From: Robert Klemme <bob.news@...>
Date: 2004-03-04T18:04:42+09:00
Subject: Re: dynamic assignment of instance variables


"Gerard A.W. Vreeswijk" <gv@cs.uu.nl> schrieb im Newsbeitrag
news:d7a05cff.0403040028.33612745@posting.google.com...
> Thanks for pointing out the existence of Object#instance_variable_set
> (which, of course, Ruby ri v1.8b knows about).
>
> But then I think Ruby should have a method Object#attribute_set (and
> then also a method Object#attribute_get of course), so that we can set
> accessible atributes directly so that we are able to say:
>
>    Person.new( :name => 'John',  :born => '2003') # (1)
>
> rather than
>
>    Person.new(:@name => 'John', :@born => '2003') # (2)
>
> or even
>
>    Person.new(:name= => 'John', :born= => '2003') # (3)
>
> I this respect, I think it is relevant to quote a 1993 post of Matz [
> http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/68955 ],
> where he says:
> >Subject: Re: instance_variable_set question
> >Newsgroups: comp.lang.ruby
> >Date: 2003-04-09 01:32:15 PST
> >
> >
> >Hi,
> >
> >In message "Re: instance_variable_set question"
> >    on 03/04/09, Phil Tomson <ptkwt@shell1.aracnet.com> writes:
> >|
> >|It _is_ a bit confusing, though - if you made an attr_accessor for
> @bar,
> >|you refer to it as :bar, not :@bar :
> >
> >You're specifying attribute names (i.e. exporting method names),
> which
> >does not contain "@" in them, not instance variable names.
> >attr_accessor defines methods that use instance variables which names
> >are "@"+attribute-name.
> >
> >       matz.
>
> I think a method Object#attribute_set should exist, to enable straight
> and dynamic access to attributes when we want to, and to avoid
> counterintuitive calls like (2) or (3) in such cases.
>
> What do you think?

I think that there is a subtle difference between "instance variables" and
"attributes": an instance variable is a "physical thing", i.e., an
instance does in fact have a variable defined that carries a value.  An
attribute OTOH is something seen from the outside that does not
necessarily relate to an instance variable, although attr, attr_accessor
and the like tend to make people think they are tied together.  It's
perfectly legal to have an attribute that has no 1:1 relationship with an
instance variable:

class Foo
  def initialize; @bar = []; end

  def size; @bar.size; end

  def size=(s)
    if s > size
      @bar.concat( Array.new(s-size, nil) )
    elsif s < size
      @bar.slice!(s..-1)
    end
  end
end

Apart from that someone might have redefined attr, attr_accessor etc. to
generate other methods than the default methods...

So basically, I'd leave it as it is since the naming difference reflects a
semantic difference.

Regards

    robert