From: "Mauricio Fernández" <batsman.geo@...>
Date: 2003-05-09T16:18:43+09:00
Subject: Re: Can a global be a constant?

On Fri, May 09, 2003 at 01:13:51PM +0900, Jim Freeze wrote:
> So, fundamentally, is the code in both class C and D below 
> doing the same thing?
> 
> class C 
>   class << self
>     def set(x); @x = x; end  
>     def get; puts @x; end
>   end
> end
> 
> class D
>   def self.set; @x = 5; end
>   def self.get; puts @x; end
> end

They do (well, except for the fact that D::set always sets to 5 :-)
There can be one difference, though:
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/57274
 
> If these class methods have instance variables for the 
> singleton object, are class methods not just really
> instance methods of the singleton object?

I don't think "singleton object" is standard terminology here.
Regardless of the use of class instance variables, class methods are in
fact singleton methods of the object that represents the class (i.e.,
that of type Class). You can also view them as instance methods of the
associated *singleton class*. 

batsman@tux-chan:/tmp$ expand -t2 ad.rb

class Foo
  puts "Foo.id:  #{id}"
  class << self
    def foo
    end

    puts "Foo'.id: #{id}"
    puts "Foo' instance methods: " + instance_methods.inspect
  end
  puts "Foo singleton methods:" + singleton_methods.inspect
end

batsman@tux-chan:/tmp$ ruby ad.rb
Foo.id:  537725700
Foo'.id: 537725690
Foo' instance methods: ["foo"]
Foo singleton methods:["foo"]

Take into account that you'd have to use Someclass.singleton_method_added instead
of (class << Someclass; self; end).method_added if you want notification
on addition of singleton methods.

-- 
 _           _                             
| |__   __ _| |_ ___ _ __ ___   __ _ _ __  
| '_ \ / _` | __/ __| '_ ` _ \ / _` | '_ \ 
| |_) | (_| | |_\__ \ | | | | | (_| | | | |
|_.__/ \__,_|\__|___/_| |_| |_|\__,_|_| |_|
	Running Debian GNU/Linux Sid (unstable)
batsman dot geo at yahoo dot com

How do you power off this machine?
	-- Linus, when upgrading linux.cs.helsinki.fi, and after using the machine for several months