Warning: This is the manual of the legacy Guile 2.2 series. You may want to read the manual of the current stable series instead.
Previous: Default Class Redefinition Behaviour, Up: Redefining a Class [Contents][Index]
When define-class
notices that a class is being redefined, it
constructs the new class metaobject as usual, then invokes the
class-redefinition
generic function with the old and new classes
as arguments. Therefore, if the old or new classes have metaclasses
other than the default <class>
, class redefinition behaviour can
be customized by defining a class-redefinition
method that is
specialized for the relevant metaclasses.
Handle the class redefinition from old-class to new-class,
and return the new class metaobject that should be bound to the
variable specified by define-class
’s first argument.
Implements GOOPS’ default class redefinition behaviour, as described in Default Class Redefinition Behaviour. Returns the metaobject for the new class definition.
The default class-redefinition
method, for classes with the
default metaclass <class>
, calls the following generic functions,
which could of course be individually customized.
The default remove-class-accessors!
method removes the accessor
methods of the old class from all classes which they specialize.
The default update-direct-method!
method substitutes the new
class for the old in all methods specialized to the old class.
The default update-direct-subclass!
method invokes
class-redefinition
recursively to handle the redefinition of
subclasses.
An alternative class redefinition strategy could be to leave all existing instances as instances of the old class, but accepting that the old class is now “nameless”, since its name has been taken over by the new definition. In this strategy, any existing subclasses could also be left as they are, on the understanding that they inherit from a nameless superclass.
This strategy is easily implemented in GOOPS, by defining a new metaclass,
that will be used as the metaclass for all classes to which the strategy
should apply, and then defining a class-redefinition
method that
is specialized for this metaclass:
(define-class <can-be-nameless> (<class>)) (define-method (class-redefinition (old <can-be-nameless>) (new <class>)) new)
When customization can be as easy as this, aren’t you glad that GOOPS implements the far more difficult strategy as its default!
Previous: Default Class Redefinition Behaviour, Up: Redefining a Class [Contents][Index]