(a)
class Stack {
public:
   virtual void push(int x) = 0;
   virtual int pop() = 0;
};
void foo(Stack& x) {
   // use push() and pop()...
}

(b)
template <class Stack>
void bar(Stack& x) {
   // use push and pop...
}

Example 1: (a) Using an object-oriented approach to describe the stack requirements; (b) a function template.