From: Robert Klemme <bob.news@...>
Date: 2004-07-22T18:22:04+09:00
Subject: Re: Referencing objects and program design


"Stephan K�mper" <Stephan.Kaemper@Schleswig-Holstein.de> schrieb im
Newsbeitrag news:2m86vsFj140oU1@uni-berlin.de...
> Hi all,
>
> suppose I have some kind of object which holds 'object ids' referring to
> other objects, probably objects of its own class - or even itself.
>
> Now adding an object to the list can be done (at least) in two ways:
> Pass an object to the method and let the method find out the object id,
> or pass the object id directly.

If there are no special requirements for your object ids I suggest to use
Object#id and ObjectSpace#_id2ref:

11:10:05 [ruby]: irb
irb(main):001:0> class Foo;end
=> nil
irb(main):002:0> f=Foo.new
=> #<Foo:0x10187fb8>
irb(main):005:0> id = f.id
=> 135020508
irb(main):006:0> ObjectSpace._id2ref id
=> #<Foo:0x10187fb8>
irb(main):007:0>

> Now the following lines are just an example of what I'm thinking about
> (and it's certainly not thread safe among other things).
>
> class Common_Base
>    @@curr_oid = "aa"

I'd change that to be an instance var of Common_Base:

   @curr_oid = "aa"

>    attr_reader :oid
>
>    def initialize
>      @oid = Common_Base.get_new_oid
>    end

Note that, depending on whether you want to be able to resurrect instances
from some kind of persistent storage, you may need to be able to override
the oid.

>    def Common_Base.get_new_oid
>      @@curr_oid = @@curr_oid.succ
>    end
> end
>
> class Reference_List < Common_Base
>    def initialize
>      super
>      @refs = []
>    end
>
>    def add_ref1( other )
>      @refs << other.oid unless @refs.include? other.oid
>    end
>
>    def add_ref2( oid  )
>      @refs << oid unless @refs.include? oid

You could better use a Set here.  That saves you the inefficient check
(with an Array this is O(n) where n is the number of oids stored so far;
with a Set (hash internally) it's ~ O(1)).

>    end
> end
>
> class Referenced < Common_Base
> end
>
> refl = Reference_List.new
> ref  = Referenced.new
>
> refl.add_ref1( ref )
> p refl # -> <Reference_List:0x27862b0 @oid="ab", @refs=["ac"]>
>
> refl.add_ref2( ref.oid )
> p refl # -> <Reference_List:0x27862b0 @oid="ab", @refs=["ac"]>
>
> refl.add_ref1( refl )
> p refl # -> <Reference_List:0x27862b0 @oid="ab", @refs=["ac", "ab"]>
>
>
> Now, for 'add_ref1' the method needs to know a bit more about the object
> to be added (needs to call the oid method of the added method), while
> 'add_ref2' seems to put more work on the caller of the method (needs to
> ask for the added oid).
> Anyway, right now using 'add_ref1' _seems_ to be more the
> 'tell-the-other-guy' way, but I'm not too sure about that...
> So, what's the better way to add a to-be-referenced object to the
> referencing object?

I'd go for add_ref1 - although you should uncover a bit more of the
problem you are solving.

Regards

    robert