Question

Given the code:

class A{};

class B : public virtual A{};

class C : public virtual A{};

class D : public B,public C{};

int main(){
cout<<"sizeof(D)"<<sizeof(D);
return 0;
}

Output:
sizeof(D) 8

Every class contains its own virtual pointer only not of any of its base class,
So, why the Size of class(D) is 8?

Answer 1

It depends on compiler implementation. My compiler is Visual Stdio C++ 2005.

Code like this:

int main(){
    cout<<"sizeof(B):"<<sizeof(B) << endl;
    cout<<"sizeof(C):"<<sizeof(C) << endl;
    cout<<"sizeof(D):"<<sizeof(D) << endl;
    return 0;
} 

It will output

sizeof(B):4
sizeof(C):4
sizeof(D):8

class B has only one virtual pointer. So sizeof(B)=4. And class C is also.

But D multiple inheritance the class B and class C. The compile don’t merge the two virtual table.So class D has two virtual pointer point to each virtual table.

If D only inheritance one class and not virtual inheritance. It will merge they virtual table.