From: dblack@...
Date: 2003-02-22T16:05:59+09:00
Subject: Re: Style question: using 'block_given?'

Hi --

On Sat, 22 Feb 2003, Brian Candler wrote:

> > This is the class instance variable police. Freeze! Keine Bewegung!
> >
> > ;-)
>
> It's a fair cop :-) I had a hard time getting my head round this one:
>
> - Class variables (@@data) are fine.
>
> - Instance variables in a module are fine: mix it into a class, and
>   instances of that class get those instance variables too
>
> - But @data as a class instance variable? You can't even refer to it,
>   except from within a class method. (Unlike a class variable, @@data,
>   which you can use within a normal instance method)

It's probably most useful in an accessor.  For example, if you want to
keep a running tally of how many instances of a class have been
created:

  class C
    class << self; protected; attr_accessor :instances; end
    @instances = 0

    def initialize
      self.class.instances += 1
    end
  end

  p C.instances       # 0
  C.new
  C.new
  p C.instances       # 2

> It looks like something which the language has by accident more than by
> design :-)

Only if consistent, predictable design happens by accident, which I
doubt :-)  (See Gennady's explanation.)


David

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