From: Dorren <dorrenchen@...>
Date: 2007-06-03T01:35:19+09:00
Subject: so lost on singleton, metaclass and virtual class

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?

# === begin code

# snippet from http://whytheluckystiff.net/articles/seeingMetaclassesClearly.html
class Object
  # The hidden singleton lurks behind everyone
  def metaclass; class << self; self; end; end

  def show_meta
    puts "--------- #{to_s} ------------"
    puts "oid:           #{object_id} #{self.class.name}"
    puts "class.oid:     #{self.class.object_id}
#{self.class.class.name}"
    puts "metaclass.oid: #{metaclass.object_id}
#{self.class.metaclass.class.name}"

    o = Object.new         # show methods
    meta = metaclass
    puts "object methods: #{(public_methods -
o.public_methods).sort.inspect}"
    puts " class methods: #{(self.class.public_methods -
o.class.public_methods).sort.inspect}"
    puts "  meta methods: #{(meta.public_methods -
Class.public_methods).sort.inspect}"
    puts
  end
end

# ------- fixtures for our test --------------
class A
  def initialize(v);    @v = v;           end
  def to_s;             @v;               end

  def m1;               "method1 "        end
  def self.class_m0;    "class method0";  end
end

b = A.new("b")
c = A.new("c")
b.show_meta
c.show_meta

# === end code


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

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?