定义point累来描述平面上的点,包含私有数据成员x和y.定义友元函数distance计算两点距离（说明两点p1（x1,y1）,p2（x2,y2）,则两点距离=根号（x1减去x2差的平方加上y1和y2差的平方）
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#include
class POINT
{public;
Point(double a,double b){ x=a;y=b};
fiend double square(Point &a,Point &b);
private:
double x,y;
};double square(Point &a,Point &b) { return sqr((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y))};
void main()
{Point p1(x1,y1),p2(x2,y2);
double l;
l=square(p1,p2);
}