virtual function table of multiple inheritance

Question

The sample code are as follow:

class A
{
public:
    int k;
    virtual int f();
};
class B:public virtual  A
{
public:
    virtual int a();
};
int main()
{
    cout<<sizeof(A)<<sizeof(B);
}

It prints

8 12

It seems class B has its own new virtual function table.

If class A changes to:

class A
{
public:
    virtual int f();
};

It prints

4 4

Could anyone explain the reason?

Answer 1

In your subclass B, B is a virtual subclass of A. Thus, B has a separate vtbl pointer (4 bytes) on top of what you have on subobject A. Thus,

sizeof(B object) 
= sizeof(A object) + sizeof (vtbl pointer of B)
= sizeof(int) + sizeof (vtbl pointer of A) + sizeof (vtbl pointer of B)
= 4 + 4 + 4
= 12

And,

sizeof(A object)
= sizeof(int) + sizeof (vtbl pointer of A)
= 4 + 4
= 8

If B is a normal subclass of A,

 sizeof(B object) 
    = sizeof(A object) 
    = sizeof(int) + sizeof (vtbl pointer of A) 
    = 4 + 4
    = 12

For empty class A, minimum size allocated for sizeof A object is sizeof pointer of vtbl = 4 And since A is empty in terms of instance data, virtual inheritance for empty class does not add to size of the object