From: gwtmp01@...
Date: 2006-12-22T09:09:14+09:00
Subject: Re: Class Variable Confusion


On Dec 21, 2006, at 4:44 PM, dblack@wobblini.net wrote:
> Here's another one you'll like:
>
>   @@avar = 1
>   class A
>     @@avar = "hello"
>   end
>   puts @@avar  # => hello
>
>   A.class_eval { puts @@avar }  # => hello

Let's see if I understand what is going on:

When @@var is set to 1 at the top-level it is associated with the  
class Object (a feature of the top-level context) and as such @@var  
then becomes visible to the entire class hierarchy rooted at Object  
(i.e., all objects).  When @@avar is evaluated in the class A block,  
Ruby finds @@var already defined for the hierarchy (because A is a  
subclass of Object).  So in this example all three occurrences of  
@@avar are associated with the same class variable.  If you reverse  
the assignments:

   class A
     @@avar = "hello"
   end
   @@avar = 1

   A.class_eval { puts @@avar }  # => 1
   class A
     puts @@avar		      # => hello
   end

There are now two distinct class variables named @@avar, one is  
associated with Object and another is associated with class A and  
shadows the one associated with Object.

I still don't understand why class_eval/instance_eval don't affect  
the resolution of class variables in a manner analogous to how they  
affect the resolution of instance variables.  Is this by design or  
accident?


Gary Wright