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.