This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

928. Defaulting a function that would be implicitly defined as deleted

[Voted into WP at October, 2009 meeting.]

9.5 [dcl.fct.def] paragraph 9 says,

A special member function that would be implicitly defined as deleted shall not be explicitly defaulted.

It would be more regular (and thus useful in generic programming) if such a member function were itself simply defined as deleted rather than being made ill-formed.

Proposed resolution (July, 2009):

1. Change 9.5 [dcl.fct.def] paragraph 9 as follows:

Only special member functions may be explicitly defaulted, and the implementation shall define them as if they had implicit definitions (11.4.5 [class.ctor], 11.4.7 [class.dtor], 11.4.5.3 [class.copy.ctor]). A special member function that would be implicitly defined as deleted shall not be explicitly defaulted. A special member function that would be implicitly defined as deleted may be explicitly defaulted only on its first declaration, in which case it is defined as deleted. A special member function is user-provided if...

2. Change 11.4.5 [class.ctor] paragraph 6 as follows:

A non-user-provided default constructor for a class is implicitly defined when it is used (6.3 [basic.def.odr]) to create an object of its class type (6.7.2 [intro.object]). If the implicitly-defined default constructor is explicitly defaulted but the corresponding implicit declaration would have been deleted, the program is ill-formed. The implicitly-defined or explicitly-defaulted default constructor...

3. Change 11.4.7 [class.dtor] paragraph 4 as follows:

A program is ill-formed if the class for which a destructor is implicitly defined or explicitly defaulted has: if the implicitly-defined destructor is explicitly defaulted, but the corresponding implicit declaration would have been deleted.

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

• a base class with an inaccessible destructor.

4. Change 11.4.5.3 [class.copy.ctor] paragraph 7 as follows:

...[Note: the copy constructor is implicitly defined even if the implementation elided its use (6.7.7 [class.temporary]). —end note] A program is ill-formed if the implicitly-defined copy constructor is explicitly defaulted, but the corresponding implicit declaration would have been deleted.

5. Change 11.4.5.3 [class.copy.ctor] paragraph 12 as follows:

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. A program is ill-formed if the implicitly-defined copy assignment operator is explicitly defaulted, but the corresponding implicit declaration would have been deleted.