C++ 求教高手 读程序写结果?希望能讲得详细点！！谢谢了。

#include <iostream>
using namespace std;
class Base {
public:
void fun1(){cout<<"Base\n";}
virtual void fun2(){cout<<"Base\n";}
};
class Derived:public Base {
public:
void fun1(){cout<<"Derived\n";}
void fun2(){cout <<"Derived\n";}
};
void f(Base& b){b.fun1();b.fun2();}
void main() {
Derived obj;
f(obj);
}

为什么是结果是：
Base
Derived

相关了解……