Understanding of virtual function

It is a function or method can be overridden within an inheriting class by a function with the same signature ,In other words , the purpose of virtual function is , to allow customization of derived class implementation

   “Where there is a casting there is a virtual”

Base * ptr = new Base() ; //  No need of virtual

Derived * ptr = new Derived();  // No need of virtual

Base * ptr = new Derived(); // need virtual to call correct function

Key benefits of Virtual function :

     It is allowed us to do function over riding

       

    Class Base { virtual void fn1() ; };

    Class derived: public base {void fn1() ;}

   Base* ptr = new derived() ;

    Ptr->fn1() ; this calls the derived class function fn1()

Above class become ptr->_vptr->fn1() ;

Now , consider if fn1() is not declared as virtual , then   the above call would call the Base class function .

Best part of this concepts is , you can have an array ,type of  Base* . and you can append derived classes object without worrying about function call .

 It is avoiding memory leak

  

   lets look at the below code

  

    Class Base { void ~Base() ; };

    Class derived: public base {void ~derived() ;}

   

    Base* ptr = new derived() ;

    delete ptr ;

   oops , there is a memory leak , above call only call base class destructor  .

  

wanna avoid memory leak

  Class Base { virtual void ~Base() ; };

Now , it will use vptr , and it will call the derived class destructor and subsequently it will call base class destructor

Disadvantages or Cost of virtual function

1.      If a class contains virtual function , then vtable is created , which means additional memory space required for that

2.      virtual function are mapped at runtime through vtable . hence calling virtual function is slow compared to normal function

3. vptr (virtual pointer which points to vtable) is created for every object .Additional 4 byte required to store vptr

pictorial representation of virtual function  

At last ,  If a function declared as virtual in base class , then the function in all derived class are virtual only . No need to mention it explicitly using “virtual” keyword

Some FAQ:

1. What is the size of an empty class

Ans : 1 byte , because compiler provides minimum 1 byte to the object

2. what is the size of the class  class A { virtual void fn1();};

      Ans : 4 byte ,  now vptr (virtual pointer comes in to the picture)

      

3. What is pure virtual function

 Ans : A virtual function doesn’t have body

         Virtual void fn1() = 0;

4. Can we create object of the class which has pure virtual function

Ans : No , Compiler will check the body of the function while creating object of the class , since it doesn’t have the body we can’t create the object and compiler will throw an error at the time of compilation