The generic function update-instance-for-different-class is not
intended to be called by programmers. Programmers may write
methods for it. The function update-instance-for-different-class
is called only by the function change-class.
The system-supplied primary method on
update-instance-for-different-class checks the validity of
initargs and signals an error if an initarg
is supplied that is not declared as valid. This method then
initializes slots with values according to the initargs,
and initializes the newly added slots with values according
to their :initform forms. It does this by calling the generic
function shared-initialize with the following arguments: the
a list of names of the newly added slots, and the initargs
it received. Newly added slots are those local slots for which
no slot of the same name exists in the previous class.
Methods for update-instance-for-different-class can be defined to
specify actions to be taken when an instance is updated. If only
after methods for update-instance-for-different-class are
defined, they will be run after the system-supplied primary method for
initialization and therefore will not interfere with the default
behavior of update-instance-for-different-class.
Methods on update-instance-for-different-class can be defined to
initialize slots differently from change-class. The default
behavior of change-class is described in
Section 7.2 Changing the Class of an Instance.
The arguments to update-instance-for-different-class are
computed by change-class. When change-class is invoked on
an instance, a copy of that instance is made; change-class then
destructively alters the original instance. The first argument to
update-instance-for-different-class, previous, is that
copy; it holds the old slot values temporarily. This argument has
dynamic extent within change-class; if it is referenced in any
way once update-instance-for-different-class returns, the
results are undefined. The second argument to
update-instance-for-different-class, current, is the altered
The intended use of previous is to extract old slot values by using
slot-value or with-slots or by invoking
a reader generic function, or to run other methods that were applicable to
the original class.