From: Robert Klemme <shortcutter@...>
Date: 2007-06-03T02:05:20+09:00
Subject: Re: so lost on singleton, metaclass and virtual class

On 02.06.2007 18:30, Dorren wrote:
> Q1: in pick axe book page 364, it talks about metaclass should be a
> singleton class. but when I run the following code,  class's object_id
> are the same for all instance, but the metaclass object_id are
> different for each object instance, is the code wrong or what?

The term "singleton class" has been discussed a lot.  Other names have 
been suggested but I guess "singleton class" sticks because the book 
uses it. :-)

Basically it just means that there is one singleton class per instance. 
  This class is invisible most of the time - you basically see it only 
when you do class<<self.  You can imagine that this class sits between 
the instance and the original class (the return value of self.class).

<snip/>

> Q2: from pick axe book 364-367, it used singleton, metaclass, and
> virtual class definition interchangeably. are they all the same thing
> or not?

Um, I would have to check with my copy which I don't have here with me. 
  But it is very possible.

> Q3: from page 365,
>   "To handle per-object attributes, Ruby provides a classlike some-
>     thing for each object that is sometimes called a singleton class."
> if runtime create something new for EACH object, how can it be called
> singleton?

See my explanation above.

Kind regards

	robert