How can I change this defmacro so I can setf the return

Here is the macro, a macro for cffi::mem-aref. I was trying to make it so I can setf it like you can with mem-aref, but can't figure out how to do it. Any help is appreciated.

(defmacro ? (ptr type &optional (index 0))
  `(cond ((pointerp ,ptr)
	  (return-from ? (mem-aref ,ptr ,type ,index)))
	 ((not (pointerp ,ptr))
	  (return-from ? mem-aref (c-pointer ,ptr) ,type  ,index)))
          (t 0)))

It's a question, how (setf (mem-aref ...) ...) is implemented so probably you can't write:

(defun resolve-pointer (ptr)
  (if (pointerp ptr) ptr (c-pointer ptr)))

(defmacro ? (ptr type &optional (index 0))
  `(mem-aref (resolve-pointer ,ptr) ,type ,index))

which result in:

(setf (? *ptr* *type* *index*) ...) ; => (setf (mem-aref (resolve-pointer *ptr*) *type* *index*) ...)

You don't want to put a comma before the whole first argument of mem-aref, because it will determine the type of ptr in compile-time, but the way I presented it won't probably work, because the setf machinery doesn't evaluate its first argument, whence it doesn't evaluate resolve-pointer. In that case you must teach the setf what to do by telling via defsetf or define-setf-expander, which isn't so simple, but it's good to tackle.

Can you show me how I would use desetf in this case, not sure I understood the documentation

Ok, I've tested the code above with:

(defun resolve-pointer (ptr) ptr)

and it works, no more labour is needed.

Thank you very much, it didn't even occur to me to write an external function. Its blazing fast too! Thanks:)