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slot-value [Function]

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.