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slot-value

slot-value Function

Syntax:

slot-value object slot-name → value

Arguments and Values:

object—an object.

name—a symbol.

value—an object.

Description:

The function slot-value returns the value of the slot named slot-name in the object. If there is no slot named slot-name, slot-missing is called. If the slot is unbound, slot-unbound is called.

The macro setf can be used with slot-value to change the value of a slot.

Examples:

(defclass foo () 
  ((a :accessor foo-a :initarg :a :initform 1) 
   (b :accessor foo-b :initarg :b) 
   (c :accessor foo-c :initform 3))) 
→ #<STANDARD-CLASS FOO 244020371> 
(setq foo1 (make-instance ’foo :a ’one :b ’two)) 
→ #<FOO 36325624> 
(slot-value foo1 ’a) → ONE 
(slot-value foo1 ’b) → TWO 
(slot-value foo1 ’c) → 3 
(setf (slot-value foo1 ’a) ’uno) → UNO 
(slot-value foo1 ’a) → UNO 
(defmethod foo-method ((x foo)) 
  (slot-value x ’a)) 
→ #<STANDARD-METHOD FOO-METHOD (FOO) 42720573> 
(foo-method foo1) → UNO


Exceptional Situations:

If an attempt is made to read a slot and no slot of the name slot-name exists in the object, slot-missing is called as follows:

(slot-missing (class-of instance)

instance

slot-name

’slot-value)

(If slot-missing is invoked, its primary value is returned by slot-value.)

If an attempt is made to write a slot and no slot of the name slot-name exists in the object, slot-missing is called as follows:

(slot-missing (class-of instance)

instance

slot-name

’setf

new-value)

(If slot-missing returns in this case, any values are ignored.)

The specific behavior depends on object’s metaclass. An error is never signaled if object has metaclass standard-class. An error is always signaled if object has metaclass built-in-class. The consequences are unspecified if object has any other metaclass–an error might or might not be signaled in this situation. Note in particular that the behavior for conditions and structures is not specified.

See Also:

slot-missing, slot-unbound, with-slots

Notes:

Although no implementation is required to do so, implementors are strongly encouraged to implement the function slot-value using the function slot-value-using-class described in the Metaobject Protocol.

Implementations may optimize slot-value by compiling it inline.

Expanded Reference: slot-value​

Basic Slot Access​

slot-value reads the value of a named slot from any object. The slot name is a symbol and is not evaluated (you must quote it).

(defclass point ()
  ((x :initarg :x)
   (y :initarg :y)))

(let ((p (make-instance 'point :x 10 :y 20)))
  (list (slot-value p 'x) (slot-value p 'y)))
=> (10 20)

Setting Slot Values with setf​

slot-value is a setf-able place. You can write to slots that have no accessor defined.

(defclass point ()
  ((x :initarg :x)
   (y :initarg :y)))

(let ((p (make-instance 'point :x 0 :y 0)))
  (setf (slot-value p 'x) 42)
  (setf (slot-value p 'y) 99)
  (list (slot-value p 'x) (slot-value p 'y)))
=> (42 99)

Accessing Slots Without Accessors​

slot-value is the only way to access slots when no :reader, :writer, or :accessor has been defined.

(defclass secret ()
  ((data :initarg :data)))

;; No accessor -- must use slot-value
(let ((s (make-instance 'secret :data "classified")))
  (slot-value s 'data))
=> "classified"

Using slot-value Inside Methods​

Within method bodies, slot-value provides direct access to any slot, even private ones not exposed through accessors.

(defclass counter ()
  ((count :initform 0)))

(defgeneric increment (counter))
(defgeneric get-count (counter))

(defmethod increment ((c counter))
  (incf (slot-value c 'count)))

(defmethod get-count ((c counter))
  (slot-value c 'count))

(let ((c (make-instance 'counter)))
  (increment c)
  (increment c)
  (increment c)
  (get-count c))
=> 3

Unbound Slot Access​

Accessing a slot that has no value (unbound) invokes slot-unbound, which by default signals an error.

(defclass maybe-value ()
  ((val :initarg :val)))

(let ((obj (make-instance 'maybe-value)))
  ;; The slot 'val has no :initform and no initarg was supplied
  (handler-case (slot-value obj 'val)
    (unbound-slot (c)
      (format nil "Slot ~A is unbound" (cell-error-name c)))))
=> "Slot VAL is unbound"