べつのクラスを一時的に同じインスタンスのように振る舞わせる

checkout.rb

class Object

  alias checkout_original_method_missing method_missing

  def checkout! into
    into = case into.class
           when Class then into.new
           when Module then Object.new.tap { |i| i.extend into }
           else into end
    define_singleton_method :method_missing do |f, *a, &b|
      begin
        checkout_original_method_missing f, *a, &b
      rescue NoMethodError => e
        into.send f, *a, &b
      end
    end
  end

  def checkout into
    clone.checkout! into
  end

end

class A < Array
  def hello; "hello"; end
end

o = Array.new

o.checkout! A
puts o.hello # hello