MAKEINSTANCE

MAKEINSTANCE, usually written as make-instance in Common Lisp, is the standard function in the Common Lisp Object System (CLOS) used to create a new instance of a defined class. It performs the allocation of a new object and triggers the initialization machinery to set up the object's slots and state. The primary argument is the class name or class object, followed by optional keyword arguments that specify initial values for slots.

The keyword arguments to make-instance correspond to the slot names of the class (or inherited slots). For example, (make-instance 'person :name "Alice" :age 30) creates a new person object with the name slot set to "Alice" and the age slot set to 30. If a slot has a specified :initform in its class definition and no corresponding initarg is provided, its default value is used. If an initarg is provided, the value is assigned accordingly during initialization.

During creation, make-instance delegates to the initialize-instance generic function. By default, a simple initialization occurs, but users can define methods on initialize-instance to customize initial behavior, validate values, or perform computations. The typical initialization sequence may involve run-time checks, derived-value calculations, or side effects, executed before or after slots are set depending on method combination and :after / :before qualifiers.

In practice, make-instance forms the backbone of object creation in CLOS, enabling extensibility through the method system. It separates allocation from initialization, allowing developers to customize how objects are prepared without altering the basic construction mechanism.

Example:

(defclass person ((name :initarg :name :initform "Unknown")

(age :initarg :age :initform 0)))

(make-instance 'person :name "Alice" :age 30)

See also: initialize-instance, defclass, defmethod, slot definitions.