When a redefinable class is redefined, any existing instance of the
redefined class will be modified for the new class definition before the
next time that any of the instance’s slots is referenced or set. GOOPS
modifies each instance by calling the generic function
change-class.
More generally, you can change the class of an existing instance at any
time by invoking the generic function change-class with two
arguments: the instance and the new class.
The default method for change-class decides how to implement the
change of class by looking at the slot definitions for the instance’s
existing class and for the new class. If the new class has slots with
the same name as slots in the existing class, the values for those slots
are preserved. Slots that are present only in the existing class are
discarded. Slots that are present only in the new class are initialized
using the corresponding slot definition’s init function (see slot-init-function).
Modify instance obj to make it an instance of class new. obj itself must already be an instance of a redefinable class.
The value of each of obj’s slots is preserved only if a similarly named slot exists in new; any other slot values are discarded.
The slots in new that do not correspond to any of obj’s pre-existing slots are initialized according to new’s slot definitions’ init functions.
The default change-class method also invokes another generic
function, update-instance-for-different-class, as the last thing
that it does before returning. The applied
update-instance-for-different-class method can make any further
adjustments to new-instance that are required to complete or
modify the change of class. The return value from the applied method is
ignored.
A generic function that can be customized to put finishing touches to an
instance whose class has just been changed. The default
update-instance-for-different-class method does nothing.
Customized change of class behavior can be implemented by defining
change-class methods that are specialized either by the class
of the instances to be modified or by the metaclass of the new class.