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3.4 Accessing Slots

The definition of a slot contains at the very least a slot name, and may also contain various slot options, including getter, setter and/or accessor functions for the slot.

It is always possible to access slots by name, using the various "slot-ref" and "slot-set!" procedures described in the following subsections. For example, 

(define-class <my-class> ()      ;; Define a class with slots
  (count #:init-value 0)         ;; named "count" and "cache".
  (cache #:init-value '())
  ...)

(define inst (make <my-class>))  ;; Make an instance of this class.

(slot-set! inst 'count 5)        ;; Set the value of the "count"
                                 ;; slot to 5.

(slot-set! inst 'cache           ;; Modify the value of the
  (cons (cons "^it" "It")        ;; "cache" slot.
        (slot-ref inst 'cache)))

If a slot definition includes a getter, setter or accessor function, these can be used instead of slot-ref and slot-set! to access the slot. 

(define-class <adv-class> ()     ;; Define a new class whose slots
  (count #:setter set-count)     ;; use a getter, a setter and
  (cache #:accessor cache)       ;; an accessor.
  (csize #:getter cache-size)
  ...)

(define inst (make <adv-class>)) ;; Make an instance of this class.

(set-count inst 5)               ;; Set the value of the "count"
                                 ;; slot to 5.

(set! (cache inst)               ;; Modify the value of the
  (cons (cons "^it" "It")        ;; "cache" slot.
        (cache inst)))

(let ((size (cache-size inst)))  ;; Get the value of the "csize"
  ...)                           ;; slot.

Whichever of these methods is used to access slots, GOOPS always calls the low-level getter and setter closures for the slot to get and set its value. These closures make sure that the slot behaves according to the #:allocation type that was specified in the slot definition (see allocation). (For more about these closures, see compute-get-n-set.)