为什么vector的移动ctor没有推导出noexcept（）？

Question

Why move constructor for std::vector with custom allocator does not deduce a noexcept() from allocator's behaviours?

This leads to the class that encapsulates such vector cannot form the (other) vector that can be normally moved in some <algorithm> s. Even if the underlying type meets the nessesary requirements (MoveInsertable and DefaultInsertable).

Answer 1

I assume that by "move constructor for std::vector with custom allocator" you mean the allocator-extended move constructor ie this constructor:

vector(vector&& v, const allocator_type& a);

The main reason is that if v.get_allocator() != a then the constructor must allocate more memory, which could throw bad_alloc . There is no way to know at compile-time if two allocators of a given type will always compare equal or not (I have reported this as a defect, see LWG 2108 ).

NB the standard does not require this constructor or the vector(vector&&) move constructor to be noexcept .