This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-08-11

3043. Lifetime extension for temporaries in expansion statements

(From submission #724.)

Subclause 6.8.7 [class.temporary] paragraph 7 and paragraph 8 specify:

The fourth context is when a temporary object is created in the for-range-initializer of either a range-based for statement or an enumerating expansion statement (8.7 [stmt.expand]). If such a temporary object would otherwise be destroyed at the end of the for-range-initializer full-expression, the object persists for the lifetime of the reference initialized by the for-range-initializer.

The fifth context is when a temporary object is created in the expansion-initializer of an iterating or destructuring expansion statement. If such a temporary object would otherwise be destroyed at the end of that expansion-initializer , the object persists for the lifetime of the reference initialized by the expansion-initializer, if any.

There are a number of problems with this phrasing:

• The original idea of lifetime extension in a for-range-initializer was to ensure that temporaries defining a range would survive across all iterations. For an enumerating expansion statement, there is no overarching range expression; each element of the expression-list is considered separately. Furthermore, there is no for-range-initializer in the corresponding grammar to start with.
• For an iterating expansion statement, the involved variables are constexpr, so no dangling pointers or similar could escape from the constant evaluation to start with. The necessity for temporary lifetime extension is thus unclear.

Possible resolution:

Change in 6.8.7 [class.temporary] paragraph 7 and paragraph 8 as follows:

The fourth context is when a temporary object is created in the for-range-initializer of either a range-based for statement or an enumerating expansion statement (8.7 [stmt.expand]). If such a temporary object would otherwise be destroyed at the end of the for-range-initializer full-expression, the object persists for the lifetime of the reference initialized by the for-range-initializer.

The fifth context is when a temporary object is created in the expansion-initializer of an iterating or a destructuring expansion statement. If such a temporary object would otherwise be destroyed at the end of that expansion-initializer, the object persists for the lifetime of the reference initialized by the expansion-initializer, if any.