with-slots (ANSI and GNU Common Lisp Document)

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#### 7.7.24 with-slots [Macro]

`with-slots` ({slot-entry}*) instance-form {declaration}* {form}*
⇒ {result}*

slot-entry ::=slot-name | (variable-name slot-name)

#### Arguments and Values::

slot-name—a slot name; not evaluated.

variable-name—a variable name; not evaluated.

instance-form—a form; evaluted to produce instance.

instance—an object.

declaration—a declare expression; not evaluated.

forms—an implicit progn.

results—the values returned by the forms.

#### Description::

The macro with-slots establishes a lexical environment for referring to the slots in the instance named by the given slot-names as though they were variables. Within such a context the value of the slot can be specified by using its slot name, as if it were a lexically bound variable. Both setf and setq can be used to set the value of the slot.

The macro with-slots translates an appearance of the slot name as a variable into a call to slot-value.

#### 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 thing (make-instance 'thing :x 0 :y 1)) ⇒  #<THING 62310540>
     (with-slots (x y) thing (incf x) (incf y)) ⇒  2
     (values (thing-x thing) (thing-y thing)) ⇒  1, 2
     (setq thing1 (make-instance 'thing :x 1 :y 2)) ⇒  #<THING 43135676>
     (setq thing2 (make-instance 'thing :x 7 :y 8)) ⇒  #<THING 43147374>
     (with-slots ((x1 x) (y1 y))
                 thing1
       (with-slots ((x2 x) (y2 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 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 slot-name is not the name of a slot in the instance.

#### See Also::

with-accessors , slot-value , symbol-macrolet

#### Notes::

A with-slots expression of the form:

    (with-slots (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

    (slot-entry_i ()
    (slot-value in 'slot-entry_i))

if slot-entry_i is a symbol and is

    (variable-name_i ()
    (slot-value in 'slot-name_i))

if slot-entry_i is of the form

    (variable-name_i
    slot-name_i)

Next: defclass, Previous: with-accessors, Up: Objects Dictionary