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:
- What is the size of an empty class
Ans : 1 byte , because compiler provides minimum 1 byte to the object
- what is the size of the class class A { virtual void fn1();};
Ans : 4 byte , now vptr (virtual pointer comes in to the picture)
- What is pure virtual function
Ans : A virtual function doesn’t have body
Virtual void fn1() = 0;
- 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
