From: Devin Mullins <twifkak@...>
Date: 2005-07-03T05:05:37+09:00
Subject: Re: class variable leading a double life

James Edward Gray II wrote:

> If you want it to be the same variable in both places, you need a  
> class variable:
>
> class Test
> @@var = 0
>
> ...
>
> James Edward Gray II

OK, now I'm confused. I had thought that class variables were just 
instance variables of the Test object, but it seems that the case is 
much weirder than that. (Why?)

class Test
  @fun = "hello"
  @@fun = 5
  def initialize
    p @fun
    p @@fun
    @fun = "blah"
    @@fun = "bloo"
  end
  def fun
    p @fun
    p @@fun
  end
end
def Test.fun
  p @fun
  p @@fun
  @fun = "bing"
  @@fun = "bong"
end

irb(main):021:0> Test.fun
"hello"
NameError: uninitialized class variable @@fun in Object
        from (irb):17:in `fun'
        from (irb):21

irb(main):022:0> t = Test.new
nil
5
=> #<Test:0x2aba608 @fun="blah">

irb(main):023:0> Test.fun
"hello"
NameError: uninitialized class variable @@fun in Object
        from (irb):17:in `fun'
        from (irb):23

irb(main):024:0> t.fun
"blah"
"bloo"
=> nil

1. Class variables are these odd things that are shared among every 
instance of Test.
2. Class variables of Test are not available to class methods of Test 
itself.
3. The singleton class for Test can have instance variables, but not 
class variables.*
4. Why didn't the second Test.fun call print "bing"?
5. Is there some way to share data between between a class's instance 
methods and its class methods?

I'm going to go study the PickAxe2 now, but I figured maybe I could get 
a quicker answer from some of you. :)

Devin
*I'm referring to the anonymous subclass of Class that is the class of 
the Test constant itself. She sells sea shells by the sea shore.