Private classes in Ruby, version 2

gistfile1.rb

module PrivateClass
  def self.included(mod)
    *parent, child = mod.name.split("::")
    Object.const_get(parent.join("::")).private_constant(child)
  end
end

module Container
  class PublicThing
    def hello
      PrivateThing.new.hello
    end
  end

  class PrivateThing
    include PrivateClass  # Mark this class as private at the top, not the bottom.

    def hello
      "Hello, world!"
    end
  end
  # private_constant :PrivateThing  <-- Normally you'd have to do this.
end

p Container::PublicThing.new.hello  # => "Hello, world!"
p Container::PrivateThing.new.hello # => NameError!