C++学习：继承问题
题目：把定义平面直角坐标系上的一个点的类CPoint作为基类，派生出描述一条直线的类CLine，再派生出一个矩形类CRect。要求成员函数能求出两点间的距离，矩形的周长和面积等。设计一个测试程序，并构造完整的程序。
下面程序报错：
#include <iostream.h>#include <math.h>class CPoint{private: double nPosX,nPosY;public: CPoint(double x, double y) { nPosX = x; nPosY = y; } void ShowPos() { cout<<"当前位置: x = "<<nPosX<<" , y = "<<nPosY<<endl; }};class CLine: public CPoint{private: double nLength;public: CLine() { nLength = 0; } double LineLength(double x1, double y1, double x2, double y2) { nLength = sqrt((x1-x2)*(x1-x2) + (y1-y2)*(y1-y2)); return nLength; }};class CRect: public CPoint{private: double dWideL; double dHeightL;public: CRect() { dWideL = 0; dheightL = 0; } double RectSL(double WideL, double HeightL) { return 2*(WideL + HeightL); }};int main(){ CPoint LU(0.0,1.0); CPoint LD(0.0,0.0); CPoint RU(1.0,1.0); CPoint RD(1.0,0.0); CLine Wide; CLine Length; CRect One; double AWide; double ALength; double P; AWide = Wide.LineLength(0.0,1.0,1.0,1.0); ALength = Length.LineLength(0.0,1.0,0.0,0.0); P = One.RectSL(AWide, ALength); return 0;}