Niranjan Parreira

Rules for Virtual Functions

Compile time (early binding) VS runtime (late binding) behavior of Virtual Functions

Consider the following simple program showing runtime behavior of virtual functions.

Output:

Explanation: Runtime polymorphism is achieved only through a pointer (or reference) of base class type. Also, a base class pointer can point to the objects of base class as well as to the objects of derived class. In above code, base class pointer ‘bptr’ contains the address of object ‘d’ of derived class.Late binding (Runtime) is done in accordance with the content of pointer (i.e. location pointed to by pointer) and Early binding (Compile time) is done according to the type of pointer, since print() function is declared with virtual keyword so it will be bound at runtime (output is print derived class as pointer is pointing to object of derived class) and show() is non-virtual so it will be bound during compile time (output is show base class as pointer is of base type).NOTE: If we have created a virtual function in the base class and it is being overridden in the derived class then we don’t need virtual keyword in the derived class, functions are automatically considered as virtual functions in the derived class.

Working of virtual functions (concept of VTABLE and VPTR)As discussed here, if a class contains a virtual function then compiler itself does two things.

Consider the example below:

Output:

Explanation: Initially, we create a pointer of type base class and initialize it with the address of the derived class object. When we create an object of the derived class, the compiler creates a pointer as a data member of the class containing the address of VTABLE of the derived class.

Similar concept of Late and Early Binding is used as in above example. For fun_1() function call, base class version of function is called, fun_2() is overridden in derived class so derived class version is called, fun_3() is not overridden in derived class and is virtual function so base class version is called, similarly fun_4() is not overridden so base class version is called.

NOTE: fun_4(int) in derived class is different from virtual function fun_4() in base class as prototypes of both the functions are different.

This article is contributed by Yash Singla. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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