with-accessors (Macro)

Syntax:

slot-entry::=

(variable-name accessor-name)

Arguments and Values:

variable-name—a variable name; not evaluated.

accessor-name—a function name; not evaluated.

instance-form—a form; evaluated.

declaration—a declare expression; not evaluated.

forms—an implicit progn.

results—the values returned by the forms.

Description:

Creates a lexical environment in which the slots specified by slot-entry are lexically available through their accessors as if they were variables. The macro with-accessors invokes the appropriate accessors to access the slots specified by slot-entry. Both setf and setq can be used to set the value of the slot.

Examples:

 (defclass thing ()
           ((x :initarg :x :accessor thing-x)
            (y :initarg :y :accessor thing-y)))
→ #<STANDARD-CLASS THING 250020173>
 (defmethod (setf thing-x) :before (new-x (thing thing))
   (format t "~&Changing X from ~D to ~D in ~S.~%"
           (thing-x thing) new-x thing))
 (setq thing1 (make-instance 'thing :x 1 :y 2)) → #<THING 43135676>
 (setq thing2 (make-instance 'thing :x 7 :y 8)) → #<THING 43147374>
 (with-accessors ((x1 thing-x) (y1 thing-y))
                 thing1
   (with-accessors ((x2 thing-x) (y2 thing-y))
                   thing2
     (list (list x1 (thing-x thing1) y1 (thing-y thing1)
                 x2 (thing-x thing2) y2 (thing-y thing2))
           (setq x1 (+ y1 x2))
           (list x1 (thing-x thing1) y1 (thing-y thing1)
                 x2 (thing-x thing2) y2 (thing-y thing2))
           (setf (thing-x thing2) (list x1))
           (list x1 (thing-x thing1) y1 (thing-y thing1)
                 x2 (thing-x thing2) y2 (thing-y thing2)))))
▷ Changing X from 1 to 9 in #<THING 43135676>.
▷ Changing X from 7 to (9) in #<THING 43147374>.
→ ((1 1 2 2 7 7 8 8)
     9
     (9 9 2 2 7 7 8 8)
     (9)
     (9 9 2 2 (9) (9) 8 8))

Affected By:

defclass

Exceptional Situations:

The consequences are undefined if any accessor-name is not the name of an accessor for the instance.

See Also:

with-slots, symbol-macrolet

Notes:

A with-accessors expression of the form:

(with-accessors (slot-entry1, ... slot-entryn)
instance-form form1 ... formk)

expands into the equivalent of

(let ((in instance-form))
(symbol-macrolet (Q1 ... Qn) form1 ... formk))

where Q_i is

(variable-namei () (accessor-namei in))