From: "Gennady F. Bystritsky" <gfb@...>
Date: 2002-11-15T03:44:18+09:00
Subject: Re: Another Newbie question regarding instance variables?


----- Original Message -----
From: "Joel VanderWerf" <vjoel@PATH.Berkeley.EDU>
To: "ruby-talk ML" <ruby-talk@ruby-lang.org>
Sent: Thursday, November 14, 2002 9:10 AM
Subject: Re: Another Newbie question regarding instance variables?


> dblack@candle.superlink.net wrote:
> > Hi --
> >
> > On Fri, 15 Nov 2002, Gennady F. Bystritsky wrote:
> >
> >
> >>I thought that I knew Ruby pretty well until I came accross this thread
> >>about "class instance variables". It always haunted me why it does not
work
> >>when I do
> >>
> >>   @var = value
> >>
> >>outside any method definition, meaning initialization of an instance
> >>variable. Now I see why. Hence the question: is there any practical use
for
> >>"class instance variables"? If so, would anybody post an example?
> >
> >
> >
> > One potentially good thing, though I can't claim to have any dazzling
> > examples, is that a class's instance variables can be used as true
> > class-scoped variables, as opposed to class variables which are sort
> > of tree-scoped.
>
> A slightly more elaborate example, showing how it can be useful to have
> per-class rather than per-class-tree variables:
>
>
> class A
>    class << self
>      def new(*args)
>        @count ||= 0
>        @count += 1
>        super
>      end
>
>      attr_reader :count
>    end
> end
>
> class B < A
> end
>
> 10.times { A.new }
> 20.times { B.new }
>
> p A.count # ==> 10
> p B.count # ==> 20
>
> Note that count is the number of *direct* instances of the class (A or
> B) which have been created, not the number that currently exist.

This one is good, now I have a clear picture of the concept.

>
>
>
>