求问题解释，请高手帮忙！！！

#include<iostream.h>
class point
{
public:
    point(double x1=0,double y1=0)
    {
        x=x1;
        y=y1;
    }
    double display()
    {
        cout<<"("<<x<<","<<y<<")";
        return 0;
    }
    double getx()
    {
        return x;
    }
    double gety()
    {
        return y;
    }

private:
    double x,y;
};

class line
{
public:
    line(point &pp1,point &pp2):p1(pp1),p2(pp2)   
{
        k=(p1.gety()-p2.gety())/(p1.getx()-p2.getx());
    }
    void display()
    {
        cout<<"该直线斜率是：";
        cout<<"k="<<k<<endl;
        cout<<"直线上的两点坐标分别是：";
        p1.display();
        cout<<",";
        p2.display();
        cout<<endl;
    }
   
private:
    point p1,p2;
    double k;
};

void main()
{
    double a,b,c,d;
    cout<<"输出该直线上的两点："<<endl;
    cout<<"输入第一点坐标：";
    cin>>a>>b;
    cout<<"输入第二点坐标：";
    cin>>c>>d;
    point q1(a,b);
    point q2(c,d);
    line L1(q1,q2);
    L1.display();
}

 line(point &pp1,point &pp2):p1(pp1),p2(pp2)    //这句是什么意思。。。。？？是联系两个类的ma ?
类与类之间还有什么方式可以联系起来？?