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".

uses objectclass;
define :class Delegator;
    slot delegate = false;
enddefine;
 
define :class Delegate;
enddefine;
 
define :method thing(x : Delegate);
   'delegate implementation'
enddefine;
 
define :method operation(x : Delegator);
if delegate(x) and fail_safe(delegate(x), thing) then
   ;;; Return value is on the stack
else
   'default implementation'
endif;
enddefine;
 
;;; Default, without a delegate
lvars a = newDelegator();
operation(a) =>
 
;;; a delegating to itself (works because Delegator does not
;;; implement thing)
a -> delegate(a);
operation(a) =>
 
;;; delegating to a freshly created Delegate
newDelegate() -> delegate(a);
operation(a) =>