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

c++ virtual vtable vptr
CodeCodeCode picture CodeCodeCode · Jan 19, 2012 · Viewed 11.2k times · Source

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

Chris Dodd picture Chris Dodd · Jan 19, 2012

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.

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