Question

To get a bit of practice with OOP i’m trying to make a Point class (has 2 ints, x & y) and a Line class (has 2 Points).

Now when i go to build my main.cpp i get errors like..

“undefined reference to `Point::Point(float, float)’ “ and

” undefined reference to `Line::Line(Point, Point)'”

At a loss as to why, perhaps you could take a brief look at my files? It’d be much appreciated!

Main.cpp

#include "Point.hpp"
#include "Line.hpp"
#include <iostream>

using namespace std;

int main()
{
    Point p1(2.0f, 8.0f); // should default to (0, 0) as specified
    Point p2(4.0f, 10.0f);  // should override default

    p1.setX(17);


    if ( p1.atOrigin() && p2.atOrigin() )
        cout << "Both points are at origin!" << endl;
    else
    {
        cout << "p1 = ( " << p1.getX() << " , " << p1.getY() << " )" <<endl;
        cout << "p2 = ( " << p2.getX() << " , " << p2.getY() << " )" <<endl;
    }

    Line line(p1, p2);
    Point midpoint = line.midpoint();
    cout << "p1 = ( " << midpoint.getX() << " , " << midpoint.getY() << " )" <<endl;
    return 0;
}

Line.hpp

#ifndef _LINE_HPP_
#define _LINE_HPP_

#include "Point.hpp"

class Line{
public:
    Line(Point p1, Point p2);
    //void setp1(Point p1);
    //void setp2(Point p2);
    //Point getp1 finish

    Point midpoint();
    int length();

private:
    int _length;
    Point _midpoint;
    Point _p1, _p2;
};

#endif

Line.cpp

#include "Line.hpp"
#include <math.h>

Line::Line(Point p1, Point p2) : _p1(p1), _p2(p2)
{
}
Point Line::midpoint()
{
    _midpoint.setX() = (_p1.getX()+ _p2.getX()) /2;
    _midpoint.setY() = (_p1.getY()+ _p2.getY()) /2;
}
int Line::length()
{
    //a^2 + b^2 = c^2

    _length = sqrt( ( (pow( _p2.getX() - _p1.getX(), 2 ))
                     +(pow( _p2.getY() - _p1.getY(), 2 )) ) );
}

Point.hpp

#ifndef _POINT_HPP_
#define _POINT_HPP_

class Point {
public:
    Point( float x = 0, float y = 0);
    float getX() const;
    float getY() const;
    void setX(float x = 0);
    void setY(float y = 0);
    void setXY(float x = 0, float y = 0);
    bool atOrigin() const;

private:
    float _x, _y;

};

#endif

Point.cpp

#include "Point.hpp"

Point::Point(float x, float y) : _x(x), _y(y)
{
}

float Point::getX() const
{
    return _x;
}
float Point::getY() const
{
    return _y;
}
void Point::setX(float x)
{
    //if (x >= 0 &&
    _x = x;
}
void Point::setY(float y)
{
    //might want to check
    _y = y;
}
void Point::setXY(float x , float y )
{
    setX(x);
    setY(y);
}
bool Point::atOrigin() const
{
    if ( _x == 0 && _y == 0)
        return true;

    return false;
}

Answer 1

In C++, not only do you have to compile main.cpp, but you also have to compile your Line.cpp and Point.cpp files. Then, when you have them all compiled into object files, you must link the object files together. This is handled automatically by some other languages such as Java.

The exact instructions on how to do this will depend on which development environment you are using.