阅读程序4

#include <iostream>
using namespace std;
class Base
{
public:
    virtual void who()=0;
};
class FirstDerived:public Base
{
public:
    void who(){cout<<'F';}
};
class SecondDerived:public Base
{
public:
    void who(){cout<<'S';}
};
int main()
{
    FirstDerived first_obj;
    SecondDerived second_obj;
    Base &bref=first_obj;
    bref.who();
    bref=second_obj;
    bref.who();
    Base *bp;
    bp=&first_obj;
    bp->who();
    bp=&second_obj;
    bp->who();
    return 0;
}