Mechanism of Vptr and Vtable in C++

Question

In C++, during dynamic binding, consider the following example...

class Base
{
  virtual void fun()
  {
     cout<<"Base";
  }      
};

class Derived : public Base
{
   void fun()
   {
     cout<<"Derived";
   }
};

int main()
{
  Base *bptr;
  Derived d;
  bptr=&d;
  bptr->fun();
}

The output of the above function is "Derived" due to the declaration of virtual keyword/dynamic binding.

As per my understanding, a virtual table (Vtable) would be created which contains the address of the virtual functions. In this case the virtual table created for the derived class points to the inherited virtual fun(). And bptr->fun() will be getting resolved to bptr->vptr->fun();. This points to the inherited base class function itself. I am not completely clear on how the derived class function is called?

Answer 1

Just went through this link virtual table and _vptr

It says that the workflow will be like ..

1. base_ptr->base_vptr----> to check the access of virtual function in base class.

2. base_ptr->derived_vptr->virtual_function()---> to call/invoke the virtual function.

Hence the derived class virtual function is called.. Hope you find it helpful.

Answer 2

And bptr->fun() will be getting resolved to bptr->vptr->fun();. This points to the base class function itself.

Wrong. The Derived instance's vptr (a hidden field in each instance) points to the Derived vtable.

Answer 3

The Standard doesn't specify the mechanism by which polymorphism is implemented. All the Standard says is how it should work -- not how compiler vendors should implement it.

That being said, you have it pretty much right as far as GCC under Linux and MSVC under Windows is concerned, and I would expect most other compilers to be similar.