Common Lisp the Language, 2nd Edition

28.1.9.6. Initialize-Instance

The generic function initialize-instance is called by make-instance to initialize a newly created instance. It uses standard method combination. Methods for initialize-instance can be defined in order to perform any initialization that cannot be achieved with the simple slot-filling mechanisms.

During initialization, initialize-instance is invoked after the following actions have been taken:

• The defaulted initialization argument list has been computed by combining the supplied initialization argument list with any default initialization arguments for the class.

• The validity of the defaulted initialization argument list has been checked. If any of the initialization arguments has not been declared valid, an error is signaled.

• A new instance whose slots are unbound has been created.

The generic function initialize-instance is called with the new instance and the defaulted initialization arguments. There is a system-supplied primary method for initialize-instance whose parameter specializer is the class standard-object. This method calls the generic function shared-initialize to fill in the slots according to the initialization arguments and the :initform forms for the slots; the generic function shared-initialize is called with the following arguments: the instance, t, and the defaulted initialization arguments.

Note that initialize-instance provides the defaulted initialization argument list in its call to shared-initialize, so the first step performed by the system-supplied primary method for shared-initialize takes into account both the initialization arguments provided in the call to make-instance and the defaulted initialization argument list.

Methods for initialize-instance can be defined to specify actions to be taken when an instance is initialized. If only :after methods for initialize-instance are defined, they will be run after the system-supplied primary method for initialization and therefore they will not interfere with the default behavior of initialize-instance.

The Object System provides two functions that are useful in the bodies of initialize-instance methods. The function slot-boundp returns a boolean value that indicates whether a specified slot has a value; this provides a mechanism for writing :after methods for initialize-instance that initialize slots only if they have not already been initialized. The function slot-makunbound causes the slot to have no value.