Understanding Polymorphism and Virtual Functions in Object-Oriented Programming, Study notes of Object Oriented Programming

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Session ObjectivesSession Objectives

Define the word “ polymorphism”

Virtual Function

virtual Base class

Abstract class

Without Using Virtual Keyword #include<iostream.h> class base { public: void getdata(); void display(); }; class derivedB: public base { private: long int rollno; char name[20]; public: void getdata(); void display(); }; void base::getdata() { cout<<"base class getdata function"; } void base::display() { cout<<"base class display function"; } void derivedB::getdata() { cout<<"Enter the name"<<endl; cin>>name; cout<<"Enter the roll no"<<endl; cin>>rollno; } void derivedB::display() { cout<<"The name is"<<name<<endl; cout<<"The roll no is"<<rollno<<endl; } void main() { clrscr(); base *ptr; derivedB obj; ptr=&obj; ptr->getdata(); ptr->display(); getch(); }

 Virtual means existing in effect but not in reality.

 (^) It provide a way for a program to decide, when it is running, what function to call.  (^) It allows greater flexibility in performing the same kinds of action on different kinds of objects.  (^) It actually holds pointers to a variety of derived types  (^) It is defined and declared in the base class and is then used by several derived functions by getting up pointers.

With Using “virtual” Keyword #include<iostream.h> #include<conio.h> class base { public: virtual void getdata()=0; virtual void display()=0; }; class derivedB: public base { private: long int rollno; char name[20]; public: void getdata(); void display(); }; void derivedB::getdata() { cout<<"Enter the name"<<endl; cin>>name; cout<<"Enter the roll no"<<endl; cin>>rollno; } void derivedB::display() { cout<<"The name is"<<name<<endl; cout<<"The roll no is"<<rollno<<endl; } void main() { clrscr(); base *ptr; derivedB obj; ptr=&obj; ptr->getdata(); ptr->display(); getch(); }

 (^) This function should be declared in the base class specifications  (^) It can be overloaded  (^) It should be a member function of a class  (^) There can be virtual destructors  (^) Constructors cannot be declared as virtual functions  (^) This function may or may not have function body  (^) It cannot be an ordinary stand alone function  (^) These functions are to be accessed by base class pointers  (^) If there is no code defined for a virtual function in derived class, then base class function is invoked

Polymorphism Using Array #include<iostream.h> #include<conio.h> class base { public: virtual void show()=0; }; class base1:public base { public: void show(){ cout<<"\n base1 called";} }; class base2:public base { public: void show(){ cout<<"\n base2 called"; } };

void main()

base *list[2];

base1 b;

base2 b1;

clrscr();

list[0]=&b;

list[1]=&b1;

list[0]->show();

list[1]->show();

getch();

Abstract classes A class that contains atleast one pure virtual function is said to be an abstract class. NOTE : you can use an abstract class only act as a base class for deriving other classes but does not create objects #include<iostream.h> #include<conio.h> class book { private : int x; public : book() { x=0; } virtual void getdata()=0; virtual void putdata()=0; }; void main() { clrscr(); cout<<"end"; getch(); }

 The function call resolved at runtime is called as dynamic

binding

 The virtual function that has no body is called as pure

virtual function

 The virtual function that is equated to zero is called as a

null virtual function

 If a class contains one or more pure virtual functions, it is

an abstract base class.