Is GCC\'s Destruction of a Returned `std::initializer_list` Array Correct?

Lifetime of Returned std::initializer_list Array

The lifetime of an std::initializer_list array returned from a function has sparked discussion regarding the behavior in different compilers. The question arises whether GCC's destruction of the array at the end of the return full-expression is correct.

Analysis of the Standard

According to C 11 §6.6.3/2, a return statement with a braced-init-list leads to the returned object or reference being copy-list-initialized from the initializer list. This process initializes a temporary initializer_list object and its underlying array storage from the initializer list, and then another initializer_list is initialized from the first.

The lifetime of the array, as defined in 8.5.4/6, matches the lifetime of the initializer_list object. This can be understood through an example where the initializer_list object and array have a full-expression lifetime for a std::vector initialized with a braced-init-list.

Return Value of a Braced-Init-List

When a bare list enclosed in braces is returned, copy-list-initialization is used. This is similar to the syntax nocopy X = { 3 }, which does not involve a copy and is identical to the example in 8.5.4/6 where the array's lifetime is extended.

Issues with GCC and Clang

However, there is a discrepancy in implementation between GCC and Clang. GCC destroys the array before returning, while Clang preserves a named initializer_list but never destroys the objects in the list.

Recommendation

It is advised to avoid using std::initializer_list to pass values around with the expectation of them persisting. Uncertainties in the standard and compiler implementations make it unreliable for preserving object lifetime.

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