Delegates: Difference between revisions

=={{header|Common Lisp}}==

In CLOS, methods exist apart from classes, and are specialized based on the types of their arguments. This example defines two classes (delegator and delegate), and a thing generic method which is specialized in three ways: (1) for 'any' argument, providing a default method; (2) for delegators, where thing is recursively applied to the delegator's delegate (if there is one); and (3) for delegates.

<lang lisp>(defgeneric thing (object)

(:documentation "Thing the object."))

(defmethod thing (object)

"default implementation")

(defclass delegator ()

((delegate

:initarg :delegate

:reader delegator-delegate)))

(defmethod thing ((delegator delegator))

"If delegator has a delegate, invoke thing on the delegate,

otherwise return \"no delegate\"."

(if (slot-boundp delegator 'delegate)

(thing (delegator-delegate delegator))

"no delegate"))

(defclass delegate () ())

(defmethod thing ((delegate delegate))

"delegate implementation")

(let ((d1 (make-instance 'delegator))

(d2 (make-instance 'delegator :delegate nil))

(d3 (make-instance 'delegator :delegate (make-instance 'delegate))))

(assert (string= "no delegate" (thing d1)))

(assert (string= "default implementation" (thing d2)))

(assert (string= "delegate implementation" (thing d3))))</lang>