18.3 Common problems when using the MOP
18.3.1 Inheritance across metaclasses
Usually an inherited class is of the same metaclass as the parent class.
For other kinds of inheritance, you need to define a method on
validate-superclass which returns true when called with the respective metaclasses. For example:
(defclass mclass-1 (standard-class)
(defclass mclass-2 (standard-class)
(defclass a ()
(defclass b (a)
validate-superclass method, the last form signals an error because
mclass-1 is an invalid superclass of
18.3.2 Accessors not using structure instance protocol
By default, defclass creates optimized standard accessors which do not call slot-value-using-class.
This optimization is controlled by the defclass option
:optimize-slot-access, which defaults to
There is an illustration of this effect of
:optimize-slot-access in the example below.
18.3.3 The MOP in delivered images
Issues with MOP code that occur only in delivered LispWorks images are documented in the section "Delivery and the MOP" in the
LispWorks Delivery User Guide
LispWorks User Guide and Reference Manual - 13 Feb 2015