[gecode-users] Extending Integer Variable

[Christian Schulte]

Dear Vincent,

let me first elaborate a little bit how Gecode's variables and propagators
are structured before I actually attempt to give an answer.

Gecode has three important concepts regarding variables:
 - variables: they are used for modeling, and the only operations they
provide are good for modeling. In particular,
   variables to not provide operations to update their domain. Examples are
IntVar, BoolVar, etc. Variables are
   hence provide nothing but an interface to variable implementations
(actually, variables are implemented as
   a pointer to variable implementations).
 - variable implementations: they are the real objects storing the domain
and which propagators and advisors must be
   executed when the domain changes. They themselves are constructed in two
steps: a domain independent stub that is 
   automatically generated from a simple description (just a few lines). The
stub is concerned with events, how
   propagators can be subscribed to, and linking the variable
implementations to the Gecode kernel etc. The real 
   variable implementation then inherits from the generated stub and adds
the variable domain and so on.
 - views: like variables they are nothing but an interface for variable
implementations. However, views are used
   by propagators and branching and provide operations to update variable
domains, subscribe propagators to changes,
   and so on. More importantly, for the same variable implementation several
views exist: a MinusView for a variable
   implementation x, for example, provides an interface that behaves as if
the operations on the MinusView are executed
   on -x rather than x. Examples are IntView, BoolView, MinusView,
OffsetView, ... For more on views there is paper
   on the Gecode webpage.

Propagators themselves have two parts: a post function and the actual
propagator. A post function (such as rel, distinct, linear, etc.) is used
for modeling and hence takes variables as arguments. It then chooses the
right propagator and the right views (converting variables to views by
passing the underlying reference to the variable implementation).

Propagators are typically parametric (as templates in C++) with respect to
the view they are using: for example, the Max propagator is instantiated for
the max posting function with an IntView whereas it is also instantiated
with a MinusView for the min posting function.

Sorry for the long blurb, let's move to the answer: as Gecode is designed,
you can reuse propagators provided that you implement: a variable
implementation by yourself (that's what you want to do), variables for them
(so that you can model), and views for them (so that you can reuse the
propagators), post functions (because they talk about your variables).

This is a lot of effort! If you want to make something that is used (rather
than an experiment), then one could also make views and variables parametric
wrt the variable implementation used. That is easy but requires intimate
Gecode knowledge. Then, in principal, one could also make the post functions
parametric but this is almost impossible in reality as they should provide
simple static linkage as they constitute the basic interface to Gecode.

So it really depends on what you want to do. For an experiment, hack the
variable implementation. For something, first do the experiment ;-)

What do you want to do? Do you think it me be generally useful to be
included in Gecode anyway?

All the best
Christian

--
Christian Schulte, www.ict.kth.se/~cschulte/


-----Original Message-----
From: users-bounces at gecode.org [mailto:users-bounces at gecode.org] On Behalf
Of Vincent Barichard
Sent: Wednesday, April 02, 2008 4:08 PM
To: gecode list
Subject: [gecode-users] Extending Integer Variable

Hi,

I am trying to extend IntVar in order to use it in my self(-made algorithms.
I would like to use an unaltered Gecode, but I need to add an "int" member
in 
the IntVar implementation. I don't want to make my own Integer Variable
class 
because I also need to use the algorithms designed in Gecode.
Is there a clean way to extend this class (by inheritance maybe ....) and
add 
a member without altering Gecode ?

Thank you for your assistance.
Best regards,

Vincent
-- 
Vincent Barichard        Université d'Angers (LERIA)
Tel: 02 41 73 52 06      Département Informatique
Fax: 02 41 73 50 73      H203

_______________________________________________
Gecode users mailing list
users at gecode.org
https://www.gecode.org/mailman/listinfo/gecode-users