From: "David A. Black" <dblack@...>
Date: 2007-11-08T20:18:44+09:00
Subject: Re: Metaclasses

Hi --

On Thu, 8 Nov 2007, Neil Curzon wrote:

> Hi, all.  I'm trying to understand chapter 24 of Programming Ruby.
> I'm using "metaid.rb" from why the lucky stiff's site to help with the
> examples. (http://whytheluckystiff.net/articles/seeingMetaclassesClearly.html)
>
> Everything is pretty straightforward except for the diagrams of
> inheritance of metaclasses.  Figure 24.2 indicates that for a direct
> subclass of Object, its metaclass's superclass should be Object's
> metaclass.  When I try this out in irb, I don't get the expected
> result.
>
> $ irb -r metaid
> irb(main):001:0> String.metaclass
> => #<Class:String>
> irb(main):002:0> String.metaclass.superclass
> => #<Class:Class>
> irb(main):003:0> String.metaclass.superclass == Object.metaclass
> => false
>
> Surprisingly, when I try it in JRuby, I do get the expected result.
>
> $ jirb -r metaid
> irb(main):001:0> String.metaclass
> => #<Class:String>
> irb(main):002:0> String.metaclass.superclass
> => #<Class:Object>
> irb(main):003:0> String.metaclass.superclass == Object.metaclass
> => true

It actually depends which version of Ruby you use. The superclass
thing worked as expected in 1.8.2; then it stopped working (I'm not
sure why); and in 1.9 it's working again. ("Not working" doesn't mean
that the subclasses couldn't call the methods defined in their
superclasses' singleton classes, just that the superclass thing itself
was not in place.)

$ cat super.rb
class Object
   def singleton_class
     class << self; self; end
   end
end

p String.singleton_class.superclass == Object.singleton_class

$ /usr/local/lib/ruby-1.8.2/bin/ruby -v super.rb 
ruby 1.8.2 (2004-12-25) [i686-linux]
true
$ ruby -v super.rb
ruby 1.8.6 (2007-03-13 patchlevel 0) [i686-linux]
false
$ /usr/local/lib/ruby-svn/bin/ruby -v super.rb 
ruby 1.9.0 (2007-11-07 patchlevel 0) [i686-linux]
true


David

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