How do I eliminate the linker errors in my program?

Question

Well, I'm writing this code for my class and I'm getting linker errors LNK2019 and LNK1120 using Visual Studio. I'm not all too sure why it's doing what it's doing, but I digress.

Header file:

        #ifndef PointClass   
        #define PointClass 
        #include <iostream>

namespace PointClass
    {          
     class point
      {
        public:
        // CONSTRUCTOR
        point(double initial_x = 0.0, double initial_y = 0.0);
        // MODIFICATION MEMBER FUNCTIONS
        void shift(double x_amount, double y_amount);
        void rotate90( );
        // CONSTANT MEMBER FUNCTIONS
        double get_x( ) const { return x; } 
        double get_y( ) const { return y; }
        // FRIEND FUNCTION
        friend std::istream& operator >>(std::istream& ins, point& target);
        double x, y; // x and y coordinates of this point
};

// NONMEMBER FUNCTIONS for the point class 
double distance(const point& p1, const point& p2);
point middle(const point& p1, const point& p2);
point operator +(const point& p1, const point& p2);
bool operator ==(const point& p1, const point& p2);
bool operator !=(const point& p1, const point& p2);
std::ostream& operator <<(std::ostream & outs, const point& source);
        }

        #endif 

And the CPP file:

        #include <iostream>
        #include <math.h>
        #include "point.h"
        using namespace std;

        namespace PointClass
        {
           point::point(double initial_x, double initial_y)
 {
    x = initial_x;   // Constructor sets point to a given position
    y = initial_y;
 }

void point::shift(double x_amount, double y_amount)
 {
    x += x_amount;
    y += y_amount;   
 }

void point::rotate90( )
 {
    double new_x;
    double new_y;
    new_x = y;  // For a 90 degree clockwise rotation the new y is -1
    new_y = -x; // times original x, and the new x is the original y
    x = new_x;
    y = new_y;   
  }

bool operator ==(const point& p1, const point& p2)
 {
    return
        (p1.get_x( ) == p2.get_x( )) 
        &&
        (p1.get_y( ) == p2.get_y( ));
 }

bool operator !=(const point& p1, const point& p2)
  {
    return !(p1 == p2);
  }

point operator +(const point& p1, const point& p2)
  {
    double x_sum, y_sum;

    // Compute the x and y of the sum:
    x_sum = (p1.get_x( ) + p2.get_x( ));
    y_sum = (p1.get_y( ) + p2.get_y( ));
    point sum(x_sum, y_sum);
    return sum;
  }

ostream& operator <<(ostream& outs, const point& source)
 {
    outs << source.get_x( ) <<  " "  << source.get_y( );
    return outs;
 }

istream& operator >>(istream& ins, point& target)
{
    ins >> target.x >> target.y;
    return ins;
 }

int rotations_needed(point p)
  {
    int answer;

    answer = 0;
    while ((p.get_x( ) < 0) || (p.get_y( ) < 0))
        {
          p.rotate90( );
          ++answer;
        }
    return answer;
  }

void rotate_to_upper_right(point& p)
  {
    while ((p.get_x( ) < 0) || (p.get_y( ) < 0))
      p.rotate90( );
  }

double distance(const point& p1, const point& p2)
{
    double a, b, c_squared;

    // Calculate differences in x and y coordinates
    a = p1.get_x( ) - p2.get_x( ); // Difference in x coordinates
    b = p1.get_y( ) - p2.get_y( ); // Difference in y coordinates

    // Pythagorean Theorem to calculate square of distance between points
    c_squared = a*a + b*b;

    return sqrt(c_squared); // sqrt calculates square root
  }

         point middle(const point& p1, const point& p2)
{
    double x_midpoint, y_midpoint;

    // Compute the x and y midpoints
    x_midpoint = (p1.get_x( ) + p2.get_x( )) / 2;
    y_midpoint = (p1.get_y( ) + p2.get_y( )) / 2;

    // Construct a new point and return it
    point midpoint(x_midpoint, y_midpoint);
    return midpoint;
         }
        }

Is there anything I can do to fix it?

Answer 1

You seem to have a namespace called PointClass, and headers guards using PointClass. Consider changing that for a start.

#ifndef PointClass // Rename this, it might be clashing with your namespace. 
#define PointClass
namespace PointClass

Answer 2

Read the error message:

Linker Error 2019: unresolved external symbol 'symbol' referenced in function 'function'

It gives you the name of a global variable or (more likely) a function that is called without definition, i.e. you didn't write a function body for it or did not add the source with its definition to your project.

The error LNK1120 is a consequence of the first error.

Without the complete error message it's hard to tell. As @Darcy pointed out, you should change the names in #ifdefand #define since

#define PointClass

tells the preprocessor to substitute this name with nothing in the following source code. This results in a unnamed local namespace. All names defined inside this local namespace are accessible from inside that compilation unit (cpp file) only.

Choose a unique name for the "include guard" which should not occure anywhere else in the program. A possible pattern could imitate the name of the header file:

#define POINT_H