// File: C++Examples/Stacks/stack.h 
#ifndef __stack
#define __stack
#include <iostream>

// This is the abstract class 
class stack {
public:
   stack () {}
   virtual bool is_empty(void) const // = 0;
   {std::cout <<
   "Subclass must implement is_empty" << std::endl;
	   return true;}
   virtual void push(int) // = 0;
   {std::cout << 
   "Subclass must implement push" << std::endl;
   }
   virtual int  pop(void) // = 0;
   // concept demo'd: pure virtual member functions
   {std::cout << 
   "Subclass must implement pop" << std::endl;
   return 0;
   }


   virtual stack* clone(void) const // = 0;
   // concept demo'd: copy of abstract object
   //                 ("clone" is the Java copy)
   {std::cout << 
    "Subclass must implement clone" << std::endl;
    return new stack(*this); // return something reasonable
   }


   virtual ~stack() {};
   // concept demo'd: virtual destructor
};

std::ostream& operator<<(std::ostream&, const stack&);
#endif