3.4.1 Instance Slots

Any slot, regardless of its allocation, can be queried, referenced and set using the following four primitive procedures.

primitive procedure: slot-exists? obj slot-name

Return #t if obj has a slot with name slot-name, otherwise #f.

primitive procedure: slot-bound? obj slot-name

Return #t if the slot named slot-name in obj has a value, otherwise #f.

slot-bound? calls the generic function slot-missing if obj does not have a slot called slot-name (see section slot-missing).

primitive procedure: slot-ref obj slot-name

Return the value of the slot named slot-name in obj.

slot-ref calls the generic function slot-missing if obj does not have a slot called slot-name (see section slot-missing).

slot-ref calls the generic function slot-unbound if the named slot in obj does not have a value (see section slot-unbound).

primitive procedure: slot-set! obj slot-name value

Set the value of the slot named slot-name in obj to value.

slot-set! calls the generic function slot-missing if obj does not have a slot called slot-name (see section slot-missing).

GOOPS stores information about slots in class metaobjects. Internally, all of these procedures work by looking up the slot definition for the slot named slot-name in the class metaobject for (class-of obj), and then using the slot definition's “getter” and “setter” closures to get and set the slot value.

The next four procedures differ from the previous ones in that they take the class metaobject as an explicit argument, rather than assuming (class-of obj). Therefore they allow you to apply the “getter” and “setter” closures of a slot definition in one class to an instance of a different class.

[ *fixme* I have no idea why this is useful! Perhaps when a slot in (class-of obj) shadows a slot with the same name in one of its superclasses? There should be an enlightening example here. ]

primitive procedure: slot-exists-using-class? class obj slot-name

Return #t if the class metaobject class has a slot definition for a slot with name slot-name, otherwise #f.

primitive procedure: slot-bound-using-class? class obj slot-name

Return #t if applying slot-ref-using-class to the same arguments would call the generic function slot-unbound, otherwise #f.

slot-bound-using-class? calls the generic function slot-missing if class does not have a slot definition for a slot called slot-name (see section slot-missing).

primitive procedure: slot-ref-using-class class obj slot-name

Apply the “getter” closure for the slot named slot-name in class to obj, and return its result.

slot-ref-using-class calls the generic function slot-missing if class does not have a slot definition for a slot called slot-name (see section slot-missing).

slot-ref-using-class calls the generic function slot-unbound if the application of the “getter” closure to obj returns an unbound value (see section slot-unbound).

primitive procedure: slot-set-using-class! class obj slot-name value

Apply the “setter” closure for the slot named slot-name in class to obj and value.

slot-set-using-class! calls the generic function slot-missing if class does not have a slot definition for a slot called slot-name (see section slot-missing).