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：

C/C++ code

#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;}

错误信息为： error C2512: 'CPoint' : no appropriate default constructor available

[解决办法]
子类构造时候需要先调用父类的构造函数，因此子类的构造函数中需要告知如何构造父类，如果父类具有默认构造函数，则可以不写
[解决办法]
1楼正解
Cline的构造函数可以这么写
CLine(double x, double y):CPoint(x,y)
{
nPosX = x;
nPosY = y;
}