关于派生的问题 求指教

#include <iostream>
#include <math.h>
using namespace std;
class point
{
protected:
    double x1;
    double y1;
double x2;
double y2;
//int color;                            //颜色属性
    //int width;                            //线宽属性
public:
point(double,double,double,double);
    void display();                       //显示坐标
    double distance(point &a);            //计算两点间距离
    double getx(){return x1;}
    double gety(){return y1;}                        //返回x,y的值
};
double point::distance(point &a)
{
double v;
v=sqrt((x1- x2)* (x1- x2)+( y1- y2)* ( y1- y2));
    cout<<"两点距离："<<v<<endl;
return 0;
}
void point::display()
{
cout<<x1<<","<<y1<<endl<<x2<<","<<y2<<endl;
}
point::point(double xn1,double yn1,double xn2,double yn2)
{
cout<<"调point构造函数"<<endl;
x1=xn1;
y1=yn1;
x2=xn2;
y2=yn2;
}
class line:public point      //由点类派生的线段类
{
protected:
double x;
    double y;
 
public:
line(double,double, double,double);
    void display();         //显示线的信息
    double distance();      //计算两点间距离
};
line::line(double q,double w, double e,double r):point(double xn1,double yn1,double xn2,double yn2){};
{
cout<<"调line构造函数"<<endl;
}
void main()
{
point a(7.8,9.8,34.5,67.8);
a.distance(a);
  point b(7.8,9.8,34.5,67.8);
line c();
//a.display();
}

唉。。。我不会写了

最后要实现这个：