Using Forwardable in Ruby

By: James Edward Gray II in Ruby Tutorials on 2009-03-03

Forwardable makes building a new class based on existing work, with a proper interface, almost trivial. We want to rely on what has come before obviously, but with delegation we can take just the methods we need and even rename them as appropriate. In many cases this is preferable to inheritance, which gives us the entire old interface, even if much of it isn't needed.

  class Queue
    extend Forwardable

    def initialize
      @q = [ ]    # prepare delegate object
    end

    # setup prefered interface, enq() and deq()...
    def_delegator :@q, :push, :enq
    def_delegator :@q, :shift, :deq

    # support some general Array methods that fit Queues well
    def_delegators :@q, :clear, :first, :push, :shift, :size
  end

  q = Queue.new
  q.enq 1, 2, 3, 4, 5
  q.push 6

  q.shift    # => 1
  while q.size > 0
    puts q.deq
  end

  q.enq "Ruby", "Perl", "Python"
  puts q.first
  q.clear
  puts q.first

Prints:

  2
  3
  4
  5
  6
  Ruby
  nil

SingleForwardable

   printer = String.new
   printer.extend SingleForwardable        # prepare object for delegation
   printer.def_delegator "STDOUT", "puts"  # add delegation for STDOUT.puts()
   printer.puts "Howdy!"

Prints: