RE: accessing default values in classes

["Peter N. Saurugger" <peter-s@molbio.cbs.umn.edu>]

Here is my original question:

   I would like to find out whether there is a defined way of accessing the
   default values in classes. The accessors seem to work only with instances
   for me.

   I probably should point out why I would like to have this feature: I am
   currently building a blackboard-based system. This system reasons about
   strings of instances of pre-defined classes. Under certain circumstances 
   (e.g. data input) I need to create an object which matches to the default 
   value of a certain slot. It seems that I have to create a dummy instance 
   for each class to be able to access these values ...?


Here is a suggestion by kanderso@dino.bbn.com :

	Slot descriptions are kept on the class, so you can access the default
	value with something like:

	(defun default-value (slot-name class)
	  (slotd-initform (find-slotd 'name (class-slots class))))

	Alternatively you could create a default instance, as you suggested,
	and then just get a slot value using an accessor.

	k

-- this works nicely in the case where I used the :initform slot option,
but slotd-initform does not access default-values (a class-option). Anyway,
this brought me on the track to a possible route, using the accessor
class-options on a class-object.


Here is another suggestion by John E. Collins (us015029@mmm.serc.3m.com)

	Isn't this what class-prototype is for?  It seems to me that
	    (class-prototype some-class-object)
	returns something that looks like a "default" instance of the class in
	question.  You can then access it as a normal instance.  I have
	several times used this to get at class-allocation slots when I didn't
	have ready access to an instance of the class.


-- This doesn't work in the version of pcl (Victoria day) I have up.
The method class-prototype was redefined to use allocate-instance
instead of make-instance (as it had been before). ALLOCATE-INSTANCE
leaves the slots of the returned prototype unbound.

Thank you for your suggestions,

--Peter N. Saurugger
Molecular Biology Computing Center
UofMN