[Gambas-devel] "Overriding" virtual classes

[Tobias Boege]

On Mon, 23 Jun 2014, Beno?t Minisini wrote:
> Le 23/06/2014 19:15, Tobias Boege a ?crit :
> > Hi Benoit,
> >
> > for the Graph class in gb.data, I thought of having a non-creatable Graph
> > class (we had that topic already in the past) which is the interface
> > necessary for all concrete graph implementations. This class exposes a
> > Graph.Edges property to a virtual .Graph.Edges class whose interface and
> > default implementation are also specified.
> >
> > As I said earlier, I really want to have it this way because it enables the
> > programmer to makde a decision on what graph data structure they need to use
> > for their application and it also makes modelling non-Graph data as a Graph
> > very easy. An example (which I want to implement as a Gambas example for
> > Graphs) would be to have an Image object. Each pixel is a vertex and edges
> > are implicitly defined between neighbouring pixels of the same colour. If
> > you do a breadth-first search on this graph from a pixel, you are actually
> > doing a flood-fill on the Image.
> >
> > This is where the virtual classes become an unbelievably handy thing: you
> > don't have to create explicit vertex and edge *objects* from the Image but
> > you can pipe the user requests through the virtual object's interface and
> > operate on the backing data.
> >
> > If I now create a GraphMatrix class basing on adjacency matrices which
> > Inherits Graph, I also need to point the Graph.Edges property to some
> > concrete .GraphMatrix.Edges class which operates on the adjacency matrix.
> >
> > Is there a way to do this or can it be implemented? I could store the needed
> > function pointers myself in a separate data structure and let the standard
> > .Graph.Edges class reference those but this would not help Gambas programmers
> > if they want to implement their Graph classes non-natively, right?
> >
> > So, I'm asking for a way to replace a virtual class with another virtual
> > class in the inheritance process. [ Maybe you can make an exception in the
> > inheritance checking routines since virtual classes are a thing of native
> > components and native components writers should be supposed to know what
> > they're doing? :-) ]
> >
> > Regards,
> > Tobi
> >
> 
> And if ".GraphMatrix.Edges" inherits ".Graph.Edges" too, doesn't it 
> solve the problem?
> 

Oh! I didn't know it was possible to derived virtual classes from others.
Great feature!

A try will show how far I can get with the solution I just sent in another
mail. If it's not satisfying, I'll return to this thread...

Regards,
Tobi

-- 
"There's an old saying: Don't change anything... ever!" -- Mr. Monk