Returning an object from a method in C++

Question

I belive the answer to this question is really simple, but I just can't get this thing working properly. I have basically created two classes; one for points and one for polygons. Polygons consists of a dynamic list of points.

However, when i try to overload the + operator in the points class and make it return a polygon of the two points i get som weird output and a "Debug assertion failed" after i close the console window.

Here is the + operator overloading method:

CPolygon CPoint::operator + (CPoint pointToAdd) {
    CPolygon polygon;
    polygon.addPoint(this);
    polygon.addPoint(pointToAdd);

    cout << polygon.toString();

    return polygon;
}

When i now try to use this method I get the following output for instance:

(5, 2, 3) - (1, 1, 2)

(444417074, -33686019, , -1555471217) - (-1424299942, 0, 0)

The first of the ouputed lines is from the method itself, whereas the 2nd line is from the place where the polygon is being returned to.

I really have noe idea what is happening to my polygon object on the way from being inside the method to its returning way to the calling code.

I'd be very thankful if anyone could give me a bit of insight on this one :)

EDIT

Here are the addPoint methods of the polygon class:

void CPolygon::addPoint(CPoint pointToAdd) {
    nrOfPoints++;

    // Create a temp array of the new size
    CPoint * tempPoints = new CPoint[nrOfPoints];

    // Copy all points to the temp array
    for(int i = 0; i < (nrOfPoints - 1); i++) {
        tempPoints[i] = points[i];
    }

    // Add the new point to the end of the array
    tempPoints[nrOfPoints - 1] = pointToAdd;

    // Delete the old array and set the temp array as the new array
    delete[] points;
    points = tempPoints;
}

void CPolygon::addPoint(CPoint * pointToAdd) {
    addPoint(* pointToAdd);
}

Answer 1

I did as PlasmaHH and urzeit suggested above and implemented the copy constructor for the polygon class, and guess what, it solved the problem! :) Thanks to everyone who has been helping me out!

The copy constructor for the polygon class looks like this:

CPolygon::CPolygon(const CPolygon & polygon) :
    nrOfPoints(polygon.nrOfPoints)
{
    points = new CPoint[nrOfPoints];

    // Add all the points from the polygon to be copied
    for(int i = 0; i < nrOfPoints; i++) {
        points[i] = polygon.points[i];
    }
}

Answer 2

Your code:

CPolygon CPoint::operator + (CPoint pointToAdd) {
    CPolygon polygon;


    polygon.addPoint(this);
    polygon.addPoint(pointToAdd);

    cout << polygon.toString();

    return polygon;
}

I see strange that for polygon.addPoint(this) you are adding a pointer to CPolygon, since this is a pointer. Should you use polygon.addPoint(*this) instead? In polygon.addPoint(pointToAdd) you are adding by value or reference.

If you need further help, please, add all the prototypes for function CPolygon::addPoint.

Answer 3

Your polygon object is declared on stack, you will lose reference to it after the operator scope!

Try to declare it:

CPolygon* polygon = new Polygon(...);

and your signature should look like:

CPolygon* CPoint::operator + (CPoint pointToAdd) 

Indeed using raw pointers it's a bad idea and you have to take care of it outside the scope, a better solution will be to user smart pointers:

std::auto_ptr<CPolygon> CPoint::operator + (CPoint pointToAdd) 
{
    std::auto_ptr<CPolygon> polygon(new CPolygon);
    // do stuff here
    return polygon;
}

// ...

{
  std::auto_ptr<CPolygon> polygon = firstPoint + secondPoint;
  // working with CPolygon

  // auto_ptr frees polygon 
}