From: Logan Capaldo <logancapaldo@...>
Date: 2006-02-01T05:12:52+09:00
Subject: Re: question about class variables and instance variables


On Jan 31, 2006, at 2:29 PM, Eric D. wrote:

> Hello,
>
> 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 ?
>
>
>
>
> Thanks in advance
> --
> Eric D.
>
> -- 
> Posted via http://www.ruby-forum.com/.
>

Classes are objects too. @strange is an instance variable of the  
object A (which has class, Class). @@strange is a class variable of  
the class A. The difference is that instances of A can peek at the  
class vars. They can't peek at A's instance vars. They are in two  
separate namespaces. The other difference is that class variables  
aren't so much class variables as they are class hiearchy vars. EG:

class A
  @@strange = 1
end

class B < A
    puts @@strange
end

#prints 1

class C
    @strange = 1
end

class D < C
    puts @strange
end

# prints nil