Re: Solving the data inheritance problem
Kaba wrote:
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.
See the post for James Kanze. The derived class uses the interface
given by Shape to decide its need for control points.
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
This way you can't move the control points without knowing the
concrete subclass (unless you make the handling functions virtual
but then code is replicated).
I don't see where you get code replication. A circle has two control
points, a pentagon has five. For a pentagon, which is which is
irrelevant, as long as they each have a unique identity. For a
circle, the centre point is fundamentally different than the edge
point. You couldn't replicate code if you wanted to.
You want the rest of the code to be able to see each shape as
something having a bunch of control points. The way to do that is to
add (virtual) methods for that to Shape, and then have each subclass
implement them, depending on which control points it has.
It occurs to me that maybe we're looking at this from the wrong end.
It seems to me that these control points are actually more important
than the shapes they are part of. How about making a Document class
that has a set of control points, which each have a pointer to the
Shape they are part of, and a set of shapes, which would have
pointers back to their control points rather than containing them.
A new shape is created and owned by a Document, which keeps track of
it and its control points. All it would need to know is how many
control points there are for a shape, which is easily added as a
constant to the derived class. The Document can then move a control
point without even knowing which object it belongs to or what the
type of that object is.
Lourens
--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]