Virtual Pointer

Question

What is virtual pointer? Hi, ALL, Today I had an telephone interview and got a question: "What is virtual pointer?" I stumbled on this one, so after it was finished, I tried Google. Unfortunately, it gave me a virtual table references only.

So what is virtual pointer in plain English? How do you define it?

Thank you.

Answer 1

There is no such thing as a "virtual pointer."

There are a few things the interviewer might have meant:

• A pointer to a polymorphic class
• A pointer to the vtable of a polymorphic class (credit @Maxim)
• A pointer within the vtable of a polymorphic class
• A smart pointer object with an overridden operator->
• A pointer to a virtual member function (credit @ Matthieu M)

But as far as "virtual pointer" is concerned, there's no such thing.

Answer 2

C++ compiler creates a hidden class member called virtual-pointer or in short vptr when there are one or more virtual functions. This vptr is a pointer that points to a table of function pointers. This table is also created by compiler and called virtual function table or vtable. Each row of the vtable is a function pointer pointing to a corresponding virtual function.

To accomplish late binding, the compiler creates this vtable table for each class that contains virtual functions and for the class derived from it. The compiler places the addresses of the virtual functions for that particular class in ‘vtable’.

When virtual function call is made through a base-class pointer, the compiler quietly inserts code to fetch the VPTR and look up the function address in the VTABLE, thus calling the right function and causing late/dynamic binding to take place.