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;
}
};