From: "David A. Black" <dblack@...>
Date: 2009-11-30T06:21:10+09:00
Subject: Re: Class variables, instance variables, singleton; Ruby v. C++

Hi --

On Sat, 28 Nov 2009, Ralph Shnelvar wrote:

> DAB> And of course this is one of the (many) problems with class variables:
> DAB> they make obscure the otherwise rather simple notion that classes can
> DAB> have instance variables because classes are objects and any object can
> DAB> have instance variables.
>
> David ... this may be the higher-level gestalt that I was missing.  It
> sure sounds like it.

I have often made the observation that the answer to 75% of all
questions about Ruby is: "Because classes are also objects." :-)

Rick has explained most or all of what you were asking about. With
regard to class variables, I would encourage you to keep them somewhat
separate in your mind from the rest of the Ruby object and
variable/identifier model. Class variables are kind of layered on top,
as an expedient for getting class objects and their instances to share
data. Since classes are objects and their instances are *different*
objects, having them share data via variables is just a variation on
the theme of global variables; indeed, class variables are essentially
hierarchy-scoped globals.

Instance variables are a completely unrelated and totally different
matter: they are the mechanism for per-object state. Every object has
its own supply of instance variables, and by definition, no object can
see the instance variables of any other object -- even if object 1 is
a class and object 2 is an instance of that class. This is enforced
via the "self" mechanism: whenever you see an instance variable, you
are seeing an instance variable that belongs to self (whatever self is
at that moment).

I've always felt that it's unfortunate that class variables start with
"@@", because that conveys the impression that they're somehow akin to
instance variables. They're not; they're actually kind of the opposite
of instance variables, because they exist in order to blur the borders
between objects rather than to clarify them. They really should have
been "$$var" :-) Many of us would not mourn them if they disappeared
from Ruby.


David

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