Virtual Functions and Polymorphism

Polymorphism 

Polymorphism stands for 'many forms'. The concept of polymorphism is already implemented using the overloaded functions and operators. This is called early binding or static linking as the compiler already knows about the overloaded functions before runtime.  Run Time Polymorphism is achieved through Virtual Functions. 

This pointer  

The keyword ‘this’ represents a pointer that points to the object for which this function was called. 

 For example :       the function call  

                             A.max();  

will set the pointer this to the address of the object A. The pointer this acts as an implicit argument to all the member functions. 

Virtual functions 

When we use same function name in both the base and derived classes, the function in base class is declared as virtual preceding its normal declaration. When a function is made virtual, C++ determines which function to use at run time based on the type of object pointed to by the base pointer. Thus, by making base pointer point to different objects, we can execute different versions of the virtual function. 

Rules for virtual functions 

1. The virtual functions must be members of some class.
2. They cannot be static members. 
3. They are accessed by using object pointers. 
4. We cannot have virtual constructors, but we can have virtual destructors. 
5. Virtual functions are defined in base class, they need not be redefined in derived class. 6. A virtual function can be a friend of another class.