1、相同點：
Allocators having the same type were assumed to be equal so that memory allocated by one
allocator could be deallocated by another allocator of the same type.
It was not possible to change the memory resource at runtime.
Allocators had limitations in the ability to hold a state, such as bookkeeping information or
information about where to allocate next.
2、區別：
Allocators were not swapped when the containers were swapped.
Allocators were not handled consistently, because copy constructors copied allocators, whereas
the assignment operator did not. Thus, the copy constructor did not have the same effect as
calling the default constructor and the assignment operator afterward.
