Movatterモバイル変換

[Voted into the WP at the February, 2012 meeting;moved to DR at the October, 2012 meeting.]

Proposed resolution (August, 2011):

1. Change 9.5 [dcl.init] paragraph 7 as follows:

Tovalue-initialize an object of typeT means:

• ifT is a (possibly cv-qualified) class type(Clause 11 [class]) witheither no defaultconstructor (11.4.5 [class.ctor]) or adefaultconstructor that is user-providedor deletedconstructor (11.4.5 [class.ctor]), then theobject is default-initializeddefault constructor forT is called (and the initialization is ill-formed ifT has no accessible default constructor);

• ifT is a (possibly cv-qualified) non-union class typewithout a user-providedor deleted default constructor,then the object is zero-initialized and, ifT'simplicitly-declared default constructor isT hasa non-trivialdefault constructor,thatconstructor is called.default-initialized;

• ...

2. Change 9.5.5 [dcl.init.list] paragraph 3 as follows:

List-initialization of an object or reference of typeT isdefined as follows:

• If the initializer list has no elements andTis a class type with a default constructor, the object isvalue-initialized.

• Otherwise, ifIfT is anaggregate, aggregate initialization is performed (9.5.2 [dcl.init.aggr]). [Example:...

• Otherwise, if the initializer list has no elements andT is a class type with a default constructor, the object isvalue-initialized.

• ...

This resolution also resolves issues1324and1368.

According to 9.5 [dcl.init] paragraph 7,

Tovalue-initialize an object of typeT means:

• ifT is a (possibly cv-qualified) class type(Clause 11 [class]) with a user-provided constructor(11.4.5 [class.ctor]), then the default constructor forT is called (and the initialization is ill-formed ifT has no accessible default constructor);

• ifT is a (possibly cv-qualified) non-union class typewithout a user-provided constructor, then the object iszero-initialized and, ifT's implicitly-declared defaultconstructor is non-trivial, that constructor is called.

• ifT is an array type, then each element isvalue-initialized;

• otherwise, the object is zero-initialized.

This suggests that for

  struct A { A() = delete; };  union B { A a };  int main()  {    B();  }

aB temporary is created and zero-initialized, even thoughits default constructor is deleted. We should strike "non-union" andalso the "if...non-trivial" condition, since we can have a trivialdeleted constructor.