From: Christoph Rippel <crippel@...>
Date: 2001-03-14T03:32:45+09:00
Subject: [ruby-talk:12578] Re: class variables

> From: ts [mailto:decoux@moulon.inra.fr]
[...]
> C> The question is how get a handle on the meta class this is what
> C>   MetaFred = ObjectSpace._id2ref (
> C> class << Fred
> C> id
> C> end )
> C> does.   Now I can define MetaFred method  which has access to this variable
> 
> C> def MetaFred.var
> C>      @var
> C> end
> 
> C> that is
> 
> C> p MetaFred.var          #  => 234
> 
>  I'm probably stupid but why you don't write ?
> 
>    class Fred
>       @var = 234
>    end
> 
>    def Fred.var
>       @var
>    end
> 
>    p Fred.var

Because they are NOT the same - if you accidentally write

class << Fred
@will_vanish = "bye"
end

were to think (you know this of course) the variables lives - it isn't an 
instance variable of an object of type Fred, nor is it an instance variable
of the class object Fred - it must live somewhere else - this is the 
metaclass (manipulating things on the first meta-level might actually be
useful beyond this it is obviously nonsense;-)

> 
>  variable with the prefix '@@' was introduced because it's was not possible
> to access a class variable in a method, i.e.
> 
>    class Fred
>       def toto
>          @var # this is always an instance variable
>       end
>    end
Funny, I just used exactly this behavior of class variables in my 
interlude post ...

[...]
Christoph