C++学习：继承问题

[南气子水]

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;
}

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

[昵称很不好取]

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

[a707000646]

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;
}