From: "Mauricio Fernández" <batsman.geo@...>
Date: 2003-09-11T07:26:52+09:00
Subject: Re: Subclassing self

On Thu, Sep 11, 2003 at 06:00:00AM +0900, Fredrik Jagenheim wrote:
> On Wed, Sep 10, 2003 at 05:06:28AM +0900, Mark J. Reed wrote:
> > 
> > Instead of using the class << self notation, you could do this:
> > 
> > 	class TheClass
> > 	    def TheClass::foo
> > 
> > But that can get tedious when defining multiple class methods,
> > plus you run the risk of missing a change if you rename the class
> > later.
> 
> I was just shown:
> 
>     class TheClass
>         def self.foo
> 
> This way you don't risk the missing a change when you rename the
> class. You still get the tedious typing though. :)
> 
> However, what does 'class << self' really mean? Why is this one
> of the 'obvious' ways to define class methods? 

class << obj opens the singleton class of obj. In a class context,
self points to the Class object, therefore in

class A
	self  # this is A
	class << self   # same as class << A but needs not be changed if
					# if A is renamed
		# we're in A's singleton class
	end
end

Class methods are in fact class singleton methods (ie. singleton methods
of the object of class Class). You sometimes want to use the class << self
idiom to create attribute accessors for the class, etc:

class A
	class << self
		attr_accessor :foo
	end
end

A.foo = 1
A.foo  # => 1

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