VTABLE_and_VPTR.md

Virtual Method Table and Virtual Table Pointer

As you might know, we have two types of polymorphism in C++:

1. Static Polymorphism : is done at compile time according to the type.
2. Dynamic Polymorphism: is done at run-time in accordance with the content of pointer.

The Virtual Method Table or vtable is a mechanism used for supporting run-time method binding.

Let say we have the following classes:

class base
{
public:

    void virtual foo()
    {
        std::cout<<"foo base" <<std::endl;
    }

    void print()
    {
        std::cout<<"print base" <<std::endl;
    }

};


class derived : public base
{
public:

    void foo() override
    {
        std::cout<<"foo derived" <<std::endl;
    }
    void print()
    {
        std::cout<<"print derived" <<std::endl;
    }
};

Now if we execute the followings in our main:

base *bptr;
derived d;
bptr = &d;

virtual function, bound at run-time, we got the foo() from derived

bptr->foo();

Non-virtual function, bound at compile time, we get print() from base:

bptr->print();

Virtual Method Table (vtable)

Every class that has virtual function will get a table name vtable. It is a table of only "virtual function" memory addresses and it is maintained per class. vtableis created by compiler at compile time.

Virtual Table Pointer (vptr)

when you create an object that which has virtual function, compiler adds a hidden member to your class called __vptr. It is pointer to the vtable, and it is maintained per object instance. vptr is inherited to all the derived classes. If you create multiple from a class, the __vptr in all of them point to the same vtable of a particular class.

   ____________________                                              _____________________________________
   |    base           |                                             |   vtable for class base            |
   |  _vptr------------|-------------------------------------------->|                                    |
   | virtual foo()<----|---------------------------------------------|--address of base version of foo()  |
---|->virtual bar()<---|---------------------------------------------|--address of base version of bar()  |
|  |___________________|                                             |____________________________________|
|
|
|
|
|
|   ____________________                                             ________________________________________________
|   |  derived         |                                             |   vtable for class derived                    |
|   |  _vptr-----------|-------------------------------------------->|                                               |
|   |         foo()<---|---------------------------------------------|--address of derived implementation of foo()   |
|   |         print()  |                                      |------|--address of base implementation of bar()      |
|   |__________________|                                      |      |_______________________________________________|
|                                                             |
|-------------------------------------------------------------|

Refs: 1, 2, 3

source code