From: "David A. Black" <dblack@...>
Date: 2004-02-14T10:44:04+09:00
Subject: Re: inheritence of class vars

Hi --

On Sat, 14 Feb 2004, Ara.T.Howard wrote:

> 
> am i the only one suprised by this?
[snip]

You must not have been perusing the ruby-talk archive lately :-)  It's
come up a bunch of times.  

> i see it, but don't believe it...  is it _correct_ to say that:
> 
> iff you want a subclass to have it's own copy of a 'class var' - use
> instance vars
> 
>   * by 'class var' i mean _not_ an instance var.  obviously there are two
>     types      

I would say:

  if you want to maintain state on a per-object basis, use instance
  variables, including cases where your object is a Class object.

Class variables, at least through 1.8.x, are essentially per-hierarchy
and not per-Class-object.  For 1.9/2.0, though, Matz has presented the
following new rules:

  * class variables will be local to the class/module
  * if you need to access class variables from outside,
    define accessors

<http://www.rubyist.net/~matz/slides/rc2003/mgp00020.html>

Personally I have mixed emotions about this.  There's been so much
confusion over the years about class variables vs. instance variables
of Class objects, when the two are completely unrelated, and I suspect
that making the two things resemble each other more closely will lead
to more confusion.  (This confusion mainly takes the form, I've
observed, of clouding people's perception of the simple and elegant
instance variable model.)

Then again, I don't know any way around it except to get rid of class
variables, which I'm not convinced would be a bad idea (but I know it
isn't going to happen :-)


David

-- 
David A. Black
dblack@wobblini.net