include <iostream>

using namespace std;
class Base
{
public:
virtual void Show()
{
cout << "Base::Show()" << endl;
}
};

class Derive : public Base
{
public:
virtual void Show()
{
cout << "Derive::Show()" << endl;
}
};
void fun1(Base *ptr)
{
ptr->Show();
}
void fun2(Base &ref)
{
ref.Show();
}
void fun3(Base b)
{
b.Show();
}

int main()
{
Base b, *p = new Derive;
Derive d;
fun1(p);
fun2(b);
fun3(d);

system("pause");
return 0;

}

show()是虚函数；
*p是D对象，因此fun1(p)--》Derive;
b是B对象，因此fun2(b) --》Base;
d是D对象，B是D的父类，调用fun3(d)会将d转成B对象，因此，输出Base；
Derive::Show()
Base::Show()
Base::Show()