From: Dave Thomas <Dave@...>
Date: 2001-03-13T14:35:41+09:00
Subject: [ruby-talk:12543] Re: class variables

"Christoph Rippel" <crippel@primenet.com> writes:

> In general nice it might be nice if your book could point out in
> a  comprehensive fashion the somewhat confusing differences
> between a
> 
> -- variable local to an object

  instance variables

> -- class variable

  class variables

> -- variable local to the Class

  I assume you mean instance variables  within the class

> -- variable local to the Meta Class

  I have no idea what this is :)



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

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 internally� are 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.

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 :)


Regards


Dave




Footnotes: 
�  or class proxies for included modules.

�  for example, the class of a Class, and classes generated for
specific objects (class <<a)