From: "David A. Black" <dblack@...>
Date: 2005-11-05T21:49:00+09:00
Subject: Re: what does "class << self" do?

Hi --

On Sat, 5 Nov 2005, petermichaux@yahoo.com wrote:

> Hi,
>
> I'm looking at the rails activerecord base.rb file it's main structure
> is shown below. Why have a class inside a class like this? What
> advantages are there?
>
> Thanks,
> Peter
>
> class Base
>
>    # methods ...
>
>    class << self # Class methods
>      # methods ...
>    end
>
>    # methods ...
>
> end

Every object in Ruby can have methods of its own, in addition to the
methods it gets from being an instance of its class.  Those
"singleton" methods (singleton in the sense of belonging to only one
object, not all instances of a class) live in the object's "singleton
class."

The notation:

   class << obj
     # ...
   end

is how you open up a definition block for the singleton class of
obj.  Once you're inside that block, it's just like being inside any
other class definition block -- except that you're defining methods
that only obj will be able to call.

Here's an example:

   class C        # class definition for all instances of C
     def speak
       puts "Hi"
     end
   end

   c = C.new
   class << c     # singleton class of this particular instance of C
     def speak
       puts "Hello"
     end
   end

   another_c = C.new
   another_c.speak    # Hi
   c.speak            # Hello

The example you gave was along the lines of:

   class C
     class << self
       def m
       (etc.)

In this case (which is a very common one), the object whose singleton
class you're accessing is the Class object C itself.  ("self" is equal
to C at the top level of the class definition body.)  So you're
creating methods that will only be callable with C as a receiver:

   C.m

Those are class methods.  Or, to put it another way, class methods are
singleton methods of Class objects.

You can also define a method for a specific object by doing this:

   def c.speak

or this, if it's a class:

   def C.speak

or (also for a class):

   class C
     def self.speak

In other words, in most cases you don't have to manually open up the
singleton class.  (But see Mauricio's point about the difference in
constant scope, though in most cases that won't be an issue.)

For more on this, see http://www.wobblini.net/~dblack/i0nas.txt


David

-- 
David A. Black
dblack@wobblini.net