From: "David A. Black" <dblack@...>
Date: 2005-10-03T12:07:25+09:00
Subject: Re: class/singleton methods

Hi --

On Mon, 3 Oct 2005, Mark Volkmann wrote:

> These seem to be identical. Are they?
>
> class Foo
>  def Foo.bar
>    # some code
>  end
> end
>
> class Foo
>  def self.bar
>    # some code
>  end
> end
>
> If they are identical, is one form generally preferred over the other?

The advantage of the self version is that it might be a little easier
to maintain (e.g., if you change the name of the class).

> So a "class method" is the same as a "singleton method" on a Class object?

Basically, yes.  I think the main difference is that they have a bit
of language-level "special case" status.  There's the class_methods
method, which makes them "official".  Also, because of the way
classes' singleton classes work, a subclass can actually call the
singleton methods of its parent class:

   class C
     def self.x
       puts "Calling x on #{self}"
     end
   end

   class D < C
   end

   D.x   # Calling x on D

As far as I know that's the only situation where an object can call
another object's singleton method.  That may be enough to warrant a
different term for them.  I have to say, though, I've seen the class
method/singleton method relation serve as the "ah ha!" moment for
many, many people grappling with singleton stuff in Ruby (even if it
turns out to be a somewhat special case).


David

-- 
David A. Black
dblack@wobblini.net