The Chicken-and-Egg Problem of Classes and Objects in Ruby (with a Twist of Metaprogramming)

Ruby’s object model has a famous philosophical puzzle at its core: every class is an object, but every object is an instance of a class. So which came first — the class or the object?

The Puzzle

In Ruby, Class is a class. But Class is also an object — an instance of itself. And Object is a class, but it’s also an instance of Class. It’s turtles all the way down.

Class.class   # => Class
Object.class  # => Class
Class.superclass # => Module
Module.superclass # => Object
Object.superclass # => BasicObject
BasicObject.superclass # => nil

The Bootstrap

Ruby’s C runtime bootstraps this circular dependency by creating BasicObject, Object, Module, and Class simultaneously before any Ruby code runs. The interpreter “cheats” — it sets up the object graph manually in C, then hands control over to Ruby.

The Metaprogramming Twist

Because classes are objects, you can manipulate them at runtime just like any other object:

Dog = Class.new do
  def bark
    "Woof!"
  end
end

Dog.new.bark # => "Woof!"

This makes Ruby’s metaprogramming unusually powerful. You can define classes, add methods, and reshape the object hierarchy all at runtime.