下面程序用来求直角三角形斜边长度。

#include <iostream.h>

#include <math.h>

class Point

{private:

double x,y;

__________

public:

Point(double i=0,double j=0)

{x=i;y=j;}

Point(Point &p)

{x=p.x;y=p.y;}

};

class Line

{private:

Point p1,p2;

public:

Line(Point &xp1,Point &xp2):________{}

double GetLength();

};

double Line::GetLength()

{double dx=p2.x-p1.x;

double dy=p2.y-p1.y;

return sqrt(dx*dx+dy*dy);

}

void main()

{ Point p1,p2(6,8);

Line L1(p1,p2);

cout<<L1.GetLength()<<endl;

}

答案：friend Line;,p1(xp1),p2(xp2)

［解析］友元类的使用，定义Line是Point类的友元类，成员对象的初始化采用列表的形式。