Search the FAQ Archives

3 - A - B - C - D - E - F - G - H - I - J - K - L - M
N - O - P - Q - R - S - T - U - V - W - X - Y - Z - Internet FAQ Archives

FAQ: Object-oriented Programming in Lisp 5/7 [Monthly posting]
Section - [5-3] How can I list all the CLOS instances in a class?

( Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Single Page )
[ Usenet FAQs | Web FAQs | Documents | RFC Index | Airports ]

Top Document: FAQ: Object-oriented Programming in Lisp 5/7 [Monthly posting]
Previous Document: [5-2] How can I write a function that can access defstruct slots by name? I would like to write something like
Next Document: [5-4] How can I store data and CLOS instances (with possibly circular references) on disk so that they may be
See reader questions & answers on this topic! - Help others by sharing your knowledge

There is no built-in way to enumerate the instances of a class.  If you are
only interested in listing the instances of classes that you have defined,
it is not very difficult to implement it as part of your class definition.
Add a shared slot, e.g. ALL-INSTANCES, with an initial value of NIL, to the
class definition.  Then write an after-method on INITIALIZE-INSTANCE for
this class, which pushes the instance being initialized onto ALL-INSTANCES.
Note that this must be done separately for each class that wants to maintain
such a list; it can't be encapsulated in a mixin class, because all its
dependent classes would share the same ALL-INSTANCES slot.  A compromise
would be to use a mixin to define the INITIALIZE-INSTANCE after-method (and
any other general-purpose methods that use the slot), but not the shared
slot; it would be up to the descendant classes to define the slot at the
level of the class hierarchy that is appropriate. You could also try
defining the classes that need instance-recording as instances of a
metaclass that holds the instance registry on the class object. The
recording behavior could then be built-in to an after method on
initialize-instance for the root class of the metaclass, or even
allocate-instance. To allow for garbage collection of old instances,
you will also need to define a generic function to remove the recorded
instances from the list of instances.

User Contributions:

Comment about this article, ask questions, or add new information about this topic: