c++ - 为什么没有 std::uses_allocator<std::optional>？

Question

对于 std::variant 有 uses_allocator 的特化，但对于 std::optional 没有。为什么？

Answer 1

来自Standard 23.6.3.1 :

Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value.

因此不需要分配器。

尽管变体 23.7.3.1 也是如此:

Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the contained value.

显然，uses_allocator 专门用于 std::variant was removed for C++17 (HT:@K-ballo!)。