From: dblack@...
Date: 2003-08-21T21:56:59+09:00
Subject: Re: Class variables - a surprising result

Hi --

On Thu, 21 Aug 2003, [iso-8859-1] Ged Byrne wrote:

> class Sup
>  @@x = "A"
>  def test
>   print @@x
>  end
> end
>
> class Sub1 < Sup
>  @@x = "B"
> end
>
> class Sub2 < Sup
>  @@x = "C"
> end
>
> Sup.new.test
> Sub1.new.test
> Sub2.new.test
> >>> CCC
>
> I think I know whats going on.
>
> When a variable is first used, that is where it is
> declared.  Unlike Java there is no separate
> declaration.  If you like, declaration is automatic
> when the variable is first reference.
>
> So Sup is declared.  Class variable @@x is referenced
> and associated with Sup.
>
> Sup1 is declared.  @@x is referenced.  @@x is not
> found in the current class, so super.@@x is
> referenced.
> @@x is found, and updated.

I think it's simpler than that: you're just assigning to a variable.
It's like:

  x = "A"
  # ...
  x = "B"

@@x is simply the same variable both times -- that's how it's scoped.

> Note thae @@x = "B" is part of the classes
> declaration, and so will always be invoked, and that
> the 'message' will be passed back to Sup, thus
> updating the value of @@x in Sup.
>
> Same thing happens all over again in Sup2.
>
> Here is the code to get the desired result.
>
> class Sup
>  @@x = nil
>  writer: @@x

There no such thing:

  $ ruby cvars.rb
  cvars.rb:3: syntax error
   writer: @@x
	  ^

[...]

> So whats the best approach in ruby?  To keep a class
> hash that contains a value associated with each class?

Over on IRC (irc.freenode.net, channel #ruby-lang) we've just
been chatting about class instance variables, which are the
best way to achieve true per-class state maintenance.  They've
been talked about here a number of times; you can search on
www.ruby-talk.org or Google.  Or join us on IRC :-)


David

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