Virtual Functions

Virtual Functions

Definition : "A virtual function is a member function that is declared within a base class and redefined by a derived class."

 

• The virtual keyword is preceded to the function name. The virtual function can be redefined in the derived class. Thus using virtual functions we can have one interface, multiple functions performing different task. This feature is called polymorphism. The virtual function within the base class defines the form of the interface to that function. Each redefinition of the virtual function by a derived class indicate some different task related to derived class.

• When a virtual function is redefined by a derived class, the keyword virtual is not needed. The virtual function written in base class acts as interface and the function defined in derived classes act as different forms of the same function.

This property of virtual function brings the runtime polymorphism. The following program shows this property ‒

#include <iostream>

using namespace std;

class base

{

   public:

   int i;

   base(int x)//constructor

   {

      i = x;

   }

   virtual void function()      // The function is with keyword virtual

   {

      cout << "Using base version of function():";

      cout << i << "\n";

   }

};

class derived1: public base

{

   public:

   //constructor for object creation

   derived1 (int x):base(x){}

   void function()

   {

      cout<<"Using derived1's version of function(): ";

      cout<i+i << "\n";

   }

};

class derived2: public base

{

   public:

   derived2(int x):base(x){ }//constructor for object creation

   void function()

   {

      cout << "Using derived2's version of function():

      cout << i*i << "\n";

   }

}:

int main()

{

   base *P;

   int num

   cout<<"\n Enter Some number "

   cin>>num;

   base obj(num);

   derived1 d1_obj(num);

   derived2 d2_ obj(num);

   p=&obj;://base object's address

   p‒>function(); //base class function

   p = &d1_obj://derived1 object's address

   p‒>function(); //derived1 class function()

   p = &d2_obj;//derived2 object's address.

   p‒>function(); //derived2 class function().

   return 0;

}

Output

Enter Some number 5

Using base version of function(): 5

Using derived1's version of function(): 10

Using derived2's version of function(): 25

 

• In above program we have written one interface function in base class preceded by the keyword virtual. Note that the derived class derived1 is also having function who is performing addition operation. This time this function is not preceded by keyword virtual. Similarly we have written another derived class derived2 in which function is performing the task of multiplication.

• Using the object pointer we can access different forms of the functions.

 

Example: 1

Write a C++ program to create a base class house. There are two classes called door and window available. The house class has members which provide information related to the area of construction, doors and windows detail. It delegates the responsibility of computing cost of doors and window construction to door and window classes respectively. Write a C++ program to model the above relationship and find the cost of constructing the house.

Solution :

#include<iostream>

using namespace std;

class House

{

public:

   int area;

double rate;

   int noofdoors;

   int noofwindows;

   void GetVal()

   {

      cout<<"\n Enter the area for construction(in sq.feet): ";

      cin>> area;

      cout<<"\n Enter the rate per sq.feet: ";

      cin>>rate;

      cout<<"\n Enter the number of dooors: ";

      cin>>noofdoors;

      cout<<"\n Enter the number of windows: ";

      cin>>noofwindows;

}

double AreaCost()

{

   return (area*rate);

}

};

class door:virtual public House.

{

public:

   double costofdoor;

   void GetDoorVal()

   {

      cout<<"\n Enter the cost of each door: ";

      cin>>costofdoor;

   }

   double DoorCost()

   {

      return (costofdoor*noofdoors);

   }

};

class window:virtual public House

{

public:

double costofwindow;

void GetWindowVal()

{

   cout<<"\n Enter the cost of each window: ";

   cin>>costofwindow;

}

double WindowCost()

{

   return (costofwindow*noofwindows);

}

};

class derived : public door,public window

{

public:

double ConstructionCost()

{

   double total;

   double c1=WindowCost();

   double c2=DoorCost();

   double c3= AreaCost();

   total=c1+c2+c3;

   return total;

}

};

int main()

{

   derived obj;

   obj.GetVal();

   obj.GetDoorVal();

   obj.GetWindowVal();

   cout<<"\n The construction cost is: "<<obj.ConstructionCost();

   return 0;

}

Output

Enter the area for construction(in sq feet): 500

Enter the rate per sq feet: 100

Enter the number of dooors: 4

Enter the number of windows: 2

Enter the cost of each door: 40

Enter the cost of each window: 20

The construction cost is: 50200

 

Example: 2

Consider an example of a book shop which sells book and video tapes. These two classes are inherited from the base class called media. The media class has command data members such as tile and publication. The book class has data members for storing number of pages in book and tape class has the playing time in a tape. Each class will have member function such as read() and show() in the base class, these members have to be defined as virtual functions, write a program which models the class hierarchy for the book shop and process the objects of these classes using pointers to the base class.

Solution :

#include <iostream>

using namespace std;

class Media

{

public:

   char Title[10],publication[20];

virtual void Read()

{

   cout<<"\n Enter the title: ";

   cin>>Title;

   cout<<"\n Enter the publication: ";

   cin>>publication;

}

virtual void show()

{

   cout<<"Title: "<<Title<<endl;

   cout<<"Publication: "<<publication<<endl;

}

};

class Book: public Media

{

public:

int pages;

virtual void Read(int num)

{

   pages num;

}

virtual void show()

{

   cout << "Number of pages of book: ";

   cout << pages << "\n";

}

};

class Tape: public Media

{

public:

int playtime;

virtual void Read(int num)

{

   playtime num;

}

virtual void show()

{

   cout << "Playing time in tape: "

   cout << playtime << "\n";

}

};

int main()

{

   Media *p;

   Media obj;

   int num;

   Book b;

   Tape t;

   obj.Read();

   obj.show();

   cout<<"\nEnter number of pages: ";

   cin>>num;

   b.Read(num);

   p=&b;

   p‒>show();

   cout<<"\nEnter Time in minutes: ";

   cin>>num;

   t.Read(num);

   p=&t;

   p‒>show();

   return 0;

}

Output

Enter the title: AAA

Title: AAA

Enter the publication: BBB

Publication: BBB

Enter number of pages: 100

Number of pages of book: 100

Enter Time in minutes: 3

Playing time in tape: 3

 

Review Questions

1. State the rules for virtual functions. Write a C++ program to declare a virtual function and   demonstrate it.

2. Explain run time polymorphism with example program in C++.

3. Explain about implementation of runtime polymorphism in C++ with an example.