error: ld returned 1 exit status| C++编译时的问题

Question

#include <iostream>
#include <cmath>
using namespace std;

class Point
{
   private:
       double x,y;
   public:
       Point()
       {
           x=0;
           y=0;
       }
       Point(int x_,int y_)
       {
           x=x_;
           y=y_;
       }
       Point(const Point &p)
       {
           x=p.x;
           y=p.y;
       }
       set(int x_,int y_)
       {
           x=x_;
           y=y_;
       }
       double Distance(Point& Point) 
       {
           return sqrt(pow(x-Point.x,2)+pow(y-Point.y,2)); 
       }
       void output()
       {
           cout<<"("<<x<<","<<y<<")"<<endl;
       }
};

class Line
{
   private:
       static int num;
       Point p1,p2;
       int color;
   public:
       Line(); 
       Line(int p1_x,int p1_y,int p2_x,int p2_y, int c);
       Line(Point p1, Point p2, int c);
       Line(const Line&l)
       {
           p1_x=l.p1_x;
           p2=l.p2;
           color=l.color;
       }
       void set_line(int p1_x,int p1_y,int p2_x,int p2_y);
       void set_line(Point q1, Point q2);
       void display();
};

Line::Line(): p1(0,0), p2(0,0), color(0)
{ 
   num++;
}

Line::Line(int p1_x,int p1_y,int p2_x,int p2_y, int c): p1(p1_x, p1_y), p2(p2_x, p2_y), color(c)
{ 
   num++;
}

Line::Line(Point p1, Point p2, int c): p1(p1), p2(p2), color(c)
{ 
   num++;
}

void Line::set_line(int p1_x,int p1_y,int p2_x,int p2_y)
{ 
   p1.set(p1_x, p1_y);
   p2.set(p2_x, p2_y);
}

void Line::set_line(Point q1, Point q2)
{
   p1 = q1;
   p2 = q2;
}

void Line::display()
{
   cout<<"起点:";
   p1.output();
   cout<<"终点:";
   p2.output();
   cout<<"颜色:"<<color<<endl;
   cout<<"长度:"<<p1.Distance(p2)<<endl;
   cout<<"线段的个数:"<<num<<endl;
   cout<<endl;
}

int main()
{
Line l1(10, 20, 30, 40, 5);
l1.display();
Point p1(23, 56), p2(14, 37);
Line l2(p1,p2,50);
l2.display();
l2.set_line(p1, p2);
l2.display();
Line l3(l1);
l3.display();
Line l4;
l4.display();
l4.set_line(10, 20, 30, 40);
l4.display();
} 

Answer 1

int main()没有返回值，加return 0就好了