If `Allocator::select_on_container_copy_construction` exists and has the right signature, the allocator will be constructed from its result.
If若 `Allocator::select_on_container_copy_construction` exists and has the right signature, the allocator will be constructed from its result.存在且签名正确,则分配器将由其返回值构造。
If若 `Allocator::select_on_container_copy_construction`exists and has the right signature, the allocator will be constructed from its result.存在且签名正确,则分配器将由其返回值构造。