Implementing a simple RPN calculator with std::stack

The std::stack is an adapter class which lets the user push objects onto it like on a real stack of objects, and pop objects down from it again. In this section, we construct a reverse polish notation (RPN) calculator around that data structure, in order to show how to use it.

The RPN is a notation that can be used to express mathematical expressions in a way that is really simple to parse. In RPN, 1 + 2 is 1 2 +. Operands first, then the operation. Another example: (1 + 2) * 3 would be 1 2 + 3 * in RPN and that already shows why it is easier to parse, as we do not need parentheses to define subexpressions.