From: Dave Thomas <Dave@...>
Date: 2001-03-04T06:32:23+09:00
Subject: [ruby-talk:11948] Re: singleton classes?

Kevin Smith <sent@qualitycode.com> writes:

> Johann Hibschman wrote:
> >Could someone either explain singleton classes to me or point me to a
> >good explanation?  
> 
> Here's a pretty good description:
> http://rampages.onramp.net/~huston/dp/singleton.ht
> ml

Actually, the word 'singleton' in Ruby has two totally different
meanings. In the OO sense, it's as you describe: a class that can only 
have one instance: every request for an object of that class will
return the same object instance.

The second mean in to do with Ruby's ability to create unique,
per-object classes. For example, I might want to create an anonymous
subclass for just a particular object:

   a = [ 1, 2, 3 ]    # create an array object
   b = [ 1, 2, 3 ]    # and another
   class << a
     def [](n)
       "element is " + at(n).to_s
     end
   end

   puts a[2]       #=> element is 3
   puts  b[2]      #=> 3

Why would you want to do this?

Sometimes it's simply a notational convenience. Othertimes, you
genuinely need to be able to add features to objects. For example, you 
might be adding objects to some time of queue in a simulation. Rather
than have them all be a subclass of, say, QueueObject, you could
instead simply add in the facilties you need on the fly:

  class << anObj
    include Queueable
  end

That way any object can be added to a queue.


I have to admit I use this rarely, but it's always a nice solution to
have in your back pocket.


Regards


Dave