跪求：C++里的一个问题(关于子类与父类的覆盖与隐藏)

亚洲猴王

#include<iostream.h>
class Base
{
public:
virtual void xfn(int i){
  cout<<"Base::xfn(int i)"<<endl;
}
void yfn(float f){
  cout<<"Base::yfn(float f)"<<endl;
}
void zfn(){
  cout<<"Base::zfn()"<<endl;
}
};
class Derived:public Base
{
public:
     void xfn(int i){
  cout<<"Derived::xfn(int i)"<<endl;
}
    void yfn(int c){
  cout<<"Derived::yfn(int c)"<<endl;
}
void zfn(){
  cout<<"Derived::zfn()"<<endl;
}
};
void main()
{
  Derived d;
  Base *pB = &d;
  Derived *pD = &d;
  pB->xfn(5);
  pD->xfn(5);
     pB->yfn(3.14f);
  pD->yfn(3.14f);
  pB->zfn();
  pD->zfn();
}
为什么显示结果是：
Derived::xfn(int i)
Derived::xfn(int i)
Base::yfn(float f)
Derived::yfn(int c)
Base::zfn()
Derived::zfn()