forwardable.rb

  forwardable.rb -
      $Release Version: 1.1$
      $Revision: 14912 $
      by Keiju ISHITSUKA(keiju@ishitsuka.com)
      original definition by delegator.rb
      Revised by Daniel J. Berger with suggestions from Florian Gross.

      Documentation by James Edward Gray II and Gavin Sinclair

Introduction

This library allows you delegate method calls to an object, on a method by method basis.

Notes

Be advised, RDoc will not detect delegated methods.

forwardable.rb provides single-method delegation via the def_delegator() and def_delegators() methods. For full-class delegation via DelegateClass(), see delegate.rb.

Examples

Forwardable

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

Forwardable can be used to setup delegation at the object level as well.

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

Prints:

   Howdy!