While working through a problem I found it would be immensely useful to be able to enumerate all of the current subclasses of a particular class. After thinking about this for a while I settled on a good old friend of mine, ObjectSpace.

For those not familiar with the ObjectSpace module, it is a means to inspect and access the items being tracked by Ruby's garbage collector. This means it has a hook into every living object, and more dangerously, every near-death object.

ObjectSpace provides a method for enumerating instances of a specific class, specifically named each_object which takes a class. With Ruby all classes are in fact instances of the Class class. This allows us to enumerate every available class by passing it to the enumerator like so:

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ObjectSpace.each_object(Class).to_a

Alright so we now have an array of every single class that could possibly be instantiated, how do we narrow it down to just the ones we're interested in? Once again Ruby provides with the ancestors method, combine that with a select and we can quickly narrow it down. You can see it in the following example:

[1] pry(main)> TargetSubclass = Class.new(String)
=> TargetSubclass
[2] pry(main)> ObjectSpace.each_object(Class).select { |k| k.ancestors.include?(String) }
=> [String, TargetSubclass]

Hmm, that's not quite right though. We have found all the subclasses but we've also grabbed the parent class. With one small modification we eliminate that as well.

[1] pry(main)> TargetSubclass = Class.new(String)
=> TargetSubclass
[2] pry(main)> ObjectSpace.each_object(Class).select { |k| k.ancestors.include?(String) && k != String }
=> [TargetSubclass]

That line is rather long though, and I generally like to avoid multiple tests in a select block. There is a tad bit of syntactic sugar provided by Ruby allowing us to accomplish the same thing, our final example is ultimately the solution I went with:

[1] pry(main)> TargetSubclass = Class.new(String)
=> TargetSubclass
[2] pry(main)> ObjectSpace.each_object(Class).select { |k| k < String }
=> [TargetSubclass]

Putting this into a method:

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def subclasses(klass)
  ObjectSpace.each_object(Class).select { |k| k < klass }
end

If you were so inclined you could extend the Class class with a method to make this available anywhere like so:

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class Class
  def self.subclasses
    ObjectSpace.each_object(Class).select { |k| k < self }
  end
end

I'm personally not a fan of extending any of the core classes unless absolutely necessary, but too each there own.