class SimpleDelegator

A concrete implementation of Delegator, this class provides the means to delegate all supported method calls to the object passed into the constructor and even to change the object being delegated to at a later time with #__setobj__.

class User
  def born_on
    Date.new(1989, 9, 10)
  end
end

require 'delegate'

class UserDecorator < SimpleDelegator
  def birth_year
    born_on.year
  end
end

decorated_user = UserDecorator.new(User.new)
decorated_user.birth_year  #=> 1989
decorated_user.__getobj__  #=> #<User: ...>

A SimpleDelegator instance can take advantage of the fact that SimpleDelegator is a subclass of Delegator to call super to have methods called on the object being delegated to.

class SuperArray < SimpleDelegator
  def [](*args)
    super + 1
  end
end

SuperArray.new([1])[0]  #=> 2

Here's a simple example that takes advantage of the fact that SimpleDelegator's delegation object can be changed at any time.

class Stats
  def initialize
    @source = SimpleDelegator.new([])
  end

  def stats(records)
    @source.__setobj__(records)

    "Elements:  #{@source.size}\n" +
    " Non-Nil:  #{@source.compact.size}\n" +
    "  Unique:  #{@source.uniq.size}\n"
  end
end

s = Stats.new
puts s.stats(%w{James Edward Gray II})
puts
puts s.stats([1, 2, 3, nil, 4, 5, 1, 2])

Prints:

Elements:  4
 Non-Nil:  4
  Unique:  4

Elements:  8
 Non-Nil:  7
  Unique:  6