The following code example is taken from the book
C++ Templates - The Complete Guide, 2nd Edition
by David Vandevoorde, Nicolai M. Josuttis, and Douglas Gregor,
Addison-Wesley, 2017
© Copyright David Vandevoorde, Nicolai M. Josuttis, Douglas Gregor 2017
template<typename T>
class Stack {
private:
std::deque<T> elems; // elements
public:
void push(T const&); // push element
void pop(); // pop element
T const& top() const; // return top element
bool empty() const { // return whether the stack is empty
return elems.empty();
}
// assign stack of elements of type T2
template<typename T2>
Stack& operator= (Stack<T2> const&);
// to get access to private members of \TStack<T2> for any type T2:
template<typename> friend class Stack;
};