将下面类定义补充完整,类Point是坐标点,Line是两点间的直线,Distance求出两点间的距离。class Line;class Point{private:int x,y; ____;public:Point(int i=0,int j=0){ x=i;y=j:}};class Line{private:Point p1,p2;public:Line(Point xpl,Point xp2):pl(xp1),p2(xp2){};double Distance();};double Line::Distance(){ double dx=p2.x-pl.x;____;return sqrt(dx*dx+dy*dy);}