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".

(defprotocol Thing
  (thing [_]))
 
(defprotocol Operation
  (operation [_]))
 
(defrecord Delegator [delegate]
  Operation
  (operation [_] (try (thing delegate) (catch IllegalArgumentException e "default implementation"))))
 
(defrecord Delegate []
  Thing
  (thing [_] "delegate implementation"))