// file:    version4.cc
// author:  keller
// purpose: demonstrating use of virtual methods
//
// description:
// There is a base class and two derived classes.  
// Each overloads a method 'op'.
//
// In this version, pure virtual is used (signified by = 0) in the base
// class.  This version works, because we don't try to instantiate any
// base objects directly; all objects are created by virtue of instantiating 
// derived classes.

#include <iostream>
#include <string>

class Base
{
public:

virtual string op() = 0;		// "pure virtual"
};


class Derived_1 : public Base
{
public:

string op()
  {
  return "op() of Derived_1";
  }
};


class Derived_2 : public Base
{
public:

string op()
  {
  return "op() of Derived_2";
  }
};


// Note that the test objects are passed by reference.  If we passed the 
// parameter to testAsBase by value, a copy constructor would get called, 
// which would change the result, since only the base portion would get copied,
// and the base method would always get called.

void testAsBase(Base &object)
  {
  cout << "testAsBase: " << object.op() << endl;
  }

void testAsDerived_1(Derived_1 &object)
  {
  cout << "testAsDerived_1: " << object.op() << endl;
  }

void testAsDerived_2(Derived_2 &object)
  {
  cout << "testAsDerived_2: " << object.op() << endl;
  }


main()
  {
  Derived_1 ob_1;
  Derived_2 ob_2;

  testAsBase(ob_1);
  testAsBase(ob_2);

  testAsDerived_1(ob_1);
  testAsDerived_2(ob_2);
  }


/* output of version 4:			output of version 2:

					testAsBase: op() of Base
testAsBase: op() of Derived_1		testAsBase: op() of Derived_1	
testAsBase: op() of Derived_2		testAsBase: op() of Derived_2
testAsDerived_1: op() of Derived_1	testAsDerived_1: op() of Derived_1
testAsDerived_2: op() of Derived_2	testAsDerived_2: op() of Derived_2

*/