Inheriting class methods from modules / mixins in Ruby

Question

It is known that in Ruby, class methods get inherited:

class P
  def self.mm; puts \'abc\' end
end
class Q < P; end
Q.mm # works

However

Answer 1

A common idiom is to use included hook and inject class methods from there.

module Foo
  def self.included base
    base.send :include, InstanceMethods
    base.extend ClassMethods
  end

  module InstanceMethods
    def bar1
      'bar1'
    end
  end

  module ClassMethods
    def bar2
      'bar2'
    end
  end
end

class Test
  include Foo
end

Test.new.bar1 # => "bar1"
Test.bar2 # => "bar2"