frien class are used when two or more classes are designed 
to work together and virtual base class aids in multiple 
inheritance

friend class is used to share private data between 2 or 
more classes the function declared as freind are not called 
using any object it is called like normal .the arguments to 
such functions is normally object of a class.
virtual class is used for run time polymorphism when object 
is linked to procedure call at run time