This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-10-26

1502. Value initialization of unions with member initializers

[Moved to DR at the April, 2013 meeting.]

According to 9.4 [dcl.init] paragraph 7,

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type ( Clause 11 [class]) with either no default constructor (11.4.5 [class.ctor]) or a default constructor that is user-provided or deleted, then the object is default-initialized;

• if T is a (possibly cv-qualified) non-union class type without a user-provided or deleted default constructor, then the object is zero-initialized and, if T has a non-trivial default constructor, default-initialized;

• if T is an array type, then each element is value-initialized;

• otherwise, the object is zero-initialized.

In an example like

  union U { int a = 5; };
  int main() { return U().a; }

this means that the value returned is 0, because none of the first three bullets apply. Should the “non-union” restriction be dropped from the second bullet?

Proposed resolution (October, 2012):

Change 9.4 [dcl.init] paragraph 7 as follows:

To value-initialize an object of type T means:

• ...

• if T is a (possibly cv-qualified) non-union class type without a user-provided or deleted default constructor, then the object is zero-initialized and, if T has a non-trivial default constructor, default-initialized;

• ...