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Monday, August 06, 2007

Introduction to Virtual Functions

Virtual functions are special member functions of a class which may be re-defined in the derived classes. It is used to give specific meaning to the base class member function with respect to the derive class.

Virtual functions can be thought of as a function name reserved in the bas class which may be re-defined in the derived classes as per the need so that every derived class has the same function that performs specific (as redefined in the derived class) action.

Let’s now have a look at a simple program to show virtual functions inaction:


  // Virtual functions
  #include <iostream.h>

  // base class
  class base
  {
  public:
    // precede the function name
    // with the 'virtual' keyword
    // to make it a virtual function
    virtual void func()
    {
      cout<<"Base's func()\n";
    }
  };

  // derived class
  class derived:public base
  {
  public:
    // redefinition of the 
    // function
    void func()
    {
      cout<<"Derived's func()\n";
    }
  };

  // main
  void main()
  {
    base b;
    derived d;

    // notice that both are calling
    // the same function but different
    // functions gets called as per
    // the class to which the object
    // belongs to
    b.func();
    d.func();
  }

OUTPUT:

  Base's func()
  Derived's func()

The redefinition of the virtual function in the derived class is known as overriding


As you can see there is nothing confusing, the virtual function is a general member function and is defined as such, only difference being that it’s preceded by the virtual keyword that gives it the special property.

NOTE: By redefining a virtual function, all its previous meaning (as was defined in the base class) is lost.

As in the example program the base class defines the virtual function to print “Base's func()” and the derived class overrides it to print “Derived's func()”. So when we call the overridden function it only prints what was defined in the redefinition hence the original meaning of the function is lost.

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