The virtual keyword means that method,property or function 
can be overridden

virtual function can be used to override the propertis of a 
function.it is normaly used in inheritance. it is part of  
polymerphism.

virtual function is ended with null 

virtual function=o

It allows one to write generalized code that works with the 
base class pointer which can dynamically point to the 
derieved class objects. Virtual function allows you to 
implement runtime polymorphism. It allows the function call 
to be binded with function definition as late as runtime.

Also read: concept of VPTR and VTABLE from Thinking in C++

When derived class overrides the base class method by 
redefining the same function, then if client wants to 
access redefined the method from derived class through a 
pointer from base class object, then you must define this 
function in base class as virtual function.

The objects of the polymorphic class, addressed by 
pointers, change at runtime and respond differently for the 
same message.Such mechanism requres postponment of binding 
of a function call to the member function [ VIRTUAL] untill 
runtime.

A virtual function is a function member of a class, 
declared using the "virtual" keyword. A pointer to a 
derived class object may be assigned to a base class 
pointer, and a virtual function called through the pointer. 
If the function is virtual and occurs both in the base 
class and in derived classes, then the right function will 
be picked up based on what the base class pointer "really" 
points at.

A virtual function is basically a like normal function but 
we can override the function in the derived classes.
Now when we want to access like (fn)

Base * bptr = new Derived();
bptr->fn(); then function of the derived class will be 
called only if the function is marked as virtual in the 
base class.

i.e the virtual keywork tells to use the function of the 
class to which it is pointing to and not to the Base class. 
This is called runtime polymorphism.

The virtual function is nothing but the function in the 
base class with the virtual keyword, that allows us to 
derive only once in the class which is going to accsess it 
while it is the child of two different classes which are 
the child of the same base class.This is nothing but the 
dynamic binding.

A virtual function is a function which is precedded by the 
key word derived is declared in the base class.It is 
normally used to achieve run time polymerphism.That means 
we can override the base class function in the derived 
class.We can invoke the virtual function through the base 
class pointer depending upon the contents to which the base 
class pointer points to.
               Generally the implementation of virtual 
class was found in inheritance where the base class 
function is override in the derived class.

virtual function is used to prevent from more times 
calling.That means if a function is declared as virtual 
then its def will be diferent in base class & derived class