8.12.3 Customizing Class Redefinition

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 <redefinable-class>, class redefinition behaviour can be customized by defining a class-redefinition method that is specialized for the relevant metaclasses.

generic: class-redefinition

Handle the class redefinition from old to new, and return the new class metaobject that should be bound to the variable specified by define-class’s first argument.

method: class-redefinition (old <top>) (new <class>)

Not all classes are redefinable, and not all previous bindings are classes. See Redefinable Classes. This default method just returns new.

method: class-redefinition (old <redefinable-class>) (new <redefinable-class>)

This method implements GOOPS’ default class redefinition behaviour, as described in Default Class Redefinition Behaviour. Returns the metaobject for the new class definition.

The class-redefinition method for classes with metaclass <redefinable-class> calls the following generic functions, which could of course be individually customized.

generic: remove-class-accessors! old

The default remove-class-accessors! method removes the accessor methods of the old class from all classes which they specialize.

generic: update-direct-method! method old new

The default update-direct-method! method substitutes the new class for the old in all methods specialized to the old class.

generic: update-direct-subclass! subclass old new

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> (<redefinable-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!