C++ virtual function

#include<iostream>
using namespace std;
class A
{

};
class B
{
    int i;
};

class C
{
    void foo();
};
class D
{
    virtual void foo();
};

class E
{
    int i ;
    virtual void foo();
};
class F
{
    int i;
    void foo();
};
class G
{
    void foo();
    int i;
    void foo1();
};

class H
{
    int i ;
    virtual void foo();
    virtual void foo1();
};
int main()
{
    cout <<"sizeof(class A) : " << sizeof(A) << endl ;
    cout <<"sizeof(class B) adding the member int i : " << sizeof(B) << endl ;
    cout <<"sizeof(class C) adding the member void foo() : " << sizeof(C) << endl ;
    cout <<"sizeof(class D) after making foo virtual : " << sizeof(D) << endl ;
    cout <<"sizeof(class E) after adding foo virtual , int : " << sizeof(E) << endl ;
    cout <<"sizeof(class F) after adding foo  , int : " << sizeof(F) << endl ;
    cout <<"sizeof(class G) after adding foo  , int : " << sizeof(G) << endl ;
    G g;
    cout <<"sizeof(class G) after adding foo  , int : " << sizeof(g) << endl ;
    cout <<"sizeof(class H) after adding int 2 virtual " << sizeof(H) << endl ;
    return 0;
}

Output result:
sizeof(class A) : 1
sizeof(class B) adding the member int i : 4
sizeof(class C) adding the member void foo() : 1
sizeof(class D) after making foo virtual : 8
sizeof(class E) after adding foo virtual , int : 16
sizeof(class F) after adding foo , int : 4
sizeof(class G) after adding foo , int : 4
sizeof(class G) after adding foo , int : 4
sizeof(class H) after adding int 2 virtual 16

sizeof(class A) == sizeof(class C), since class is empty and non-virtual function doesn’t take any space.