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

2850. Unclear storage duration for function parameter objects

[Accepted as a DR at the March, 2024 meeting.]

(From submission #490.)

Function parameter objects have automatic storage duration and are not temporary objects (see also issue 2849). However, it is unclear how long the storage for function parameter objects lasts.

Furthermore, for temporary objects that are destroyed at the end of the full-expression, it is unclear how the destruction is ordered with respect to temporary objects destroyed at the end of the full-expression.

Proposed resolution (approved by CWG 2024-03-20):

1. Change in and combine 6.7.6.4 [basic.stc.auto] paragraph 1 and 2 as follows:

   Variables that belong to a block or parameter scope and are not explicitly declared static, thread_local, or extern have automatic storage duration. The storage for these entitiessuch variables lasts until the block in which they are created exits. [Note 1: These variables are initialized and destroyed as described in 8.8 [stmt.dcl]. -- end note]

   Variables that belong to a parameter scope also have automatic storage duration. The storage for a function parameter lasts until immediately after its destruction (7.6.1.3 [expr.call]).

2. Change in 6.7.7 [class.temporary] paragraph 8 as follows:

   The destruction of a temporary whose lifetime is not extended beyond the full-expression in which it was created is sequenced before the destruction of every temporary which is constructed earlier in the same full-expression. Let x and y each be either a temporary object whose lifetime is not extended, or a function parameter. If the lifetimes of x and y end at the end of the same full-expression, and x is initialized before y, then the destruction of y is sequenced before that of x. If the lifetime of two or more temporaries with lifetimes extending beyond the full-expressions in which they were created ends at the same point, these temporaries are destroyed at that point in the reverse order of the completion of their construction. In addition, the destruction of such temporaries shall take into account the ordering of destruction of objects with static, thread, or automatic storage duration (6.7.6.2 [basic.stc.static], 6.7.6.3 [basic.stc.thread], 6.7.6.4 [basic.stc.auto]); that is, if obj1 is an object with the same storage duration as the temporary and created before the temporary is created the temporary shall be destroyed before obj1 is destroyed; if obj2 is an object with the same storage duration as the temporary and created after the temporary is created the temporary shall be destroyed after obj2 is destroyed.

3. Change in 7.6.1.3 [expr.call] paragraph 6 as follows:

   ... It is implementation-defined whether the lifetime of a parameter ends is destroyed when the function in which it is defined returns exits (8.7.4 [stmt.return], 14.3 [except.ctor]) or at the end of the enclosing full-expression; parameters are always destroyed in the reverse order of their construction. The initialization and destruction of each parameter occurs within the context of the full-expression (6.9.1 [intro.execution]) where the function call appears.

4. Change in 8.8 [stmt.dcl] paragraph 2 as follows:

   A block variable with automatic storage duration (6.7.6.4 [basic.stc.auto]) is active everywhere in the scope to which it belongs after its init-declarator . Upon each transfer of control (including sequential execution of statements) within a function from point P to point Q, all block variables with automatic storage duration that are active at P and not at Q are destroyed in the reverse order of their construction. Then, all block variables with automatic storage duration that are active at Q but not at P are initialized in declaration order; unless all such variables have vacuous initialization (6.7.4 [basic.life]), the transfer of control shall not be a jump. [ Footnote: ... ] When a declaration-statement is executed, P and Q are the points immediately before and after it; when a function returns, Q is after its body.