Nested template type : template parameter « template « C++ Tutorial

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2. template
3. template parameter

/* The following code example is taken from the book
 * "C++ Templates - The Complete Guide"
 * by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002
 *
 * (C) Copyright David Vandevoorde and Nicolai M. Josuttis 2002.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.
 */
#include <iostream>
#include <string>
#include <cstdlib>
#include <vector>



#include <deque>
#include <stdexcept>
#include <memory>

template <typename T,
          template <typename ELEM, 
                    typename = std::allocator<ELEM> >
                    class CONT = std::deque>
class Stack {
  private:
    CONT<T> elems;         // elements

  public:
    void push(T const&);   // push element
    void pop();            // pop element
    T 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, 
             template<typename ELEM2, 
                      typename = std::allocator<ELEM2>
                     >class CONT2>
    Stack<T,CONT>& operator= (Stack<T2,CONT2> const&);
};

template <typename T, template <typename,typename> class CONT>
void Stack<T,CONT>::push (T const& elem)
{
    elems.push_back(elem);    // append copy of passed elem
}

template<typename T, template <typename,typename> class CONT>
void Stack<T,CONT>::pop ()
{
    if (elems.empty()) {
        throw std::out_of_range("Stack<>::pop(): empty stack");
    }
    elems.pop_back();         // remove last element
}

template <typename T, template <typename,typename> class CONT>
T Stack<T,CONT>::top () const
{
    if (elems.empty()) {
        throw std::out_of_range("Stack<>::top(): empty stack");
    }
    return elems.back();      // return copy of last element
}

template <typename T, template <typename,typename> class CONT>
 template <typename T2, template <typename,typename> class CONT2>
Stack<T,CONT>&
Stack<T,CONT>::operator= (Stack<T2,CONT2> const& op2)
{
    if ((void*)this == (void*)&op2) {    // assignment to itself?
        return *this;
    }

    Stack<T2,CONT2> tmp(op2);        // create a copy of the assigned stack

    elems.clear();                   // remove existing elements
    while (!tmp.empty()) {           // copy all elements
        elems.push_front(tmp.top());
        tmp.pop();
    }
    return *this;
}


int main()
{
    try {
        Stack<int>   intStack;       // stack of ints
        Stack<float> floatStack;     // stack of floats

        // manipulate int stack
        intStack.push(42);
        intStack.push(7);

        // manipulate float stack
        floatStack.push(7.7);

        // assign stacks of different type
        floatStack = intStack;

        // print float stack
        std::cout << floatStack.top() << std::endl;
        floatStack.pop();
        std::cout << floatStack.top() << std::endl;
        floatStack.pop();
        std::cout << floatStack.top() << std::endl;
        floatStack.pop();
    }
    catch (std::exception const& ex) {
        std::cerr << "Exception: " << ex.what() << std::endl;
    }

    // stack for ints using a vector as an internal container
    Stack<int,std::vector> vStack;
    //...
    vStack.push(42);
    vStack.push(7);
    std::cout << vStack.top() << std::endl;
    vStack.pop();
}

7
42
Exception: Stack<>::top(): empty stack
7