From: Robert Klemme <bob.news@...>
Date: 2005-07-04T06:35:44+09:00
Subject: Re: class variable leading a double life

Devin Mullins <twifkak@comcast.net> wrote:
> Robert Klemme wrote:
>
>> Class variables (the ones with @@) have some strange properties that
>> sometimes lead to surprising behavior.
>
> Indeed. I figured out why my code sample didn't work. Ruby considers
> it a "toplevel singleton method." If I define a class method inside
> the class definition, it works, but if I define it outside (as I did
> in my email), it's looks to the closest class definition outward --
> in this case, the thing irb wraps my code in. This is one of those
> odd cases where irb acts differently from Ruby-raw, too. Also, Ruby
> didn't warn me about toplevel singleton access like they said it
> should. I noticed it does, though, if I use class << Test syntax.
>
> This make sense, now. def Test.blah is a singleton method, and Ruby
> doesn't know anything about class scope when it's looking at this
> definition, because it's treating Test like any old object on which to
> define a singleton method. It needs to be stuck inside a class
> Blah...end block to know where the class variables lie.
>
> Still icky, though. (See below for my RCR-lite.)
>
>> I generally recommend to not use them.  Instance variables in classes
>> are much simpler to handle and much clearer and cleaner IMHO.
>>
>> Just a simple example where each new instance gets assigned a serial
>> number:
>
> Thanks for the example. I'm just wondering why
>
> class Foo
>  def blah
>    @@cnt += 1
>  end
> end
>
> couldn't have been a shortcut for [an optimization of]:
>
> class Foo
>  class << self
>    attr_accessor :cnt
>  end
>  def blah
>    self.class.cnt += 1
>  end
> end
>
> And why instead class variables were done the way they were.
>
> And if I don't get a decent answer, I'll go complain about it in my
> imaginary blog! So take that, matz!
>
> Devin

As far as I remember even Matz agrees that class variables are awkward.  I 
think they are on their way out - at least in Ruby 2.

Btw, for extra fun mix inheritance with class variables and change the order 
in which they are defined between super and sub classes! :-)

Regards

    robert