2. Defining classes.

You define a class with DEFCLASS :

(DEFCLASS class-name (superclass-name*)
  (slot-description*)
  class-option*)

For simple things, forget about class options.

A slot-description has the form (slot-name slot-option*) , where each option is a keyword followed by a name, expression, or whatever. The most useful slot options are

:ACCESSOR

function-name

:INITFORM

expression

:INITARG

symbol

(Initargs are usually keywords.)

DEFCLASS is similar to DEFSTRUCT. The syntax is a bit different, and you have more control over what things are called. For instance, consider the DEFSTRUCT:

(defstruct person
  (name 'bill)
  (age 10))

DEFSTRUCT would automatically define slots with expressions to compute default initial values, access-functions like PERSON-NAME to get and set slot values, and a MAKE-PERSON that took keyword initialization arguments (initargs) as in

(make-person :name 'george :age 12)

A DEFCLASS that provided similar access functions, etc, would be:

(defclass person ()
  ((name :accessor person-name
         :initform 'bill
         :initarg :name)
   (age :accessor person-age
        :initform 10
        :initarg :age)))

Note that DEFCLASS lets you control what things are called. For instance, you don't have to call the accessor PERSON-NAME. You could call it NAME.

In general, you should pick names that make sense for a group of related classes rather than rigidly following the DEFSTRUCT conventions.

You do not have to provide all options for every slot. Maybe you don't want it to be possible to initialize a slot when calling MAKE-INSTANCE (for which see below). In that case, don't provide an :INITARG. Or maybe there isn't a meaningful default value. (Perhaps the meaningful values will always be specified by a subclass.) In that case, no :INITFORM.

Note that classes are objects. To get the class object from its name, use (FIND-CLASS name). Ordinarily, you won't need to do this.