一些疑问

看到书上的代码之后，产生了一些疑问，还请大佬指教一下
程序如下：

#include<iostream>
using namespace std;
class Base1{
    public:
        virtual void display()const;
};
void Base1::display()const{
    cout<<"Base1::display()"<<endl;
}
class Base2:public Base1{
    public:
        void display()const;
};
void Base2::display()const{
    cout<<"Base2::display()"<<endl;
}
class Derived:public Base2{
    public:
        void display()const;
};
void Derived::display()const{
    cout<<"Derived::display()"<<endl;
}
void fun(Base1 *ptr){
    ptr->display();
}
int main()
{
    Base1 base1;
    Base2 base2;
    Derived derived;
    fun(&base1);
    fun(&base2);
    fun(&derived);
    return 0;
}#include<iost