#include <iostream>

 using namespace std;

 class Base

 {

 public :

     void display()

     {

         cout << "Base display" << endl;

     }

     void (Base :: **VTable)();//指向虚表的函数指针

     int a;

 }; 

 class Derived : public Base

 {

 public :

     void display()

     {

         cout << "derived display" << endl;

     }

 };

 void test(Base *pointer)

 {

     void (Base :: *pFunc)() = (void(Base :: *)())pointer->VTable[];

     (pointer->*pFunc)();

 }

 int main()

 {

     //多态是通过多余的存储空间存储了虚函数的地址，通过函数指针来实现虚函数的调用

     //1.分配空间+4，成员变量从+4的位置开始了。最上面的控件保留一个指针。

     //2.为该指针指向的地址分配一块存储空间

     //3.将所有虚函数的地址存储到这块存储空间中，这个存储空间称为虚表

　　　　//初始化工作，也就是编译器帮我们做的事

     Derived *derived = new Derived();

     derived->VTable = (void(Base :: **)())new int[];

     derived->VTable[] =(void(Base :: *)()) &Derived :: display;//初始化工作

     test(derived);

     Base *base = new Base();

     base->VTable = (void(Base :: **)())new int[];

     base->VTable[] = (void(Base :: *)()) &Base :: display;

     test(base);

     system("pause");

     return ;

 }

 #include <iostream>

 using namespace std;

 class Base

 {

 public :

     void display()

     {

         cout << "Base display" << endl;

     }

     int a;

 }; 

 class Derived : public Base

 {

 public :

     void display()

     {

         cout << "derived display" << endl;

     }

 };

 int main()

 {

     cout << "Base's size is " << sizeof(Base) << endl;

     system("pause");

     return ;

 }

#include <iostream>

using namespace std;

class Base

{

public :

    virtual void display()

    {

        cout << "Base display" << endl;

    }

    int a;

}; 

class Derived : public Base

{

public :

    void display()

    {

        cout << "derived display" << endl;

    }

};

int main()

{

    cout << "Base's size is " << sizeof(Base) << endl;

    system("pause");

    return ;

}