Inheriting question...

[defiler_z]

Hi ...
I have 3 classes : CBase(char *name)->C2DPoint(float x, float y)->C3DPoint(x, y, z coordinates). I have predefined all operators(+, -, *, /, +=, -=, *=, /=, <<, >>, =, copy constructor - all working properly).
In C3DPoint class I have :

C3DPoint::C3DPoint(const C2DPoint &ob) : C2DPoint(ob)

Here an example of the main() function :

int main()
{
      C2DPoint a;
      C3DPoint b;

      cout << b + a;     // working because of the method above

      cout << a + b;     // also working but i have no such method in 
                                  //C2DPoint class and the result is object 
                                  // of C2DPoint...it should be C3DPoint object
                                  // in fact , it should not work at all I think
}

I hope that I've posted the problem clearly enough ... if it isn't I can attach the files ....
Thanks in advance

[wien]

So C3DPoint inherits C2DPoint publically, and C2DPoint has a + operator defined? In that case, it's not really that strange. Remember that public inheritance means IS-A, and because of that C3DPoint IS-A C2DPoint. This makes the operator + defined for C2DPoint fit perfectly when you add a C2DPoint and a C3DPoint.

To tell you the truth, I don't really think public inheritance is the right relation between the two classes here. A C3DPoint is not a C2DPoint in my opinion. It only looks very similar. At best you could say it HAS-A C2DPoint, in which case you would use a member to model the relationship.

[cilu]

Moreover, I don't see how you can naturally add a 2D point to a 3D point...

[defiler_z]

Moreover, I don't see how you can naturally add a 2D point to a 3D point...

You just move over x and y coordinates .. z coordinate stays the same.

A C3DPoint is not a C2DPoint in my opinion.

C3DPoint just add a z coordinate ... if I create 2D point it is 2D .. if I create 3D point it first create 2D and then add the z coordinate and become 3D....

I attach the source code if you have time to look at it ... I need to know why this happens and how can i prevent that ...
Thanks again

[Graham]

I have to agree with wien: a 3D point is not a 2D point. This example is similar to the circle/ellipse example of bad inheritance. Just because it looks that way mathematically, doesn't mean that it's a good idea to use inheritance in this way.

[cilu]

You just move over x and y coordinates .. z coordinate stays the same.

Why not move the y and z coordinates and keep the x unchanged?

Anyway, please use zip to compress files, not rar. Zip is a cross-platform format. For instance, I don't have WinRar, I can't extract your files.

[marten_range]

Also, aside from the fact that I find it strange having C3DPoint inherit C2DPoint , since these classes will most often be used as values you set yourself up for slicing problems by publically inheriting C2DPoint.

If I have a number of fixed dimensions I would prefer a point (vector) to be a template class with a numerical template parameter that decides the dimension of the point (vector).

template<std::size_t n>
struct vector
{
    // ... methods
    double m_vector[n];
};

[Graham]

Minor niggle: I wouldn't call it vector, though.

[defiler_z]

Well,
thanks for the answers ... i figured out what i needed ...