【c++】虚函数对对象大小的影响

#include "stdafx.h"
#include<iostream>
using namespace std;

class Base{   //4
public:
	virtual void fun1(){}
};

class Derived1: public Base//  4+4 8
{
public:
	char a[3];

};
class Derived2: public Base//父类4+char a[3] 4  8
{
public:
	virtual void fun2(){}
	char a[3];

};
class Derived3: public virtual Base//父类4+char a[3] 4+指向父类的指针 12</span>

{
public:
	char a[3];

};
class Derived4: public virtual Base//父类4+char a[3] 4+指向父类的指针 +新的虚函数表的指针16

{
public:
	char a[3];
	virtual void fun3(){}
};

int _tmain(int argc, _TCHAR* argv[])
{
	cout << "4  " << sizeof(Base) << endl;
	cout << "Derived1  8  " << sizeof(Derived1) << endl;
	cout << "Derived2  8  " << sizeof(Derived2) << endl;
	cout << "Derived3  12  " << sizeof(Derived3) << endl;
	cout << " Derived4  16  " << sizeof(Derived4) << endl;
	system("pause");
	return 0;
}