From: Gary Wright <gwtmp01@...>
Date: 2007-05-01T12:10:34+09:00
Subject: Re: Class Level Variables


On Apr 30, 2007, at 9:32 PM, German Monfort wrote:
> OK, I now understand the difference.
>
> Probably I'm too new to programming to understand the full power of  
> class
> variables, and I found this concept a little bit dangerous because  
> you can
> affect the whole behavior of a class by simply changing a class  
> variable in
> any other subclass ... so I try not to use class variables for noiw.

Well Ruby's class variables are unusual in their semantics, even veteran
programmers get confused by them.

The usual recommendation is to use 'class instance variables' when you
want to have state associated with a particular class object (vs. a
class hierarchy.)  The easiest way to do that is to create attributes
via the singleton class:

class A
   class <<self
     attr_accessor :alpha
   end
end

class B < A
end

A.alpha = 4
puts A.alpha		# 4

puts B.alpha		# nil
B.alpha = 5
puts B.alpha            # 5

puts A.alpha            # 4

In this way you define the accessor once but the state is unique to each
class and to each subclass.


Gary Wright