std::experimental::promise object, using the supplied
allocator to allocate memory for the associated asynchronous result.
template<typename Allocator> promise(std::allocator_arg_t, Allocator const& alloc);
std::experimental::promise instance with
an associated asynchronous result of type
that is not ready. The memory for the asynchronous
result is allocated through the allocator
Any exception thrown by the allocator when attempting to allocate memory for the asynchronous result.