Number of Virtual tables and Virtual Pointers in a C++ Program

Question

Let say we have below program:

class A
{     public:
      virtual fun(){};
};
class B:public A
{     public:
     virtual fun(){};
};
int main()
{
     A a1;
     B b1;
 }

My question is how many vtables and how many vptrs will be created, when we run this program?

Answer 1

Its heavily implementation dependent, but generally you'll get one vtable object per class that has any virtual functions (classes with no virtual functions or bases don't need them), and one vptr per object of a class with a vtable (pointing at the class's vtable).

Things get more complex if you have multiple inheritance and virtual base classes -- which can be implemented many ways. Some implementations use an addition vtable per additional base class (so you end up with a vtable per base class per class), while others use a single vtable with extra info in it. This may result in needing multiple vptrs per object.

The virtual keyword in B is irrelevant -- if the function is virtual in the base class, it will be virtual in the derived classes regardless.

Answer 2

Note that this is strictly implementation dependent.
C++ Standard does not talk of vptr or vtable, the virtual mechanism is left out as an implementation detail for compilers. So practically, compilers can implement it without using vptr or vtable.However, almost all known compilers implement it using vptr and vtable.

Given the above, to answer your question:

Each class will have its own virtual table.
While each object has its own virtual pointer.