From: Francois Goret <fg@...>
Date: 2002-12-06T22:34:02+09:00
Subject: Re: Class-instance variables access

Hi Greg,

Thanks for your help. In fact the question was a little bit different: you can 
define IN A CLASS what's Guy called "class instance variables". I was stunned 
by this a few days ago, as I was assuming too that there are only "class 
variables" and "instance variables", the ones you described. 

Class instance variables seems to be a third kind of animal: variables stored 
in the class (not in it's instances), and who does support the following 
construct:

class B
   def B.test
      @x
    end
end

class A < B
  @x = 12    # not @@x = 12 
end

A.test
=> 12

If you do the same with @@x, it doesn't work as the class method B.test will 
try to access the non-existant @@x IN B, not in A.

So I'm really puzzled by these class instance variables... and I didn't find a 
way to access their values from an instance.

Thanks again for your help,

Francois

On Friday 06 December 2002 20:08, Greg Millam wrote:
> > I try to understand the difference between 'class variables' and 'class
> > instance variables', following a post by Guy Decoux a few days ago.
>
> If you've ever worked with C or Java, a class variable is like a variable
> declared 'static'
>
> class A
>   def intialize(y)
>     @x = y
>     @@x = y
>   end
>   def test
>     puts "@x: #{@x}"
>     puts "@@x: #{@@x}"
>   end
> end
>
> foo = A.new("foo")
> bar = A.new("bar")
>
> foo.test
> bar.test
>
> - You get:
> @x: foo
> @@x: bar
> @x: bar
> @@x: bar
>
> @x is an instance variable that each instance stores it's own version of
> @@x is a class variable that is stored in the class definition
>
> @x can be different from instance to instance, @@x is the same for all
> instances.