Re: Solving the data inheritance problem

From:

Lourens Veen <lourens@rainbowdesert.net>

Newsgroups:

comp.lang.c++.moderated

Date:

6 Dec 2006 12:10:42 -0500

Message-ID:

<631b2$4576bdf0$8259a2fa$23412@news1.tudelft.nl>

Kaba wrote:

For a concrete example of the need for a common implementation is
the following:

Let a Shape be something that can be drawn. The drawing of a Shape
is controlled by control points. For example a circle would be
controlled by two control points: first marks the center of the
circle and the second marks some point on the circle. These are the
kind of Shapes familiar from many vector graphic programs, such as
Visio. The behaviour of the control points should be uniform
(mainly, you can move them around) and should not be dependent on
the specific subclass.

This demonstrates a class that has two different properties:
1) It has something that is dependent on a particular subclass: the
drawing. Maybe one could call this "abstract similarity".

2) It has something that is is not dependent on a particular
subclass: the handling of the control points. Maybe one could call
this "concrete similarity".

The problem is to implement this concrete similarity in a good way.

So what does a Shape contain? Control points? How many of them? You
don't know that either without knowing what kind of shape it is.
What's wrong with the following?

class ControlPoint {
private:
    double x, y;
public:
    // handling
};

struct Shape {
    virtual void draw() const = 0;
};

struct Circle : public Shape {
    ControlPoint centre, edge;

    virtual void draw() const;
};

struct Triangle : public Shape {
    ControlPoint point[3];

    virtual void draw() const;
};

// implementation here

Lourens

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