About Sizeof Object with with and w/o virtual function

Question

Can somebody help me with what all data and in which order is stored in class object?

#include <iostream>

using namespace std;

class A{

    public:
    A(){}
    virtual ~A(){}
};

class B{

    public:
    B(int i):_i(i){}
    ~B(){}

    private:
        int _i;
};

class C{

    public:
        C(int i):_i(i){}
        virtual ~C(){}

    private:
        int _i;
};
int main()
{
   cout << "SIZEOF A : " << sizeof(A) << endl;
   cout << "SIZEOF B : " << sizeof(B) << endl;
   cout << "SIZEOF C : " << sizeof(C) << endl;

   return 0;
}

Output:

SIZEOF A : 8 SIZEOF B : 4 SIZEOF C : 16

Why Sizeof(C) is 16 and not 12 ?