Next: Modifying the Structure Up: Programmer Interface Concepts Previous: Definitions of Make-Instance
Common Lisp the Language, 2nd Edition
Next: Modifying the
Structure Up: Programmer
Interface Concepts Previous: Definitions of Make-Instance
A class that is an instance of standard-class can be
redefined if the new class will also be an instance of
standard-class. Redefining a class modifies the existing
class object to reflect the new class definition; it does not create a
new class object for the class. Any method object created by a
:reader, :writer, or :accessor
option specified by the old defclass form is removed from
the corresponding generic function. Methods specified by the new
defclass form are added.
When the class C is redefined, changes are propagated to its
instances and to instances of any of its subclasses. Updating such an
instance occurs at an implementation-dependent time, but no later than
the next time a slot of that instance is read or written. Updating an
instance does not change its identity as defined by the eq
function. The updating process may change the slots of that particular
instance, but it does not create a new instance. Whether updating an
instance consumes storage is implementation-dependent.
Note that redefining a class may cause slots to be added or deleted. If a class is redefined in a way that changes the set of local slots accessible in instances, the instances will be updated. It is implementation-dependent whether instances are updated if a class is redefined in a way that does not change the set of local slots accessible in instances.
The value of a slot that is specified as shared both in the old class and in the new class is retained. If such a shared slot was unbound in the old class, it will be unbound in the new class. Slots that were local in the old class and that are shared in the new class are initialized. Newly added shared slots are initialized.
Each newly added shared slot is set to the result of evaluating the
captured :initform form for the slot that was specified in
the defclass form for the new class. If there is no
:initform form, the slot is unbound.
If a class is redefined in such a way that the set of local slots
accessible in an instance of the class is changed, a two-step process of
updating the instances of the class takes place. The process may be
explicitly started by invoking the generic function
make-instances-obsolete. This two-step process can happen
in other circumstances in some implementations. For example, in some
implementations this two-step process will be triggered if the order of
slots in storage is changed.
The first step modifies the structure of the instance by adding new
local slots and discarding local slots that are not defined in the new
version of the class. The second step initializes the newly added local
slots and performs any other user-defined actions. These steps are
further specified in the next two sections.
Next: Modifying the
Structure Up: Programmer
Interface Concepts Previous: Definitions of Make-Instance
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