Module

The Forwardable module provides delegation of specified methods to a designated object, using the methods def_delegator and def_delegators.

For example, say you have a class RecordCollection which contains an array @records. You could provide the lookup method record_number(), which simply calls [] on the @records array, like this:

require 'forwardable'

class RecordCollection
  attr_accessor :records
  extend Forwardable
  def_delegator :@records, :[], :record_number
end

We can use the lookup method like so:

r = RecordCollection.new
r.records = [4,5,6]
r.record_number(0)  # => 4

Further, if you wish to provide the methods size, <<, and map, all of which delegate to @records, this is how you can do it:

class RecordCollection # re-open RecordCollection class
  def_delegators :@records, :size, :<<, :map
end

r = RecordCollection.new
r.records = [1,2,3]
r.record_number(0)   # => 1
r.size               # => 3
r << 4               # => [1, 2, 3, 4]
r.map { |x| x * 2 }  # => [2, 4, 6, 8]

You can even extend regular objects with Forwardable.

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

Another example

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 preferred 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

This should output:

2
3
4
5
6
Ruby
nil

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.

Constants

Version of forwardable.rb

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Attributes
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ignored

Instance Methods

Define method as delegator instance method with an optional alias name ali. Method calls to ali will be delegated to accessor.method.

class MyQueue
  extend Forwardable
  attr_reader :queue
  def initialize
    @queue = []
  end

  def_delegator :@queue, :push, :mypush
end

q = MyQueue.new
q.mypush 42
q.queue    #=> [42]
q.push 23  #=> NoMethodError

Shortcut for defining multiple delegator methods, but with no provision for using a different name. The following two code samples have the same effect:

def_delegators :@records, :size, :<<, :map

def_delegator :@records, :size
def_delegator :@records, :<<
def_delegator :@records, :map

Takes a hash as its argument. The key is a symbol or an array of symbols. These symbols correspond to method names. The value is the accessor to which the methods will be delegated.