The Art of the Metaobject Protocol- P2: The work described here is synthetic in nature, bringing together techniques and insights from several branches of computer. | How CLOS is Implemented 27 Object identity Different calls to make-instance must create distinct objects . non-eq . Slot storage There must be a place to store the current bindings of each instance s slots. Classification It must be possible to determine the class that a given object is an instance of. Reclassification change-class must change the class of an instance without changing its identity. For example the door instance of color-rectangle is laid out along the lines of Figure . The actual implementation of the low-level instance representation is not important to this presentation. We simply assume the existence of an abstract data type called standard instance with the following functional interface 12 allocate-std-instance class slot-storage returns a brand new standard instance whose class is class and whose slot storage is slot-storage . std-instance-p object returns true only when object is a standard instance . the result of a call to allocate-std-instance and not some other Common Lisp object like a cons or a symbol . std-instance-class instance provides access to the class of the standard instance instance . std-instance-slots instance provides access to the slot storage for the standard instance instance . allocate-slot-storage size initial-value returns a new chunk of storage big enough to hold the bindings of size number of slots all are initialized to initial-value . slot-contents slot-storage location provides access to the binding of the slot at a given location in slot-storage . Locations are numbered from zero. Class color-rectangle Slots clearp nil cyan 60 height 84 magenta 0 width 38 yellow 55 Figure The structure of an instance of color-rectangle. Determining the Class of an Instance Every object in Common Lisp is an instance of a CLOS class. Internally implementing generic function invocation and other operations requires that we be able to determine 12For the curious see std-instance 281 . 28 Chapter 1 the class of an
