6.4 The Resource Facility

6.4.1 Resource examples

;; Make a simple, empty resource of 10-element arrays.
> (setq simp-array-resource (make-resource "10 element arrays"
                :constructor #'(lambda () (make-array 10))))
#<Resource 10 element arrays 0 objects>

;; Obtain an element from the resource.
> (resource-allocate simp-array-resource)
#<Simple-Vector T 10 B3789B>

;; Now put this object back.
> (resource-deallocate simp-array-resource)
#<Resource 10 element arrays 104CC7F6>

;; When you allocate from the resource, you get the same object 
;; back.
> (resource-allocate simp-array-resource)
#<Simple-Vector T 10 B3789B>

;; If you allocate again, you get a different one.
> (resource-allocate simp-array-resource)
#<Simple-Vector T 10 B39773>

;; Put them both back, so there are two objects remembered.
> (resource-deallocate simp-array-resource **)
#<Resource 10 element arrays 104CC7F6>

> simp-array-resource
#<Resource 10 element arrays 104CC7F6>

;; Now clear all the objects, so there are none.
> (clear-resource simp-array-resource)
0

> simp-array-resource
#<Resource 10 element arrays 104CC7F6>

;; Make a complicated resource.  It accepts a parameter and gives 
;; you an array with 10 elements initialized to the parameter
;;  value. When you return an array, the elements are set to NIL;
;; otherwise, they might point to some object and keep it from
;;  being garbage collected.  The resource is created with three
;;  arrays preallocated and initialized to contain 0.
> (setq comp-array-resource (make-resource "10 element initialized arrays"
              :constructor
           #'(lambda (init) (make-array 10 :initial-element init))
              :initial-copies 3
              :initial-args '(0)
              :initialization-function #'(lambda (obj init)
                                           (fill obj init))
              :cleanup-function #'(lambda (obj)
                                    (fill obj nil))))
#<Resource 10 element initialized arrays 104CCDDE

;; Temporarily allocate an array with elements set to 'X.
> (using-resource (x comp-array-resource 'x)
    (let ((*print-array* t)) (format t "~S" x)))
#(X X X X X X X X X X)
NIL

;; This resource has a new cons cell as its initial element. When
;; the array is deallocated, the cons cell is eligible for garbage
;; collection.
> (using-resource (x comp-array-resource (cons 'a 'b))
    (let ((*print-array* t)) (format t "~S" x)))
#((A . B) (A . B) (A . B) (A . B) (A . B) (A . B) (A . B) (A . B) 
(A . B) (A . B))
NIL

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