malloc-free allocation -- here: instance methods

Hi,

Berkeley DB's C-level interface uses DB->get(...) instead of a fixed function 
like DB_get(...)
How could one interface to this? A possible way is explored here.

                        On instance methods
One could use
(def-c-struct DB
  (get (c-function (:arguments ...) ...))
  (put (c-function ...))
  ...many others function slots)
to have CLISP create callable foreign-function objects that match C's DB->put().

(def-call-out db-create ; which you already must have
  (:arguments (DB (c-ptr c-pointer) :out)
              ...))
Then
(funcall
  (slot 'get ; create foreign-function object out of DB->get slot
   (foreign-value
    (foreign-address-variable ; which nobody has but me
     ; - much like cast: foreign-address -> foreign-variable
     (nth-value 1 (db-create ...)) ; that DB pointer
     'DB)))
  db-txn key data flags)
One should memoize the foreign-function object, instead of creating a new one 
for each call.

However, duplicating the DB structure in Lisp, if the BDB C structure changes, 
Lisp doesn't know -- not nice!  Unreliable :(

A wrapper around the latter remembers me of typical OO frameworks for Scheme, 
where you pass the function name as argument (to a closure, which does the 
dispatch):
(DB-invoke 'get DB-handle ...args) or
(DB-invoke DB-handle 'get ...args)

In CL/CLOS, people tend to prefer
(DB-get DB-handle ...args)
(DB-put DB-handle ...args)

It could still be defined as:
(defmethod DB-get ((DB-handle DB-class) &rest args)
  (apply (memoizing-DB-foreign-function-factory DB-handle 'get) args))
Of course, DB-get could be turned into reader slots of the DB-class CLOS miror 
-- use slots for presumably often used functions like get and put.

[memoizing-]DB-foreign-function-factory could be implemented in C, to avoid the 
problem about the C DB struct being changed by the BDB people, without Lisp 
noticing this change. This would prevent getting out of sync with BDB/
db.h

Regards,
        Jorg Hohle.


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