to imply that all the assignment operators are considered. OTOH, there has already been a core DR (221) about a missing distinction between "assignment operator" and "compound assignment operators". Is there a similar defect here?
Steve Adamczyk: The standard is ambiguous. However, I think the ARM was fairly clear about "assignment operators" meaning only "=", and I find that Cfront 3.0.1 accepts the test case (with typename changed to class). I don't know whether that's good or bad, but it's at least a precedent. Given the change of Core Issue 221, if we do nothing further conversions are valid on += and therefore this case is valid.
Note that "t2++;" is unquestionably valid, so one could also argue for the status quo (post-221) on the basis of consistency.
Notes from the October 2003 meeting:
We believe the example is well-formed, and no change other than that in issue 221 is needed.
The rules for selecting candidate functions in copy-list-initialization (13.3.1.7 [over.match.list]) differ from those of regular copy-initialization (13.3.1.4 [over.match.copy]): the latter specify that only the converting (non-explicit) constructors are considered, while the former include all constructors but state that the program is ill-formed if an explicit constructor is selected by overload resolution. This is counterintuitive and can lead to surprising results. For example, the call to the function object p in the following example is ambiguous because the explicit constructor is a candidate for the initialization of the operator's parameter:
struct MyList {
explicit MyStore(int initialCapacity);
};
struct MyInt {
MyInt(int i);
};
struct Printer {
void operator()(MyStore const& s);
void operator()(MyInt const& i);
};
void f() {
Printer p;
p({23});
}
Rationale (March, 2011):
The current rules are as intended.
There is a moderately serious problem with the definition of overload resolution. Consider this example:
struct B;
struct A {
A(B);
};
struct B {
operator A();
} b;
int main() {
(void)A(b);
}
This is pretty much the definition of "ambiguous," right? You want to convert a B to an A, and there are two equally good ways of doing that: a constructor of A that takes a B, and a conversion function of B that returns an A.
What we discover when we trace this through the standard, unfortunately, is that the constructor is favored over the conversion function. The definition of direct-initialization (the parenthesized form) of a class considers only constructors of that class. In this case, the constructors are the A(B) constructor and the (implicitly-generated) A(const A&) copy constructor. Here's how they are ranked on the argument match:
| A(B) | exact match (need a B, have a B) |
| A(const A&) | user-defined conversion (B::operator A used to convert B to A) |
In other words, the conversion function does get considered, but it's operating with, in effect, a handicap of one user defined conversion. To put that a different way, this problem is a problem of weighting, not a problem that certain conversion paths are not considered.
I believe the reason that the standard's approach doesn't yield the "intuitive" result is that programmers expect copy constructor elision to be done whenever reasonable, so the intuitive cost of using the conversion function in the example above is simply the cost of the conversion function, not the cost of the conversion function plus the cost of the copy constructor (which is what the standard counts).
Suggested resolution:
In a direct-initialization overload resolution case, if the candidate function being called is a copy constructor and its argument (after any implicit conversions) is a temporary that is the return value of a conversion function, and the temporary can be optimized away, the cost of the argument match for the copy constructor should be considered to be the cost of the argument match on the conversion function argument.
Notes from 10/01 meeting:
It turns out that there is existing practice both ways on this issue, so it's not clear that it is "broken". There is some reason to feel that something that looks like a "constructor call" should call a constructor if possible, rather than a conversion function. The CWG decided to leave it alone.
Can p->f, where f refers to a set of overloaded functions all of which are static member functions, be used as an expression in an address-of-overloaded-function context? A strict reading of this section suggests "no", because "p->f" is not the name of an overloaded function (it's an expression). I'm happy with that, but the core group should decide and should add an example to document the decision, whichever way it goes.
Rationale (10/99): The "strict reading" correctly reflects the intent of the Committee, for the reason given, and no clarification is required.
It is unclear whether the following code is well-formed or not:
class A { };
struct B : public A
{
void foo ();
void foo (int);
};
int main ()
{
void (A::*f)() = (void (A::*)())&B::foo;
}
13.4 [over.over] paragraph 1 says,
The function selected is the one whose type matches the target type required in the context. The target can be
- ...
- an explicit type conversion (5.2.3 [expr.type.conv], 5.2.9 [expr.static.cast], 5.4 [expr.cast]).
This would appear to make the program ill-formed, since the type in the cast is different from that of either interpretation of the address-of-member expression (its class is A, while the class of the address-of-member expression is B)
However, 13.4 [over.over] paragraph 3 says,
Nonstatic member functions match targets of type "pointer-to-member-function;" the function type of the pointer to member is used to select the member function from the set of overloaded member functions.
The class of which a function is a member is not part of the "function type" (8.3.5 [dcl.fct] paragraph 4). Paragraph 4 is thus either a misuse of the phrase "function type," a contradiction of paragraph 1, or an explanation of what "matching the target type" means in the context of a pointer-to-member target. By the latter interpretation, the example is well-formed and B::foo() is selected.
Bill Gibbons: I think this is an accident due to vague wording. I think the intent was
The function selected is the one which will make the effect of the cast be that of the identity conversion.
Mike Miller: The "identity conversion" reading seems to me to be overly restrictive. It would lead to the following:
struct B {
void f();
void f(int);
};
struct D: B { };
void (D::* p1)() = &D::f; // ill-formed
void (D::* p2)() = (void (B::*)()) &D::f; // okay
I would find the need for an explicit cast here surprising, since the downcast is a standard conversion and since the declaration of p1 certainly has enough information to disambiguate the overload set. (See also issue 203.)
Bill Gibbons: There is an interesting situation with using-declarations. If a base class member function is present in the overload set in a derived class due to a using-declaration, it is treated as if it were a derived class member function for purposes of overload resolution in a call (13.3.1 [over.match.funcs] paragraph 4):
For non-conversion functions introduced by a using-declaration into a derived class, the function is considered to be a member of the derived class for the purpose of defining the type of the implicit object parameter
There is no corresponding rule for casts. Such a rule would be practical, but if the base class member function were selected it would not have the same class as that specified in the cast. Since base-to-derived pointer to member conversions are implicit conversions, it would seem reasonable to perform this conversion implicitly in this case, so that the result of the cast has the right type. The usual ambiguity and access restrictions on the base-to-derived conversion would not apply since they do not apply to calling through the using-declaration either.
On the other hand, if there is no special case for this, we end up with the bizarre case:
struct A {
void foo();
};
struct B : A {
using A::foo;
void foo(int);
}
int main() {
// Works because "B::foo" contains A::foo() in its overload set.
(void (A::*)())&B::foo;
// Does not work because "B::foo(int)" does not match the cast.
(void (A::*)(int))&B::foo;
}
I think the standard should be clarified by either:
Adding a note to 13.4 [over.over] saying that using-declarations do not participate in this kind of overload resolution; or
Modifying 13.4 [over.over] saying that using-declarations are treated as members of the derived class for matching purposes, and if selected, the resulting pointer to member is implicitly converted to the derived type with no access or ambiguity errors. (The using-declaration itself has already addressed both of these areas.)
Rationale (10/00): The cited example is well-formed. The function type, ignoring the class specification, is used to select the matching function from the overload set as specified in 13.4 [over.over] paragraph 3. If the target type is supplied by an explicit cast, as here, the conversion is then performed on the selected pointer-to-member value, with the usual restrictions on what can and cannot be done with the converted value (5.2.9 [expr.static.cast] paragraph 9, 5.2.10 [expr.reinterpret.cast] paragraph 9).
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 are
operator ?:(bool, const Integer &, const Integer &)
operator ?:(bool, Integer, Integer)
which are both perfect matches.
(Not a problem in FDIS, but misleading.)
Rationale: The description of the conditional operator in 5.16 [expr.cond] handles the lvalue case before the prototype is considered.
Rationale (August, 2011):
The specification is as desired.
Given an example like
template<const int I> struct S {
decltype(I) m;
};
what is the type of m? 14.1 [temp.param] paragraph 5 is clear on the question:
The top-level cv-qualifiers on the template-parameter are ignored when determining its type.
It's not clear that this is the desired outcome, however, particularly in light of the resolution of issue 1130. (This does not make any difference for the current language, as a non-type template parameter is a prvalue and non-class prvalues are never cv-qualified. It would have an impact, however, if a future revision of the language were to allow literal class types as non-type template parameters, so if a change is needed, it might be a good idea to do it now.)
Rationale (November, 2010):
As noted, the treatment of cv-qualification of the type of non-type template parameters is irrelevant because they are currently always non-class prvalues. If the language is extended to allow literal class types, a change to the handling of cv-qualification would be upwardly compatible, so nothing needs to be done now.
The example in 14.1 [temp.param] paragraph 15 contains the line
template<T... Values> apply { }; // Values is a non-type template parameter pack
// and a pack expansion
This should presumably be struct apply or some such.
Rationale (August, 2011)
This is an editorial issue that has been transmitted to the project editor.
The discussion in of the use of typename with a qualified-id in a template parameter-declaration in 14.2 [temp.names] paragraph 2 is confusing:
typename followed by an unqualified-id names a template type parameter. typename followed by a qualified-id denotes the type in a non-type parameter-declaration.
This rule would be clearer if the unqualified-id case were described in terms of resolving the ambiguity of declaring a template parameter name versus referring to a type-name from the enclosing scope, and if the qualified-id case referred to the use of the typename keyword with dependent types in 14.6 [temp.res]. An example would also be helpful.
Rationale (April, 2006):
The CWG felt that the wording was already clear enough.
According to 14.5.2 [temp.mem] paragraph 4,
A specialization of a member function template does not override a virtual function from a base class.Bill Gibbons: I think that's sufficiently surprising behavior that it should be ill-formed instead.
As I recall, the main reason why a member function template cannot be virtual is that you can't easily construct reasonable vtables for an infinite set of functions. That doesn't apply to overrides.
Another problem is that you don't know that a specialization overrides until the specialization exists:
struct A {
virtual void f(int);
};
struct B : A {
template<class T> void f(T); // does this override?
};
But this could be handled by saying:
template<int I> struct X;
struct A {
virtual void f(A<5>);
};
struct B : A {
template<int I, int J> void f(A<I+J>); // does not overrride
};
void g(B *b) {
X<t> x;
b->f<3,2>(x); // specialization B::f(A<5>) makes program ill-formed
}
So I think there are reasonable semantics. But is it useful?
If not, I think the creation of a specialization that would have been an override had it been declared in the class should be an error.
Daveed Vandevoorde: There is real code out there that is written with this rule in mind. Changing the standard on them would not be good form, IMO.
Mike Ball: Also, if you allow template functions to be specialized outside of the class you introduce yet another non-obvious ordering constraint.
Please don't make such a change after the fact.
John Spicer: This is the result of an explicit committee decision. The reason for this rule is that it is too easy to unwittingly override a function from a base class, which was probably not what was intended when the template was written. Overriding should be a conscious decision by the class writer, not something done accidentally by a template.
Rationale (10/99): The Standard correctly reflects the intent of the Committee.
Notes from October 2002 meeting:
This was reopened because of a discussion while reviewing possible extensions.
Notes from April 2003 meeting:
This was discussed again, and the consensus was that we did not want to make a change, and in particular we did not want to make it an error and risk breaking existing code.
The list of contexts in which pack expansions can occur, in 14.5.3 [temp.variadic] paragraph 4, does not include a function call, in spite of the comments in the example there that assume that a function call is such a context.
Rationale (August, 2010):
initializer-list, mentioned in 14.5.3 [temp.variadic], is used in the argument list of a function call.
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.
Library issue 225 poses the following questions:
For example, a programmer might want to provide a version of std::swap that would be used for any specialization of a particular class template. It is possible to do that for specific types, but not for all specializations of a template.
The problem is due to the fact that programmers are forbidden to add overloads to namespace std, although specializations are permitted. One suggested solution would be to allow partial specialization of function templates, analogous to partial specialization of class templates.
Library issue 225 contains a detailed proposal for adding partial specialization of function templates (not reproduced here in the interest of space and avoiding multiple-copy problems). This Core issue is being opened to provide for discussion of the proposal within the core language working group.
Notes from 10/00 meeting:
A major concern over the idea of partial specialization of function templates is that function templates can be overloaded, unlike class templates. Simply naming the function template in the specialization, as is done for class specialization, is not adequate to identify the template being specialized.
In view of this problem, the library working group is exploring the other alternative, permitting overloads to be added to functions in namespace std, as long as certain restrictions (to be determined) are satisfied.
(See also documents N1295 and N1296 and issue 285.)
Notes from 10/01 meeting:
The Core Working Group decided to ask the Library Working Group for guidance on whether this feature is still needed to resolve a library issue. The answer at present is "we don't know."
Rationale (October, 2004):
The Core Working Group decided that the Evolution Working Group is the appropriate forum in which to explore the desirability and form of this feature.
Note (March, 2008):
The Evolution Working Group recommended closing this issue with no further consideration. See paper J16/07-0033 = WG21 N2173.
Issue 1:
14.5.6.2 [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:
14.5.6.2 [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:
14.5.6.2 [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.
Additional note (April, 2011):
These points appear to have been addressed by previous resolutions, so presumably the issue is now NAD.
Rationale (August, 2011):
As given in the preceding note.
The following appears to be well-formed, with templates foo() being distinct since any type T will produce an invalid type for the second parameter for at least one foo() when T is replaced within the non-deduced context:
template <typename T>
bool *foo(T *, enum T::u_type * = 0) { return 0; }
template <typename T>
char *foo(T *, struct T::u_type * = 0) { return 0; }
struct A { enum u_type { I }; };
int main(void) { foo((A*)0); }
In particular, while determining the signature for the function templates foo(), an elaborated-type-specifier qualifies as part of the decl-specifier-seq under 8.3.5 [dcl.fct] paragraph 5 in determining the type of a parameter in the parameter-type-list (absent additional wording). Also, the return type is included in the signature of a function template.
Implementations do not appear to support this case and the ability to do so brings little value since type traits such as is_enum and is_class cannot be defined using this and equivalent functionality can be achieved using the aforementioned type traits.
Rationale (August, 2010):
The specification is as intended; compilers should handle cases like these.
A gcc hacker recently sent in a patch to make the compiler give an error on code like this:
template <template <typename> class T> struct A { };
template <typename U> struct B
{
A<B> *p;
};
presumably because the DR from issue 176
says that we decide whether or not
B is to be treated as a template depending on whether a
template-argument-list is supplied. I think this is a drafting oversight,
and that B should also be treated as a template when passed as a template
template parameter. The discussion in the issue list only talks about
making the name usable both as a class and as a template.
John Spicer: This case was explicitly discussed and it was agreed that to use the injected name as a template template parameter you need to use the non-injected name.
A (possibly unstated) rule that I've understood about template arguments is that the form of the argument (type/nontype/template) is based only on the argument and not on the kind of template parameter. An example is that "int()" is always "function taking no arguments returning int" and never a convoluted way of saying zero.
In a similar way, we now decide whether or not something is a template based only on the form of the argument.
I think this rule is important for two kinds of cases. The first case involves explicit arguments of function templates:
template <template <typename> class T> void f(){} // #1
template <class T> void f(){} // #2
template <typename U> struct B {
void g() {
f<B>();
}
};
int main() {
B<int> b;
b.g();
}
With the current rules, this uses B as a type argument to template #2.
The second case involves the use of a class template for which the template parameter list is unknown at the point where the argument list is scanned:
template <class T> void f(){}
template <typename U> struct B {
void g() {
f< U::template X<B> >(); // what is B?
}
};
struct Z1 {
template <class T> struct X {};
};
struct Z2 {
template <template <class> class T> struct X {};
};
int main() {
B<Z1> b1;
b1.g();
B<Z2> b2;
b2.g();
}
If B could be used as a template name we would be unable to decide how to treat B at the point that it was scanned in the template argument list.
So, I think it is not an oversight and that it should be left the way it is.
Notes from the 4/02 meeting:
It was agreed that this is Not a Defect.There is some question as to whether 14.6.2 [temp.dep] paragraph 3 applies to the definition of an explicitly-specialized member of a class template:
In the definition of a class template or a member of a class template, if a base class of the class template depends on a template-parameter, the base class scope is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member.
Consider an example like the following:
template <class T>
struct A {
void foo() {}
};
template <class T>
struct B: A<T> {
int bar();
};
int foo() { return 0; }
template <>
int B<int>::bar() { return foo(); }
int main() {
return B<int>().bar();
}
Does foo in the definition of B<int>::bar() refer to ::foo() or to A<int>::foo()?
Rationale (April, 2006):
An explicitly-specialized member of a class template is not, in fact, a member of a class template but a member of a particular specialization of that template. The special treatment of lookup vis-a-vis dependent base classes in 14.6.2 [temp.dep] thus does not apply, and base class members are found by unqualified name lookup.
Is the comma expression in the following dependent?
template <class T> static void f(T)
{
}
template <class T> void g(T)
{
f((T::x, 0));
}
struct A {
static int x;
};
void h()
{
g(A());
}
According to the standard, it is, because 14.6.2.2 [temp.dep.expr] says that an expression is dependent if any of its sub-expressions is dependent, but there is a question about whether the language should say something different. The type and value of the expression are not really dependent, and similar cases (like casting T::x to int) are not dependent.
Mark Mitchell: If the first operand is dependent, how do we know it does not have an overloaded comma operator?
Rationale (October, 2004):
CWG agreed that such comma expressions are and ought to be dependent, for the reason expressed in Mark Mitchell's comment.
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.
A related question to that raised in issue 488 is whether member function templates must be instantiated if the compiler can determine that they will not be needed by the function selected by overload resolution. That is explicitly specified for class templates in 14.7.1 [temp.inst] paragraph 5:
If the overload resolution process can determine the correct function to call without instantiating a class template definition, it is unspecified whether that instantiation actually takes place.
Should the same be true for member function templates? In the example from issue 488,
struct S {
template <typename T> S(const T&);
};
void f(const S&);
void f(int);
void g() {
enum E { e };
f(e); // ill-formed?
}
a compiler could conceivably determine that f(int) would be selected by overload resolution (because it involves only an integral promotion, while the alternative requires a user-defined conversion) without instantiating the declaration of the S constructor. Should the compiler have that freedom?
Rationale (April, 2005):
In order for this question to come up, there would need to be a “gap” between the the normal rules and the rules for template argument deduction failure. The resolution for issue 488 will close the only such gap of which the CWG is aware. The issue can be reopened if other such cases turn up.
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 [intro.defs] “undefined behavior.”
The current language specification allows suppression of implicit instantiations of templates via an explicit instantiation declaration; if all uses of a particular specialization follow an explicit instantiation declaration for that specialization, and there is one explicit instantiation definition in the program, there will be only a single copy of that instance. However, the Standard does not require the presence of an explicit instantiation declaration prior to use, so implementations must still be prepared (using weak symbols, for example) to handle multiple copies of the instance at link time. This can be a significant overhead, particularly in shared libraries where weak symbols must be resolved at load time. Requiring the presence of an explicit instantiation declaration in every translation unit in which the specialization is used would allow the compiler to emit strong symbols for the explicit instantiation definition and reduce the overhead.
On the other hand, the current definition allows use of multiple independent libraries with explicit instantiation directives of the same specializations (from a common third-party library, for instance), as well as incremental migration of libraries to use of explicit instantiation declarations rather than requiring all libraries to be updated at once.
Rationale (August, 2010):
CWG prefers the current specification.
According to 14.7.2 [temp.explicit] paragraph 9,
Except for inline functions and class template specializations, explicit instantiation declarations have the effect of suppressing the implicit instantiation of the entity to which they refer.
This means that an implementation cannot do inline expansion of an extern template function or member function, because that would require its instantiation. As a result, adding an explicit instantiation declaration can affect performance, even though the user only intended to suppress out-of-line copies of functions.
Rationale (August, 2010):
If implementations are allowed to do speculative instantiation for the purpose of inlining, there could be silent changes of meaning depending on whether the instantiation is done or not.
[N1065 issue 1.19] An explicit specialization declaration may not be visible during instantiation under the template compilation model rules, even though its existence must be known to perform the instantiation correctly. For example:
translation unit #1
template<class T> struct A { };
export template<class T> void f(T) { A<T> a; }
translation unit #2
template<class T> struct A { };
template<> struct A<int> { }; // not visible during instantiation
template<class T> void f(T);
void g() { f(1); }
Rationale:
This issue was addressed in the FDIS and should have been closed.
Is this valid C++? The question is whether a member constant can be specialized. My inclination is to say no.
template <class T> struct A {
static const T i = 0;
};
template<> const int A<int>::i = 42;
int main () {
return A<int>::i;
}
John Spicer: This is ill-formed because
9.4.2 [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.
Rationale (10/99): The Standard is already sufficiently clear on this question.
John Spicer: Certain access checks are suppressed on explicit instantiations. 14.7.2 [temp.explicit] paragraph 8 says:
The usual access checking rules do not apply to names used to specify explicit instantiations. [Note: In particular, the template arguments and names used in the function declarator (including parameter types, return types and exception specifications) may be private types or objects which would normally not be accessible and the template may be a member template or member function which would not normally be accessible. ]I was surprised that similar wording does not exist (that I could find) for explicit specializations. I believe that the two cases should be handled equivalently in the example below (i.e., that the specialization should be permitted).
template <class T> struct C {
void f();
void g();
};
template <class T> void C<T>::f(){}
template <class T> void C<T>::g(){}
class A {
class B {};
void f();
};
template void C<A::B>::f(); // okay
template <> void C<A::B>::g(); // error - A::B inaccessible
void A::f() {
C<B> cb;
cb.f();
}
Mike Miller: According to the note in 14.3 [temp.arg] paragraph 3,
if the name of a template-argument is accessible at the point where it is used as a template-argument, there is no further access restriction in the resulting instantiation where the corresponding template-parameter name is used.
(Is this specified anywhere in the normative text? Should it be?)
In the absence of text to the contrary, this blanket permission apparently applies to explicitly-specialized templates as well as to implicitly-generated ones (is that right?). If so, I don't see any reason that an explicit instantiation should be treated differently from an explicit specialization, even though the latter involves new program text and the former is just a placement instruction to the implementation.
Proposed Resolution (4/02):
In 14.7.2 [temp.explicit] delete paragraph 8:
The usual access checking rules do not apply to names used to specify explicit instantiations. [Note: In particular, the template arguments and names used in the function declarator (including parameter types, return types and exception specifications) may be private types or objects which would normally not be accessible and the template may be a member template or member function which would not normally be accessible. ]
In 14.7 [temp.spec] add the paragraph deleted above as paragraph 7 with the changes highlighted below:
The usual access checking rules do not apply to names used to specify explicit instantiations or explicit specializations. [Note: In particular, tThe template arguments and names used in the function declarator (including parameter types, return types and exception specifications) may be private types or objects which would normally not be accessible and the template may be a member template or member function which would not normally be accessible. ]
Rationale (October 2002):
We reconsidered this and decided that the difference between the two cases (explicit specialization and explicit instantiation) is appropriate. The access rules are sometimes bent when necessary to allow naming something, as in an explicit instantiation, but explicit specialization requires not only naming the entity but also providing a definition somewhere.
The Standard does not describe how to handle an example like the following:
template <class T> int f(T, int);
template <class T> int f(int, T);
template<> int f<int>(int, int) { /*...*/ }
It is impossible to determine which of the function templates is being specialized. This problem is related to the discussion of issue 229, in which one of the objections raised against partial specialization of function templates is that it is not possible to determine which template is being specialized.
Notes from 10/01 meeting:
It was decided that while this is true, it's not a problem. You can't call such functions anyway; the call would be ambiguous.
The Standard currently specifies (7.1.3 [dcl.typedef] paragraph 9, 14.3.1 [temp.arg.type] paragraph 4) that an attempt to create a reference to a reference (via a typedef or template type parameter) is effectively ignored. The same is not true of an attempt to form a pointer to a reference; that is, assuming that T is specified to be a reference type,
template <typename T> void f(T t) {
T& tr = t; // OK
T* tp = &t; // error
}
It would be more consistent to allow pointers to references to collapse in the same way that references to references do.
Rationale (February, 2008):
In the absence of a compelling need, the CWG felt that it was better not to change the existing rules. Allowing this case could cause a quiet change to the meaning of a program, because attempting to create a pointer to a reference type is currently a deduction failure.
Additional discussion (May, 2009):
Consider the following slightly extended version of the example above:
template <typename T> void f(T t) {
T& tr = t; // OK
T* tp = &t; // error
auto * ap = &t; // OK!
}
This means that a template that expects a reference type will need to use auto just to work around the failure to collapse pointer-to-reference types. The result might, in fact, be subtly different with auto, as well, in case there is an overloaded operator& function that doesn't return exactly T*. This contradicts one of the main goals of C++0x, to make it simpler, more consistent, and easier to teach.
Rationale (July, 2009):
The CWG reaffirmed its early decision. In general, templates will need to be written differently for reference and non-reference type parameters. Also, the Standard library provides a facility, std::remove_reference, that can be used easily for such cases.
Rationale (August, 2011):
The specification is as intended.
Consider:
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&).
Rationale (10/99): The Standard correctly reflects the intent of the Committee.
(October 2002) This is resolved by issue 214.
In the following example,
template<typename T> void f(const T&); // #1
template<typename T> void f(T&&); // #2
void g() {
const int x = 5;
f(x);
}
the call f(x) is ambiguous by the current rules. For #1, T is deduced as int, giving
f<int>(const int&)
For #2, because of the special case for T&& in 14.8.2.1 [temp.deduct.call] paragraph 3, T is deduced as const int&; application of the reference-collapsing rules in 8.3.2 [dcl.ref] paragraph 6 to the substituted parameter type yields
f<const int&>(const int&)
These are indistinguishable in overload resolution, resulting in an ambiguity. It's not clear how this might be addressed.
Rationale (August, 2010):
The two functions are distinguished by partial ordering, so the call is not actually ambiguous.
Consider the following example:
template <typename T> void f(T** p, void (*)()); // #1
template <typename T> void f(T* p, void (&)()); // #2
void x();
void g(int** p) {
f(p, x); // #3
}
The question is whether the call at #3 is ambiguous or not.
The synthesized declarations for overload resolution are:
void f<int>(int**, void(*)()); // From #1
void f<int*>(int**, void(&)()); // From #2
Neither of these is a better match on the basis of conversion sequences (the function-to-pointer conversion and the reference binding have “exact match” rank), and both are function template specializations, so the tiebreaker in 13.3.3 [over.match.best] paragraph 1 comes down to whether #1 is more specialized than #2 or vice versa.
The determination of whether one of these templates is more specialized than the other is done (as described in 14.5.6.2 [temp.func.order]) by synthesizing a type for the template parameter of each function template (call them @1 and @2, respectively), substituting that synthesized type for each occurrence of the template parameter in the function type of the template, and then performing deduction on each pair of corresponding function parameters as described in 14.8.2.4 [temp.deduct.partial].
For the first function parameter, #1 is more specialized: deduction succeeds with P=T* and A=@1**, giving T=@1*, but it fails with P=T** and A=@2*. For the second parameter, deduction fails in both directions, with P=void(*)() and A=void() as well as with P=void() and A=void(*)() (the reference is dropped from both the parameter and argument types, as described in 14.8.2.4 [temp.deduct.partial] paragraph 5). This means that neither parameter type is at least as specialized as the other (paragraph 8).
According to 14.8.2.4 [temp.deduct.partial] paragraph 10,
If for each type being considered a given template is at least as specialized for all types and more specialized for some set of types and the other template is not more specialized for any types or is not at least as specialized for any types, then the given template is more specialized than the other template. Otherwise, neither template is more specialized than the other.
According to this rule, #1 is not more specialized than #2 because it is not “at least as specialized” for the second parameter type, so the call at #3 is ambiguous.
Results vary among implementations, with some rejecting the call as ambiguous and others resolving it to #1.
Would it be better to say that a function template F1 is more specialized than F2 if at least one of F1's types is more specialized than the corresponding F2 type and none of F2's types is more specialized than the corresponding F1 type? That would be simpler and, for examples like this, arguably more intuitive. The rationale for this change would be that if, for a given parameter pair, neither is more specialized than the other, then that parameter pair simply says nothing about whether one of the templates is more specialized than the other, rather than indicating that the templates cannot be ordered.
(See also issue 455.)
Rationale (October, 2009):
The consensus of the CWG is that this issue, in which corresponding parameters cannot be compared but the functions are equivalent in terms of overload resolution, arises so infrequently in practice that no change is warranted at this time.
Consider the following:
template <typename T> struct X {}; // #1
template <typename T> struct X<const T>; //#2
template struct X<int&>; //#3
Which specialization are we instantiating in #3? The "obvious" answer is #1, because "int&" doesn't have a top level cv-qualification. However, there's also an argument saying that we should actually be instantiating #2. The argument is: int& can be taken as a match for either one (top-level cv-qualifiers are ignored on references, so they're equally good), and given two equally good matches we must choose the more specialized one.
Is this a valid argument? If so, is this behavior intentional?
John Spicer: I don't see the rationale for any choice other than #1. While it is true that if you attempt to apply const to a reference type it just gets dropped, that is very different from saying that a reference type is acceptable where a const-qualified type is required.
If the type matched both templates, the const one would be more specialized, but "int&" does not match "const T".
Nathan Sidwell: thanks for bringing this one to the committee. However this is resolved, I'd like clarification on the followup questions in the gcc bug report regarding deduced and non-deduced contexts and function templates. Here're those questions for y'all,
template <typename T> void Foo (T *); // #1 template <typename T> void Foo (T const *); // #2 void Baz (); Foo (Baz); // which? template <typename T> T const *Foo (T *); // #1 void Baz (); Foo (Baz); // well formed? template <typename T> void Foo (T *, T const * = 0); void Baz (); Foo (Baz); // well formed?
BTW, I didn't go trying to break things, I implemented the cv-qualifier ignoring requirements and fell over this. I could find nothing in the standard saying 'don't do this ignoring during deduction'.
Issue 226 removed the original prohibition on default template-arguments for function templates. However, the note in 14.8.2.5 [temp.deduct.type] paragraph 19 still reflects that prohibition. It should be revised or removed.
Rationale (February, 2010):
The problematic note was removed as a by-product of the changes for variadic templates, document N2284.
The grammar should be changed so that constructor function-try-blocks must end with a throw-expression.
Rationale (04/00):
No change is needed. It is already the case that flowing off the end of a function-try-block of a constructor rethrows the exception, and return statements are prohibited in constructor function-try-blocks (15.3 [except.handle] paragraphs 15-16.
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?
Rationale (10/99): Neither std::exit(int) nor std::abort() destroys temporary objects, so the exception temporary is not destroyed when no handler is found. The original exception object is destroyed when it is replaced by an unexpected() handler. The Standard is sufficiently clear on these points.
15.3 [except.handle] paragraph 3 contains the following text:
A handler is a match for a throw-expression with an object of type E if
- The handler is of type cv T or cv T& and E and T are the same type (ignoring the top-level cv- qualifiers), or
- the handler is of type cv T or cv T& and T is an unambiguous public base class of E, or
- the handler is of type cv1 T* cv2 and E is a pointer type that can be converted to the type of the handler by either or both of
- a standard pointer conversion (4.10 [conv.ptr]) not involving conversions to pointers to private or protected or ambiguous classes
- a qualification conversion
I propose to alter this text to allow to catch exceptions with ambiguous public base classes by some of the public subobjects. I'm really sure that if someone writes:
try {
// ...
}
catch (Matherr& m) {
// ...
}
he really wants to catch all Matherrs rather than to allow
some of the Matherrs to escape:
class SomeMatherr : public Matherr { /* */ };
struct TrickyBase1 : public SomeMatherr { /* */ };
struct TrickyBase2 : public SomeMatherr { /* */ };
struct TrickyMatherr : public TrickyBase1, TrickyBase2 { /* */ };
According to the standard TrickyMatherr will leak through the catch (Matherr& m) clause. For example:
#include <stdio.h>
struct B {};
struct B1 : B {};
struct B2 : B {};
struct D : B1, B2 {}; // D() has two B() subobjects
void f() { throw D(); }
int main()
{
try { f(); }
catch (B& b) { puts("B&"); } // passed
catch (D& d) { puts("D&"); } // really works _after_ B&!!!
}
Also I see one more possible solution: to forbid objects with ambiguous base classes to be "exceptional objects" (for example Borland C++ goes this way) but it seems to be unnecessary restrictive.
Notes from the 10/01 meeting:
The Core Working Group did not feel this was a significant problem. Catching either of the ambiguous base classes would be surprising, and giving an error on throwing an object that has an ambiguous base class would break existing code.
If control reaches the end of handler in a destructor's function-try-block, the exception is rethrown (15.3 [except.handle] paragraph 15). Because of the danger of destructors that throw exceptions, would it be better to treat this case as an implicit return; statement, as in a function body? There could be a transitional period, perhaps using conditionally-supported behavior or the like, before mandating the change.
Rationale (October, 2006):
The CWG felt that the current behavior is clearly specified and reflects the intention of the Committee at the time the rules were adopted. Possible changes to these rules should be pursued through the Evolution Working Group.
15.4 [except.spec] paragraph 13 contains the following text. I believe 'implicitLY' marked below should be replaced with 'implicit.'
An implicitly declared special member function (clause 12 [special]) shall have an exception-specification. If f is an implicitly declared default constructor, copy constructor, destructor, or copy assignment operator, its implicit exception-specification specifies the type-id T if and only if T is allowed by the exception-specification of a function directly invoked by f's implicitly definition; f shall allow all exceptions if any function it directly invokes allows all exceptions, and f shall allow no exceptions if every function it directly invokes allows no exceptions. [Example:
struct A { A(); A(const A&) throw(); ~A() throw(X); }; struct B { B() throw(); B(const B&) throw(); ~B() throw(Y); }; struct D : public A, public B { // Implicit declaration of D::D(); // Implicit declaration of D::D(const D&) throw(); // Implicit declaration of D::~D() throw (X,Y); };Furthermore, if A::~A() or B::~B() were virtual, D::~D() would not be as restrictive as that of A::~A, and the program would be ill-formed since a function that overrides a virtual function from a base class shall have an exception-specification at least as restrictive as that in the base class. ]
The example code shows structs whose destructors have exception specifications which throw certain types. There is no defect here, but it doesn't sit well with our general advice elsewhere that destructors should not throw. I wish I could think of some other way to illustrate this section.
Notes from October 2002 meeting:
This was previously resolved by an editorial change.
It is unclear whether std::unexpected is called before or after the destruction of function arguments, partially-constructed bases and members (when called from a constructor or destructor), etc.
Rationale (October, 2009):
The point at which std::unexpected is called is specified in 15.5.2 [except.unexpected] paragraph 1:
If a function with an exception-specification throws an exception that is not listed in the exception-specification, the function std::unexpected() is called (D.11 [exception.unexpected]) immediately after completing the stack unwinding for the former function.
That is, it will be called after any local automatic objects and temporaries are destroyed and before any other objects, such as function arguments, are destroyed. (See 5.2.2 [expr.call] paragraph 4: “The initialization and destruction of each parameter occurs within the context of the calling function.”)
The consensus at the Pittsburgh (March, 2010) meeting, as reflected in the adoption of paper N3050, was that it was preferable for violation of a noexcept guarantee to call std::terminate; previous versions of the paper had called for undefined behavior in this case. Not everyone was convinced that this was a good decision, however; this issue is intended to facilitate further investigation and discussion of the question with the benefit of more time and resources than were available during the deliberations at the meeting.
Rationale (August, 2010):
CWG reaffirmed the explicit decision of the Committee.
Destructors that throw can easily cause programs to terminate, with no possible defense. Example: Given
struct XY { X x; Y y; };
Assume that X::~X() is the only destructor in the entire program that can throw. Assume further that Y construction is the only other operation in the whole program that can throw. Then XY cannot be used safely, in any context whatsoever, period — even simply declaring an XY object can crash the program:
XY xy; // construction attempt might terminate program:
// 1. construct x -- succeeds
// 2. construct y -- fails, throws exception
// 3. clean up by destroying x -- fails, throws exception,
// but an exception is already active, so call
// std::terminate() (oops)
// there is no defense
So it is highly dangerous to have even one destructor that could throw.
Suggested Resolution:
Fix the above problem in one of the following two ways. I prefer the first.
Fergus Henderson: I disagree. Code using XY may well be safe, if X::~X() only throws if std::uncaught_exception() is false.
I think the current exception handling scheme in C++ is certainly flawed, but the flaws are IMHO design flaws, not minor technical defects, and I don't think they can be solved by minor tweaks to the existing design. I think that at this point it is probably better to keep the standard stable, and learn to live with the existing flaws, rather than trying to solve them via TC.
Bjarne Stroustrup: I strongly prefer to have the call to std::terminate() be conforming. I see std::terminate() as a proper way to blow away "the current mess" and get to the next level of error handling. I do not want that escape to be non-conforming — that would imply that programs relying on a error handling based on serious errors being handled by terminating a process (which happens to be a C++ program) in std::terminate() becomes non-conforming. In many systems, there are — and/or should be — error-handling and recovery mechanisms beyond what is offered by a single C++ program.
Andy Koenig: If we were to prohibit writing a destructor that can throw, how would I solve the following problem?
I want to write a class that does buffered output. Among the other properties of that class is that destroying an object of that class writes the last buffer on the output device before freeing memory.
What should my class do if writing that last buffer indicates a hardware output error? My user had the option to flush the last buffer explicitly before destroying the object, but didn't do so, and therefore did not anticipate such a problem. Unfortunately, the problem happened anyway. Should I be required to suppress this error indication anyway? In all cases?
Herb Sutter (June, 2007): IMO, it's fine to suppress it. The user had the option of flushing the buffer and thus being notified of the problem and chose not to use it. If the caller didn't flush, then likely the caller isn't ready for an exception from the destructor, either. You could also put an assert into the destructor that would trigger if flush() had not been called, to force callers to use the interface that would report the error.
In practice, I would rather thrown an exception, even at the risk of crashing the program if we happen to be in the middle of stack unwinding. The reason is that the program would crash only if a hardware error occurred in the middle of cleaning up from some other error that was in the process of being handled. I would rather have such a bizarre coincidence cause a crash, which stands a chance of being diagnosed later, than to be ignored entirely and leave the system in a state where the ignore error could cause other trouble later that is even harder to diagnose.
If I'm not allowed to throw an exception when I detect this problem, what are my options?
Herb Sutter: I understand that some people might feel that "a failed dtor during stack unwinding is preferable in certain cases" (e.g., when recovery can be done beyond the scope of the program), but the problem is "says who?" It is the application program that should be able to decide whether or not such semantics are correct for it, and the problem here is that with the status quo a program cannot defend itself against a std::terminate() — period. The lower-level code makes the decision for everyone. In the original example, the mere existence of an XY object puts at risk every program that uses it, whether std::terminate() makes sense for that program or not, and there is no way for a program to protect itself.
That the "it's okay if the process goes south should a rare combination of things happen" decision should be made by lower-level code (e.g., X dtor) for all apps that use it, and which doesn't even understand the context of any of the hundreds of apps that use it, just cannot be correct.
Additional note (April, 2011):
The addition of the noexcept specifier, along with changes to make many destructors noexcept by default, may have sufficiently addressed these concerns. CWG should consider changing this to NAD or extension status.
Rationale (August, 2011):
As given in the preceding note.
The term "throw exception" seems to sometimes refer to an expression of the form "throw expr" and sometimes just to the "expr" portion thereof.
As a result it is not quite clear to me whether when "uncaught_exception()" becomes true: before or after the temporary copy of the value of "expr".
Is there a definite consensus about that?
Rationale: The standard is sufficiently clear; the phrase "to be thrown" indicates that the throw itself (which includes the copy to the temporary object) has not yet begun. The footnote in 15.5.1 [except.terminate] paragraph 1 reinforces this ordering.
See also issue 475.
In language imported directly from the C Standard, 16.2 [cpp.include] paragraph 5 says,
The implementation provides unique mappings for sequences consisting of one or more nondigits (2.11 [lex.name]) followed by a period (.) and a single nondigit.
This is clearly intended to support C header names like stdio.h. However, C++ has header names like cstdio that do not conform to this pattern but still presumably require “unique mappings.”
Proposed resolution (April, 2006):
Change 16.2 [cpp.include] paragraph 5 as indicated:
The implementation provides unique mappings between the delimited sequence and the external source file name for sequences consisting of one or more nondigits or digits (2.11 [lex.name]), optionally followed by a period (.) and a single nondigit...
(Clark Nelson will discuss this revision with WG14.)
Additional notes (October, 2006):
WG14 takes no position on this proposed change.
Rationale (September, 2008):
It is unclear what effect the provision of “unique mappings” has or if a conforming program could detect the failure of an implementation to do so. There has been a significant effort to synchronize this clause with the corresponding section of the C99 Standard, and given the lack of perceptible impact of the proposed change, there is insufficient motivation to introduce a new divergence in the wording.
The grammar in Appendix A does not indicate a grammar sentence symbol and is therefore formally not a grammar.
Rationale (04/01):
Appendix A does not claim to be a formal grammar. The specification is clear enough in its current formulation.
Annex C lists C compatibility issues. One item not in the annex came up in a discussion in comp.std.c++.
Consider this C and C++ code:
const int j = 0;
char* p = (char*)j;
Rationale (10/99): Because j is not a constant expression in C, this code fragment has implementation-defined behavior in C. There is no incompatibility with C resulting from the fact that C++ defines this behavior.
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):
I don't think we discussed (c).
Rationale (04/99): The substantive part of this issue is covered by Core issue 48
Given the following test case:
enum E { e1, e2, e3 };
void f(int, E e = e1);
void f(E, E e = e1);
void g() {
void f(long, E e = e2);
f(1); // calls ::f(int, E)
f(e1); // ?
}
First note that Koenig lookup breaks the concept of hiding functions through
local extern declarations as illustrated by the call `f(1)'. Should
the WP show this as an example?
Second, it appears the WP is silent as to what happens with the call `f(e1)': do the different default arguments create an ambiguity? is the local choice preferred? or the global?
Tentative Resolution (10/98) In 3.4.2 [basic.lookup.argdep] paragraph 2, change
If the ordinary unqualified lookup of the name finds the declaration of a class member function, the associated namespaces and classes are not considered.to
If the ordinary unqualified lookup of the name finds the declaration of a class member function or the declaration of a function at block scope, the associated namespaces and classes are not considered.
Rationale (04/99): The proposal would also apply to local using-declarations (per Mike Ball) and was therefore deemed undesirable. The ambiguity issue is dealt with in Core issue 1
5.3.3 [expr.sizeof] paragraph 1 says,
The sizeof operator shall not be applied... to an enumeration type before all its enumerators have been declared...
This prevents use of sizeof with an opaque enumeration type, even though the underlying type of such enumerations is known.
Rationale (May, 2009):
Duplicate of issue 803.
Should it be allowed to use an object of a class type having a single conversion function to an integral type as an array size in the first bound of the type in an array new?
struct A {
operator int();
} a;
int main () {
new int[a];
}
There are similar accommodations for the expression in a delete (5.3.5 [expr.delete] paragraph 1) and in a switch (6.4.2 [stmt.switch] paragraph 2). There is also widespread existing practice on this (g++, EDG, MSVC++, and Sun accept it, and even cfront 3.0.2).
Rationale (October, 2004):
Duplicate of issue 299.
Does the Standard require that the deallocation function will be called if the destructor throws an exception? For example,
struct S {
~S() { throw 0; }
};
void f() {
try {
delete new S;
}
catch(...) { }
}
The question is whether the memory for the S object will be freed or not. It doesn't appear that the Standard answers the question, although most people would probably assume that it will be freed.
Notes from 04/01 meeting:
There is a widespread feeling that it is a poor programming practice to allow destructors to terminate with an exception (see issue 219). This question is thus viewed as a tradeoff between efficiency and supporting "bad code." It was observed that there is no way in the current language to protect against a throwing destructor, since the throw might come from a virtual override.
It was suggested that the resolution to the issue might be to make it implementation-defined whether the storage is freed if the destructor throws. Others suggested that the Standard should require that the storage be freed, with the understanding that implementations might have a flag to allow optimizing away the overhead. Still others thought that both this issue and issue 219 should be resolved by forbidding a destructor to exit via an exception. No consensus was reached.
Rationale (October, 2008):
It was noticed that issue 353, an exact duplicate of this one, was independently opened and resolved.
According to 8.3 [dcl.meaning] paragraph 1, the declarator in the definition or explicit instantiation of a namespace member can only be qualified if the definition or explicit instantiation appears outside the member's namespace:
A declarator-id shall not be qualified except for the definition of a member function (9.3 [class.mfct]) or static data member (9.4 [class.static]) outside of its class, the definition or explicit instantiation of a function or variable member of a namespace outside of its namespace, or the definition of a previously declared explicit specialization outside of its namespace, or the declaration of a friend function that is a member of another class or namespace (11.3 [class.friend]). When the declarator-id is qualified, the declaration shall refer to a previously declared member of the class or namespace to which the qualifier refers, and the member shall not have been introduced by a using-declaration in the scope of the class or namespace nominated by the nested-name-specifier of the declarator-id.
There is no similar restriction on a qualified-id in a class definition (9 [class] paragraph 5):
If a class-head contains a nested-name-specifier, the class-specifier shall refer to a class that was previously declared directly in the class or namespace to which the nested-name-specifier refers (i.e., neither inherited nor introduced by a using-declaration), and the class-specifier shall appear in a namespace enclosing the previous declaration.
An elaborated-type-specifier in an explicit instatiation containing a qualified-id is also not prohibited from appearing in the namespace nominated by its nested-name-specifier (14.7.2 [temp.explicit] paragraph 2):
An explicit instantiation shall appear in an enclosing namespace of its template. If the name declared in the explicit instantiation is an unqualified name, the explicit instantiation shall appear in the namespace where its template is declared.
(This asymmetry is due to the removal of inappropriate mention of classes in 8.3 [dcl.meaning] by issue 40 and a failure to insert the intended restrictions elsewhere.)
An example of this inconsistency is:
namespace N {
template <class T> struct S { };
template <class T> void foo () { }
template struct N::S<int>; // OK
template void N::foo<int>(); // ill-formed
}
It is not clear that any purpose is served by the “outside of its namespace” restriction on declarators in definitions and explicit instantiations; if possible, it would be desirable to reconcile the treatment of declarators and class names by removing the restriction on declarators (which appears to be widespread implementation practice, anyway).
Rationale (April, 2006):
This is the same as issue 482.
8.3.5 [dcl.fct] paragraph 5 specifies that cv-qualifiers are deleted from parameter types. However, it's not clear what this should mean for function templates. For example,
template<class T> struct A {
typedef A arr[3];
};
template<class T> void f(const typename A<T>::arr) { }
template void f<int>(const A<int>::arr); // #1
template <class T> struct B {
void g(T);
};
template <class T> void B<T>::g(const T) { } // #2
If cv-qualifiers are dropped, then the explicit instantiation in #1 will fail to match; if cv-qualifiers are retained, then the definition in #2 does not match the declaration.
Rationale (August, 2010):
This is a duplicate of issue 1001.
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.
Notes from October 2002 meeting:
The example here is flawed. It's not clear what is being requested. One possibility is the extension introduced by issue 226. Other meanings don't seem to be useful.
It is not clear whether the following declaration is well-formed:
struct S { int i; } s = { { 1 } };
According to 8.5.1 [dcl.init.aggr]
paragraph 2, a brace-enclosed initializer is permitted for a
subaggregate of an aggregate; however, i is a scalar, not an
aggregate. 8.5 [dcl.init]
paragraph 13
says that a standalone declaration like
int i = { 1 };
is permitted, but it is not clear whether this says anything about the
form of initializers for scalar members of aggregates.
This is (more) clearly permitted by the C89 Standard.
Rationale (May, 2008):
Issue 632 refers to exactly the same question and has a more detailed discussion of the considerations involved.
The grammar for member-declaration in 9.2 [class.mem] does not allow an alias-declaration as a class member. This seems like an oversight.
Rationale (August, 2010):
This issue is a duplicate of 924.
Although accepted by several compilers and used in popular code, the grammar currently does not permit the use of a dependent template name in a base-specifier or mem-initializer-id, for example:
template<typename T, typename U>
struct X : T::template apply<U> { };
There does not seem to be a good reason to reject this usage.
Rationale (August, 2010):
This issue is a duplicate of issue 314.
Consider the following example:
struct B { void f(){} };
class N : protected B { };
struct P: N { friend int main(); };
int main() {
N n;
B& b = n; // R
b.f();
}
This code is rendered well-formed by bullet 3 of 11.2 [class.access.base] paragraph 4, which says that a base class B of N is accessible at R if
R occurs in a member or friend of a class P derived from N, and an invented public member of B would be a private or protected member of P
This provision circumvents the additional restrictions on access to protected members found in 11.4 [class.protected] — main() could not call B::f() directly because the reference is not via an object of the class through which access is obtained. What is the purpose of this rule?
Rationale (April, 2010):
This is a duplicate of issue 472.
The following example is ambiguous according to the Standard:
struct Y {
operator int();
operator double();
};
void f(Y y) {
double d;
d = y; // Ambiguous: Y::operator int() or Y::operator double()?
}
The reason for the ambiguity is that 13.6 [over.built] paragraph 18 says that there are candidate functions double& operator=(double&, int) and double& operator=(double&, double) (among others). In each case, the second argument is converted by a user-defined conversion sequence (13.3.3.1.2 [over.ics.user]) where the initial and final standard conversion sequences are the identity conversion — i.e., the conversion sequences for the second argument are indistinguishable for each of these candidate functions, and they are thus ambiguous.
Intuitively one might expect that, because it converts directly to the target type in the assignment, Y::operator double() would be selected, and in fact, most compilers do select it, but there is currently no rule to distinguish between these user-defined conversions. Should there be?
Additional note (May, 2008):
Here is another example that is somewhat similar:
enum En { ec };
struct S {
operator int();
operator En();
};
void foo () {
S() == 0; // ambiguous?
}
According to 13.6 [over.built] paragraph 12, the candidate functions are
where R is int and L is every promoted arithmetic type. Overload resolution proceeds in two steps: first, for each candidate function, determine which implicit conversion sequence is used to convert from the argument type to the parameter type; then compare the candidate functions on the basis of the relative costs of those conversion sequences.
In the case of operator==(int, int) there is a clear winner: S::operator int() is chosen because the identity conversion int -> int is better than the promotion En -> int. For all the other candidates, the conversion for the first parameter is ambiguous: both S::operator int() and S::operator En() require either an integral conversion (for integral L) or a floating-integral conversion (for floating point L) and are thus indistinguishable.
These additional candidates are not removed from the set of viable functions, however; because of 13.3.3.1 [over.best.ics] paragraph 10, they are assigned the “ambiguous conversion sequence,” which “is treated as a user-defined sequence that is indistinguishable from any other user-defined conversion sequence.” As a result, all the viable functions are indistinguishable and the call is ambiguous. Like the earlier example, one might naively think that the exact match with S::operator int() and bool operator==(int, int) would be selected, but that is not the case.
Rationale (August, 2010):
Duplicate of issue 260.
John Spicer: The standard does say that a namespace scope template has external linkage unless it is a function template declared "static". It doesn't explicitly say that the linkage of the template is also the linkage of the instantiations, but I believe that is the intent. For example, a storage class is prohibited on an explicit specialization to ensure that a specialization cannot be given a different storage class than the template on which it is based.
Mike Ball: This makes sense, but I couldn't find much support in the document. Sounds like yet another interpretation to add to the list.
John Spicer: The standard does not talk about the linkage of instantiations, because only "names" are considered to have linkage, and instances are not really names. So, from an implementation point of view, instances have linkage, but from a language point of view, only the template from which the instances are generated has linkage.
Mike Ball: Which is why I think it would be cleaner to eliminate storage class specifiers entirely and rely on the unnamed namespace. There is a statement that specializations go into the namespace of the template. No big deal, it's not something it says, so we live with what's there.
John Spicer: That would mean prohibiting static function templates. I doubt those are common, but I don't really see much motivation for getting rid of them at this point.
"export" is an additional attribute that is separate from linkage, but that can only be applied to templates with external linkage.
Mike Ball: I can't find that restriction in the standard, though there is one that templates in an unnamed namespace can't be exported. I'm pretty sure that we intended it, though.
John Spicer: I can't find it either. The "inline" case seems to be addressed, but not static. Surely this is an error as, by definition, a static template can't be used from elsewhere.
Rationale: Duplicate of Core issue 69.
The description of how the partial ordering of template functions is determined in 14.5.6.2 [temp.func.order] paragraphs 3-5 does not make any provision for nondeduced template parameters. For example, the function call in the following code is ambiguous, even though one template is "obviously" more specialized than the other:
template <class T> T f(int);
template <class T, class U> T f(U);
void g() {
f<int>(1);
}
The reason is that neither function parameter list allows template
parameter T to be deduced; both deductions fail, so neither
template is considered more specialized than the other and the
function call is ambiguous.
One possibility of addressing this situation would be to incorporate explicit template arguments from the call in the argument deduction using the transformed function parameter lists. In this case, that would result in finding the first template to be more specialized than the second.
Rationale (04/00):
This issue is covered in a more general context in issue 214.
Section 14.6 [temp.res] paragraph 4 uses the following example to show that qualified name lookup described in Section 3.4.3 [basic.lookup.qual] applies even in the presence of "typename":
struct A {
struct X { } ;
int X ;
} ;
template<class T> void f(T t) {
typename T::X x ; // ill-formed: finds the data member X
// not the member type X
}
This example is confusing because the definition of the template function itself is not ill formed unless it is instantiated with "A" as the template parameter. In other words, the example should be modified to something like:
struct A {
struct X { } ;
int X ;
} ;
struct B {
struct X { } ;
} ;
template<class T> void f(T t) {
typename T::X x ;
}
void foo() {
A a ;
B b ;
f(b) ; // OK -- finds member type B::X.
f(a) ; // ill-formed: finds the data member A::X not
// the member type A::X.
}
Notes from October 2002 meeting:
This is a duplicate of Core Issue 345.
15.4 [except.spec] paragraph 1 says,
An exception-specification shall appear only on a function declarator in a function, pointer, reference or pointer to member declaration or definition.This wording forbids exception specifications in declarations where they might plausibly occur (e.g., an array of function pointers). This restriction seems arbitrary. It's also unclear whether this wording allows declarations such as
void (*f())() throw(int); // returns a pointer to a function
// that might throw "int"
At the same time, other cases are allowed by the wording in paragraph 1 (e.g., a pointer to a pointer to a function), but no checking for such cases is specified in paragraph 3. For example, the following appears to be allowed:
void (*p)() throw(int);
void (**pp)() throw() = &p;
Rationale (10/99): Duplicate of issues 87 and 92.
The example in 18.6.1.3 [new.delete.placement] reads:
[Example: This can be useful for constructing
an object at a known address:
char place[sizeof(Something)];
Something* p = new (place) Something();
—end example]
This example has potential alignment problems. One way to correct it would
be to change the definition of place to read:
char* place = new char[sizeof(Something)];
Rationale (10/99): This is an issue for the Library Working Group.
In the example in _N2914_.14.10.1.1 [concept.fct] paragraph 10,
concept EqualityComparable<typename T> {
bool operator==(T, T);
bool operator!=(T x, T y) { return !(x == y); }
}
is the call to operator== in the default implementation well-formed, or is another requirement needed to allow the arguments to be passed by value? If another requirement is needed, should it be added in this example, or should the rules for implicit requirements be changed so that the example is well-formed?
According to _N2914_.14.11.1.1 [temp.req.sat] paragraph 2, a concept requirement is satisfied if a concept map with the same name and template argument list is found by concept map lookup. The point at which the name of a concept map is inserted into its scope is, according to _N2914_.14.10.2 [concept.map] paragraph 2, immediately following its concept-id. This enables a requirement on a member of a concept map to be satisfied by the concept map in which it appears, for example:
concept C2<typename T>{}
concept D2<typename T> {
typename type;
requires C2<type>;
}
template<D2 T>
concept_map C2<T>{}
concept_map D2<int> {
typedef int type; // Okay
}
However, these rules might lead to problems with the concept maps that the compiler tries but fails to generate for auto concepts. Presumably a compiler might insert the name of the generated concept map into the containing scope, so that it can satisfy its own requirements, but then if some other requirement cannot be satisfied and thus the concept map is not defined after all (_N2914_.14.10.2 [concept.map] paragraph 11), the name must then be removed again. It might be clearer to make the point of definition for a concept map after the closing brace and just have a special case for how the concept map is handled within its own definition.
On a related note, the current specification seems unclear about whether a failure to generate a concept map for an auto concept means that no further attempts will be made to generate it. Consider the following example:
auto concept A<typename X> { /* */ }
auto concept B<typename X> : A<X> { void enabler(X); }
template <A T> void f(T x); // #1
template <B T> void f(T x); // #2
class C
{
// a class that satisfies A<C> but not B<C>
// because no enabler(X) in scope
};
int foo()
{
C x;
f(x); // #3
}
void enabler(C);
int bar()
{
C x;
f(x); // #4
}
At #3, the concept map for B cannot be generated, so the call invokes #1. There doesn't appear to be anything currently that indicates that the reference at #4 should not once again attempt to generate the concept map for B, which will succeed this time and make the call invoke #2. It seems preferable that both calls should invoke #1, but that does not seem to be the effect of the current wording.
Given a concept and an unconstrained template, e.g.,
auto concept HasFoo<typename T> {
void foo(T&);
}
template<typename T>
struct SomeThing {
void bar(){}
};
how can one write a concept map template that adapts all specializations of SomeThing to concept HasFoo? Because a concept map template is a constrained context, referring to SomeThing violates the prohibition against using a specialization of an unconstrained template.
Surrounding the entire concept map template with late_check would appear not to work; the location of the late_check is in the unconstrained context, and late_check is ignored in unconstrained contexts.
One possibility would be to allow late_check to appear in the concept_map syntax.
_N2914_.14.10.2.1 [concept.map.fct] paragraph 3 says,
Construct an expression E (as defined below) in the scope of the concept map.
This is the wrong context for this expression. Requirement members are visible to name lookup, and they are obviously not a desirable lookup result; names within the concept map should be invisible during the evaluation of E. Presumably this should read,
...in the scope in which the concept map is defined.
_N2914_.14.10.2.1 [concept.map.fct] paragraph 5 says,
Each satisfied associated function (or function template) requirement has a corresponding associated function candidate set. An associated function candidate set is a candidate set (_N2914_.14.11.3 [temp.constrained.set]) representing the functions or operations used to satisfy the requirement. The seed of the associated function candidate set is determined based on the expression E used to determine that the requirement was satisfied.
If the evaluation of E involves overload resolution at the top level, the seed is the function (13.3.1 [over.match.funcs]) selected by the outermost application of overload resolution (clause 13 [over]).
Otherwise, if E is a pseudo destructor call (5.2.4 [expr.pseudo]), the seed is a pseudo-destructor-name.
Otherwise, the seed is the initialization of an object.
It is not clear that this takes built-in operators into account. For example:
concept C<class T, class U> {
typename R;
R operator+( T, U );
}
concept_map C<int, double> {}
It is possible that under some circumstances an expression created under the rules of _N2914_.14.10.2.1 [concept.map.fct] might be syntactically ambiguous with declarations, in which case they would be interpreted as declarations and not expressions. It would be helpful to have an explicit statement to the effect that the expressions created by these rules shall always be interpreted as expressions and never as declarations.
Given the following example:
auto concept C<typename T, typename U> {
Returnable U;
typename type = T&&;
U::U(type);
}
_N2914_.14.10.2.2 [concept.map.assoc] paragraph 5 says,
If an associated type or class template (_N2914_.14.10.1.2 [concept.assoc]) has a default value, a concept map member satisfying the associated type or class template requirement shall be implicitly defined by substituting the concept map arguments into the default value.
It is not clear what the order of processing should be between this step and the formation of the expression in _N2914_.14.10.2.1 [concept.map.fct]. Deduction of the associated type (in _N2914_.14.10.2.2 [concept.map.assoc] paragraph 4) isn't used in this example, but in general requires the expression, but the expression can't be created without the definition of the associated type. Perhaps the approach should be to attempt to define the expression, fail for want of the associated type, apply the default, and then try to define the expression again. Whatever the answer, this needs to be spelled out more clearly.
The example in _N2914_.14.10.3.2 [concept.refine.maps] paragraph 3 reads:
concept C<typename T> { }
concept D<typename T, typename U> : C<T> { }
template<typename T> struct A { };
template<typename T> concept_map D<A<T>, T> { }
...
Since all concept maps templates are constrained templates, we know that we're in a constrained context at the point of the concept_map keyword. Then the first argument to D is A<T>, and A is an unconstrained template, so this is ill-formed by _N2914_.14.11 [temp.constrained] paragraph 5:
Within a constrained context, a program shall not require a template specialization of an unconstrained template for which the template arguments of the specialization depend on a template parameter.
Suggestion: make A a constrained template, e.g.,
template<std::ObjectType T> struct A { };
Additional notes (May, 2009):
There are other examples that exhibit the same problem. For example, _N2960_.14.6.8 [temp.concept.map] paragraph 7 has this example:
concept Stack<typename X> {
typename value_type;
value_type& top(X&);
// ...
}
template<typename T> struct dynarray {
T& top();
};
template<> struct dynarray<bool> {
bool top();
};
template<typename T>
concept_map Stack<dynarray<T>> {
typedef T value_type;
T& top(dynarray<T>& x) { return x.top(); }
}
dynarray needs to be constrained. Similarly, in _N2914_.14.10.2.2 [concept.map.assoc] paragraph 3, in the example
concept Allocator<typename Alloc> {
template<class T> class rebind_type;
}
template<typename T>
class my_allocator {
template<typename U> class rebind_type;
};
template<typename T>
concept_map Allocator<my_allocator<T>> {
template<class U> using rebind_type = my_allocator<T>::rebind_type;
}
my_allocator must be constrained. (Note also the missing template argument in the target of the template alias declaration.)
Trivially copyable type was added in 3.9 [basic.types], so we think that it is necessary to add a concept for trivially copyable type like TriviallyCopyableType.
Notes from the March, 2009 meeting:
It is not clear whether this should be supported here or in _N2914_.20.2.9 [concept.copymove], similar to TriviallyCopyConstructible and TriviallyCopyAssignable.
_N2914_.14.11 [temp.constrained] paragraph 5 says,
Within a constrained context, a program shall not require a template specialization of an unconstrained template for which the template arguments of the specialization depend on a template parameter.
This would appear to indicate that an example like the following is ill-formed:
auto concept C<class T> {};
template<template<class> class T, C U>
struct Y {
Y() {
T<U> x; // Well-formed?
}
};
because T' is not a constrained template archetype. However, this is not the intended outcome. The wording needs to be clarified on this point (and an example and a note explaining the rationale would be helpful).
(See also issues 849 and 851.)
It should be possible to support boolean constant expressions as requirements without resorting to defining the True concept in the library. Boolean expressions are very likely to be constraints when dealing with non-type template parameters and variadic templates, and constraints in these cases should feel just as natural as constraints on the type system.
The use of && as the separator for a list of requirements has shown itself to be a serious teachability problem. The mental model behind && treats concepts as simple predicates, which ignores the role of concepts in type-checking templates. The more programmers read into the && (and especially try to fake || with && and !), the harder it is for them to understand the role of concept maps. Simply changing the separator to , would eliminate a significant source of confusion.
The example in _N2914_.14.11.1.1 [temp.req.sat] paragraph 6 reads,
concept C<typename T> { }
concept D<typename T> { }
namespace N2 {
template<C T> void f(T); // #1
template<C T> requires D<T> void f(T); // #2
template<C T> void g(T x) {
f(x);
}
...
The call f(x) is ill-formed without a constraint indicating that x can be passed by value.
Consider the following example:
auto concept A<class T> {
int f(T);
}
auto concept B<class T> {
int f(T)
template<class U>
requires A<U> // brings f(U') into scope
auto g(T p, U q) ->
decltype(f(p) + f(q));
// Both B<T>::f(T) and A<U>::f(U) needed here
// but B<T>::f(T) is hidden by A<U>::f(U)
// (declared in the same scope as g's template parameters)
}
This is similar to the case that motivated 9.2 [class.mem] paragraph 19:
A constrained member is treated as a constrained template (_N2914_.14.11 [temp.constrained]) whose template requirements include the requirements specified in its member-requirement clause and the requirements of each enclosing constrained template.
See also _N2914_.14.10.1.1 [concept.fct] paragraph 10 for a similar rule for default implementations.
A more general version of this merging of requirements is needed, but it does not appear to exist. _N2914_.14.11.1.2 [temp.req.impl] would seem to be the logical place for such a rule.
The example at the end of _N2914_.14.11.2 [temp.archetype] paragraph 13 reads,
auto concept CopyConstructible<typename T> {
T::T(const T&);
}
template<CopyConstructible T> struct vector;
auto concept VectorLike<typename X> {
typename value_type = typename X::value_type;
X::X();
void X::push_back(const value_type&);
value_type& X::front();
}
template<CopyConstructible T>
requires VectorLike<vector<T>> // vector<T> is an archetype (but not an instantiated archetype)
void f(const T& value) {
vector<T> x; // OK: default constructor in VectorLike<vector<T> >
x.push_back(value); // OK: push_back in VectorLike<vector<T> >
VectorLike<vector<T>>::value_type& val = x.front(); // OK: front in VectorLike<vector<T> >
}
However, x.push_back(value) is, in fact, ill-formed: there is no relationship between VectorLike<vector<T>>::value_type and T in this example. The function needs one further requirement, e.g., std::SameType<VectorLike<vector<T>>::value_type, T> to allow use of the function parameter value as the argument of the push_back call.
Suppose we have
template<std::ObjectType T>
T* f(T* p) {
return ++p; // Presumably ok
}
5.3.2 [expr.pre.incr] paragraph 1 requires that “The type of the operand shall be an arithmetic type or a pointer to a completely-defined effective object type.” At ++p in this example, the type archetype T' is considered to be completely-defined because
A type archetype is considered to be completely defined when it is established
(_N2914_.14.11.2.1 [temp.archetype.assemble] paragraph 1) and 14.7.2 [temp.explicit] paragraph 7 says that an archetype becomes established when
the archetype is used in a context where a complete type is required
So far, so good. Consider use of f(T*) with an incomplete type, for instance:
struct A; // A is not defined yet.
A* g(A* p) {
return f(p);
}
During template argument deduction against the template f(T*), we find that there is a concept map for std::ObjectType<A> because std::ObjectType is a compiler-supported concept, and because A is an object type (3.9 [basic.types]), so the compiler provides the concept map implicitly. Type deduction succeeds, but then we get an instantiation-time error on ++p because A is incomplete.
I see two potential solutions:
We can remove built-in operations for ptr-to-effective-object-type, so that you would have to explicitly require something like std::HasPreincrement<T*> before using ++ on values of type T* in f(T*). Then A's lack of completeness would be indicated when we try to satisfy those requirements automatically (and not at instantiation time).
Alternatively, we can introduce the notion of a compiler-supported concept std::CompleteType<T>, and amend _N2914_.14.11.2.1 [temp.archetype.assemble] so that a type archetype is only considered to be completely-defined if it has that requirement. This would imply that f(T*) above is ill-formed at ++p because T would then be an incomplete effective object type; the user could fix this by inserting requires std::CompleteType<T> after the template-parameter-list, and then the call f(p) would be ill-formed because std::CompleteType<A> would not be satisfied.
If there is no requirement for a destructor for a type, according to _N2914_.14.11.2.1 [temp.archetype.assemble] paragraph 5 its archetype will have a deleted destructor. As a result, several examples in the current wording are ill-formed:
_N2914_.14.10.2 [concept.map] paragraph 11: the add function.
_N2914_.14.10.3.1 [concept.member.lookup] paragraph 3: the f function. (Also missing the copy constructor requirement.)
_N2914_.14.10.3.1 [concept.member.lookup] paragraph 5: the h function. (Also missing the copy constructor requirement.)
_N2914_.14.11.1 [temp.req] paragraph 3: the f function.
_N2914_.14.11.1.1 [temp.req.sat] paragraph 6: the g function. (Also missing the copy constructor requirement.)
_N2914_.14.11.2 [temp.archetype] paragraph 15: the distance function.
_N2914_.14.11.2.1 [temp.archetype.assemble] paragraph 2: the foo function.
_N2914_.14.11.2.1 [temp.archetype.assemble] paragraph 3: the f function.
_N2914_.14.11.4 [temp.constrained.inst] paragraph 4: needed for difference_type.
One possibility would be to add the destructor requirement directly in these examples. Another might be to use std::CopyConstructible instead of a local concept. Yet another would be to consider an implicit requirement for a destructor for std::Returnable and std::VariableType.
The example in _N2914_.14.11.4 [temp.constrained.inst] paragraph 4 is ill-formed. The call to advance(i, 1) in f attempts to pass 1 to a parameter declared as Iter::difference_type, but there is nothing that says that int is compatible with Iter::difference_type.
According to _N2960_.3.3.9 [basic.scope.req] paragraph 2,
In a constrained context (_N2914_.14.11 [temp.constrained]), the names of all associated functions inside the concepts named by the concept requirements in the template's requirements are declared in the same scope as the constrained template's template parameters.
This does not appear to cover the case when the requirement appears in a concept definition:
auto concept B<class T> {
void f( T );
}
auto concept C<class T> {
typename U = int;
requires B<U>; // Is void f(U) placed in the scope of C?
void g(U x) {
f(x); // Ok, finds the 'f(U)' implicitly declared as a
// result of the associated requirement B<U>.
}
}
void f(int);
void g() {
C<char>::f(42); // Ok?
}
This program should be well-formed, but the current wording does not make that clear.
Another question that must be addressed is if C also contained an explicit declaration of f(U), either before or after B<U>, and whether it would need a satisfier within a concept map to C.
(See also issue 866.)
_N2960_.6.9 [stmt.late] paragraph 2 consists of the following example:
concept Semigroup<typename T> {
T::T(const T&);
T operator+(T, T);
}
concept_map Semigroup<int> {
int operator+(int x, int y) { return x * y; }
}
template<Semigroup T>
T add(T x, T y) {
T r = x + y; // uses Semigroup<T>::operator+
late_check {
r = x + y; // uses operator+ found at instantiation time (not considering Semigroup<T>::operator+)
}
return r;
}
The second comment is correct but incomplete, because the assignment operator is also found at instantiation time. The assignment would be ill-formed outside the late_check block, because the Semigroup concept has no copy assignment operator. The comment should be extended accordingly.
According to 3.4.3 [basic.lookup.qual] paragraph 7,
In a constrained context (_N2914_.14.11 [temp.constrained]), a name prefixed by a nested-name-specifier that nominates a template type parameter T is looked up as follows: for each template requirement C<args> whose template argument list references T, the name is looked up as if the nested-name-specifier referenced C<args> instead of T (_N2960_.3.4.3.3 [concept.qual]), except that only the names of associated types are visible during this lookup. If an associated type of at least one requirement is found, then each name found shall refer to the same type. Otherwise, if the reference to the name occurs within a constrained context, the name is looked up within the scope of the archetype associated with T (and no special restriction on name visibility is in effect for this lookup).
In an example like,
concept A<typename T> {
typename assoc_type;
}
concept B<typename T> {
typename assoc_type;
}
template<typename T>
requires A<T>
B<T::assoc_type>::assoc_type f();
it is not clear whether the argument T::assoc_type of B “references” T or not.
James Widman: In our mental model (and in our intentions while drafting), we still have a (non-archetype) dependent type for the T in your example, and, even after the SameType requirement is seen, we also have a distinct dependent type to represent A<T>::assoc_type (which itself is distinct from the type of the entity named assoc_type that lives in the scope of the concept A). And those two dependent types (A<T>::assoc_type and T) will both alias the same type archetype when that archetype is established (see the paragraph on establishment in _N2914_.14.11.2 [temp.archetype]).
I think 3.4.3 [basic.lookup.qual] paragraph 6 will read more easily if we change the “references a template parameter” verbiage to a generalized “dependent type” verbiage. (We shied away from that in the past because we wanted to say that there's nothing “dependent” within a constrained context. That's because we wanted to say that all name references are bound to something, overload resolution is done, etc. So certainly there are no instances of deferred name lookup or deferred overload resolution within a constrained context. But we still need to be able to say when a type, template, value or concept instance depends on a template parameter.) I propose we change this wording to read,
In a constrained context (_N2914_.14.11 [temp.constrained]), the identifier of an unqualified-id prefixed by a nested-name-specifier that nominates a dependent type T is looked up as follows: for each template requirement C<args> such that either T or an equivalent type (_N2914_.14.11.1 [temp.req]) is a template argument to C, the identifier of the unqualified-id is looked up as if the nested-name-specifier nominated C<args> instead of T (_N2960_.3.4.3.3 [concept.qual]), except that only the names of associated types and class templates (_N2914_.14.10.1.2 [concept.assoc]) are visible during this lookup. If an associated type or class template of at least one requirement is found, then the unqualified-id shall refer either to the same type or to an equivalent type when its identifier is looked up in each of the concepts of the other requirements where T is a template argument. [Note: no part of the procedure described in the preceding part of this paragraph results in the establishment of an archetype (_N2914_.14.11.2 [temp.archetype]). However, in the event that the unqualified-id is a template-id, one of its template arguments could contain some construct that would force archetype establishment. —end note] Otherwise, the name is looked up within the scope of the archetype aliased by T (and no special restriction on name visibility is in effect for this lookup). [Note: this establishes the archetype of T (if it was not established already). —end note]
(It looks like we have a wording nit to fix in the archetype establishment paragraph: it talks about a type archetype coming into existence “when it is used [in some way].” It seems odd to say that something is used in a particular way before it exists. We should instead say something like “when a necessarily-dependent type that would alias the archetype is used [in some way].”)
(It might also be nice to have a cleanup in the paragraph that introduces the notion of std::SameType and “equivalent types” (_N2914_.14.11.1 [temp.req] paragraph 3) so that the congruence relation is part of the normative text rather than a note.)
3.5 [basic.link] does not specify whether concept names have linkage or not.
Given an example like:
auto concept Conv<typename T, typename U> {
U::U(T&&);
U::U(const U&);
U::~U();
};
template<typename U, typename T>
requires Conv<T*, U*>
U* f(T* p) {
return static_cast<T*&&>(p);
}
There is currently no normative wording that makes a T* convertible to a U* in the return statement.
One possible approach would be to take the concept map archetype as specifying an additional case for the pointer conversions in 4.10 [conv.ptr].
If we cannot bind references/take address of functions in concept_maps, does that mean we cannot use generic bind in constrained templates? Launch threads with expressions found via concept map lookup? Hit problems creating std::function objects? Does the problem only occur if we use qualified lookup to explicitly name a concept map? Does it only kick in if we rely on the implicit function implementation provided by a concept_map, so some types will work and others won't for the same algorithm?!
Additional note, June, 2009:
Here is an example illustrating the question:
auto concept Fooable<T> {
void foo(T);
}
struct test_type {
void foo() {
cout << "foo test_type\n";
}
};
concept_map Fooable<test_type> {
void foo(test_type& t) {
t.foo();
}
}
void foo(int x) {
cout "foo int\n";
}
template<typename T>
requires Fooable<T>
function<void(T)> callback(T t) {
void(*fn)(T) = foo;
return fn;
}
int main() {
auto fn1 = factory(test_type{});
auto fn2 = factory(0);
fn1(test_type{});
fn2(0);
return 0;
}
The expansion of the range-based for statement is given in 6.5.4 [stmt.ranged] paragraph 1 as:
{
auto && __range = ( expression );
for ( auto __begin = std::Range<_RangeT>::begin(__range),
__end = std::Range<_RangeT>::end(__range);
__begin != __end;
++__begin ) {
for-range-declaration = *__begin;
statement
}
}
In a non-templated context, the concept map to std::Range has been dropped, so the operators and initialization will be whatever they would normally be; if the concept map replaced those with some customized version (e.g., if the iterator's ++ were supposed to skip odd-numbered elements), that customized meaning would be lost.
What we really want are the operators associated with the concept map to std::Iterator that was used to satisfy the associated requirement (std::Iterator<iterator>) within std::Range<_RangeT> (in whatever concept map was used to satisfy std::Range<_RangeT>). That is, if the grammar permitted it, we want something like
std::Range<_RangeT>::concept_map
::std::Iterator<std::Range<_RangeT>::iterator>::operator++(_begin)
Another alternative would be, if issue 856 is resolved by injecting the declaration of associated functions into concept definitions, something like
std::Range<_RangeT>::operator++(__begin)
Paper N2762 changed 6.7 [stmt.dcl] paragraph 3 from
...unless the variable has trivial type (3.9 [basic.types])...
to
...unless the variable has scalar type, class type with a trivial default constructor and a trivial destructor, a cv-qualified version of one of these types, or an array of one of the preceding types...
However, this change overrode the colliding change from N2773 that would have changed it to read
...unless the variable has effective trivial type...
The revised wording needs to be changed to allow for archetypes with the appropriate requirements.
If we write
concept C<class T> {}
template<C T>
struct B {
B f();
virtual void g() = 0;
};
... it seems reasonable to expect a diagnostic about B<T>::f() not because it doesn't require std::Returnable<B<T>> (which I think should not draw an error), but because g() is a pure virtual function.
Now how about this:
template<C T>
struct G {
B<T> f() { return B<T>(); }
};
Here, I'd like to see an error not because we lack the requirement std::Returnable<B<T>>, but because, when we instantiate B<T'> (as the current wording indicates we must within the definition of G<T>::f()), it turns out to be an abstract class.
Now, it could be that when we instantiate G, we get a different partial specialization of B, and that partial specialization could have a pure virtual member. So you might see an instantiation-time error. But partial specializations present dangers like this anyway.
I suggest we make the rule about Returnable<T> apply only in the case where T is not an instantiated archetype. The rationale is that with an instantiated archetype, it's possible to see at template definition time whether the type is abstract, whereas with a non-instantiated archetype, the only known attributes come from requirements.
I suspect we need similar changes for the declarator section. E.g., for a class template A, we shouldn't need to explicitly require VariableType<A<T>> if we want to declare a variable of type A<T>. Instead, we just instantiate A<T'> (as would be naturally required at the point of definition of a variable of type A<T'>), and issue errors when appropriate like we do with ordinary classes today.
According to 9.2 [class.mem] paragraph 19,
A non-template member-declaration that has a member-requirement (_N2914_.14.11.1 [temp.req]) is a constrained member and shall occur only in a class template (14.5.1 [temp.class]) or nested class thereof. The member-declaration for a constrained member shall declare a member function. A constrained member is treated as a constrained template (_N2914_.14.11 [temp.constrained]) whose template requirements include the requirements specified in its member-requirement clause and the requirements of each enclosing constrained template.
Furthermore, 14.5.2 [temp.mem] paragraph 9 says,
A member template of a constrained class template is itself a constrained template (_N2914_.14.11 [temp.constrained])...
and illustrates this statement with the following example:
concept C<typename T> { void f(const T&); }
concept D<typename T> { void g(const T&); }
template<C T> class A {
requires D<T> void h(const T& x) {
f(x); // OK: C<T>::f
g(x); // OK: D<T>::g
}
};
If these passages are taken at face value and a constrained member function is, in fact, “treated as a... template,” there are negative consequences. For example, according to 12.8 [class.copy] paragraph 2, a member function template is never considered to be a copy constructor, so a constrained constructor that is in the form of a copy constructor does not suppress the implicit declaration and definition of a default copy constructor. Also, according to 14.5.2 [temp.mem] paragraph 3, a member function template cannot be virtual, so it is not possible to specify a member-requirement clause for a virtual function.
Presumably these consequences are unintended, so the wording that suggests otherwise should be revised to make that clear.
12.3.2 [class.conv.fct] paragraph 1 says,
A conversion function is never used to convert a (possibly cv-qualified) object to the (possibly cv-qualified) same object type (or a reference to it), to a (possibly cv-qualified) base class of that type (or a reference to it), or to (possibly cv-qualified) void.
Does this mean that the following example is ill-formed?
auto concept Convertible<typename T, typename U> {
operator U(const T&);
}
template <typename T, typename U>
requires Convertible<T, U>
U convert(const T& t) { return t; }
int main() {
convert<int>(42);
}
13.6 [over.built] paragraph 11 says,
For every quintuple (C1, C2, T, CV1, CV2), where C2 is a class type, C1 is the same type as C2 or is a derived class of C2, T is an effective object type or a function type, and CV1 and CV2 are cv-qualifier-seqs, there exist candidate operator functions of the form
CV12 T& operator->*(CV1 C1*, CV2 T C2::*);
where CV12 is the union of CV1 and CV2.
C1 and C2 should be effective class types (cf 5.5 [expr.mptr.oper] paragraph 3.
Also, should the relationship between those two classes be expressed as std::SameType or std::DerivedFrom requirements?
Is it possible to export a concept map template? The current wording suggests it is possible, but it is not entirely clear what it would mean.
Notes from the March, 2009 meeting:
Export is only useful for non-inline function templates and static data members of class templates, so it does not make sense to export a concept map template.
The grammar for constrained-template-parameter given in 14.1 [temp.param] paragraph 1 is:
The identifier naming the parameter is optional in the first two productions but not in the latter two productions. Is there a reason for this discrepancy?
There is currently no way to distinguish between templates that differ only by their requirements when naming a specialization. For example:
auto concept A<class T> {}
auto concept B<class T> {}
template<class T> requires A<T> void f(T); // #1
template<class T> requires B<T> void f(T); // #2
template <> void f(int); // Which one?
(See also issue 868.)
There is no way to specify a concept map in the name of a specialization. It would be useful to be able to do something like
void g(int n) {
f<int : N::concept_map A<int>>(n);
}
(See also issue 867.)
The requirements for matching of template template parameters and template template arguments given in 14.3.3 [temp.arg.template] do not mention constraints, leaving questions about whether examples like the following are well-formed:
auto concept C<class T> {};
template <template <C T> class U, C V> struct A{};
template <class T> struct X {};
A<X,int> ax; // Well-formed?
template <template <class T> class U, C V> struct B{};
template <C T> struct Y {};
B<Y,int> by; // Well-formed?
template <template <class T> class U> struct D{};
template <C T> struct Z {};
D<Z> dz; // Well-formed?
(See also issue 848.)
If the requirements of a constrained special member function are not satisfied, the result is that the member function is not declared (14.5.1 [temp.class] paragraph 5). This allows the special member function to be implicitly declared and defined, which will likely result in an ill-formed program or one with the wrong semantics.
Although the current wording does specify the outcome, it is not immediately apparent what the result of an example like the following should be:
template<std::ObjectType T>
struct S {
requires std::CopyConstructible<T>
S(const S&) = default;
};
The outcome (that S will have an implicitly-declared copy constructor that is defined as deleted in specializations in which T is not copy-constructible) would be clearer with the addition of two notes. First, it would be helpful if 14.5.1 [temp.class] paragraph 5, which currently reads,
A constrained member (9.2 [class.mem]) in a class template is declared only in class template specializations in which its template requirements (_N2914_.14.11.1 [temp.req]) are satisfied (_N2914_.14.11.1.1 [temp.req.sat])...
had a note or footnote to the effect,
When a constrained member of a template is a special member function, and when, in an instantiation, the member is not declared because its requirements are not satisfied, the special member is considered not to have been “explicitly declared” (i.e., the member is not user-declared); therefore a declaration may still be implicitly generated as specified in clause 12 [special].
The fact that the implicitly-declared copy constructor in this case is defined as deleted would be clearer if somewhere in the second list in 12.8 [class.copy] paragraph 5, which currently reads
...An implicitly-declared copy constructor for a class X is defined as deleted if X has:
a variant member with a non-trivial copy constructor and X is a union-like class,
a non-static data member of class type M (or array thereof) that cannot be copied because overload resolution (13.3 [over.match]), as applied to M's copy constructor, results in an ambiguity or a function that is deleted or inaccessible from the implicitly-declared copy constructor, or
a direct or virtual base class B that cannot be copied because overload resolution (13.3 [over.match]), as applied to B's copy constructor, results in an ambiguity or a function that is deleted or inaccessible from the implicitly-declared copy constructor.
there were a cross-reference to _N2914_.14.11.2.1 [temp.archetype.assemble], whose third paragraph reads,
If no requirement specifies a copy constructor for a type T,
a copy constructor is implicitly declared (12.8 [class.copy])
in the archetype of T with the following signature:
T(const T&) = delete;
The relationship of requirements with template aliases is not clear in the current wording. For example, something like
auto concept C{};
template <class T> struct A{};
template <C T> using B = A<T>;
is presumably allowed by the current wording of 14.5.7 [temp.alias] but, unless a good use case is presented, should probably be prohibited.
On the other hand, _N2914_.14.11 [temp.constrained] paragraph 5,
Within a constrained context, a program shall not require a template specialization of an unconstrained template for which the template arguments of the specialization depend on a template parameter.
might be considered to forbid an example like
template <C T> struct X {};
template <class T> using Y = X<T>;
template <std::VariableType T>
void f(Y<T>); // Error?
although it should probably be allowed. (Note, however, that 14.5.7 [temp.alias] paragraph 2,
When a template-id refers to the specialization of a template alias, it is equivalent to the associated type obtained by substitution of its template-arguments for the template-parameters in the type-id of the template alias.
could be viewed as allowing this example, depending on how the word “equivalent” is understood.)
The text should be amended to clarify the resolution of these questions. (See also issue 848.)
Any use of a pointer to deleted storage, even if the pointer is not dereferenced, produces undefined behavior (3.7.4.2 [basic.stc.dynamic.deallocation] paragraph 4). The reason for this restriction is that, on some historical architectures, deallocating an object might free a memory segment, resulting in a hardware exception if a pointer referring to that segment were loaded into a pointer register, and on those architectures use of a pointer register for moving and comparing pointers was the most efficient mechanism for these operations.
It is not clear whether current or foreseeable architectures still require such a draconian restriction or whether it is feasible to relax it only to forbid a smaller range of operations. Of particular concern is the use of atomic pointers, which might be used in race conditions involving deallocation, where the loser of the race might encounter this undefined behavior.
(See also issue 312.)
Rationale (April, 2007):
The current specification is clear and was well-motivated. Analysis of whether this restriction is still needed should be done via a paper and discussed in the Evolution Working Group rather than being handled by CWG as an issue/defect.
Currently both implicit (4.11 [conv.mem]) and explicit (5.2.9 [expr.static.cast]) conversions of pointers to members permit only cases in which the type of the member is the same except for cv-qualification. It would seem reasonable to allow conversions in which one member type is a base class of the other. For example:
struct B { };
struct D: B { };
struct X {
D d;
};
struct Y: X { };
B Y::* pm = &X::d; // Currently ill-formed: type of d is D, not B
(See also issue 170.)
Consider the following:
namespace N {
struct A { };
template<typename T>
T func(const A&) { return T(); }
}
void f() {
N::A a;
func<int>(a); // error
}
Although argument-dependent lookup would allow N::func to be found in this call, the < is taken as a less-than operator rather than as the beginning of a template argument list. If the use of the template keyword for syntactic disambiguation were permitted for unqualified-ids, this problem could be solved by prefixing the function name with template, allowing the template-id to be parsed and argument-dependent lookup to be performed.
Rationale (July, 2009):
This suggestion would need a full proposal and discussion by the EWG before the CWG could consider it.
In the current specification of lambda expressions, a name appearing in a lambda-capture must refer to a local variable or reference with automatic storage duration (5.1.2 [expr.prim.lambda] paragraph 3). This restriction seems unnecessary and possibly confusing.
One possibility would be to extend the syntax of the lambda-capture to be something like
v = expr
with the meaning that the closure object would have a member named v initialized with the value expr. With this extension, the current syntax could be viewed as an abbreviation for
v = v
Rationale, March, 2009:
This idea was discussed and rejected by the EWG.
auto and lambda return types use slightly different rules for determining the result type from an expression. auto uses the rules in 14.8.2.1 [temp.deduct.call], which explicitly drops top-level cv-qualification in all cases, while the lambda return type is based on the lvalue-to-rvalue conversion, which drops cv-qualification only for non-class types. As a result:
struct A { };
const A f();
auto a = f(); // decltype(a) is A
auto b = []{ return f(); }; // decltype(b()) is const A
This seems like an unnecessary inconsistency.
John Spicer:
The difference is intentional; auto is intended only to give a const type if you explicitly ask for it, while the lambda return type should generally be the type of the expression.
Daniel Krügler:
Another inconsistency: with auto, use of a braced-init-list can deduce a specialization of std::initializer_list; it would be helpful if the same could be done for a lambda return type.
Although value-initialization is defined for array types and the () initializer is permitted in a mem-initializer naming an array member of a class, the syntax T() (where is an array type) is explicitly forbidden by 5.2.3 [expr.type.conv] paragraph 2. This is inconsistent and the syntax should be permitted.
Rationale (July, 2009):
The CWG was not convinced of the utility of this extension, especially in light of questions about handling the lifetime of temporary arrays. This suggestion needs a proposal and analysis by the EWG before it can be considered by the CWG.
The current specification of the alignof operator (5.3.6 [expr.alignof]) allows it to be applied only to types, not to objects. Since the align attribute may be applied to objects, and since existing practice permits querying the alignment of objects, it should be considered whether to allow this in Standard C++ as well.
The relational operators have unspecified results when comparing pointers that refer to objects that are not members of the same object or elements of the same array (5.9 [expr.rel] paragraph 2, second bullet). This restriction (which dates from C89) stems from the desire not to penalize implementations on architectures with segmented memory by forcing them essentially to simulate a flat address space for the purpose of these comparisons. If such an implementation requires that objects and arrays to fit within a single segment, this restriction enables pointer comparison to be done simply by comparing the offset portion of the pointers, which could be much faster than comparing the full pointer values.
The problem with this restriction in C++ is that it forces users of the Standard Library containers to use less<T*> instead of the built-in < operator to provide a total ordering on pointers, a usage that is inconvenient and error-prone. Can the existing restriction be relaxed in some way to allow the built-in operator to provide a total ordering? (John Spicer pointed out that the actual comparison for a segmented architecture need only supply a total ordering of pointer values, not necessarily the complete linearization of the address space.)
Rationale (April, 2007):
The current specification is clear and was well-motivated. Analysis of whether this restriction is still needed should be done via a paper and discussed in the Evolution Working Group rather than being handled by CWG as an issue/defect.
The current wording of 7.3.1.2 [namespace.memdef] and 14.7.3 [temp.expl.spec] requires that an explicit specialization be declared either in the same namespace as the template or in an enclosing namespace. It would be convenient to relax that requirement and allow the specialization to be declared in a non-enclosing namespace to which one or more if the template arguments belongs.
[Picked up by evolution group at October 2002 meeting.]
How can we write a function template, or member function of a class template that takes a C linkage function as a parameter when the function type depends on one of the template parameter types?
extern "C" void f(int);
void g(char);
template <class T> struct A {
A(void (*fp)(T));
};
A<char> a1(g); // okay
A<int> a2(f); // error
Another variant of the same problem is:
extern "C" void f(int);
void g(char);
template <class T> void h( void (*fp)(T) );
int main() {
h(g); // okay
h(f); // error
}
Somehow permit a language linkage to be specified as part of a function parameter declaration. i.e.
template <class T> struct A {
A( extern "C" void (*fp)(T) );
};
template <class T> void h( extern "C" void (*fp)(T) );
Suggested resolution: (Bill Gibbons)
The whole area of linkage needs revisiting. Declaring calling convention as a storage class was incorrect to begin with; it should be a function qualifier, as in:
void f( void (*pf)(int) c_linkage );
instead of the suggested:
void f( extern "C" void (*pf)(int) );
I would like to keep calling convention on the "next round" issues list,
including the alternative of using function qualifiers.
And to that end, I suggest that the use of linkage specifiers to specify calling convention be deprecated - which would make any use of linkage specifiers in a parameter declaration deprecated.
Martin Sebor: 7.5 [dcl.link], paragraph 4 says that "A linkage-specification shall occur only in namespace scope..." I'm wondering why this restriction is necessary since it prevents, among other things, the use of the functions defined <cmath> in generic code that involves function objects. For example, the program below is ill-formed since std::pointer_to_binary_function<> takes a pointer to a function with extern "C++" linkage which is incompatible with the type of the double overload of std::pow.
Relaxing the restriction to allow linkage specification in declarations of typedefs in class scope would allow std::pointer_to_binary_function<> ctor to be overloaded on both types (i.e., extern "C" and extern "C++"). An alternative would be to allow for the linkage specification to be deduced along with the type.
#include <cmath>
#include <functional>
#include <numeric>
int main () {
double a[] = { 1, 2, 3 };
return std::accumulate (a, a + 3, 2.0,
std::pointer_to_binary_function<double, double, double>(std::pow));
}
(See also library issue 2068.)
Now that the restriction against local classes being used as template arguments has been lifted, they are more useful, yet they are still crippled. For some reason or oversight, the restriction against local classes being templates or having member templates was not lifted. Allowing local classes to have member templates facilitates generic programming (the reason for lifting the other restriction), especially when it comes to the visitor-pattern (see the boost::variant documentation and the following example) as implemented in boost and the boost::MPL library (since functors have to be template classes in mpl, and higher-order functors have to have member templates to be useful). A local class with a member template would allow this desirable solution:
#include <boost/variant.hpp>
int main() {
struct times_two_generic: public boost::static_visitor<> {
template <typename T> void operator()(T& operand) const {
operand += operand;
}
};
std::vector<boost::variant<int, std::string>> vec;
vec.push_back(21);
vec.push_back("hello ");
times_two_generic visitor;
std::for_each(vec.begin(), vec.end(), boost::apply_visitor(visitor));
}
Is there any compelling reason not to allow this code? Is there any compelling reason not to allow local classes to be templates, have friends, or be able to define their static data members at function scope? Wouldn't this symmetry amongst local and non-local classes make the language more appealing and less embarrassing?
It would be nice to allow template alias within a function scope, and possibly a scoped concept map. As these affect name lookup and resolution, rather than defining new callable code, they are not seen to present the same problems that prevented class and function templates in the past.
Rationale (July, 2009):
This suggestion needs a paper and discussion in EWG before CWG can consider it.
14.7.3 [temp.expl.spec] paragraph 2 requires that explicit specializations of member templates be declared in namespace scope, not in the class definition. This restriction does not apply to partial specializations of member templates; that is,
struct A {
template<class T> struct B;
template <class T> struct B<T*> { }; // well-formed
template <> struct B<int*> { }; // ill-formed
};
There does not seem to be a good reason for this inconsistency.
It would be useful to be able to deduce the type of a function template argument from a corresponding default function argument expression, for example:
template <class T> int f(T = 0);
int x = f();
A more realistic use case would be
template <class T, class U>
int f(T x, U y = pair<T, T>());
Ideally one would also like
template <class T, class U>
int f(T x, U y = g(x));
These capabilities are part of the Boost parameter library, so there should not be issues of implementability.