A delegate is a helper object used by another object. The delegator may send the delegate certain messages, and provide a default implementation when there is no delegate or the delegate does not respond to a message. This pattern is heavily used in Cocoa framework on Mac OS X. See also wp:Delegation pattern.

Objects responsibilities:

Delegator:

 Keep an optional delegate instance.
 Implement "operation" method, returning the delegate "thing" if the delegate respond to "thing", or the string "default implementation".
Delegate:

 Implement "thing" and return the string "delegate implementation"
Show how objects are created and used. First, without a delegate, then with a delegate that does not implement "thing", and last with a delegate that implements "thing".

class Delegator
   attr_accessor :delegate
   def operation
      if @delegate.respond_to?(:thing)
         @delegate.thing
      else
         'default implementation'
      end
   end
end
 
class Delegate
   def thing
      'delegate implementation'
   end
end
 
if __FILE__ == $PROGRAM_NAME
 
   # No delegate
   a = Delegator.new
   puts a.operation # prints "default implementation"
 
   # With a delegate that does not implement "thing"
   a.delegate = 'A delegate may be any object'
   puts a.operation # prints "default implementation"
 
   # With delegate that implements "thing"
   a.delegate = Delegate.new
   puts a.operation # prints "delegate implementation"
end