#include <iostream.h>class rectangle{                                   //定义一个矩形基类public :    int l;    int w;    rectangle(){l=3,w=2;}    virtual void out(){                       //输出长和宽        cout<<"rectangle.l="<<l<<endl;        cout<<"rectangle.w="<<w<<endl;    }};class squar : public rectangle {                 ////由基类以公有方式导子类型    void out(){        int max;        (l<w)?max=w:max=l;                //将长方形中较长的边设为                   w=l=max;                          //正方形的边        cout<<"squar.l=squar.w="<<max<<endl;    }};  void main(){    rectangle re,*pre;    squar sq;    pre=&re;               pre->out();    pre=&sq;                       //使基类指针指向子类    pre->out();}请问各位:   为什么将 公有 导出改为 私有 时编译出错   谢了!