Classy Ruby (a whole new world of objects)


There are a million Ruby tutorials that start with something like this:

class Animal


Yet so few that actually explain what’s really going on when we define a new class.

So after six or so years programming Ruby, I thought I’d take a closer look.

And it turns out the truth is hidden in plain sight.

This is from the Ruby docs

When a new class is created, an object of type Class is initialized and assigned to a global constant.

After reading that, the other way of defining a class now makes perfect sense.

Animal =

But it does beg a whole new bunch of questions.

Like if Animal is an instance of a Class, then the logical conclusion is that a class method is really just an instance method on the Class object.

But that’s impossible, right? Because if every user-defined class would be an instance of the Class object, every class method must be instance method on Class itself! There’d be all kinds of naming collisions.


Yeah, that’s not what’s happening.

So how the devil does Ruby define a class method?


Enter the conceptual minefield of metaclasses.

You see, it turns out that when we define our Animal class (that initializes a new instance of Class), we also get a magical second object that is the singleton class of Animal.

It’s called a Singleton Class because you can only ever instantiate one instance of it – and hence it’s the perfect candidate for defining our instance method (that will parade around like a class method).

And the beautiful thing about Ruby is proving all this stuff actually happens is as simple as opening a new irb session.

So let’s define our class, this time with a class method of count.

class Animal
  def self.count

Let’s prove Animal is an instance of Class, it has a singleton class, and that singleton class has an instance method of count.

Animal.class # => Class
Animal.singleton_class # => #<Class:Animal>
Animal.singleton_class.instance_methods.grep(/count/) # => [:count]


Doing some of this stuff, and poking around in an irb session was humbling. It made me realise how little I knew about a language I thought I had a decent grasp of. But the joy of learning is that it always comes with the promise of another season.


And so I have heaps of gratitude for the following wonderful things on the internet:

  1. The Case of the Missing Method – a talk by Nadia Odunayo
  2. Ruby has no Class Methods – an article by Edison Yap