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




.

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