2016-11-09-ruby-187-define-method-with-bound-method.rb

require 'test/unit'

class Parent
  class << self
    def my_class_method
      'class-method'
    end
    def bound_class_method
      method(:my_class_method)
    end
  end
end

class Child < Parent
end

class ClassMethodTest < Test::Unit::TestCase
  def test_should_define_method_using_bound_class_method
    Parent.class_eval do
      class << self
        # In Ruby 1.8.7 passing the bound Method to `define_method` results in
        # "TypeError: singleton method bound for a different object"
        # when calling this method from a subclass of the parent (i.e. when
        # calling `Child.new_class_method`).
        define_method(:new_class_method, Parent.bound_class_method)

        # The following implementation works in all ruby versions 1.8.7 up to 2.3
        # define_method(:new_class_method) { Parent.bound_class_method.call }
      end
    end

    assert_equal 'class-method', Parent.my_class_method
    assert_equal 'class-method', Child.my_class_method

    assert_equal 'class-method', Parent.new_class_method
    assert_equal 'class-method', Child.new_class_method
  end
end