Answer / yuva

You use virtual functions when you want to override a
certain behavior(read method) for your derived class than
the one implemented for the Base class and you want to do so
at run-time through an pointer to Base class.

The classical example is when you have a base class called
Shape and concrete shapes(classes) which derive from it.
Each concrete class overrirdes(implements a virtual method)
called Draw().

The class hierarchy as follows:

Class hierarchy

The following snippet shows the usage of the example, it
creates an array of Shape class pointers wherein each points
to distinct derived class object. At run-time invoking
Draw() method results in calling of the method overriden by
that derived class and the particular Shape is drawn(rather
rendered).

Shape *basep[] = { &line_obj, &tri_obj,
&rect_obj, &cir_obj};
for (i = 0; i < NO_PICTURES; i++)
basep[i] -> Draw ();

The above program just uses the pointer to the Base class
storing addresses of the Derived class objects provides a
loose coupling in the way that the program does not have to
change drastically if a new concrete derived class of shape
is added anytime because the are minimal code segments which
actually use(depend) on the concrete Shape type.

The above is a good example of Open Closed Principle of the
famous SOLID design principles.

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