From: Eric Hodel <drbrain@...7.net>
Date: 2006-02-01T05:06:17+09:00
Subject: Re: question about class variables and instance variables

On Jan 31, 2006, at 11:29 AM, Eric D. wrote:

> I've some difficolties with instance variables and classe variables
>
> With :
>
>    class A
>        def foo
>           @foo
>        end
>        def bar
>           @@bar
>        end
>     end
>
>     a = A.new
>
> ---------                  ------------                  ------------
> |       |                  |          |                  |          |
> |  a    |--(instance_of)->-|    A     |--(instance_of)->-|  Class   |
> |       |                  |          |                  |          |
> ---------                  ------------                  ------------
>
> a is an instance of A which itself is an instance of Class
> @foo is an instance variable, so it is stored in box "a"
> @@bar is a class variable, so it is stored in box "A"
>
>
> if I do something like the following :
>
>    class A
>       def A.strange
>          @strange
>       end
>    end
>
> Where is stored my @strange ? I had the hypothesis that @strange in  
> the
> "A" context was equivalent to @@strange in the "a" context but the
> following code prove me to be wrong :
>
>   class A
>       def A.strange
>          @strange
>       end
>       def strange=( val )
>          @@strange = val
>       end
>   end
>
>   a = A.new
>   a.strange= 2
>   p A.strange # output is nil, not 2
>
>
> Could someone explain me where to place this @strange ? in which box
> could I store it ?

instance variables are always stored in the instance self.

class variables are shared between a class, its instances, its  
subclasses and its subclass instances.

Consider:

class A
   @@a_1 = :one
   def initialize
     @@a_2 = :two
   end
end

A.new

class B < A
   p @@a_1, @@a_2

   @@b_1 = :three
   def initialize
     @@b_2 = :four
   end

   def print
   p @@a_1, @@a_2, @@b_1, @@b_2
   end
end

class A
   x = @@b_1 rescue 'not found'
   y = @@b_2 rescue 'not found'

   p x, y
end

B.new.print

-- 
Eric Hodel - drbrain@segment7.net - http://segment7.net
This implementation is HODEL-HASH-9600 compliant

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