虚函数内容，看不懂，求解析，帮帮忙

#include <iostream>
using namespace std;
 
class Base1 {
public:
            virtual void f() { cout << "Base1::f" << endl; }
            virtual void g() { cout << "Base1::g" << endl; }
            virtual void h() { cout << "Base1::h" << endl; }
 
};
 
class Base2 {
public:
            virtual void f() { cout << "Base2::f" << endl; }
            virtual void g() { cout << "Base2::g" << endl; }
            virtual void h() { cout << "Base2::h" << endl; }
};
 
class Base3 {
public:
            virtual void f() { cout << "Base3::f" << endl; }
            virtual void g() { cout << "Base3::g" << endl; }
            virtual void h() { cout << "Base3::h" << endl; }
};
 
 
class Derive : public Base1, public Base2, public Base3 {
public:
            virtual void f() { cout << "Derive::f" << endl; }
            virtual void g1() { cout << "Derive::g1" << endl; }
};
 
 
typedef void(*Fun)(void);//定义了一个函数指针
 
int main()
{
            Fun pFun = NULL;
 
            Derive d;
            int** pVtab = (int**)&d;
 
            //Base1's vtable
            //pFun = (Fun)*((int*)*(int*)((int*)&d+0)+0);
            pFun = (Fun)pVtab[0][0];
            pFun();
 
            //pFun = (Fun)*((int*)*(int*)((int*)&d+0)+1);
            pFun = (Fun)pVtab[0][1];
            pFun();
 
            //pFun = (Fun)*((int*)*(int*)((int*)&d+0)+2);
            pFun = (Fun)pVtab[0][2];
            pFun();
 
            //Derive's vtable
            //pFun = (Fun)*((int*)*(int*)((int*)&d+0)+3);
            pFun = (Fun)pVtab[0][3];
            pFun();
 
            //The tail of the vtable
            pFun = (Fun)pVtab[0][4];
            cout<<pFun<<endl;
 
 
            //Base2's vtable
            //pFun = (Fun)*((int*)*(int*)((int*)&d+1)+0);
            pFun = (Fun)pVtab[1][0];
            pFun();
 
            //pFun = (Fun)*((int*)*(int*)((int*)&d+1)+1);
            pFun = (Fun)pVtab[1][1];
            pFun();
 
            pFun = (Fun)pVtab[1][2];
            pFun();
 
            //The tail of the vtable
            pFun = (Fun)pVtab[1][3];
            cout<<pFun<<endl;
 
 
 
            //Base3's vtable
            //pFun = (Fun)*((int*)*(int*)((int*)&d+1)+0);
            pFun = (Fun)pVtab[2][0];
            pFun();
 
            //pFun = (Fun)*((int*)*(int*)((int*)&d+1)+1);
            pFun = (Fun)pVtab[2][1];
            pFun();
 
            pFun = (Fun)pVtab[2][2];
            pFun();
 
            //The tail of the vtable
            pFun = (Fun)pVtab[2][3];
            cout<<pFun<<endl;
 
            return 0;
}