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make-instance

make-instance Standard Generic Function

Syntax:

make-instance class &rest initargs &key &allow-other-keys → instance

Method Signatures:

make-instance (class standard-class) &rest initargs

make-instance (class symbol) &rest initargs

Arguments and Values:

class—a class, or a symbol that names a class.

initargs—an initialization argument list.

instance—a fresh instance of class class.

Description:

The generic function make-instance creates and returns a new instance of the given class.

If the second of the above methods is selected, that method invokes make-instance on the arguments (find-class class) and initargs.

The initialization arguments are checked within make-instance.

The generic function make-instance may be used as described in Section 7.1 (Object Creation and Initialization).

Exceptional Situations:

If any of the initialization arguments has not been declared as valid, an error of type error is signaled.

See Also:

defclass, class-of, allocate-instance, initialize-instance, Section 7.1 (Object Creation and Initialization)

Expanded Reference: make-instance​

Basic Instance Creation​

make-instance creates a new instance of a class. The first argument is either a class name (a symbol) or a class object. Additional keyword arguments supply initialization values for slots that have declared :initarg names.

(defclass point ()
  ((x :initarg :x :accessor point-x :initform 0)
   (y :initarg :y :accessor point-y :initform 0)))

(let ((p (make-instance 'point :x 3 :y 4)))
  (list (point-x p) (point-y p)))
=> (3 4)

Default Values with :initform​

When an initarg is not supplied, the slot receives its :initform default value. Slots without either remain unbound.

(defclass config ()
  ((host  :initarg :host  :accessor config-host :initform "localhost")
   (port  :initarg :port  :accessor config-port :initform 8080)
   (debug :initarg :debug :accessor config-debug)))

(let ((c (make-instance 'config)))
  (list (config-host c) (config-port c)))
=> ("localhost" 8080)

;; Override defaults
(let ((c (make-instance 'config :host "example.com" :port 443)))
  (list (config-host c) (config-port c)))
=> ("example.com" 443)

Using a Class Object Instead of a Symbol​

You can pass a class object directly to make-instance instead of a symbol.

(defclass point ()
  ((x :initarg :x :accessor point-x :initform 0)
   (y :initarg :y :accessor point-y :initform 0)))

(let* ((cls (find-class 'point))
       (p (make-instance cls :x 10 :y 20)))
  (list (point-x p) (point-y p)))
=> (10 20)

Creating Instances of Subclasses​

make-instance works with inherited slots and default-initargs from superclasses.

(defclass shape ()
  ((color :initarg :color :accessor shape-color))
  (:default-initargs :color :black))

(defclass circle (shape)
  ((radius :initarg :radius :accessor circle-radius)))

(let ((c (make-instance 'circle :radius 5)))
  (list (circle-radius c) (shape-color c)))
=> (5 :BLACK)

(let ((c (make-instance 'circle :radius 5 :color :red)))
  (list (circle-radius c) (shape-color c)))
=> (5 :RED)

Interaction with initialize-instance​

After allocating the instance, make-instance calls initialize-instance. Custom :after methods on initialize-instance can perform validation or derived slot computation.

(defclass rectangle ()
  ((width  :initarg :width  :accessor rect-width)
   (height :initarg :height :accessor rect-height)
   (area   :accessor rect-area)))

(defmethod initialize-instance :after ((r rectangle) &key)
  (setf (rect-area r) (* (rect-width r) (rect-height r))))

(let ((r (make-instance 'rectangle :width 3 :height 4)))
  (rect-area r))
=> 12