虚函数的疑问.

#include<iostream.h>
    class Base {
    public:
        Base(int x,int y)
        {  a=x; b=y; }
        virtual void show()      //定义虚函数show()
        { cout<<"Base----------\n"; cout<<a<<" "<<b<<endl;}
         private:
        int a,b;  };
    class Derived : public Base    {
    public:
        Derived(int x,int y,int z):Base(x,y){c=z; }
        void show()          //重新定义虚函数show()
        { Base::show();cout<< "Derived---------\n"; cout<<c<<endl;}
        private:
        int c;
    };
void main()
{
     Base mb(60,60),*pc;
     Derived mc(10,20,30);
     pc=&mb;
     pc->show();     //调用基类Base的show()版本
     pc=&mc;
     pc->show();     //调用派生类Derived的show()版本
 }

有什么办法能让红色部分输出为60,60,也就是基类对象的值.
派生类的构造函数把他初始化为10,20了