From: dblack@...
Date: 2006-03-17T10:13:26+09:00
Subject: Re: Scope of an @variable

Hi --

On Fri, 17 Mar 2006, Benjohn Barnes wrote:

>
> On 15 Mar 2006, at 00:54, Jeff Cohen wrote:
>
>> rtilley wrote:
>>> This may be wrong, but I like to think of @@ as a class variable and @
>>> as a method variable. @@ is accessible by any method in the class and @
>>> is accessible by the method only.
>> 
>> You're close. @ are instance variables.  Any instance method in your
>> class can
>> access them.
>
> You can also access @ variables from a class method - ie ClassName.function; 
> @a_variable; end. They are local to each class within a hierarchy, rather 
> than @@ which would be shared among classes in the hierarchy.
>
> So:
>
> class X
> 	def X.var;
> 		@var
> 	end
>
> 	def X.var=(v)
> 		@var=v
> 	end
> end
>
> class Y<X;end
>
> X.var=10
> Y.var=20
> X.var	=> 10
> Y.var	=> 20

Nice, isn't it?  It's really just this:

   obj = Object.new
   def obj.var
     @var
   end

   def obj.var=(v)
     @var = v
   end

with obj being a Class object.  Plus the special-cased thing where the
subclasses get to call it (because the singleton class of X is
considered the superclass of the singleton class of Y).

You can also do:

   class X
     class << self      # singleton class of X
       attr_accessor :var
     end
   end


David

-- 
David A. Black (dblack@wobblini.net)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

"Ruby for Rails" chapters now available
from Manning Early Access Program! http://www.manning.com/books/black