I do appreciate all your attention to my types GC issue so far but when Wim gave me this untested code: (defmethod translate-to-foreign ((lisp-value cv-matrix) (c-type cv-mat))      (values  (c-pointer lisp-value) lisp-value)) I thought it worked but I still had my mem-aref macro defined to access the c-pointer value so I didn't get a good test? for example I'm referring to the lisp wrapper for this C function now, but it is identical to the cv-matrix one: Scalar* cv_create_Scalar(double val0, double val1, double val2, double val3) {     return new Scalar(val0, val1, val2, val3); } I now have my lisp wrapper and types/finalizers for the above c function defined like this: (defcfun ("cv_create_Scalar" %scalar) scalar   (val0 :double)   (val1 :double)   (val2 :double)   (val3 :double)) (define-foreign-type scalar ()   ((garbage-collect  :reader garbage-collect :initform nil :initarg                      :garbage-collect))   (:actual-type :pointer)   (:simple-parser scalar)) (defclass cv-scalar ()   ((c-pointer :reader c-pointer :initarg :c-pointer))) (defmethod translate-to-foreign ((lisp-value cv-scalar) (c-type scalar))    (values  (c-pointer lisp-value) lisp-value))  <--the new version Wim gave me untested (defmethod translate-from-foreign (c-pointer (c-type scalar))   (let ((scalar (make-instance 'cv-scalar :c-pointer c-pointer)))     (when (garbage-collect c-type)       (tg:finalize scalar (lambda () (del-scalar c-pointer))))     scalar)) When I run (defparameter a (%scalar 1d0 2d0 3d0 4d0))  and (mem-aref a :double)  I still get: The value #<CV-SCALAR {10046BC833}> is not of type   SB-SYS:SYSTEM-AREA-POINTER. but when I run  (mem-aref (c-pointer a) :double)  it works. What do I have to change in my types so I can mem-aref  "A" normally, like this (mem-aref a :double)...Any help is much appreciated:)