From: Gary Wright <gwtmp01@...>
Date: 2011-03-07T11:41:54+09:00
Subject: Re: Class Variables in EigenClass


On Mar 6, 2011, at 7:55 PM, JP Billaud wrote:

> Actually it seems that for some reasons the @@test class variables ended 
> up in the Object class:
> 
> puts Object.class_variables
>> @@test
> 
> Anyway I am still confused about this... Help would very appreciated.

The snarky answer is: "Don't use class variables".  They really are hard to understand and generally don't have the semantics you expect or need.

The non-snarky answer is that when you do:

egA.class_eval do
 @@test = 5
end

Ruby does *not* evaluate '@@test' relative to self, which would be egA, but instead evaluates it relative to the lexical scope that is in effect, which in your code is the top level scope.  Class variables evaluated at the top level scope are instantiated within Object.

Most people expect class variables to only be visible to the class they are instantiated in but they are actually visible to their 'home' class as well as all subclasses.  Since every class is a subclass of Object, in your example, @@test becomes visible in every class and in every instance.

Gary Wright