Re: Class slots in CLOS

Blake McBride wrote:

Greetings,

It seems to me that there are two ways to define class slots (slots common to all instances of the class) in CLOS as follows:

1. via the use of :allocation :class slot option

2. By the creation and use of a meta-class that is to house the shared slots for a given class or set of classes.

I suppose my main question is what is the difference. I'm pretty sure method #1 is easier to declare and access to the class slots is easier (just like instance slots). Is one faster than the other.

I understand that using method #2 is more flexible and theoretically pure. Or, is method #1 just a shorthand for method #2?

No, method #2 is not more flexible. There is an important technical difference, which also implies a semantical difference.

The technical difference is that with method #1, you need an instance of a class to access the class slot, while with method #2, the slots are actually not class slots, but instance slots of the class metaobject, so you need the class metaobject to access the slots.

In code:

[1]

(defclass foo ()
((foo-slot :allocation :class :initform 0 :reader foo-slot)))

(defvar *foo* (make-instance 'foo))

(foo-slot *foo*)
=> yields class slot 'foo-slot of class foo.

[2]

(defclass foo-class ()
((foo-slot :initform 0 :reader foo-slot)))

(defclass foo-object ()
()
(:metaclass foo-class))

(defvar *foo* (make-instance 'foo-object))

(foo-slot (class-of *foo*))
=> yields instance slot 'foo-slot of the foo-class metaobject


Among other things, there is a well-defined inheritance and overriding relationship for class slots, including the interactions between class slots and instance slots, while for method [2], there is no inheritance at all between the slots of the class metaobjects in a class hierarchy.


Pascal

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