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

2821. Lifetime, zero-initialization, and dynamic initialization

Consider:

  int i = i;

According to 6.9.3.2 [basic.start.static] paragraph 2, i is zero-initialized and then dynamic initialization is applied. However, it is unclear whether the lifetime of i has started at the point when the dynamic initialization occurs. According to 6.7.4 [basic.life] paragraph 1:

... The lifetime of an object of type T begins when:

• storage with the proper alignment and size for type T is obtained, and
• its initialization (if any) is complete (including vacuous initialization) (9.4 [dcl.init]),

except ...

Possible directions:

• Zero-initialization starts the lifetime, and the dynamic initialization just modifies the value, similar to a delegating constructor.
• The lifetime doesn't begin until the dynamic initialization is complete, but we allow scalar objects to be "under construction" (11.9.5 [class.cdtor]).
• Zero-initialization starts the lifetime, and dynamic initialization transparently replaces the object.

The example in 6.9.3.2 [basic.start.static] paragraph 3 suggests the first option.

CWG 2024-01-19

The first option is is intended.

Possible resolution:

Split and change 6.7.4 [basic.life] paragraph 1 as follows:

... The lifetime of an object of type T begins when:

• storage with the proper alignment and size for type T is obtained, and
• its any initialization (if any) is complete (including vacuous initialization) (9.4 [dcl.init]),

except that if the object is a union member or subobject thereof, its lifetime only begins if that union member is the initialized member in the union (9.4.2 [dcl.init.aggr], 11.9.3 [class.base.init]), or as described in 11.5 [class.union], 11.4.5.3 [class.copy.ctor], and 11.4.6 [class.copy.assign], and except as described in 20.2.10.2 [allocator.members]. [ Note: In particular, the lifetime of a complete class object begins when the non-delegating constructor returns (11.9.3 [class.base.init]). The lifetime of a complete object with static or thread storage duration begins when static initialization is complete (6.9.3.2 [basic.start.static]). -- end note ]

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