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7.7.23 with-accessors [Macro]

with-accessors ({slot-entry}*) instance-form {declaration}* {form}*
=> {result}*

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-entry_1 ...slot-entry_n) instance-form form_1 ...form_k)

expands into the equivalent of

     
     (let ((in instance-form))
 (symbol-macrolet (Q_1... Q_n) form_1 ...form_k))

where Q_i is

     (variable-name_i ()
     (accessor-name_i in))