Document number:  PL22.16/08-0198 = WG21 N2688
Date:  2008-06-29
Project:  Programming Language C++
Reference:  ISO/IEC IS 14882:2003
Reply to:  William M. Miller
 Edison Design Group, Inc.
 wmm@edg.com


C++ Standard Core Language Active Issues, Revision 56


This document contains the C++ core language issues on which the Committee (J16 + WG21) has not yet acted, that is, issues with status "Ready," "Review," "Drafting," and "Open."

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

Section references in this document reflect the section numbering of document PL22.16/08-0981 = WG21 N2588.

The purpose of these documents is to record the disposition of issues that have come before the Core Language Working Group of the ANSI (J16) and ISO (WG21) C++ Standard Committee.

Some issues represent potential defects in the ISO/IEC IS 14882:2003 document and corrected defects in the earlier ISO/IEC 14882:1998 document; others refer to text in the working draft for the next revision of the C++ language, commonly known as C++0x, and not to any Standard text. Issues are not necessarily formal ISO Defect Reports (DRs). While some issues will eventually be elevated to DR status, others will be disposed of in other ways. (See Issue Status below.)

The most current public version of this document can be found at http://www.open-std.org/jtc1/sc22/wg21. Requests for further information about these documents should include the document number, reference ISO/IEC 14882:2003, and be submitted to the InterNational Committee for Information Technology Standards (INCITS), 1250 Eye Street NW, Suite 200, Washington, DC 20005, USA.

Information regarding how to obtain a copy of the C++ Standard, join the Standard Committee, or submit an issue can be found in the C++ FAQ at http://www.comeaucomputing.com/csc/faq.html. Public discussion of the C++ Standard and related issues occurs on newsgroup comp.std.c++.


Revision History

Issue status

Issues progress through various statuses as the Core Language Working Group and, ultimately, the full J16 and WG21 committees deliberate and act. For ease of reference, issues are grouped in these documents by their status. Issues have one of the following statuses:

Open: The issue is new or the working group has not yet formed an opinion on the issue. If a Suggested Resolution is given, it reflects the opinion of the issue's submitter, not necessarily that of the working group or the Committee as a whole.

Drafting: Informal consensus has been reached in the working group and is described in rough terms in a Tentative Resolution, although precise wording for the change is not yet available.

Review: Exact wording of a Proposed Resolution is now available for an issue on which the working group previously reached informal consensus.

Ready: The working group has reached consensus that the issue is a defect in the Standard, the Proposed Resolution is correct, and the issue is ready to forward to the full Committee for ratification as a proposed defect report.

DR: The full Committee has approved the item as a proposed defect report. The Proposed Resolution in an issue with this status reflects the best judgment of the Committee at this time regarding the action that will be taken to remedy the defect; however, the current wording of the Standard remains in effect until such time as a Technical Corrigendum or a revision of the Standard is issued by ISO.

TC1: A DR issue included in Technical Corrigendum 1. TC1 is a revision of the Standard issued in 2003.

WP: A DR issue whose resolution is reflected in the current Working Paper. The Working Paper is a draft for a future version of the Standard.

Dup: The issue is identical to or a subset of another issue, identified in a Rationale statement.

NAD: The working group has reached consensus that the issue is not a defect in the Standard. A Rationale statement describes the working group's reasoning.

Extension: The working group has reached consensus that the issue is not a defect in the Standard but is a request for an extension to the language. The working group expresses no opinion on the merits of an issue with this status; however, the issue will be maintained on the list for possible future consideration as an extension proposal.

Issues with "Ready" Status

637. Sequencing rules and example disagree

Section: 1.9  [intro.execution]     Status: ready     Submitter: Ofer Porat     Date: 2 June 2007

In 1.9 [intro.execution] paragraph 16, the following expression is still listed as an example of undefined behavior:

    i = ++i + 1;

However, it appears that the new sequencing rules make this expression well-defined:

  1. The assignment side-effect is required to be sequenced after the value computations of both its LHS and RHS (5.17 [expr.ass] paragraph 1).

  2. The LHS (i) is an lvalue, so its value computation involves computing the address of i.

  3. In order to value-compute the RHS (++i + 1), it is necessary to first value-compute the lvalue expression ++i and then do an lvalue-to-rvalue conversion on the result. This guarantees that the incrementation side-effect is sequenced before the computation of the addition operation, which in turn is sequenced before the assignment side effect. In other words, it yields a well-defined order and final value for this expression.

It should be noted that a similar expression

    i = i++ + 1;

is still not well-defined, since the incrementation side-effect remains unsequenced with respect to the assignment side-effect.

It's unclear whether making the expression in the example well-defined was intentional or just a coincidental byproduct of the new sequencing rules. In either case either the example should be fixed, or the rules should be changed.

Clark Nelson: In my opinion, the poster's argument is perfectly correct. The rules adopted reflect the CWG's desired outcome for issue 222. At the Portland meeting, I presented (and still sympathize with) Tom Plum's case that these rules go a little too far in nailing down required behavior; this is a consequence of that.

One way or another, a change needs to be made, and I think we should seriously consider weakening the resolution of issue 222 to keep this example as having undefined behavior. This could be done fairly simply by having the sequencing requirements for an assignment expression depend on whether it appears in an lvalue context.

James Widman: How's this for a possible re-wording?

In all cases, the side effect of the assignment expression is sequenced after the value computations of the right and left operands. Furthermore, if the assignment expression appears in a context where an lvalue is required, the side effect of the assignment expression is sequenced before its value computation.

Notes from the February, 2008 meeting:

There was no real support in the CWG for weakening the resolution of issue 222 and returning the example to having undefined behavior. No one knew of an implementation that doesn't already do the (newly) right thing for such an example, so there was little motivation to go out of our way to increase the domain of undefined behavior. So the proposed resolution is to change the example to one that definitely does have undependable behavior in existing practice, and undefined behavior under the new rules.

Also, the new formulation of the sequencing rules approved in Oxford contained the wording that by and large resolved issue 222, so with the resolution of this issue, we can also close issue 222.

Proposed resolution (March, 2008):

Change the example in 1.9 [intro.execution] paragraph 16 as follows:

    i = v[i++];             // the behavior is undefined
    i = 7, i++, i++;        // i becomes 9
    i = ++i i++ + 1;        // the behavior is undefined
    i = i + 1;              // the value of i is incremented


639. What makes side effects “different” from one another?

Section: 1.9  [intro.execution]     Status: ready     Submitter: James Widman     Date: 26 July 2007

Is the behavior undefined in the following example?

    void f() {
         int n = 0;
         n = --n;
    }

1.9 [intro.execution] paragraph 16 says,

If a side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined.

It's not clear to me whether the two side-effects in n=--n are “different.” As far as I can tell, it seems that both side-effects involve the assignment of -1 to n, which in a sense makes them non-“different.” But I don't know if that's the intent. Would it be better to say “another” instead of “a different?”

On a related note, can we include this example to illustrate?

    void f( int, int );
    void g( int a ) { f( a = -1, a = -1 ); } // Undefined?

Proposed resolution (March, 2008):

Change 1.9 [intro.execution] paragraph 16 as follows:

...If a side effect on a scalar object is unsequenced relative to either a different another side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined. [Example:

    void f(int, int);
    void g(int i, int* v) {
        i = v[i++];         // the behavior is undefined
        i = 7, i++, i++;    // i becomes 9

        i = ++i + 1;        // the behavior is undefined
        i = i + 1;          // the value of i is incremented

        f(i = -1, i = -1);  // the behavior is undefined
    }

—end example] When calling...



677. Deleted operator delete and virtual destructors

Section: 12.4  [class.dtor]     Status: ready     Submitter: Mike Miller     Date: 15 February, 2008

Deallocation functions can't be virtual because they are static member functions; however, according to 12.5 [class.free] paragraph 7, they behave like virtual functions when the class's destructor is virtual:

Since member allocation and deallocation functions are static they cannot be virtual. [Note: however, when the cast-expression of a delete-expression refers to an object of class type, because the deallocation function actually called is looked up in the scope of the class that is the dynamic type of the object, if the destructor is virtual, the effect is the same.

Because the intent is to make any use of a deleted function diagnosable at compile time, a virtual deleted function can neither override nor be overridden by a non-deleted function, as described in 10.3 [class.virtual] paragraph 14:

A function with a deleted definition (8.4 [dcl.fct.def]) shall not override a function that does not have a deleted definition. Likewise, a function that does not have a deleted definition shall not override a function with a deleted definition.

One would assume that a similar kind of prohibition is needed for deallocation functions in a class hierarchy with virtual destructors, but it's not clear that the current specification says that. 8.4 [dcl.fct.def] paragraph 10 says,

A program that refers to a deleted function implicitly or explicitly, other than to declare it, is ill-formed.

Furthermore, the deallocation function is looked up at the point of definition of a virtual destructor (12.4 [class.dtor] paragraph 11), and the function found by this lookup is considered to be “used” (3.2 [basic.def.odr] paragraph 2). However, it's not completely clear that this “use” constitutes a “reference” in the sense of 8.4 [dcl.fct.def] paragraph 10, especially in a program in which an object of a type that would call that deallocation function is never deleted.

Suggested resolution:

Augment the list of lookup results from a virtual destructor that render a program ill-formed in 12.4 [class.dtor] paragraph 10 to include a deleted function:

If the result of this lookup is ambiguous or inaccessible, or if the lookup selects a placement deallocation function or a function with a deleted definition (8.4 [dcl.fct.def]), the program is ill-formed.

Proposed resolution (June, 2008):

Change 12.4 [class.dtor] paragraph 10 as follows:

If the result of this lookup is ambiguous or inaccessible, or if the lookup selects a placement deallocation function or a function with a deleted definition (8.4 [dcl.fct.def]), the program is ill-formed.


679. Equivalence of template-ids and operator function templates

Section: 14.4  [temp.type]     Status: ready     Submitter: Richard Corden     Date: 1 March, 2008

In order for two template-ids to refer to the same function, 14.4 [temp.type] paragraph 1, bullet 1 requires that

This makes it impossible for two template-ids referring to operator function templates to be equivalent, because only simple-template-ids have a template-name, and a template-id referring to an operator function template is not a simple-template-id (14.2 [temp.names] paragraph 1).

Suggested resolution:

Change 14.4 [temp.type] paragraph 1, bullet 1 to read,

Proposed resolution (June, 2008):

Change 14.4 [temp.type] paragraph 1, first bullet, as follows:



649. Optionally ill-formed extended alignment requests

Section: 3.11  [basic.align]     Status: ready     Submitter: Mike Miller     Date: 12 Aug 2007

The requirements on an implementation when presented with an alignment-specifier not supported by that implementation in that context are contradictory: 3.11 [basic.align] paragraph 9 says,

If a request for a specific extended alignment in a specific context is not supported by an implementation, the implementation may reject the request as ill-formed. The implementation may also silently ignore the requested alignment.

In contrast, 7.1.7 [dcl.align] paragraph 2, bullet 4 says simply,

with no provision to “silently ignore” the requested alignment. These two passages need to be reconciled.

If the outcome of the reconciliation is to grant implementations the license to accept and ignore extended alignment requests, the specification should be framed in terms of mechanisms that already exist in the Standard, such as undefined behavior and/or conditionally-supported constructs; “ill-formed” is a category that is defined by the Standard, not something that an implementation can decide.

Notes from the February, 2008 meeting:

The consensus was that such requests should be ill-formed and require a diagnostic. However, it was also observed that an implementation need not reject an ill-formed program; the only requirement is that it issue a diagnostic. It would thus be permissible for an implementation to “noisily ignore” (as opposed to “silently ignoring”) an unsupported alignment request.

Proposed resolution (June, 2008):

Change 3.11 [basic.align] paragraph 9 as follows:

If a request for a specific extended alignment in a specific context is not supported by an implementation, the implementation may reject the request as program is ill-formed. The implementation may also silently ignore the requested alignment. [Note: aAdditionally, a request for runtime allocation of dynamic memory storage for which the requested alignment cannot be honored may shall be treated as an allocation failure. —end note]


594. Coordinating issues 119 and 404 with delegating constructors

Section: 3.8  [basic.life]     Status: ready     Submitter: Tom Plum     Date: 30 August 2006

In ISO/IEC 14882:2003, the second bullet of 3.8 [basic.life] paragraph 1 reads,

if T is a class type with a non-trivial constructor (12.1 [class.ctor]), the constructor call has completed.

Issue 119 pointed out that aggregate initialization can be used with some classes with a non-trivial implicitly-declared default constructor, and that in such cases there is no call to the object's constructor. The resolution for that issue was to change the previously-cited wording to read,

If T is a class type with a non-trivial constructor (12.1 [class.ctor], the initialization is complete.

Later (but before the WP was revised with the wording from the resolution of issue 119), issue 404 changed the 2003 wording to read,

If T is a class type and the constructor invoked to create the object is non-trivial (12.1 [class.ctor]), the constructor call has completed.

thus reversing the effect of issue 119, whose whole purpose was to cover objects with non-trivial constructors that are not invoked.

Through an editorial error, the post-Redmond draft (N1905) still contained the original 2003 wording that should have been replaced by the resolution of issue 119, in addition to the new wording from the resolution:

if T is a class type and the constructor invoked to create the object is non-trivial (12.1 [class.ctor]), the constructor call has completed. the initialization is complete.

Finally, during the application of the edits for delegating constructors (N1986), this editing error was “fixed” by retaining the original 2003 wording (which was needed for the application of the change specified in N1986), so that the current draft (N2009) reads,

if T is a class type and the constructor invoked to create the object is non-trivial (12.1 [class.ctor]), the principal constructor call 12.6.2 [class.base.init]) has completed.

Because the completion of the call to the principal constructor corresponds to the point at which the object is “fully constructed” (15.2 [except.ctor] paragraph 2), i.e., its initialization is complete, I believe that the exact wording of the issue 119 resolution would be correct and should be restored verbatim.

Proposed resolution (June, 2008):

Change 3.8 [basic.life] paragraph 1 as follows:

The lifetime of an object is a runtime property of the object. An object is said to have non-trivial initialization if it is of a class or aggregate type and it or one of its members is initialized by a constructor other than a trivial default constructor. [Note: Initialization by a trivial copy constructor is non-trivial initialization. —end note] The lifetime of an object of type T begins when:

The lifetime of an object of type T ends when...



634. Conditionally-supported behavior for non-POD objects passed to ellipsis redux

Section: 5.2.2  [expr.call]     Status: ready     Submitter: Howard Hinnant     Date: 6 Jun 2007

Issue 506 changed passing a non-POD class type to an ellipsis from undefined behavior to conditionally-supported behavior. As a result, an implementation could conceivably reject code like the following:

    struct two {char _[2];};

    template <class From, class To>
    struct is_convertible
    {
    private:
            static From f;

            template <class U> static char test(const U&);
            template <class U> static two test(...);
    public:
            static const bool value = sizeof(test<To>(f)) == 1;
    };

    struct A
    {
         A();
    };

    int main()
    {
         const bool b = is_convertible<A,int>::value;  // b == false
    }

This technique has become popular in template metaprogramming, and no non-POD object is actually passed at runtime. Concepts will eliminate much (perhaps not all) of the need for this kind of programming, but legacy code will persist.

Perhaps this technique should be officially supported by allowing implementations to reject passing a non-POD type to ellipsis only if it appears in a potentially-evaluated expression?

Notes from the July, 2007 meeting:

The CWG agreed with the suggestion to allow such calls in unevaluated contexts.

Proposed resolution (September, 2007):

Change 5.2.2 [expr.call] paragraph 7 as follows:

...Passing an a potentially-evaluated argument of non-trivial class type (clause 9 [class]) with no corresponding parameter is conditionally-supported, with implementation-defined semantics...


671. Explicit conversion from a scoped enumeration type to integral type

Section: 5.2.9  [expr.static.cast]     Status: ready     Submitter: Daveed Vandevoorde     Date: 22 December 2007

There appears to be no provision in the Standard for explicit conversion of a value of a scoped enumeration type to an integral type, even though the inverse conversion is permitted. That is,

    enum class E { e };
    static_cast<E>(0);    // #1: OK
    static_cast<int>(e);  // #2: error

This is because values of scope enumeration types (intentionally) cannot be implicitly converted to integral types (4.5 [conv.prom] and 4.7 [conv.integral]) and 5.2.9 [expr.static.cast] was not updated to permit #2, although #1 is covered by paragraph 8.

Proposed resolution (June, 2008):

Add the following as a new paragraph following 5.2.9 [expr.static.cast] paragraph 8:

A value of a scoped enumeration type (7.2 [dcl.enum]) can be explicitly converted to an integral type. The value is unchanged if the original value can be represented by the specified type. Otherwise, the resulting value is unspecified.


659. Alignment of function types

Section: 5.3.6  [expr.alignof]     Status: ready     Submitter: Alisdair Meredith     Date: 7 November 2007

The specification for the alignof operator (5.3.6 [expr.alignof]) does not forbid function types as operands, although it probably should.

Proposed resolution (March, 2008):

The issue, as described, is incorrect. The requirement in 5.3.6 [expr.alignof] is for “a complete object type,” so a function type is already forbidden. However, the existing text does have a problem in this requirement in that it does not allow a reference type, as anticipated by paragraph 3. Consequently, the proposal is to change 5.3.6 [expr.alignof] paragraph 1 as indicated:

An alignof expression yields the alignment requirement of its operand type. The operand shall be a type-id representing a complete object type or a reference to a complete object type.


647. Non-constexpr instances of constexpr constructor templates

Section: 7.1.5  [dcl.constexpr]     Status: ready     Submitter: Mike Miller     Date: 12 Aug 2007

According to 7.1.5 [dcl.constexpr] paragraph 5,

If the instantiated template specialization of a constexpr function template would fail to satisfy the requirements for a constexpr function, the constexpr specifier is ignored and the specialization is not a constexpr function.

One would expect to see a similar provision for an instantiated constructor template (because the requirements for a constexpr function [paragraph 3] are different from the requirements for a constexpr constructor [paragraph 4]), but there is none; constexpr constructor templates are not mentioned.

Suggested resolution:

Change the wording of 7.1.5 [dcl.constexpr] paragraph 5 as indicated:

If the instantiated template specialization of a constexpr function template would fail to satisfy the requirements for a constexpr function or constexpr constructor, as appropriate to the function template, the constexpr specifier is ignored and the specialization is not a constexpr function or constexpr constructor.

Proposed resolution (June, 2008):

[Drafting note: This resolution goes beyond the problem described in the issue discussion, which is one aspect of the general failure of the existing wording to deal consistently with the distinctions between constexpr functions and constexpr constructors. The wording below attempts to rectify that problem systematically.]

  1. Change 7.1.5 [dcl.constexpr] paragraph 2 as follows:

    2. A constexpr specifier used in a function declaration the declaration of a function that is not a constructor declares that function to be a constexpr function. Similarly, a constexpr specifier used in a constructor declaration declares that constructor to be a constexpr constructor. Constexpr functions and constexpr constructors are implicitly inline (7.1.2 [dcl.fct.spec]). A constexpr function shall not be virtual (10.3).

  2. Change 7.1.5 [dcl.constexpr] paragraph 3 as follows:

    3. The definition of a constexpr function shall satisfy the following constraints:

    [Example:...

  3. Change 7.1.5 [dcl.constexpr] paragraph 4 as follows:

    4. The definition of a constexpr constructor shall satisfy the following constraints:

    A trivial copy constructor is also a constexpr constructor. [Example: ...

  4. Change 7.1.5 [dcl.constexpr] paragraph 5 as follows:

    5. If the instantiated template specialization of a constexpr function template would fail to satisfy the requirements for a constexpr function or constexpr constructor, the constexpr specifier is ignored and the specialization is not a constexpr function.

  5. Change 7.1.5 [dcl.constexpr] paragraph 6 as follows:

    6. A constexpr specifier used in for a non-static member function definition that is not a constructor declares that member function to be const (9.3.1 [class.mfct.non-static]). [Note: ...


648. Constant expressions in constexpr initializers

Section: 7.1.5  [dcl.constexpr]     Status: ready     Submitter: Mike Miller     Date: 12 Aug 2007

The current wording of 7.1.5 [dcl.constexpr] paragraph 7 seems not quite correct. It reads,

A constexpr specifier used in an object declaration declares the object as const. Such an object shall be initialized, and every expression that appears in its initializer (8.5 [dcl.init]) shall be a constant expression.

The phrase “every expression” is intended to cover multiple arguments to a constexpr constructor and multiple expressions in an aggregate initializer. However, it could be read (incorrectly) as saying that non-constant expressions cannot appear as subexpressions in such initializers, even in places where they do not render the full-expression non-constant (i.e., as unevaluated operands and in the unselected branches of &&, ||, and ?:). Perhaps this problem could be remedied by replacing “every expression” with “every full-expression?”

Proposed resolution (June, 2008):

Change 7.1.5 [dcl.constexpr] paragraph 7 as follows:

A constexpr specifier used in an object declaration declares the object as const. Such an object shall be initialized, and every expression that appears in its initializer (8.5) initialized. If it is initialized by a constructor call, the constructor shall be a constexpr constructor and every argument to the constructor shall be a constant expression. Otherwise, every full-expression that appears in its initializer shall be a constant expression. Every implicit conversion used...


651. Problems in decltype specification and examples

Section: 7.1.6.2  [dcl.type.simple]     Status: ready     Submitter: Daveed Vandevoorde     Date: 16 Aug 2007

The second bullet of 7.1.6.2 [dcl.type.simple] paragraph 4 reads,

The reference to “that function” is imprecise; it is not the actual function called at runtime but the statically chosen function (ignoring covariant return types in virtual functions).

Also, the examples in this paragraph have errors:

  1. The declaration of struct A should end with a semicolon.

  2. The lines of the form decltype(...); are ill-formed; they need a declarator.

Proposed Resolution (October, 2007):

Change 7.1.6.2 [dcl.type.simple] paragraph 4 as follows:

The type denoted by decltype(e) is defined as follows:

The operand of the decltype specifier is an unevaluated operand (clause 5 [expr]).

[Example:

    const int&& foo();
    int i;
    struct A { double x; };
    const A* a = new A();
    decltype(foo()) x1;      // type is const int&&
    decltype(i) x2;          // type is int
    decltype(a->x) x3;       // type is double
    decltype((a->x)) x4;     // type is const double&

—end example]



660. Unnamed scoped enumerations

Section: 7.2  [dcl.enum]     Status: ready     Submitter: Daveed Vandevoorde     Date: 15 November 2007

The current specification of scoped enumerations does not appear to forbid an example like the following, even though the enumerator e cannot be used:

    enum class { e };

This might be covered by 7 [dcl.dcl] paragraph 3,

In a simple-declaration, the optional init-declarator-list can be omitted only when declaring a class (clause 9 [class]) or enumeration (7.2 [dcl.enum]), that is, when the decl-specifier-seq contains either a class-specifier, an elaborated-type-specifier with a class-key (9.1 [class.name]), or an enum-specifier. In these cases and whenever a class-specifier or enum-specifier is present in the decl-specifier-seq, the identifiers in these specifiers are among the names being declared by the declaration (as class-names, enum-names, or enumerators, depending on the syntax). In such cases, and except for the declaration of an unnamed bit-field (9.6 [class.bit]), the decl-specifier-seq shall introduce one or more names into the program, or shall redeclare a name introduced by a previous declaration.

which, when combined with paragraph 2,

A declaration occurs in a scope (3.3 [basic.scope]); the scope rules are summarized in 3.4 [basic.lookup]. A declaration that declares a function or defines a class, namespace, template, or function also has one or more scopes nested within it. These nested scopes, in turn, can have declarations nested within them. Unless otherwise stated, utterances in clause 7 [dcl.dcl] about components in, of, or contained by a declaration or subcomponent thereof refer only to those components of the declaration that are not nested within scopes nested within the declaration.

appears to rule out the similar class definition,

    struct { int m; };

However, a scoped enumeration is not listed in paragraph 2 among the constructs containing a nested scope (although 3.3.7 [basic.scope.enum] does describe “enumeration scope”); furthermore, an enumerator-definition is not formally a “nested declaration.” If unusable scoped enumeration definitions are to be banned, these shortcomings in 7 [dcl.dcl] paragraph 2 must be addressed. (A note in 7.2 [dcl.enum] mentioning that unnamed scoped enumerations are not allowed would also be helpful.)

Notes from the February, 2008 meeting:

The consensus was to require that the identifier be present in an enum-specifier unless the enum-key is enum.

Proposed resolution (June, 2008):

Change 7.2 [dcl.enum] paragraph 2 as follows:

...The enum-keys enum class and enum struct are semantically equivalent; an enumeration type declared with one of these is a scoped enumeration, and its enumerators are scoped enumerators. The optional identifier shall not be omitted in the declaration of a scoped enumeration. The type-specifier-seq of an enum-base...


686. Type declarations/definitions in type-specifier-seqs and type-ids

Section: 8.1  [dcl.name]     Status: ready     Submitter: Jens Maurer     Date: 21 March, 2008

The restrictions on declaring and/or defining classes inside type-specifier-seqs and type-ids are inconsistent throughout the Standard. This is probably due to the fact that nearly all of the sections that deal with them attempt to state the restriction afresh. There are three cases:

  1. 5.3.4 [expr.new], 6.4 [stmt.select], and 12.3.2 [class.conv.fct] prohibit “declarations” of classes and enumerations. That means that

        while (struct C* p = 0) ;

    is ill-formed unless a prior declaration of C has been seen. These appear to be cases that should have been fixed by issue 379, changing “class declaration” to “class definition,” but were overlooked.

  2. 5.1.1 [expr.prim.lambda], 7 [dcl.dcl], and 8.3.5 [dcl.fct] (late-specified return types) do not contain any restriction at all.

  3. All the remaining cases prohibit “type definitions,” apparently referring to classes and enumerations.

Suggested resolution:

Add something like, “A class or enumeration shall not be defined in a type-specifier-seq or in a type-id,” to a single place in the Standard and remove all other mentions of that restriction (allowing declarations via elaborated-type-specifier).

Mike Miller:

An alias-declaration is just a different syntax for a typedef declaration, which allows definitions of a class in the type; I would expect the same to be true of an alias-declaration. I don't have any particularly strong attachment to allowing a class definition in an alias-declaration. My only concern is introducing an irregularity into what are currently exact-match semantics with typedefs.

There's a parallel restriction in many (but not all?) of these places on typedef declarations.

Jens Maurer:

Those are redundant, as typedef is not a type-specifier, and should be removed as well.

Proposed resolution (March, 2008):

  1. Delete the indicated words from 5.2.7 [expr.dynamic.cast] paragraph 1:

    2. ...Types shall not be defined in a dynamic_cast....

  2. Delete the indicated words from 5.2.8 [expr.typeid] paragraph 4:

    3. ...Types shall not be defined in the type-id....

  3. Delete the indicated words from 5.2.9 [expr.static.cast] paragraph 1:

    4. ...Types shall not be defined in a static_cast....

  4. Delete the indicated words from 5.2.10 [expr.reinterpret.cast] paragraph 1:

    5. ...Types shall not be defined in a reinterpret_cast....

  5. Delete the indicated words from 5.2.11 [expr.const.cast] paragraph 1:

    6. ...Types shall not be defined in a const_cast....

  6. Delete paragraph 5 of 5.3.3 [expr.sizeof]:

    7. Types shall not be defined in a sizeof expression.

  7. Delete paragraph 5 of 5.3.4 [expr.new]:

    8. The type-specifier-seq shall not contain class declarations, or enumeration declarations.

  8. Delete paragraph 4 of 5.3.6 [expr.alignof]:

    9. A type shall not be defined in an alignof expression.

  9. Delete paragraph 3 of 5.4 [expr.cast]:

    10. Types shall not be defined in casts.

  10. Delete the indicated words from 6.4 [stmt.select] paragraph 2:

    11. ...The type-specifier-seq shall not contain typedef and shall not declare a new class or enumeration....

  11. Add the indicated words to 7.1.6 [dcl.type] paragraph 3:

    12. At least one type-specifier that is not a cv-qualifier is required in a declaration unless it declares a constructor, destructor or conversion function. [Footnote: ... ] A type-specifier-seq shall not define a class or enumeration unless it appears in the type-id of an alias-declaration (7.1.3 [dcl.typedef]).

  12. Delete the indicated words from 12.3.2 [class.conv.fct] paragraph 1:

    13. ...Classes, enumerations, and typedef-names shall not be declared in the type-specifier-seq....

  13. Delete the indicated words from 15.3 [except.handle] paragraph 1:

    14. ...Types shall not be defined in an exception-declaration.

  14. Delete paragraph 6 of 15.4 [except.spec]:

    15. Types shall not be defined in exception-specifications.

[Drafting note: no changes are required to 5.1.1 [expr.prim.lambda], 7.1.3 [dcl.typedef], 7.1.7 [dcl.align], 7.2 [dcl.enum], 8.3.5 [dcl.fct], 14.1 [temp.param], or 14.2 [temp.names].]





Issues with "Review" Status

608. Determining the final overrider of a virtual function

Section: 10.3  [class.virtual]     Status: review     Submitter: Mike Miller     Date: 7 December 2006

According to 10.3 [class.virtual] paragraph 2:

Then in any well-formed class, for each virtual function declared in that class or any of its direct or indirect base classes there is a unique final overrider that overrides that function and every other overrider of that function. The rules for member lookup (10.2 [class.member.lookup]) are used to determine the final overrider for a virtual function in the scope of a derived class but ignoring names introduced by using-declarations.

I think that description is wrong on at least a couple of counts. First, consider the following example:

    struct A { virtual void f(); };
    struct B: A { };
    struct C: A { void f(); };
    struct D: B, C { };

What is the “unique final overrider” of A::f() in D? According to 10.3 [class.virtual] paragraph 2, we determine that by looking up f in D using the lookup rules in 10.2 [class.member.lookup]. However, that lookup determines that f in D is ambiguous, so there is no “unique final overrider” of A::f() in D. Consequently, because “any well-formed class” must have such an overrider, D must be ill-formed.

Of course, we all know that D is not ill-formed. In fact, 10.3 [class.virtual] paragraph 10 contains an example that illustrates exactly this point:

struct A {
    virtual void f();
};
struct B1 : A {     // note non-virtual derivation
    void f();
};
struct B2 : A {
    void f();
};
struct D : B1, B2 { // D has two separate A subobjects
};

In class D above there are two occurrences of class A and hence two occurrences of the virtual member function A::f. The final overrider of B1::A::f is B1::f and the final overrider of B2::A::f is B2::f.

It appears that the requirement for a “unique final overrider” in 10.3 [class.virtual] paragraph 2 needs to say something about sub-objects. Whatever that “something” is, you can't just say “look up the name in the derived class using 10.2 [class.member.lookup].”

There's another problem with using the 10.2 [class.member.lookup] lookup to specify the final overrider: name lookup just looks up the name, while the overriding relationship is based not only on the name but on a matching parameter-type-list and cv-qualification. To illustrate this point:

    struct X {
        virtual void f();
    };
    struct Y: X {
        void f(int);
    };
    struct Z: Y { };

What is the “unique final overrider” of X::f() in A? Again, 10.3 [class.virtual] paragraph 2 says you're supposed to look up f in Z to find it; however, what you find is Y::f(int), not X::f(), and that's clearly wrong.

Proposed Resolution (December, 2006):

Change 10.3 [class.virtual] paragraph 2 as follows:

Then in any well-formed class, for each virtual function declared in that class or any of its direct or indirect base classes there is a unique final overrider that overrides that function and every other overrider of that function. The rules for member lookup (10.2 [class.member.lookup]) are used to determine the final overrider for a virtual function in the scope of a derived class but ignoring names introduced by using-declaration s. A virtual member function vf of a class C is a final overrider unless the most derived class (1.8 [intro.object]) of which C is a base class (if any) declares or inherits another member function that overrides vf. In a derived class, if a virtual member function of a base class subobject has more than one final overrider, the program is ill-formed.


580. Access in template-parameters of member and friend definitions

Section: 11  [class.access]     Status: review     Submitter: John Spicer     Date: 16 May 2006

The resolution of issue 372 leaves unclear whether the following are well-formed or not:

    class C {
        typedef int I;                // private
        template <int> struct X;
        template <int> friend struct Y;
    }

    template <C::I> struct C::X { };  // C::I accessible to member?

    template <C::I> struct Y { };     // C::I accessible to friend?

Presumably the answer to both questions is “yes,” but the new wording does not address template-parameters.

Proposed resolution (June, 2008):

Change 11 [class.access] paragraph 6 as follows:

...For purposes of access control, the base-specifiers of a class, the template-parameters of a template-declaration, and the definitions of class members that appear outside of the class definition are considered to be within the scope of that class...


585. Friend template template parameters

Section: 11.4  [class.friend]     Status: review     Submitter: James Widman     Date: 15 June 2006

After the adoption of the wording for extended friend declarations, we now have this new paragraph in 11.4 [class.friend]:

A friend declaration that does not declare a function shall have one of the following forms:

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 11.4 [class.friend] paragraph 3 as follows:

A friend declaration that does not declare a function shall have one of the following forms:

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:...



462. Lifetime of temporaries bound to comma expressions

Section: 12.2  [class.temporary]     Status: review     Submitter: Steve Adamczyk     Date: April 2004

Split off from issue 86.

Should binding a reference to the result of a "," operation whose second operand is a temporary extend the lifetime of the temporary?

  const SFileName &C = ( f(), SFileName("abc") );

Notes from the March 2004 meeting:

We think the temporary should be extended.

Proposed resolution (October, 2004):

Change 12.2 [class.temporary] paragraph 2 as indicated:

... In all these cases, the temporaries created during the evaluation of the expression initializing the reference, except the temporary that is the overall result of the expression [Footnote: For example, if the expression is a comma expression (5.18 [expr.comma]) and the value of its second operand is a temporary, the reference is bound to that temporary.] and to which the reference is bound, are destroyed at the end of the full-expression in which they are created and in the reverse order of the completion of their construction...

[Note: this wording partially resolves issue 86. See also issue 446.]

Notes from the April, 2005 meeting:

The CWG suggested a different approach from the 10/2004 resolution, leaving 12.2 [class.temporary] unchanged and adding normative wording to 5.18 [expr.comma] specifying that, if the result of the second operand is a temporary, that temporary is the result of the comma expression as well.

Proposed Resolution (November, 2006):

Add the indicated wording to 5.18 [expr.comma] paragraph 1:

... The type and value of the result are the type and value of the right operand; the result is an lvalue if its right operand is an lvalue, and is a bit-field if its right operand is an lvalue and a bit-field. If the value of the right operand is a temporary (12.2 [class.temporary]), the result is that temporary.


650. Order of destruction for temporaries bound to the returned value of a function

Section: 12.2  [class.temporary]     Status: review     Submitter: Mike Miller     Date: 14 Aug 2007

In describing the order of destruction of temporaries, 12.2 [class.temporary] paragraphs 4-5 say,

There are two contexts in which temporaries are destroyed at a different point than the end of the full-expression...

The second context is when a reference is bound to a temporary... A temporary bound to the returned value in a function return statement (6.6.3 [stmt.return]) persists until the function exits.

The following example illustrates the issues here:

    struct S {
        ~S();
    };

    S& f() {
        S s;            // #1
        return
            (S(),       // #2
             S());      // #3
    }

If the return type of f() were simply S instead of S&, the two temporaries would be destroyed at the end of the full-expression in the return statement in reverse order of their construction, followed by the destruction of the variable s at block-exit, i.e., the order of destruction of the S objects would be #3, #2, #1.

Because the temporary #3 is bound to the returned value, however, its lifetime is extended beyond the end of the full-expression, so that S object #2 is destroyed before #3.

There are two problems here. First, it is not clear what “until the function exits” means. Does it mean that the temporary is destroyed as part of the normal block-exit destructions, as described in 6.6 [stmt.jump] paragraph 2:

On exit from a scope (however accomplished), destructors (12.4 [class.dtor]) are called for all constructed objects with automatic storage duration (3.7.2 [basic.stc.auto]) (named objects or temporaries) that are declared in that scope, in the reverse order of their declaration.

Or is the point of destruction for #3 after the destruction of the “constructed objects... that are declared [emphasis mine] in that scope” (because temporary #3 was not “declared”)? I.e., should #3 be destroyed before or after #1?

The other problem is that, according to the recollection of one of the participants responsible for this wording, the intent was not to extend the lifetime of #3 but simply to emphasize that its lifetime ended before the function returned, i.e., that the result of f() could not be used without causing undefined behavior. This is also consistent with the treatment of this example by many implementations; MSVC++, g++, and EDG all destroy #3 before #2.

Suggested resolution:

Change 12.2 [class.temporary] paragraph 5 as indicated:

A The lifetime of a temporary bound to the returned value in a function return statement (6.6.3 [stmt.return]) persists until the function exits is not extended; it is destroyed at the end of the full-expression in the return statement.

Proposed resolution (June, 2008):

Change 12.2 [class.temporary] paragraph 5 as follows (converting the running text into a bulleted list and making the indicated edits to the wording):

... The temporary to which the reference is bound or the temporary that is the complete object of a subobject to which the reference is bound persists for the lifetime of the reference except: as specified below.

The destruction of a temporary whose lifetime is not extended...



542. Value initialization of arrays of POD-structs

Section: 12.6  [class.init]     Status: review     Submitter: Alisdair Meredith     Date: 27 October 2005

12.6 [class.init] paragraph 2 says,

When an array of class objects is initialized (either explicitly or implicitly), the constructor shall be called for each element of the array, following the subscript order;

That implies that, given

    struct POD {
      int x;
    };

    POD data[10] = {};

this should call the implicitly declared default ctor 10 times, leaving 10 uninitialized ints, rather than value initialize each member of data, resulting in 10 initialized ints (which is required by 8.5.1 [dcl.init.aggr] paragraph 7).

I suggest rephrasing along the lines:

When an array is initialized (either explicitly or implicitly), each element of the array shall be initialized in turn, following the subscript order;

This would allow for PODs and other classes with a dual nature under value/default initialization, and cover copy initialization for arrays too.

Proposed resolution (October, 2006):

Change 12.6 [class.init] paragraph 3 as follows:

When an array of class objects is initialized (either explicitly or implicitly) and the elements are initialized by constructor, the constructor shall be called for each element of the array, following the subscript order; see 8.3.4 [dcl.array].


257. Abstract base constructors and virtual base initialization

Section: 12.6.2  [class.base.init]     Status: review     Submitter: Mike Miller     Date: 1 Nov 2000

Must a constructor for an abstract base class provide a mem-initializer for each virtual base class from which it is directly or indirectly derived? Since the initialization of virtual base classes is performed by the most-derived class, and since an abstract base class can never be the most-derived class, there would seem to be no reason to require constructors for abstract base classes to initialize virtual base classes.

It is not clear from the Standard whether there actually is such a requirement or not. The relevant text is found in 12.6.2 [class.base.init] paragraph 6:

All sub-objects representing virtual base classes are initialized by the constructor of the most derived class (1.8 [intro.object]). If the constructor of the most derived class does not specify a mem-initializer for a virtual base class V, then V's default constructor is called to initialize the virtual base class subobject. If V does not have an accessible default constructor, the initialization is ill-formed. A mem-initializer naming a virtual base class shall be ignored during execution of the constructor of any class that is not the most derived class.

This paragraph requires only that the most-derived class's constructor have a mem-initializer for virtual base classes. Should the silence be construed as permission for constructors of classes that are not the most-derived to omit such mem-initializers?

Christopher Lester, on comp.std.c++, March 19, 2004: If any of you reading this posting happen to be members of the above working group, I would like to encourage you to review the suggestion contained therein, as it seems to me that the final tenor of the submission is both (a) correct (the silence of the standard DOES mandate the omission) and (b) describes what most users would intuitively expect and desire from the C++ language as well.

The suggestion is to make it clearer that constructors for abstract base classes should not be required to provide initialisers for any virtual base classes they contain (as only the most-derived class has the job of initialising virtual base classes, and an abstract base class cannot possibly be a most-derived class).

For example:

struct A {
  A(const int i, const int j) {};
};

struct B1 : virtual public A {
  virtual void moo()=0;
  B1() {};   // (1) Look! not "B1() : A(5,6) {};"
};

struct B2 : virtual public A {
  virtual void cow()=0;
  B2() {};   // (2) Look! not "B2() : A(7,8) {};"
};

struct C : public B1, public B2 {
  C() : A(2,3) {};
  void moo() {};
  void cow() {};
};

int main() {
  C c;
  return 0;
};

I believe that, by not expressly forbidding it, the standard does (and should!) allow the above code. However, as the standard doesn't expressly allow it either (have I missed something?) there appears to be room for misunderstanding. For example, g++ version 3.2.3 (and maybe other versions as well) rejects the above code with messages like:

	In constructor `B1::B1()':
	no matching function for call to `A::A()'
	candidates are: A::A(const A&)
         	        A::A(int, int)

Fair enough, the standard is perhaps not clear enough. But it seems to be a shame that although this issue was first raised in 2000, we are still living with it today.

Note that we can work-around, and persuade g++ to compile the above by either (a) providing a default constructor A() for A, or (b) supplying default values for i and j in A(i,j), or (c) replace the construtors B1() and B2() with the forms shown in the two comments in the above example.

All three of these workarounds may at times be appropriate, but equally there are other times when all of these workarounds are particularly bad. (a) and (b) may be very bad if you are trying to enforce string contracts among objects, while (c) is just barmy (I mean why did I have to invent random numbers like 5, 6, 7 and 8 just to get the code to compile?).

So to to round up, then, my plea to the working group is: "at the very least, please make the standard clearer on this issue, but preferrably make the decision to expressly allow code that looks something like the above"

Proposed resolution (March, 2008):

  1. Add the indicated text (moved from paragraph 6) to the end of 12.6.2 [class.base.init] paragraph 3:

    2. ...The initialization of each base and member constitutes a full-expression. Any expression in a mem-initializer is evaluated as part of the full-expression that performs the initialization. A mem-initializer where the mem-initializer-id names a virtual base class is ignored during execution of a constructor of any class that is not the most derived class.

  2. Change 12.6.2 [class.base.init] paragraph 4 as follows:

    3. If a given non-static data member or base class is not named by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer) and the entity is not a virtual base class of an abstract class, then

    [Note: An abstract base class (10.4 [class.abstract]) is never a most derived class, thus its constructors never initialize virtual base classes, therefore the corresponding mem-initializers may be omitted. —end note] After the call to a constructor for class X has completed, if a member of X is neither specified in the constructor's mem-initializers, nor default-initialized, nor value-initialized, nor given a value during execution of the compound-statement of the body of the constructor, the member has indeterminate value.

  3. Change 12.6.2 [class.base.init] paragraph 5 as follows:

    4. Initialization shall proceeds in the following order:

    [Note: the declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization. —end note]

    [Drafting note: The “shall” clauses above were rewritten to accord with the usual phrasing throughout the rest of the Standard.]

  4. Remove all the normative text in 12.6.2 [class.base.init] paragraph 6, keeping the example:

    5. All subobjects representing virtual base classes are initialized by the constructor of the most derived class (1.8 [intro.object]). If the constructor of the most derived class does not specify a mem-initializer for a virtual base class V, then V's default constructor is called to initialize the virtual base class subobject. If V does not have an accessible default constructor, the initialization is ill-formed. A mem-initializer naming a virtual base class shall be ignored during execution of the constructor of any class that is not the most derived class. [Example:...


655. Initialization not specified for forwarding constructors

Section: 12.6.2  [class.base.init]     Status: review     Submitter: Alisdair Meredith     Date: 17 October 2007

The changes for delegating constructors overlooked the need to change 12.6.2 [class.base.init] paragraph 3:

The expression-list in a mem-initializer is used to initialize the base class or non-static data member subobject denoted by the mem-initializer-id. The semantics of a mem-initializer are as follows:

The initialization of each base and member constitutes a full-expression. Any expression in a mem-initializer is evaluated as part of the full-expression that performs the initialization.

This paragraph deals only with subobjects; it needs to be made more general to apply to the complete object as well when the mem-initializer-id designates the constructor's class.

Proposed resolution (June, 2008):

Change 12.6.2 [class.base.init] paragraph 3 as follows:

The expression-list in a mem-initializer is used to initialize the base class or non-static data member subobject denoted by the mem-initializer-id. The semantics of a mem-initializer are A mem-initializer in which the mem-initializer-id names the constructor's class initializes the object by invoking the selected target constructor with the mem-initializer's expression-list. A mem-initializer in which the mem-initializer-id names a base class or non-static data member initializes the designated subobject as follows:

...

The initialization of each base and member performed by each mem-initializer constitutes a full-expression. Any expression...



111. Copy constructors and cv-qualifiers

Section: 12.8  [class.copy]     Status: review     Submitter: Jack Rouse     Date: 4 May 1999

Jack Rouse: In 12.8 [class.copy] paragraph 8, the standard includes the following about the copying of class subobjects in such a constructor:

But there can be multiple copy constructors declared by the user with differing cv-qualifiers on the source parameter. I would assume overload resolution would be used in such cases. If so then the passage above seems insufficient.

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.



535. Copy construction without a copy constructor

Section: 12.8  [class.copy]     Status: review     Submitter: Mike Miller     Date: 7 October 2005

Footnote 112 (12.8 [class.copy] paragraph 2) says,

Because a template constructor is never a copy constructor, the presence of such a template does not suppress the implicit declaration of a copy constructor. Template constructors participate in overload resolution with other constructors, including copy constructors, and a template constructor may be used to copy an object if it provides a better match than other constructors.

However, many of the stipulations about copy construction are phrased to refer only to “copy constructors.” For example, 12.8 [class.copy] paragraph 14 says,

A program is ill-formed if the copy constructor... for an object is implicitly used and the special member function is not accessible (clause 11 [class.access]).

Does that mean that using an inaccessible template constructor to copy an object is permissible, because it is not a “copy constructor?” Obviously not, but each use of the term “copy constructor” in the Standard should be examined to determine if it applies strictly to copy constructors or to any constructor used for copying. (A similar issue applies to “copy assignment operators,” which have the same relationship to assignment operator function templates.)

Proposed Resolution (February, 2008):

  1. Change 3.2 [basic.def.odr] paragraph 2 as follows:

    2. ... [Note: this covers calls to named functions (5.2.2 [expr.call]), operator overloading (clause 13 [over]), user-defined conversions (12.3.2 [class.conv.fct]), allocation function for placement new (5.3.4 [expr.new]), as well as non-default initialization (8.5 [dcl.init]). A copy constructor selected to copy class objects is used even if the call is actually elided by the implementation (12.8 [class.copy]). —end note] ... A copy-assignment function for a class An assignment operator function in a class is used by an implicitly-defined copy-assignment function for another class as specified in 12.8 [class.copy]...

  2. Delete 12.1 [class.ctor] paragraphs 10 and 11:

    3. A copy constructor (12.8 [class.copy]) is used to copy objects of class type.

    A union member shall not be of a class type (or array thereof) that has a non-trivial constructor.

  3. Replace the “example” in 12.2 [class.temporary] paragraph 1 with a note as follows:

    4. [Example: even if the copy constructor is not called, all the semantic restrictions, such as accessibility (clause 11 [class.access]), shall be satisfied. —end example] [Note: This includes accessibility (clause 11 [class.access]) for the constructor selected. —end note]

  4. Change 12.8 [class.copy] paragraph 7 as follows:

    5. A non-user-provided copy constructor is implicitly defined if it is used to initialize an object of its class type from a copy of an object of its class type or of a class type derived from its class type (3.2 [basic.def.odr]). [Footnote: See 8.5 [dcl.init] for more details on direct and copy initialization. —end footnote] [Note: the copy constructor is implicitly defined even if the implementation elided its use (12.2 [class.temporary]) the copy operation (12.8 [class.copy]). —end note] A program is ill-formed if the class for which a copy constructor is implicitly defined or explicitly defaulted has:

    • a non-static data member of class type (or array thereof) with an inaccessible or ambiguous copy constructor, or

    • a base class with an inaccessible or ambiguous copy constructor.

    Before the non-user-provided copy constructor for a class is implicitly defined...

  5. Change 12.8 [class.copy] paragraph 8 as follows:

    6. ...Each subobject is copied in the manner appropriate to its type:

    [Drafting note: 8.5 [dcl.init] paragraph 15 requires “unambiguous” and 13.3 [over.match] paragraph 3 requires “accessible,” thus no need for normative text here.]

  6. Change 12.8 [class.copy] paragraph 12 as follows:

    7. A non-user-provided copy assignment operator is implicitly defined when an object of its class type is assigned a value of its class type or a value of a class type derived from its class type it is used (3.2 [basic.def.odr]). A program is ill-formed if the class for which a copy assignment operator is implicitly defined or explicitly defaulted has: a non-static data member of const or reference type.

    • a non-static data member of const type, or

    • a non-static data member of reference type, or

    • a non-static data member of class type (or array thereof) with an inaccessible copy assignment operator, or

    • a base class with an inaccessible copy assignment operator.

  7. Change 12.8 [class.copy] paragraph 13 as follows:

    8. ... Each subobject is assigned in the manner appropriate to its type:

  8. Delete 12.8 [class.copy] paragraph 14:

    9. A program is ill-formed if the copy constructor or the copy assignment operator for an object is implicitly used and the special member function is not accessible (clause 11 [class.access]). [Note: Copying one object into another using the copy constructor or the copy assignment operator does not change the layout or size of either object. —end note]

  9. Change 12.8 [class.copy] paragraph 15 as follows:

    10. When certain criteria are met, an implementation is allowed to omit the copy construction of a class object, even if the copy constructor selected for the copy operation and/or the destructor for the object have side effects. In such cases, the implementation treats the source and target of the omitted copy operation as simply two different ways of referring to the same object, and the destruction of that object occurs at the later of the times when the two objects would have been destroyed without the optimization. [Footnote: Because only one object is destroyed instead of two, and one copy constructor is not executed, there is still one object destroyed for each one constructed. —end footnote] This elision...

  10. Change 13.3.3.1.2 [over.ics.user] paragraph 4 as follows:

    11. A conversion of an expression of class type to the same class type is given Exact Match rank, and a conversion of an expression of class type to a base class of that type is given Conversion rank, in spite of the fact that a copy constructor (i.e., a user-defined conversion function) is called for those cases.

  11. Change 15.1 [except.throw] paragraph 3 as follows:

    12. A throw-expression initializes a temporary object, called the exception object, the type of which by copy-initialization (8.5 [dcl.init]). The type of that temporary object is determined...

  12. Change 15.1 [except.throw] paragraph 5 as follows:

    13. When the thrown object is a class object, the copy constructor selected for the copy-initialization and the destructor shall be accessible, even if the copy operation is elided (12.8 [class.copy]).

  13. Change 15.3 [except.handle] paragraphs 16-17 as follows:

    14. When the exception-declaration specifies a class type, a copy constructor copy-initialization (8.5 [dcl.init]) is used to initialize either the object declared in the exception-declaration or, if the exception-declaration does not specify a name, a temporary object of that type. The object shall not have an abstract class type. The object is destroyed when the handler exits, after the destruction of any automatic objects initialized within the handler. The copy constructor selected for the copy-initialization and the destructor shall be accessible in the context of the handler, even if the copy operation is elided (12.8 [class.copy]). If the copy constructor and destructor are implicitly declared (12.8 [class.copy]), such a use in the handler causes these functions to be implicitly defined; otherwise, the program shall provide a definition for these functions.

    The copy constructor and destructor associated with the object shall be accessible even if the copy operation is elided (12.8 [class.copy]).

  14. Change the footnote in 15.5.1 [except.terminate] paragraph 1 as follows:

    15. [Footnote: For example, if the object being thrown is of a class with a copy constructor type, std::terminate() will be called if that copy constructor the constructor selected to copy the object exits with an exception during a throw. —end footnote]

(This resolution also resolves issue 111.)

[Drafting note: The following do not require changes: 5.17 [expr.ass] paragraph 4; 9 [class] paragraph 5; 9.5 [class.union] paragraph 1; 12.2 [class.temporary] paragraph 2; 12.8 [class.copy] paragraphs 1-2; 15.4 [except.spec] paragraph 14.]

Notes from February, 2008 meeting:

These changes overlap those that will be made when concepts are added. This issue will be maintained in “review” status until the concepts proposal is adopted and any conflicts will be resolved at that point.



574. Definition of “copy assignment operator”

Section: 12.8  [class.copy]     Status: review     Submitter: Steve Adamczyk     Date: 15 April 2006

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.]



653. Copy assignment of unions

Section: 12.8  [class.copy]     Status: review     Submitter: Jens Maurer     Date: 3 October 2007

How does copy assignment for unions work? For example,

  union U {
    int a;
    float b;
  };

  void f() {
    union U u = { 5 };
    union U v;
    v = u;    // what happens here?
  }

9.5 [class.union] is silent on the issue, therefore it seems that 12.8 [class.copy] applies. There is no special case for unions, thus paragraph 13 (memberwise assignment of subobjects) seems to apply. That would seem to imply these actions in the compiler-generated copy assignment operator:

  v.a = u.a;
  v.b = u.b;

And this is just wrong. For example, the lifetime of v.a ends once the second assignment reuses the memory of v.a.

We should probably prescribe “memcpy” copying for unions (both for the copy constructor and the assignment operator) unless the user provided his own special member function.

Proposed resolution (March, 2008):

  1. Change 12.8 [class.copy] paragraph 8 as follows:

    2. The implicitly-defined or explicitly-defaulted copy constructor for a non-union class X performs a memberwise copy of its subobjects...

  2. Add a new paragraph after 12.8 [class.copy] paragraph 8:

    3. The implicitly-defined or explicitly-defaulted copy constructor for a union X where all members have a trivial copy constructor copies the object representation (3.9 [basic.types]) of X. [Note: The behavior is undefined if X is not a trivial type. —end note]

  3. Change 12.8 [class.copy] paragraph 13 as follows:

    4. The implicitly-defined or explicitly-defaulted copy assignment operator for a non-union class X performs memberwise assignment of its subobjects...

  4. Add a new paragraph after 12.8 [class.copy] paragraph 13:

    5. The implicitly-defined or explicitly-defaulted copy assignment operator for a union X where all members have a trivial copy assignment operator copies the object representation (3.9 [basic.types]) of X. [Note: The behavior is undefined if X is not a trivial type. —end note]


667. Trivial special member functions that cannot be implicitly defined

Section: 12.8  [class.copy]     Status: review     Submitter: James Widman     Date: 14 December 2007

Should the following class have a trivial copy assignment operator?

    struct A {
        int& m;
        A();