1) GBoxed - это структуры, у которых нет идентичности
2) При передаче ссылок, ссылки всегда имеют ограниченное время действия
3) Структуры могут быть открытыми и закрытыми (opaque)

Основной критерий: лисповский код не должен заботиться о владении структурами. Идентичности нет.

Следствия:
1) Все GBoxed, созданные в лиспе, удаляются только лиспом (при необходимости делается копия)
2) Если foreign-функции возвращают GBoxed, то им владеет лисп (при необходимости делается копия)
3) В callback'и возможна передача по ссылке (в том случае, если не знаем, как присвоить значение исходной структуре);
в этом случае после выхода из callback'а лисповский прокси помечается как невалидный и операции с ним приводят к исключению

Для реализации надо, чтобы CFFI позволяло совершать действия по очистке в конце callback'а.

Код:

(defclass g-boxed-type ()
  ((g-type-designator :initarg :g-type
                      :initform ":G-TYPE must be specified"
                      :accessor g-boxed-type-type)))

;;Some terminology:
;; * native structure - a C structure in foreign memory that
;; has the data and is expected to be passed/received by foreign functions
;; * proxy - a Lisp object (class or a structure) that is
;; equivalent to native structure (has the same data in it). Its lifetime is indefinite
;; and it is not affected by foreign code.
;; A proxy may (but not required to) contain pointer to its own copy of a
;; native structure.
;; * reference proxy - a proxy that is whose lifetime is equal to a duration
;; of a callback. Reference proxies can only be used during a callback that created them.

;(defgeneric create-proxy (type)
;  "Creates a new proxy of a specified TYPE.")

(defgeneric create-proxy-for-native (type native-ptr)
  (:documentation "Creates a proxy that is initialized by data contained in native
structured pointed to by NATIVE-PTR.

Created proxy should not be linked to NATIVE-PTR and should have
indefinite lifetime (until garbage collector collects it). Specifically,
if proxy need a pointer to native structure, it should make a copy of
a structure.

If proxy requires finalization, finalizers should be added."))

(defgeneric create-temporary-native (type proxy)
  (:documentation "Creates a native structure (or passes a pointer to copy contained in PROXY)
that contains the same data that the PROXY contains and returns a pointer to it.

This call is always paired by call to FREE-TEMPORARY-NATIVE and calls may be nested."))

(defgeneric free-temporary-native (type proxy native-ptr)
  (:documentation "Frees the native structure that was previously created
by CREATE-TEMPORARY-NATIVE for the same PROXY.

Also reads data from native structure pointer to by NATIVE-PTR
and sets the PROXY to contain the same data.

This call is always paired by call to CREATE-TEMPORARY-NATIVE and calls may be nested."))

(defgeneric create-reference-proxy (type native-ptr)
  (:documentation "Creates a reference proxy for a native structure pointed to by NATIVE-PTR.

Reference proxy's lifetime is bound to duration of a callback. When the
callback returns the reference proxy is declared invalid and operations on it are errors.

This call is always paired by call to FREE-REFERENCE-PROXY and calls will not nest."))

(defgeneric free-reference-proxy (type proxy native-ptr)
  (:documentation "Frees a reference proxy PROXY previously created by call to
CREATE-REFERENCE-PROXY. This call should ensure that all changes on PROXY are
reflected in native structure pointed to by NATIVE-PTR.

After a call to FREE-REFERENCE-PROXY, PROXY is declared invalid and using it is an error,
operations on it should signal erros.

This call is always paired by call to CREATE-REFERENCE-PROXY."))

(define-foreign-type g-boxed-foreign ()
  ((g-type :initarg :g-type
           :initform (error ":G-TYPE must be specified")
           :reader g-boxed-foreign-g-type)
   (pass-type :initarg :pass-type
              :reader g-boxed-foreign-g-type
              :type (member :callback :normal)
              :initform :normal))
  (:actual-type :pointer))

(defun g-boxed-foreign->boxed-type (type)
  nil)

(defmethod translate-to-foreign (proxy (type g-boxed-foreign))
  (let* ((boxed-type (g-boxed-foreign->boxed-type type))
         (native-ptr (create-temporary-native boxed-type proxy)))
    (values native-ptr proxy)))

(defmethod free-translated-object (native-ptr (type g-boxed-foreign) proxy)
  (let ((boxed-type (g-boxed-foreign->boxed-type type)))
    (free-temporary-native boxed-type proxy native-ptr)))

(defmethod translate-from-foreign (native-ptr (type g-boxed-foreign))
  (let ((boxed-type (g-boxed-foreign->boxed-type type)))
    (ecase )
    (create-proxy-for-native boxed-type native-ptr)))