Common Lisp Object System

• HyperSpec: Objects
• CLtL2: Common Lisp Object System
• PCL: Generic Functions
• PCL: Classes

Classes

defclass name ([superclass]) definitions* ⇒ new-class

Defines a named class, returning the new class object as its result.

(defclass mammal ()
  (legs sound))

(defclass dog (mammal)
  ((name
     :initarg :name
     :initform (error "A dog needs a name.")
     :reader title                           ;getter
     :writer (setf name)                     ;setter
     :documentation "Dog name.")
   (sound
     :accessor sound                         ;getter/setter
     :initform "woof"))
  (:documentation "Man's best friend."))

And to pretty-print the object at the REPL:

(defmethod print-object ((dog dog) stream)
  (print-unreadable-object (dog stream :type t :identity t)
    (with-slots (name) dog
      (format stream ":NAME ~S" name))))

make-instance class ⇒ instance

(setf mydog (make-instance 'dog :name "Sparky"))

class-of instance ⇒ class

(class-of mydog)  ;⇒ #<STANDARD-CLASS DOG>

find-class symbol [error-p environment] ⇒ class

(find-class 'dog) ;⇒ #<STANDARD-CLASS DOG>

class-name class ⇒ name

Returns the name of the given class.

(class-name (find-class 'dog))                ;⇒ DOG
(setf (class-name (find-class 'dog)) 'doggie)

slot-value instance slot ⇒ value

(setf (slot-value mydog 'legs) 4)
(slot-value mydog 'legs)          ;⇒ 4
(slot-value mydog 'breed)         ;⇒ [throws error]

with-slots (slots*) instance forms* ⇒ result

(with-slots (name sound) mydog
  (list name sound))                  ;⇒ ("Sparky" "woof")

(with-slots ((n name) (l legs)) mydog
  (list n l))                         ;⇒ ("Sparky" 4)

with-accessors (accessors*) instance forms* ⇒ result

(with-accessors (sound) mydog
  (list sound))                       ;⇒ ("woof")

(with-accessors ((s sound)) mydog
  (list s))                           ;⇒ ("woof")

slot-exists-p instance name ⇒ boolean

Test if object instance has a given slot.

slot-boundp instance name ⇒ boolean

Test if a slot is bound on a given object instance. Throw error if no such slot.

slot-makunbound instance name ⇒ instance

Unbind the slot on the given object instance.

Methods

defgeneric name lambda-list …options-and-methods ⇒ generic-function

Define or modify a generic function.

(defgeneric say-hello (mammal)
  (:documentation "Say hi."))

(defgeneric say-hello (mammal)
  (:method ((m mammal)) (print "blink"))
  (:method ((d dog)) (print "wag tail")))

defmethod name lambda-list …forms* ⇒ method

Defines a method on a generic function.

(defgeneric make-sound (dog))

(defmethod make-sound ((dog dog))
  (slot-value dog 'sound))

(make-sound mydog) ;⇒ "woof"

Use setf to define a class setter method:

(defmethod (setf make-sound) (noise (dog dog))
  (setf (slot-value dog 'sound) noise))

(setf (make-sound mydog) "bark!")

initialize-instance instance …initargs &key &allow-other-keys ⇒ instance

Called by make-instance to initialize a newly created instance. Use with method combination to create a constructor-like function.

(defmethod initialize-instance :after ((dog dog) &key)
  (setf (slot-value dog 'legs) 4))

Advanced

add-method generic-function method ⇒ generic-function

Adds a method to a generic-function.

remove-method generic-function method ⇒ generic-function

Removes a method from a generic-function.

ensure-generic-function name &key… ⇒ generic-function

Define or modify a generic-function.

invalid-method-error method format-control …args ⇒ implementation-dependent

Signal error on applicable method with invalid qualifiers.

method-combination-error format-control …args ⇒ implementation-dependent

Signal error on applicable method with invalid method combination.

Method Combination

• PCL: Method Combination
• CLtL2: Method Selection and Combination

Using standard method combination:

(defgeneric bedtime (dog))

(defmethod bedtime ((d dog))
  (print 'zzzzz))

(defmethod bedtime :before ((d dog))
  (print 'lay-down))

(defmethod bedtime :after ((d dog))
  (print 'wake-up))

(defmethod bedtime :around ((d dog))
  (print 'yawn)
  (let ((val (call-next-method)))  ;⇒ save original return value
    (print 'get-up)
    val))

(bedtime mydog) ;⇒ ZZZZZ [prints YAWN, LAY-DOWN, ZZZZZ, WAKE-UP, GET-UP]

define-method-combination name lambda-list …forms* ⇒ name

Define a new method-combination type.

Combination options: progn|+|list|and|or|max|min|append|nconc Default is progn, this executes all applicable methods in most-specific to least-specific order.

(defgeneric eat (mammal)
  (:method-combination progn))

(defmethod eat progn ((m mammal))
  (print 'digest))

(defmethod eat progn ((d dog))
  (print 'lick))

(eat mydog) ;⇒ DIGEST [prints LICK, DIGEST]

next-method-p ⇒ boolean

Called within a method to determine whether a next method exists.

call-next-method …args ⇒ result

Used within a method to call the next method.

(defgeneric itch (mammal body-part))

(defmethod itch ((m mammal) body-part)
  (format t "twitch ~a" body-part))

(defmethod itch ((d dog) body-part)
  (format t "scratch ~a~%" body-part)
  (if (next-method-p)
    (call-next-method)))

(itch mydog "leg") ;⇒ [prints "scratch leg" then "twitch leg"]

call-method method-name [next-method-list] ⇒ result

From within a method, call method-name with the arguments of the generic function and with information about its next-methods; return its values.

Method Selection

Advanced

find-method generic-function method-qualifiers specializers [error-p] ⇒ method

Return suitable method, or signal error.

(find-method #'say-hello '() '(mammal)) ;⇒ #<STANDARD-METHOD SAY-HELLO (MAMMAL)>

compute-applicable-methods generic-function args ⇒ methods

List of methods suitable for args, most specific first.

(compute-applicable-methods #'say-hello (list mydog))
  ;⇒ (#<STANDARD-METHOD SAY-HELLO (DOG)> #<STANDARD-METHOD SAY-HELLO (MAMMAL)>)

function-keywords method ⇒ keys, allow-other-keys-p

Returns the keyword parameter specifiers for a method.

method-qualifiers method ⇒ qualifiers

Returns the keyword parameter specifiers for a method.

Redefine

Advanced

reinitialize-instance instance …initargs &key &allow-other-keys ⇒ instance

Change the values of local slots of an object instance according to initargs.

change-class instance new-class &key &allow-other-keys ⇒ instance

Changes the class of an instance to new-class. It destructively modifies and returns the instance.

make-instances-obsolete class ⇒ class

Update all existing instances of class using update-instance-for-redefined-class.

allocate-instance class …initargs &key &allow-other-keys ⇒ new-instance

Creates and returns a new instance of the class, without initializing it. Called by make-instance.

shared-initialize instance slot-names …initargs &key &allow-other-keys ⇒ instance

Fill the slots of an object instance using initargs and :initform forms. Called when an instance is created.

Internal

Advanced

slot-missing class instance slot-name operation [new-value] ⇒ result

Called on attempted access to non-existing slot.

slot-unbound class instance slot-name ⇒ result

Called on attempted access to unbound slot.

update-instance-for-redefined-class instance args… ⇒ result

Set any initarg slots to their corresponding values.

update-instance-for-different-class previous current … ⇒ implementation-dependent

Set slots on behalf of change-class by means of shared-initialize.

no-applicable-method generic-function …args ⇒ result

Called on invocation of generic-function on args if there is no applicable method.

no-next-method generic-function method …args ⇒ result

Called on invocation of call-next-method when there is no next method.