From: Brian Candler <b.candler@...>
Date: 2009-12-15T00:05:14+09:00
Subject: Re: about class and module

Ruby Newbee wrote:
> As you said "Objects have singleton classes", what's "singleton
> class"? could you show a description by examples?

  s1 = "a string"
  s2 = "another string"

  def s1.wibble
    puts "bibble"
  end

  s1.wibble   # => "bibble"
  s2.wibble   # => NoMethodError: undefined method `wibble'

Every object has a 'singleton class' which contains methods private to 
that object.

A class, like class Foo, is an instance of class Class. So the 'class 
methods' of Foo are just methods in the singleton class of Foo.

  class Foo; end
  def Foo.wibble
    puts "I am a singleton method"
  end
  Foo.wibble

Another way to add methods into the singleton class is like this:

  s1 = "hello"
  class << s1
    def hey; puts "hey!"; end
  end
  s1.hey

So of course this works for classes (and class methods) too:

  class Foo
    class << self
      def bar
        puts "In bar"
      end
    end
  end
  Foo.bar

The singleton class is itself an object. It's quite hard to grab hold 
of, but the above gives you a clue to the trick:

  s = "hello"
  sc = class << s; self; end

So sometimes you will see code like this:

  class << s; self; end.class_eval { define_method(:bar) { puts "bar!" } 
}

An object with a singleton class loses it when it is dup'd (otherwise it 
wouldn't be a singleton any more :-)

  a = "hello"
  def a.world; puts "world"; end
  b = a.dup
  a.world     # "world"
  b.world     # NoMethodError: undefined method `world'
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