Question

I have a class:

class Point3D : public Point{
    protected:
        float x;
        float y;
        float z;

    public:
        Point3D(){x=0; y=0; z=0;}
        Point3D(const Point3D & point){x = point.x; y = point.y; z = point.z;} 
        Point3D(float _x,float _y,float _z){x = _x; y = _y; z = _z;}

        inline const Point3D operator+(const Vector3D &);

        const Point3D & operator+(const Point3D &point){
            float xT = x + point.getX();
            float yT = y + point.getY();
            float zT = z + point.getZ();
            return Point3D(xT, yT, zT);
        }
...

When I use it that way:

Point3D point = Point3D(10,0,10);

Everything works fine.

When I write:

Point3D point = Point3D(10,0,10);
Point3D point2 = Point3D(0,0,0) + point();

Also it’s ok (point2 = point). When I add something more than (0,0,0) it’s also working.

But when I want just to:

Point3D point = Point3D(10,0,10);
someFunction( Point3D(0,0,0) + point ); //will get strange (x,y,z)

The function get value of some (in my opinion) random (x,y,z). Why?

What’s even stranger, in that similar example everything will be working again:

Point3D point = Point3D(10,0,10);
Point3D point2 = Point3D(0,0,0) + point;
someFunction( point2 );  // will get (10,0,10)

What’s the reason for that strange behaviour?

Answer 1

The operator+() is returning a dangling reference, the returned reference is referring to a Point3D instance that is destroyed when operator+() returns. Change to:

Point3D operator+(const Point3D &point) const {

so a copy is returned, and make it const as it has no reason to be changing anything.