C++ Virtual Function


C++ virtual function is a member function of a class, whose functionality can be over-ridden in its derived classes. The whole function body can be replaced with a new set of implementation in the derived class. The concept of c++ virtual functions is different from C++ Function overloading.

C++ Virtual Function – Properties:

C++ virtual function is,

  • A member function of a class
  • Declared with virtual keyword
  • Usually has a different functionality in the derived class
  • A function call is resolved at run-time

The difference between a non-virtual c++ member function and a virtual member function is, the non-virtual member functions are resolved at compile time. This mechanism is called static binding. Where as the c++ virtual member functions are resolved during run-time. This mechanism is known as dynamic binding.

C++ Virtual Function – Reasons:

The most prominent reason why a C++ virtual function will be used is to have a different functionality in the derived class.

For example a Create function in a class Window may have to create a window with white background. But a class called CommandButton derived or inherited from Window, may have to use a gray background and write a caption on the center. The Create function for CommandButton now should have a functionality different from the one at the class called Window.

C++ Virtual function – Example:

This article assumes a base class named Window with a virtual member function named Create. The derived class name will be CommandButton, with our over ridden function Create.

class Window // Base class for C++ virtual function example
{
public:
virtual void Create() // virtual function for C++ virtual function example
{
cout <<"Base class Window"<<endl;
}
};

class CommandButton : public Window
{
public:
void Create()
{
cout<<"Derived class Command Button - Overridden C++ virtual function"<<endl;
}
};

void main()
{
Window  *x, *y;

x = new Window();
x->Create();

y = new CommandButton();
y->Create();
}

The output of the above program will be,
Base class Window
Derived class Command Button

If the function had not been declared virtual, then the base class function would have been called all the times. Because, the function address would have been statically bound during compile time. But now, as the function is declared virtual it is a candidate for run-time linking and the derived class function is being invoked.

C++ Virtual function – Call Mechanism:

Whenever a program has a C++ virtual function declared, a v-table is constructed for the class. The v-table consists of addresses to the virtual functions for classes and pointers to the functions from each of the objects of the derived class. Whenever there is a function call made to the c++ virtual function, the v-table is used to resolve to the function address. This is how the Dynamic binding happens during a virtual function call.