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slot-exists-p

slot-exists-p Function

Syntax:

slot-exists-p object slot-name → generalized-boolean

Arguments and Values:

object—an object.

slot-name—a symbol.

generalized-boolean—a generalized boolean.

Description:

Returns true if the object has a slot named slot-name.

Affected By:

defclass, defstruct

See Also:

defclass, slot-missing

Notes:

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

Expanded Reference: slot-exists-p​

Checking for Slot Existence​

slot-exists-p returns true if the object has a slot with the given name, and false otherwise. It does not check whether the slot is bound.

(defclass person ()
  ((name :initarg :name)
   (age  :initarg :age)))

(let ((p (make-instance 'person :name "Alice")))
  (list (slot-exists-p p 'name)
        (slot-exists-p p 'age)
        (slot-exists-p p 'address)))
=> (T T NIL)

Guarding Against Missing Slots​

slot-exists-p is useful for writing generic code that works with objects from different classes.

(defclass base-record ()
  ((id :initarg :id :accessor record-id)))

(defclass extended-record (base-record)
  ((notes :initarg :notes :accessor record-notes)))

(defun safe-get-notes (obj)
  (if (slot-exists-p obj 'notes)
      (if (slot-boundp obj 'notes)
          (slot-value obj 'notes)
          "(no notes)")
      "(slot not available)"))

(safe-get-notes (make-instance 'extended-record :id 1 :notes "important"))
=> "important"

(safe-get-notes (make-instance 'base-record :id 2))
=> "(slot not available)"

Works with Structures Too​

slot-exists-p also works with structure instances.

(defstruct point x y)

(let ((p (make-point :x 1 :y 2)))
  (list (slot-exists-p p 'x)
        (slot-exists-p p 'z)))
=> (T NIL)