The Beta Blog

Getting the Singleton Class of a BasicObject in Ruby

Ruby objects provide the method singleton_class which returns the object's singleton class. Unfortunately, BasicObject doesn't have this because it's Object's superclass. So to get it, we need to be somewhat clever.

And having spent way too much time figuring out how to do this, I'm writing it here so a) that I don't lose it again and b) so that others will have less trouble than me. (I'm not on Medium so, um, hello from the fourth page of your Google search results.)

TL; DR, How do I do it?

In an instance, you'd do something like this:

obj = BasicObject.new
obj.instance_exec(obj) {
  class << self
    lself = self
    self.define_method(:my_singleton_class) { lself }
  end
}

Notice how I copy self to lself on line 4. That's because self will have changed when the method is called but the block that forms the body of my_singleton_class captures the local variable.

Also: this won't work on Ruby versions from sometime before 2.7 because define_method is private before then; see your version's Module documentation for define_method for a hacky workaround if it's too old.

Doing this with a BasicObject subclass is even simpler:

class Thingy < BasicObject
  def my_singleton_class
    class << self
      return self
    end
  end
end

What's it good for?

Any case where you want an object to handle a method call by doing something other than call a method. For example, a DSL or a proxy object that forwards the call to something else.

Typically, you'd create a class with no methods, then implement method_missing to catch the failing method lookups and do the right thing with them.

class Proxy
  def initialize(target) @target = target;   end
  def method_missing(name, args)
    log "Called #{name} with #{args}"
    return @target.send(name, args)
  end
end

BasicObject is the ideal base class for this because it has very few methods but if that's not enough–if you need to get rid of those few as well–you can always override (most of) them with a method that calls method_missing directly. This is straightforward when creating a subclass but there are times when it's necessary or easier to add methods to the object instead, and for that you need to get the singleton class.

In my case, I'm writing a DSL where every method whose name starts with a letter is valid; this means they all need to turn into calls to method_missing.

(Handling the case where the user uses method_missing as a name in the DSL is left as an exercise to the reader.)

What's a singleton class anyway?

So normally in OOP, an object is an instance of a class and this is the case with Ruby as well:

[]                          # => []
[].class                    # => Array
[].class.class              # => Class

But, when Ruby creates an object from a class, it also first creates another anonymous class called the singleton class. This gets inserted in the new object's inheritance heirarchy: that is, the singleton class becomes a subclass of the new object's class and the object becomes an instance of the singleton instead of the original class.

x = []                          # => []
x.class                         # => Array
x.singleton_class               # => #<Class:#<Array:0x00007fbc862d41b0>>
x.singleton_class.superclass    # => Array

This is how you can add methods to individual Ruby objects: you're actually defining them in the object's singleton class.

Fun fact: singleton classes are also objects and thus have their own singleton classes:

x.singleton_class
    # => #<Class:#<Array:0x00007fbc869b0060>>
x.singleton_class.singleton_class
    # => #<Class:#<Class:#<Array:0x00007fbc869b0060>>>
x.singleton_class.singleton_class.singleton_class
    # => #<Class:#<Class:#<Class:#<Array:0x00007fbc869b0060>>>>

This can go as deeply as you want it to.

The reason Ruby doesn't immediately fill up all available RAM with singleton classes and then die is because they are not created until the first time a program uses them. As a result, most objects don't have singleton classes at all.

Isn't this whole singleton class thing kind of overkill?

Not really.

See, Ruby is a language where everything is an object (in the OOP sense of the term), and so this means that classes are also objects. But since all objects have classes, that means each class is also an instance of a class. And so is that class. And this is if we ignore the singleton classes, which we are for the moment.

So how does this end? Well, it's pretty boring actually. Each class is an instance of the class named Class, including Class itself. Class is an instance of itself and that's all we really need.

[]                          # => []
[].class                    # => Array
[].class.class              # => Class
[].class.class.class        # => Class
[].class.class.class.class  # => Class

But wait! How do we do class methods or class instance variables:

class Thing
  def self.instance
    @instance = Thing.new unless @instance
    return @instance
  end
  # ...etc...
end

In Smalltalk, this gets done by giving each class object its own distinct class (the metaclass) to hold the methods and variable declarations. They are unnamed but you can get it with the class method just like Ruby. The metaclass's inheritance tree mirrors the class's tree (i.e. if Item is derived from Thing, then Thing.class is derived from Item.class) with class Class as the abstract base class of the heirarchy.

t class.                                => Thing
t class superclass.                     => Object
t class superclass superclass.          => nil

t class class.                          => Unnamed class ('Thing class')
t class class superclass.               => Unnamed class ('Object class')
t class class superclass superclass.    => Class

All metaclasses are instances of the class Metaclass:

t class.                                => Thing
t class class.                          => Unnamed class ('Thing class')
t class class class.                    => Metaclass

This includes Metaclass itself, which is how the loop closes:

Metaclass class                         => 'Metaclass class'
Metaclass class class                   => Metaclass

(Disclaimer: I've somewhat simplified the above. I also haven't run it.)

In Ruby, each Class instance (i.e. class) has a singleton class that holds the class methods and variables. That is, singleton classes serve as metaclasses. The nice thing about this is that it's a generalization of what Smalltalk does for classes, and it gives you instance methods for free.

This is not to say that it's necessarily a better way than Smalltalk's. There are advantages and disadvantages to each approach but I'm far too lazy to write about them here.