This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2025-04-13
1502. Value initialization of unions with member initializersSection: 9.5 [dcl.init] Status: CD3 Submitter: Richard Smith Date: 2012-05-06[Moved to DR at the April, 2013 meeting.]
According to 9.5 [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 9.5 [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;
...
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