From: Giacomo Ritucci
Subject: Mutually dependent classes
Date: Sat, 16 May 2009 10:07:50 +0000
Message-ID: <gum39m$ngd$1@lec.cs.unibo.it>

I'm writing a simple network simulator where the network-link and the
network-interface classes need to reference each other.


;;; file: network-interface.lisp ;;;;;;;;;;;;;;;;;;;;

(defclass network-interface (buffer)
  ((link
    :type network-link)))

;;; end of file: network-interface.lisp ;;;;;;;;;;;;;


;;; file: network-link.lisp ;;;;;;;;;;;;;;;;;;;;;;;;;

(defclass network-link (buffer)
  ((endpoint-A
    :type network-interface)
   (endpoint-B
    :type network-interface)))

;;; end of file: network-link.lisp ;;;;;;;;;;;;;;;;;;


How to solve the circular dependency?

I'm not sure about these possible solutions:

1) a simple "forward declaration", but not sure if redefining classes is safe
   (newbie alert!)

   ;;; file: network-interface.lisp ;;;;;;;;;;;;;;;;;;;;

   (defclass network-link (buffer)
     nil)

   (defclass network-interface (buffer)
     ((link
       :type network-link)))

   ;;; end of file: network-interface.lisp ;;;;;;;;;;;;;

2) removing the reference from the classes, using a "connector" object that
   keeps track of the connected objects (e.g. a simple adjacency list
   graph representation).

3) suggestions welcome


I'd prefer the first solution because it's simple and allows type check, but I don't
know if it's safe.

Regards,
Giacomo Ritucci

• Re: Mutually dependent classes Pascal Costanza
  • Re: Mutually dependent classes Giacomo Ritucci

From: Pascal Costanza
Subject: Re: Mutually dependent classes
Date: Sat, 16 May 2009 11:08:03 +0000
Message-ID: <777l4jF1fckdsU1@mid.individual.net>

Giacomo Ritucci wrote:
> I'm writing a simple network simulator where the network-link and the
> network-interface classes need to reference each other.
> 
> 
> ;;; file: network-interface.lisp ;;;;;;;;;;;;;;;;;;;;
> 
> (defclass network-interface (buffer)
>   ((link
>     :type network-link)))
> 
> ;;; end of file: network-interface.lisp ;;;;;;;;;;;;;
> 
> 
> ;;; file: network-link.lisp ;;;;;;;;;;;;;;;;;;;;;;;;;
> 
> (defclass network-link (buffer)
>   ((endpoint-A
>     :type network-interface)
>    (endpoint-B
>     :type network-interface)))
> 
> ;;; end of file: network-link.lisp ;;;;;;;;;;;;;;;;;;
> 
> 
> How to solve the circular dependency?
> 
> I'm not sure about these possible solutions:
> 
> 1) a simple "forward declaration", but not sure if redefining classes is safe
>    (newbie alert!)
> 
>    ;;; file: network-interface.lisp ;;;;;;;;;;;;;;;;;;;;
> 
>    (defclass network-link (buffer)
>      nil)
> 
>    (defclass network-interface (buffer)
>      ((link
>        :type network-link)))
> 
>    ;;; end of file: network-interface.lisp ;;;;;;;;;;;;;
> 
> 2) removing the reference from the classes, using a "connector" object that
>    keeps track of the connected objects (e.g. a simple adjacency list
>    graph representation).
> 
> 3) suggestions welcome
> 
> 
> I'd prefer the first solution because it's simple and allows type check, but I don't
> know if it's safe.

Redefining classes is safe, but I would rather recommend to omit the 
type declarations. They don't buy you a lot anyway.

BTW, your type declarations are probably wrong. Most of the time you 
want something like (or null network-link).


Pascal

-- 
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My website: http://p-cos.net
Common Lisp Document Repository: http://cdr.eurolisp.org
Closer to MOP & ContextL: http://common-lisp.net/project/closer/

From: Giacomo Ritucci
Subject: Re: Mutually dependent classes
Date: Sat, 16 May 2009 11:18:12 +0000
Message-ID: <gum7dk$qit$1@lec.cs.unibo.it>

Pascal Costanza <··@p-cos.net> wrote:
> Redefining classes is safe, but I would rather recommend to omit the 
> type declarations. They don't buy you a lot anyway.
> 
> BTW, your type declarations are probably wrong. Most of the time you 
> want something like (or null network-link).

Thanks.

Giacomo