The second step initializes the newly added local slots and performs any other user-defined actions. This step is implemented by the generic function update-instance-for-redefined-class, which is called after completion of the first step of modifying the structure of the instance.
The generic function update-instance-for-redefined-class takes four required arguments: the instance being updated after it has undergone the first step, a list of the names of local slots that were added, a list of the names of local slots that were discarded, and a property list containing the slot names and values of slots that were discarded and had values. Included among the discarded slots are slots that were local in the old class and that are shared in the new class.
The generic function update-instance-for-redefined-class also takes any number of initialization arguments. When it is called by the system to update an instance whose class has been redefined, no initialization arguments are provided.
There is a system-supplied primary method for update-instance-for-redefined-class whose parameter specializer for its instance argument is the class standard-object. First this method checks the validity of initialization arguments and signals an error if an initialization argument is supplied that is not declared as valid. (For more information, see Section 7.1.2 (Declaring the Validity of Initialization Arguments).) Then it calls the generic function shared-initialize with the following arguments: the instance, the list of names of the newly added slots, and the initialization arguments it received.