Error from compiler on class point slope program

I am getting an unresolved external error for the program below. Any ideas on what is causing this error. I still have to write the program to test these functions.

error LNK2019: unresolved external symbol _main referenced in function _mainCRTStartup
fatal error LNK1120: 1 unresolved externals

Code:

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

class CartesianPoint
{
	float x,y;
	public:
		CartesianPoint(float x1,float y1)
		{
			x=x1;y=y1;
		}
		void operations()
		{
			cout<<"\n Distance from x-axis : "<<y;
			cout<<"\n Distance from y-axis : "<<x;
			float d;
			d=sqrt(x*x+y*y);
			cout<<"\n Distance from origin : "<<d;
		}
};
class LineSegment
{
	float x1,y1,x2,y2;
	float mx,my;
	public:
		LineSegment(float x11,float y11,float x12,float y12)
		{
			x1=x11;y1=y11;x2=x12;y2=y12;
			CartesianPoint c1(x1,y1),c2(x2,y2);
			c1.operations();
			c2.operations();
		}
		void midpt()
		{
			mx=(x1+x2)/2;
			my=(y1+y2)/2;
			cout<<"\n Line joining ("<<x1<<","<<y1<<") and ("<<x2<<","<<y2<<") :";
			cout<<"\n The mid-point is- ("<<mx<<","<<my<<")";
		}
		void perpbis()
		{
			float m;
			m=(y2-y1)/(x2-x1);
			m=(-1)/m;
			cout<<"\n Equation of perpendicular bisector is: ";
			cout<<"\n y-"<<my<<"="<<m<<"(x-"<<mx<<")";
		}
};
class Line
{
	float m,x,y;
	public:
		Line(float m1,float x1,float y1)
		{
			m=m1;x=x1;y=y1;
		}
		void pseq()
		{
			cout<<"\n Point-Slope Equation is :";
			cout<<"\n y-"<<y<<"="<<m<<"(x-"<<x<<")";
		}
		void intercept()
		{
			
			cout<<" \n Intercept on x-axis: "<<x-y/m;
			cout<<" \n Intercept on y-axis: "<<y-m*x;
		}

};

>> I still have to write the program to test these functions.
That's it. You have to write a main function before you can link everything into an executable. You can make an empty main to get it to finish linking if you want.

or just don't link. you could select compile rather than build (if its visual studio). If its gcc just use the -c flag