From: Gary Wright <gwtmp01@...>
Date: 2007-12-22T09:29:15+09:00
Subject: Re: Singleton Method


On Dec 21, 2007, at 6:51 PM, Ryan Lewis wrote:

> What's the difference between a regular method and a singleton method?

 From the perspective of a particular object, singleton methods are
those methods that are defined within the singleton class of
the object.

Regular methods are methods that are defined within the normal
inheritance tree of the object's class.

Method lookup is done by first checking the singleton class of an
object and if not found there by checking the normal inheritance tree.

By default objects don't have a singleton class.  The singleton class
is created when it is referenced:

$ irb
 >> a = [1,2,3]
=> [1, 2, 3]
 >> a.second
NoMethodError: undefined method `second' for [1, 2, 3]:Array
         from (irb):2
         from :0
 >> class <<a
 >>   def second
 >>     self[1]
 >>     end
 >>   end
=> nil
 >> a.second
=> 2
 >> [4, 5, 6].second
NoMethodError: undefined method `second' for [4, 5, 6]:Array
         from (irb):9
         from :0
 >>

In this example, I opened up the singleton class associated
with 'a' and defined a new method, second, that returns the
second element of the array.  As you can see, this method
is only available to that one particular array and not to
all array's in general.

Gary Wright