一个指针看不懂，求教！

#include "iostream"
using namespace std;

class base{ private: int b;
public : base(int x=0){ b=x; cout<<"调用基类构造函数！"<<endl;}
         virtual void show()
         {cout<<"base::b"<<b<<endl;
          cout<<"base::show()被调用！"<<endl;
         }
};

class derived:public  base{
          private: int d;
public : derived(int y=-1){d=y; cout<<"调用派生类构造函数！"<<endl;}
                  virtual void show()
                    {cout<<"derived::d"<<d<<endl;
                     cout<<"derived::show()被调用！"<<endl;
                  }
};
void main()
{   derived d;
    void(base::*fp)();
    fp=&base::show;
    (d.*fp)();
    
}

void(base::*fp)();
    fp=&base::show;
    (d.*fp)();