8.3 Instance Creation and Slot Access

An instance (or object) of a defined class can be created with make. make takes one mandatory parameter, which is the class of the instance to create, and a list of optional arguments that will be used to initialize the slots of the new instance. For instance the following form

(define c (make <my-complex>))

creates a new <my-complex> object and binds it to the Scheme variable c.

generic: make

method: make (class <class>) initarg …

Create and return a new instance of class class, initialized using initarg ....

In theory, initarg … can have any structure that is understood by whatever methods get applied when the initialize generic function is applied to the newly allocated instance.

In practice, specialized initialize methods would normally call (next-method), and so eventually the standard GOOPS initialize methods are applied. These methods expect initargs to be a list with an even number of elements, where even-numbered elements (counting from zero) are keywords and odd-numbered elements are the corresponding values.

GOOPS processes initialization argument keywords automatically for slots whose definition includes the #:init-keyword option (see init-keyword). Other keyword value pairs can only be processed by an initialize method that is specialized for the new instance’s class. Any unprocessed keyword value pairs are ignored.

generic: make-instance

method: make-instance (class <class>) initarg …

make-instance is an alias for make.

The slots of the new complex number can be accessed using slot-ref and slot-set!. slot-set! sets the value of an object slot and slot-ref retrieves it.

(slot-set! c 'r 10)
(slot-set! c 'i 3)
(slot-ref c 'r) ⇒ 10
(slot-ref c 'i) ⇒ 3

The (oop goops describe) module provides a describe function that is useful for seeing all the slots of an object; it prints the slots and their values to standard output.

(describe c)
-|
#<<my-complex> 401d8638> is an instance of class <my-complex>
Slots are: 
     r = 10
     i = 3