I get error LNK1561

I am learning to design classes and I have found a source file of the book "C ++ for C programmers" in which an elementary implementation is made for complex numbers (I already know that the type exists complex in the STL , but what interests me is learning to design elementary classes); the fact is that the linker gives me the error LNK1561 , as if I had forgotten the function main ...

// An elementary implementation of type complex
#include <iostream>      //IO library
#include <string>        //string type

using namespace std;

class complex {
public:                   //universal access to interface

   void re_assign(double r) { real = r; }
   void im_assign(double im) { imaginary = im; }
   void print() const
   {
     cout << "(" << real << ","
          << imaginary << "i)" << endl;
   }

   friend complex operator+(complex, complex);

private:                  //restricted access to implementation

   double real, imaginary;

};

//overload +
complex operator+(complex x, complex y)
{
   complex t;
   t.real = x.real + y.real;
   t.imaginary = x.imaginary + y.imaginary;
   return t;
}

int main()
{
   complex x, y, z;
   x.re_assign(9.5);
   x.im_assign(-4.5);
   y.re_assign(4.2);
   y.im_assign(6.0);

   z = x + y;
   x.print();
   y.print();
   z.print();

   int look; cin >> look;
}