From: dblack@...
Date: 2006-05-21T20:12:47+09:00
Subject: Re: About <programming ruby, 2nc edition>, meta class and Cl

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Hi --

On Sun, 21 May 2006, uncutstone wu wrote:

> The following is quoted from ruby-1.8.4-core-rdocs
>> Classes in Ruby are first-class objects=E2=80=94each is an instance of c=
lass Class.
>> All metaclasses are instances of the class `Class=E2=80=99.
>
> This makes me confused. In my understanding, all metaclasses are
> subclass of the class 'Class' as I posted in previous post.
>
> Somebody can explain it?

The class Class has no subclasses:

   irb(main):004:0> class C < Class; end
   TypeError: can't make subclass of Class

Every class is an instance of Class itself, including singleton
classes of all flavors.

You can test this like this:

   class C
     class << self
       puts self.class
     end
   end

   =3D> Class

What happens with inheritance is this: Given the following:

   class C
   end

   class D < C
   end

C is the superclass of D, and C's singleton class (yes, I prefer to
avoid the term metaclass entirely :-) is the superclass of D's
singleton class.  So it's like this, looking at it side by side:

   class C                   class C'
   end                       end

   class D < C               class D' < C'
   end                       end

where the thing on the right happens automatically.

You can verify this:

   irb(main):013:0> class << C; object_id; end
   537746792
   irb(main):014:0> class << D; superclass.object_id; end
   537746792

This, by the way, is why you can do this:

   def C.x; puts "A singleton method of C"; end
   D.x

    =3D> "A singleton method of C"

It's strange for one object (D) to be able to call the singleton
methods of another (C) -- except that classes are treated as a special
case.  So it's as if you'd done this:

   class C'
     def x
       puts "A singleton method of C"
     end
   end

   class D' < C'
   end

So "instances" of D' will respond to that x method -- and the only
"instance" of D', by definition, is D itself.


David

--=20
David A. Black (dblack@wobblini.net)
* Ruby Power and Light, LLC (http://www.rubypowerandlight.com)
   > Ruby and Rails consultancy and training
* Author of "Ruby for Rails" from Manning Publications!
   > http://www.manning.com/black
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