From: Christoph Rippel <crippel@...>
Date: 2001-03-14T02:40:02+09:00
Subject: [ruby-talk:12575] Re: class variables

"Dave Thomas" <Dave@PragmaticProgrammer.com> wrote in message
news:m2elw2xmbw.fsf@zip.local.thomases.com...

[...]
>   I assume you mean instance variables  within the class
yes
> > -- variable local to the Meta Class
>
>   I have no idea what this is :)

Instance variable within in the meta class - i.e. the singleton class of
the class  object.

>
>
> From your examples, I'm not sure I see where the confusion is. The

Yes confusing was not the right word - it is very consistent but it
takes time to get used to variables being created on a need base
(this was very confusing to me in the beginning - you might want
to throw in an extra page of examples to illustrate this behavior
if you every make a major rewrite of your overall great book - but
my problems with this might have been singular ... )

> only possible strangeness appears to be the expectation that
>
>    class Fred
>      @var = 123
>    end
>
> sets an instance variable for objects of class Fred, rather than for
> Fred itself.

I guess this was my main point  of the post  - I find this quite usefull.
Instance variables of meta classes  are created the same way
(I don't think you will find much use for this -  I was simply
wandering  were  @var   ``lives'' )

     class << Fred
          @var = 234 # creates instance variable of meta-class
     end

The question is how get a handle on the meta class this is what
  MetaFred = ObjectSpace._id2ref (
class << Fred
id
end )
does.   Now I can define MetaFred method  which has access to this variable

def MetaFred.var
     @var
end

that is

p MetaFred.var          #  => 234

Note that  evenso

MetaFred.var    # => Class

you still have

p MetaFred == Class  # => false

Class variables on the `'singleton - level'' obviously don't
exist (but I checked to make sure anyway.)

> When I think about classes in Ruby, I have a very simple model.  There
> really is only one basic structure: an instance with some state, and
> chained off it a list of classes,each of which contains a list of
> methods. Then there's the twist: classes are object instances too, and
> so have their own state and list of classes, and so on. Classes that
> Ruby generates internallyare called singletons, and are mostly the
> same as any other class, except they're invisible.
>
> Then there's just one more thing: when defining a class, you're
> executing code in the context of that class object:
>
>   class Dave
>     p self       #=>  Dave
>   end
>
> Instance variables are always defined in terms of the current value of
> self.

What really cleared a lot things up for me was the realization that
you can only trust the id of an object - that is you have to look at

  class Christoph
   p id     # => .....
  end

> Once this picture was clear, I found that I can understand pretty
> intuitively what the scope of various constructs should be.  I tried
> to set all this down in chapter 19. However, this just represents my
> own warped way of thinking: I'm sure others will have different ways
> of visualizing the hypercube :)

Talking about hypercubes - it is actually possible to create Meta Freds
of any order - One simply has to put a handle on the previous level -
it is really  amazing that everything works out perfectly - really neat.
I don't think it possible to get a handle on the proxy class of a module
include put who knows.



[...]


Christoph