stacks are implemented as containers adaptors, which are classes that use an encapsulated object of a specific container class as its underlying container, providing a specific set of member functions to access its elements. Elements are pushed/popped from the "back" of the specific container, which is known as the top of the stack.
The underlying container may be any of the standard container class templates or some other specifically designed container class. The only requirement is that it supports the following operations: