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unbound-slot

unbound-slot Condition Type

Class Precedence List:

unbound-slot, cell-error, error, serious-condition, condition, t

Description:

The object having the unbound slot is initialized by the :instance initialization argument to make-condition, and is accessed by the function unbound-slot-instance.

The name of the cell (see cell-error) is the name of the slot.

See Also:

cell-error-name, unbound-slot-object, Section 9.1 (Condition System Concepts)

Expanded Reference: unbound-slot​

Catching unbound-slot Conditions​

The unbound-slot condition is signaled when slot-value attempts to read an unbound slot. It is a subtype of cell-error, so cell-error-name gives the slot name, and unbound-slot-instance gives the instance.

(defclass data-holder ()
  ((value :initarg :value)))

(let ((obj (make-instance 'data-holder)))
  (handler-case (slot-value obj 'value)
    (unbound-slot (c)
      (list :slot (cell-error-name c)
            :instance-class (class-name (class-of (unbound-slot-instance c)))))))
=> (:SLOT VALUE :INSTANCE-CLASS DATA-HOLDER)

Using handler-bind for Recovery​

You can use handler-bind to handle the condition without unwinding the stack.

(defclass optional-slot ()
  ((data :initarg :data)))

(defun safe-read (obj slot)
  (handler-case (slot-value obj slot)
    (unbound-slot () :unbound)))

(safe-read (make-instance 'optional-slot :data 42) 'data)
=> 42

(safe-read (make-instance 'optional-slot) 'data)
=> :UNBOUND

Class Precedence List​

unbound-slot inherits from cell-error, error, serious-condition, condition, and t.

(subtypep 'unbound-slot 'cell-error)
=> T
=> T
(subtypep 'unbound-slot 'error)
=> T
=> T