[gecode-users] Posting propagator : IntView lifetime ?

[Christian Schulte]

Hmmm, I am not sure that I understand correctly but I try to summarize:
propagators never compute with variables (that is, IntVar) but only with
IntViews. Both IntVars and IntViews are proxies to integer variable
implementations.

Posting in general works as follows: a post function takes its arguments as
IntVars (or IntVarArgs). An example of this function is linear, rel,
distinct, etc. A post function calls some post member function of a subclass
of a propagator (this is the static member function post of a propagator).
Post member functions typically take IntViews (or other views as arguments)
which means that a post function converts IntVars to IntViews (through a
copy constructor Int::IntView(const IntVar&)) or explicitly creates
ViewArrays.

A post member function then uses new together with a constructor to create
the propagator. What a constructor does is that it stores the views in the
propagator's state (so views have been created in the post function, have
been passed to the post member function, have been passed to a constructor,
and then the constructor stores them). After storing, the propagator will
also subscribe to the stored views. 

After that a propagator knows its views: the propagators uses these views in
propagate(...) but also during copy (and in the constructor used during
cloning) and eventually for cancelling subscriptions in the destructor.

So the answer is that Views live as long as propagators live and they are
created only once per propagator in the post function. The integer variable
implementation to which a view is a proxy will live as long as there is a
proxy to it (be it an IntVar or IntView).

Hope that helps
Christian

PS: We found the distinction between "post function" and "post member
function" quite useful in practice. "Post function" typically do consistency
checking and preprocessing (are all integer values in the right range, can
overflow happen). Then the "post member function" typiccaly performs tasks
that are related to the particular propagator (is there a specialized
version to be posted instead, such as ternary versus n-ary max, etc; is
there sharing and so on). Post member functions are also used by other
propagators, for example when an abs(x)=y propagator is rewritten to an
equality propagator x=y in case that x is known to be non-negative.

--
Christian Schulte, http://www.imit.kth.se/~schulte/ 

-----Original Message-----
From: users-bounces at gecode.org [mailto:users-bounces at gecode.org] On Behalf
Of jeremie.vautard
Sent: Wednesday, April 12, 2006 11:24 AM
To: users
Subject: [gecode-users] Posting propagator : IntView lifetime ?


Hi ! 

I have been checking how some of  the public static post members of
propagator classes works for posting constraints passing IntVar arguments,
however I get lost in the code. Can anyone tell me exactly how this works ?
If I understand well, for the subscription part, it is only necessary to
create the Intview, subscribe the new propagator to this view, and then, the
view can be destroyed, as the subscription called the InVarImp subscribe
method. But, concerning the domain shrinking of the variables, I have to
keep a trace of the views of the variables I possibly modify. During space
cloning, how is the update of these views managed ? Must I create new views
from the IntVars each time the propagate() method is called ? 

My point is to create a propagator which concerns two integer variables and
post it using only the intVars of the concerned two variables. What is the
best way to do this ? 


Jérémie Vautard

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