Forum: Ruby question about class variables and instance variables

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Eric D. (Guest)
on 2006-01-31 21:29
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.
Eric H. (Guest)
on 2006-01-31 22:07
(Received via mailing list)
On Jan 31, 2006, at 11:29 AM, Eric D. wrote:

>        end
> a is an instance of A which itself is an instance of Class
>    end
>       def strange=( val )
> 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 H. - removed_email_address@domain.invalid - http://segment7.net
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Logan C. (Guest)
on 2006-01-31 22:13
(Received via mailing list)
On Jan 31, 2006, at 2:29 PM, Eric D. wrote:

>        def bar
> ---------                  ------------                  ------------
>          @strange
>          @strange
>
> --
> 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
Jeremy H. (Guest)
on 2006-02-02 01:54
(Received via mailing list)
On 2006-01-31, Eric H. <removed_email_address@domain.invalid> wrote:

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

Is this really the whole story?  Because until now I always thought I
didn't understand class variables but if this is so then now I do.
Wheeeee!

But there's at least one gotcha; a class variable shadows any class
variable of the same name that you subsequently create in an ancestor
class:

    $ irb
    irb(main):001:0> class A ; end ; class B < A ; end
    => nil
    irb(main):002:0> class B ; @@x = 1 ; end ; class A ; @@x = 0 ; end
    => 0
    irb(main):003:0> class A ; puts @@x ; end ; class B ; puts @@x ; end
    0
    1
    => nil
    irb(main):004:0> exit

.... but if you create the class variable in the ancestor class first
then the descendant class inherits it instead of creating a new one:

    $ irb
    irb(main):001:0> class A ; end ; class B < A ; end
    => nil
    irb(main):002:0> class A ; @@x = 0 ; end ; class B ; @@x = 1 ; end
    => 1
    irb(main):003:0> class A ; puts @@x ; end ; class B ; puts @@x ; end
    1
    1
    => nil
    irb(main):004:0> exit

Any other subtleties?

Cheers,

Jeremy H.
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