name - a symbol.

value - an object.

The macro setf can be used with slot-value to change the value of a slot.

 (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

 (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.

Implementations may optimize slot-value by compiling it inline.