Anonymous classes in Ruby

Question

I have two questions:

1. Does method f_1 belong to the metaclass anonymous class?
2. Does method f_2 belong to the anonymous class?

related to the following code:

car = "car"

class << car
  def self.f_1
    puts "f_1"
  end
  def f_2
    puts "f_2"
  end
end

Answer 1

Well, terminology is frangible, but FWIW I would say your class wasn't really an anonymous class. As for belonging, both of these methods only exist in the car object.

I'll be honest and admit that I'm a little vague about the difference between a class method and an instance method when the class is defined against an individual object like this -- I would guess that if there is any difference, it will be an obscure one that will make your code much harder to read ;)

Update: You might find this helpful, if you've not seen it before. (Personally, it makes my head hurt, but everyone's different...)

Answer 2

Since ruby's own API uses the term "singleton class," I'd say the following are true:

1. f_1 is a class method on car's singleton class and can be called like this:

   car.singleton_class.f_1
   

2. f_2 is an instance method on car's singleton class and can be called like this:

   car.f_2
   

Answer 3

I was under the impression that an anonymous class is a class that has no name:

my_class = Class.new
my_class.name # => nil

However, the Pickaxe refers to it as a unnamed class rather than as an anonymous class.

Answer 4

A reformulation of Rob Davis' answer:

1. The method-owner of :f_1 is car.singleton_class.singleton_class.
2. The method-owner of :f_2 is car.singleton_class.

The chain car → car.singleton_class → car.singleton_class.singleton_class corresponds to the bottom row in the diagram at http://www.atalon.cz/rb-om/ruby-object-model/#sc-inheritance-sample.

Notes:

• The code does NOT create any new class (in particular, no anonymous class is created).
• In most Ruby programs, eigenclasses of eigenclasses are NOT made "actual" (see http://www.atalon.cz/rb-om/ruby-object-model/#actual-lists).