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slot-missing

slot-missing Standard Generic Function

Syntax:

slot-missing class object slot-name operation &optional new-value → {result}*

Method Signatures:

slot-missing (class t) object slot-name

operation &optional new-value

Arguments and Values:

class—the class of object.

object—an object.

slot-name—a symbol (the name of a would-be slot).

operation—one of the symbols setf, slot-boundp, slot-makunbound, or slot-value. new-value—an object.

result—an object.

Description:

The generic function slot-missing is invoked when an attempt is made to access a slot in an object whose metaclass is standard-class and the slot of the name slot-name is not a name of a slot in that class. The default method signals an error.

The generic function slot-missing is not intended to be called by programmers. Programmers may write methods for it.

The generic function slot-missing may be called during evaluation of slot-value, (setf slot-value), slot-boundp, and slot-makunbound. For each of these operations the corresponding symbol for the operation argument is slot-value, setf, slot-boundp, and slot-makunbound respectively.

The optional new-value argument to slot-missing is used when the operation is attempting to set the value of the slot.

If slot-missing returns, its values will be treated as follows:

• If the operation is setf or slot-makunbound, any values will be ignored by the caller.

• If the operation is slot-value, only the primary value will be used by the caller, and all other values will be ignored.

• If the operation is slot-boundp, any boolean equivalent of the primary value of the method might be is used, and all other values will be ignored.

Exceptional Situations:

The default method on slot-missing signals an error of type error.

See Also:

defclass, slot-exists-p, slot-value

Notes:

The set of arguments (including the class of the instance) facilitates defining methods on the metaclass for slot-missing.

Expanded Reference: slot-missing​

Default Behavior​

slot-missing is called when slot-value, (setf slot-value), slot-boundp, or slot-makunbound is used with a slot name that does not exist on the object. The default method signals an error.

(defclass simple ()
  ((x :initarg :x)))

(let ((s (make-instance 'simple :x 1)))
  (handler-case (slot-value s 'nonexistent)
    (error () :caught-error)))
=> :CAUGHT-ERROR

Custom slot-missing for Dynamic Properties​

You can define a method on slot-missing to implement a property-list-like fallback for missing slots.

(defclass flexible-object ()
  ((properties :initform (make-hash-table) :accessor obj-properties)))

(defmethod slot-missing ((class t) (obj flexible-object) slot-name
                         (operation (eql 'slot-value)) &optional new-value)
  (declare (ignore new-value))
  (gethash slot-name (obj-properties obj)))

(defmethod slot-missing ((class t) (obj flexible-object) slot-name
                         (operation (eql 'setf)) &optional new-value)
  (setf (gethash slot-name (obj-properties obj)) new-value))

(let ((obj (make-instance 'flexible-object)))
  (setf (slot-value obj 'color) :red)
  (slot-value obj 'color))
=> :RED

The operation Argument​

The operation argument tells you which function triggered the call: slot-value, setf, slot-boundp, or slot-makunbound.

(defclass logged ()
  ((data :initarg :data)))

(defmethod slot-missing ((class t) (obj logged) slot-name operation
                         &optional new-value)
  (format nil "Missing slot ~A, operation ~A" slot-name operation))

(let ((obj (make-instance 'logged :data 1)))
  (slot-value obj 'no-such-slot))
=> "Missing slot NO-SUCH-SLOT, operation SLOT-VALUE"