From: dblack@...
Date: 2002-11-15T01:32:18+09:00
Subject: Re: Another Newbie question regarding instance variables?

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.

For example:

  class A
    class << self; attr_accessor :var; end
    @var = 0
    def increment
      self.class.var += 1
      puts "New value: #{self.class.var}"
    end
  end

  a = A.new
  a.increment  # New value: 1
  b = A.new
  b.increment  # New value: 2

  class B < A
    @var = 100
  end

  c = B.new
  c.increment  # New value: 101
  b.increment  # New value: 3


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav