下面程序中Base是抽象类。请在下面程序的横线处填上适当内容，以使程序完整,并使程序的输出为:

Der1 called!

Der2 called!

#include <iostream.h>

class Base

{public:

_______________;

};

class Der1:public Base

{public:

void display(){cout<<"Der1 called!"<<endl;}

};

class Der2:public Base

{public:

void display(){cout<<"Der2 called!"<<endl;}

};

void fun(______________)

{p->display();}

void main()

{Der1 b1;

Der2 b2;

Base * p=&b1;

fun(p);

p=&b2;

fun(p);

}

答案：virtual void display()=0;，Base *p

［解析］抽象类有纯虚函数，派生类为display。结果fun函数用指针做参数。