From: dblack@...
Date: 2006-03-02T23:13:34+09:00
Subject: Re: Class method in singleton_methods?

Hi --

Adding something to my previous reply:

On Thu, 2 Mar 2006, dblack@wobblini.net wrote:

> Hi --
>
> On Thu, 2 Mar 2006, Minkoo Seo wrote:
>
[...]
>> It is interesting that a class method is actually a singleton method. I
>> know that there are tons of posting regarding metaclasses/objects.
>> Unfortunately, there are simply too many to read and understand all of
>> them. So, please forgive my naive question.
>> 
[...]
>> Am I correct?
>
> A class method is indeed a singleton method of a Class object.  The
> term "class method" is really just a convenient label for this case,
> because it occurs quite frequently.

The one way in which class methods differ from other singleton methods
is that they're the only case, as far as I know, where more than one
object can call a specific singleton method:

   class C
     def self.x
     end
   end

   class D < C
   end

   D.x     # OK, because D is a subclass of C, even though x is
           # a singleton method on another object (C)

The reason is this: C's singleton class (or "metaclass", as singleton
classes of Class objects are sometimes known) is the superclass of D's
singleton class.  Since the method x resides in C's singleton class,
it is visible to instances of that singleton class (namely C), and to
instances of descendants of that singleton class (namely, D).

In other words, when you send the message "x" to D, D looks in its
singleton class, and then in the superclass of its singleton class --
and there it finds a method "x", and executes it.


David

-- 
David A. Black (dblack@wobblini.net)
Ruby Power and Light (http://www.rubypowerandlight.com)

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