Node:Basic Class Definition, Next:, Up:Defining New Classes


3.2.1 Basic Class Definition

New classes are defined using the define-class syntax, with arguments that specify the classes that the new class should inherit from, the direct slots of the new class, and any required class options.

define-class name (super ...) slot-definition ... . options syntax
Define a class called name that inherits from supers, with direct slots defined by slot-definitions and class options options. The newly created class is bound to the variable name name in the current environment.

Each slot-definition is either a symbol that names the slot or a list, 

(slot-name-symbol . slot-options)

where slot-name-symbol is a symbol and slot-options is a list with an even number of elements. The even-numbered elements of slot-options (counting from zero) are slot option keywords; the odd-numbered elements are the corresponding values for those keywords.

options is a similarly structured list containing class option keywords and corresponding values.

The standard GOOPS class and slot options are described in the following subsections: see Class Options and Slot Options.

Example 1. Define a class that combines two pre-existing classes by inheritance but adds no new slots. 

(define-class <combined> (<tree> <bicycle>))

Example 2. Define a regular-polygon class with slots for side length and number of sides that have default values and can be accessed via the generic functions side-length and num-sides. 

(define-class <regular-polygon> ()
  (sl #:init-value 1 #:accessor side-length)
  (ns #:init-value 5 #:accessor num-sides))

Example 3. Define a class whose behavior (and that of its instances) is customized via an application-defined metaclass. 

(define-class <tcpip-fsm> ()
  (s #:init-value #f #:accessor state)
  ...
  #:metaclass <finite-state-class>)