From: Robert Klemme <bob.news@...>
Date: 2005-03-31T00:44:45+09:00
Subject: Re: Defining a Class Accessor


"James Edward Gray II" <james@grayproductions.net> schrieb im Newsbeitrag 
news:07549561ed849e497e65fdc423a92c88@grayproductions.net...
>I was playing around with an idea in another thread and ran into a 
>surprise.  Can anyone explain the following to me?
>
> irb(main):015:0> class Module
> irb(main):016:1> def cattr_accessor( *symbols )
> irb(main):017:2>     symbols.each do |sym|
> irb(main):018:3*         eval "def self.#{sym}(  ) @@#{sym} end
> irb(main):019:3"               def self.#{sym}=( value ) @@#{sym} = value 
> end"
> irb(main):020:3>     end
> irb(main):021:2> end
> irb(main):022:1> end
> => nil
> irb(main):023:0> class Accessor
> irb(main):024:1> cattr_accessor :one, :two
> irb(main):025:1> def self.fetch_one(  )
> irb(main):026:2>     @@one
> irb(main):027:2> end
> irb(main):028:1> end
> => nil
> irb(main):029:0> Accessor.one = "James"
> => "James"
> irb(main):030:0> Accessor.two = "Gray"
> => "Gray"
> irb(main):031:0> Accessor.one
> => "James"
> irb(main):032:0> Accessor.two
> => "Gray"
> irb(main):033:0> Accessor.fetch_one
> NameError: uninitialized class variable @@one in Accessor
>         from (irb):26:in `fetch_one'
>         from (irb):33
>         from (null):0
>
> Everything works as expected, save fetch_one().  Why does it not return 
> "James"?

I'm a bit stunned, too.  Maybe this is a hint:

?> Accessor.class_variables
=> []
>> Module.class_variables
=> ["@@one", "@@two"]

Although:
>> Accessor.class.ancestors
=> [Class, Module, Object, Kernel]

So, since Accessor is an instance of Class, both should be accessible IMO. 
Weired...

For me it's one more reason to never use class variables.  They are simply 
too awkward.

Kind regards

    robert


PS: But I'm curios, too, why this does not work.