由Point类派生出描述一个矩形的类Rectangle (左上角、右下角两点确定矩形) ：

 　　Rectangle类的成员数据：

① 公有继承Point类后已有1个点；　　

② 再增加1个成员对象： protected: Point p2

③ 增加存放周长的成员数据： protected: float length

④ 增加存放面积的成员数据： protected: float rectarea

 　　Rectangle类的成员函数、友元函数：//　　float distance(){return 2*(abs(x-p2.x)+abs(y-p2.y)) }

 　　　　float area(){return abs(x-p2.x)*abs(y-p2.y) } 派生类对象不可访问对象的protected的成员！！

-Derived from the Point class describes a rectangle class Rectangle (upper left, lower right corner points determine the rectangle): 　Rectangle class members of the data: ① public inheritance after the Point class has a point　② to add a member object: protected: Point p2　③ increase in circumference of the members of the data store: protected: float length　④ members to increase the data storage area: protected: float rectarea　　Rectangle class member function, friend function:// float distance () ( return 2* (abs (x-p2.x)+ abs (y-p2.y)) ) 　float area () (return abs (x-p2.x)* abs (y-p2.y) ) derived class object can not access protected members of object! !