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Delegates: Difference between revisions
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delegate implementation
</pre>
=={{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>
=={{header|D}}==
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