CLHS: Function SLOT-VALUE

#

Function SLOT-VALUE

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

Affected By: None.

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.

The following X3J13 cleanup issues, not part of the specification, apply to this section:

- SLOT-VALUE-METACLASSES:LESS-MINIMAL
- SLOT-MISSING-VALUES:SPECIFY

Copyright 1996-2005, LispWorks Ltd. All rights reserved.