From: dblack@...
Date: 2002-12-02T00:16:42+09:00
Subject: Re: class << self  (was Re: [FAQ] What do some of Ruby's symbols mean?)

Hi --

On Sun, 1 Dec 2002, Gavin Sinclair wrote:

> From: "Shannon Fang" <xrfang@hotmail.com>
>
>
> > Hi,
> > On Sun, 1 Dec 2002 23:35:30 +0900
> > ts <decoux@moulon.inra.fr> wrote:
> >
> >
> > >  If instance methods are stored in the class, where do you think that the
> > >  class methods are stored ?
> >
> > I'm sorry I can't understand the purpose of your question. Is it related
> > to understanding class << object? The level I'm interested in is for
> > application developers, why they need to use <<, does it make their life
> > easier, or...? I did not consider to make the language developer's life
> > easier.. (sorry for being selfish :-p).
> >
> > Shannon
>
>
> You know what a class method is, right?
>
>   class Example
>     def Example.foo
>       19
>     end
>   end
>
>   Example.foo           # -> 19
>
>
> The same is achieved with
>
>   class Example
>     class << self
>       def example
>         19
>       end
>     end
>   end
>

> I don't know why and I don't like it.  I'm happy enough with
> singleton objects (but what's wrong with "def obj.meth"?):

Nothing, but it can get tedious:

  class A
    def self.thing
      @thing
    end
    def self.thing=(x)
      @thing = x
    end
  end


as opposed to:

  class A
    class << self
      attr_accessor :thing
    end
  end

(You could of course just add the attribute to the object's real
class:

  class Class
    attr_accessor :thing
  end

but then every Class object (String, Array, etc.) would have this
attribute.)

The class << obj idiom is based on two fundamental things about Ruby:

1. you can open/reopen any class:

  class String ...

  class MyClass ...


2. every object (almost) has a singleton class


So class << obj is just a way of accomplishing #1 with regard to #2.
It thus provides a way to treat singleton classes very much like
generic (if that's the word) classes, which makes for what I think is
a nice consistency and seamlessness.  But there's no imperative that
you use this idiom, if you don't like it and can write around it.


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav