Программа, которая переводит арифметические выражения в postfix notation используя Stack

// FILE: stack2.h 
// TEMPLATE CLASS PROVIDED: stack<Item> (a stack of items)
//   The template parameter, Item, is the data type of the items in the stack,
//   also defined as stack<Item>::value_type.
//   It may be any of the C++ built-in types (int, char, etc.), or a class
//   with a default constructor, a copy constructor, and an assignment
//   operator. The definition stack<Item>::size_type is the data type of
//   any variable that keeps track of how many items are in a stack.
//
// CONSTRUCTOR for the stack<Item> template class:
//   stack( )
//     Postcondition: The stack has been initialized as an empty stack.
//
// MODIFICATION MEMBER FUNCTIONS for the stack<Item> class:
//   void push(const Item& entry)
//     Precondition: size( ) < CAPACITY.
//     Postcondition: A new copy of entry has been pushed onto the stack.
//
//   void pop( )
//     Precondition: size( ) > 0.
//     Postcondition: The top item of the stack has been removed.
//
// CONSTANT MEMBER FUNCTIONS for the stack<Item> class:
//   bool empty( ) const
//     Postcondition: Return value is true if the stack is empty.
//
//   size_type size( ) const
//     Postcondition: Return value is the total number of items in the stack.
//
//   Item top( ) const
//     Precondition: size( ) > 0.
//     Postcondition: The return value is the top item of the stack (but the
//     stack is unchanged. This differs slightly from the STL stack (where
//     the top function returns a reference to the item on top of the stack).
//
// VALUE SEMANTICS for the stack<Item> class:
//   Assignments and the copy constructor may be used with stack<Item>
//   objects.
//
// DYNAMIC MEMORY USAGE by the stack<Item> template class:
//   If there is insufficient dynamic memory, then the following functions
//   throw bad_alloc:
//   the copy constructor, push, and the assignment operator.
 
#ifndef MAIN_SAVITCH_STACK2_H
#define MAIN_SAVITCH_STACK2_H
#include <cstdlib>   // Provides NULL and size_t
#include "node2.h"   // Node template class from Figure 6.5 on page 308
 
namespace main_savitch_7B
{
    template <class Item>
    class stack
    {
    public:
        // TYPEDEFS 
        typedef std::size_t size_type;
        typedef Item value_type;
        // CONSTRUCTORS and DESTRUCTOR
        stack( ) { top_ptr = NULL; }
        stack(const stack& source);
        ~stack( ) { list_clear(top_ptr); }
        // MODIFICATION MEMBER FUNCTIONS
        void push(const Item& entry);
        void pop( );
        void operator =(const stack& source);
        // CONSTANT MEMBER FUNCTIONS
        size_type size( ) const
        { return main_savitch_6B::list_length(top_ptr); }
        bool empty( ) const { return (top_ptr == NULL); }
        Item top( ) const;
    private:
        main_savitch_6B::node<Item> *top_ptr;  // Points to top of stack
    };
}
 
#include "stack2.template" // Include the implementation 
#endif

// FILE: stack2.template
// TEMPLATE CLASS IMPLEMENTED: stack<Item> (see stack2.h for documentation)
// This file is included in the header file, and not compiled separately.
// INVARIANT for the stack class:
//   1. The items in the stack are stored in a linked list, with the top of the
//      stack stored at the head node, down to the bottom of the stack at the
//      final node.
//   2. The member variable top_ptr is the head pointer of the linked list.
 
#include <cassert>  // Provides assert
#include "node2.h"  // Node template class
 
namespace main_savitch_7B
{
    template <class Item>
    stack<Item>::stack(const stack<Item>& source) 
    // Library facilities used: node2.h
    {
        main_savitch_6B::node<Item> *tail_ptr; // Needed for argument of list_copy
 
        list_copy(source.top_ptr, top_ptr, tail_ptr);
    }
 
    template <class Item>
    void stack<Item>::push(const Item& entry) 
    // Library facilities used: node2.h
    {
        list_head_insert(top_ptr, entry);
    }
 
    template <class Item>
    void stack<Item>::pop( )
    // Library facilities used: cassert, node2.h
    {
        assert(!empty( ));
        list_head_remove(top_ptr);
    }
    
    template <class Item>
    void stack<Item>::operator =(const stack<Item>& source) 
    // Library facilities used: node2.h
    {
        main_savitch_6B::node<Item> *tail_ptr; // Needed for argument of list_copy
 
        if (this == &source) // Handle self-assignment
            return;
 
        list_clear(top_ptr);
        list_copy(source.top_ptr, top_ptr, tail_ptr);
    }
 
    template <class Item>
    Item stack<Item>::top( ) const 
    // Library facilities used: cassert
    {
        assert(!empty( ));
        return top_ptr->data( );
    }
}