From: Rick DeNatale <rick.denatale@...>
Date: 2008-04-10T05:07:21+09:00
Subject: Re: My error or bug in "attr_reader" ?

On Wed, Apr 9, 2008 at 3:09 PM, Hexren <me@hexren.net> wrote:
>
> Stefano Crocco wrote:
>
> > On Wednesday 09 April 2008, Hexren wrote:
> >
> > > Hi guys.
> > >
> > > The code posted below actually modifies the array in the object.
> > > Am I doing something wrong here ?
> > > If not, this should imho be included in the doc. It seems
> > > counterintuitive to me ;)
> > >
> > >
> > > Regards
> > > Oliver
> > >
> > > ----------------------------------
> > > class AttrReaderErr
> > >   attr_reader :entries
> > >   def initialize()
> > >     @entries = [0,1,2]
> > >   end
> > > end
> > >
> > > showcase = AttrReaderErr.new
> > >
> > > puts showcase.entries
> > > #should this really work ?
> > > showcase.entries[1]="foo"
> > > #it does, but looks wrong
> > > puts showcase.entries
> > > ----------------------------------
> > >
> >
> > It works correctly, once you understand exactly what's going on.
> attr_reader creates a method which returns the object stored in the instance
> variable @entries. If it were written by hand, it would be something like:
> >
> > def entries
> >  @entries
> > end
> >
> > Writing entries[1]="foo", you're calling the []= method of the object
> returned by entries, that is, the array stored in @entries. Since the []=
> method modifies the receiver, the content of @entries is modified. If you
> don't want this to happen, you need to create the entries method by hand and
> return a copy of the array:
> >
> > def entries
> >  @entries.dup
> > end
> >
> > I hope this helps
> >
> > Stefano
> >
> >
>
>
>  Yup, what you say makes sense, thanks :)
>
>  But I maintain, that the doc is kind of confusing. The fingerprint:
>  "attr(symbol, writable=false) => nil"
>
>  Very strongly implies (at least to me) that the default is, for the
> instance variable to be not writable. I am going to go, cry wolf on the doc
> mailing list :)

This is a typical newbie confusion over instance variable vs. the
objects they reference.

http://talklikeaduck.denhaven2.com/articles/2006/09/13/on-variables-values-and-objects
See in particular the section on "Mutability, and Aliasing"
-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/