From: 7stud -- <bbxx789_05ss@...>
Date: 2011-08-23T12:51:45+09:00
Subject: Re: Gritty Details of super()

I wondered if the singleton class of a singleton class inherits from the
singleton class of a parent's singleton class:


class Cat
  class << self  #Cat's singleton class
    class << self #singleton class of Cat's singleton class
      def speak
        puts 'meow'
      end
    end
  end
end

class Dog < Cat
  class << self #Dog's singleton class
    def greet
      puts 'hello'
    end

  end

end

Dog.greet
Dog.singleton_class.speak

--output:--
hello
meow


So the lookup diagram looks something like this:


  Object -> #Object      ##Object
      ^       ^            ^
      |       |            |
     Cat --> #Cat        ##Cat
      ^       ^            ^
      |       |            |
     Dog --> #Dog        ##Dog
      ^
      |
d => #d


Now, we know that every class object inherits from Class.  That means
any instance methods defined in class can be called by a class object,
e.g. new().  Therefore, #Object should be able to call instance methods
defined in Class:


            Class
              ^
              |
  Object -> #Object      ##Object
      ^       ^            ^
      |       |            |
     Cat --> #Cat        ##Cat
      ^       ^            ^
      |       |            |
     Dog --> #Dog        ##Dog
      ^
      |
d => #d


But ##Object is also a class object, so it should be able to call
instance methods defined in Class as well:

            Class        Class
              ^            ^
              |            |
  Object -> #Object      ##Object
      ^       ^            ^
      |       |            |
     Cat --> #Cat        ##Cat
      ^       ^            ^
      |       |            |
     Dog --> #Dog        ##Dog
      ^
      |
d => #d


Let's test that:


class Class
  def eat
    puts "chomp, chomp"
  end
end

class Cat
  class << self  #Cat's singleton class
    class << self #singleton class of Cat's singleton class
      def speak
        puts 'meow'
      end
    end
  end
end

class Dog < Cat
  class << self #Dog's singleton class
    def greet
      puts 'hello'
    end
  end

end

Dog.greet     #=>hello
Dog.singleton_class.speak  #=>meow
Dog.singleton_class.eat    #=>chomp, chomp
Dog.singleton_class.singleton_class.eat  #=>chomp, chomp

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