On Saturday, April 26, 2014 5:39 AM, Joeish W <joeish80829@yahoo.com> wrote: Thank you very much, I should have went back to my C ways of doing things, but it was so odd to see a struct in the OpenCv C++ interface I must have got a little cocky.  Again I really appreciate all your attention to these 3 matters today

On Saturday, April 26, 2014 4:32 AM, Willem Rein Oudshoorn <woudshoo@xs4all.nl> wrote:

Joeish W <joeish80829@yahoo.com> writes:

...

...

I have this C wrapper

DMatch* cv_create_DMatch3(int _queryIdx, int _trainIdx, float _distance) {     return new DMatch(_queryIdx, _trainIdx, _distance); }

it is a wrapper for these struct members

http://docs.opencv.org/modules/features2d/doc/common_interfaces_of_descripto...

here is my lisp wrapper for it:

(defctype dmatch :pointer)

(defcfun ("cv_create_DMatch3" dmatch3) (:pointer dmatch)   "DMatch

constructor"

...

...

  (-query-idx :int)   (-train-idx :int)   (-distance :float))

In C I can do this:

  DMatch* a = cv_create_DMatch3(1, 2, 3.0);   cout << a->distance ;

and the output is 3

but in lisp when I do this:

(defparameter a (dmatch3 1 2 3f0))

LCV> (mem-aref a :int 0) 1 LCV> (mem-aref a :int 1) 2 LCV> (mem-aref a :float 2) #<SINGLE-FLOAT quiet NaN> <--- this is output

Any help on figuring out the reason would be much appreciated

This is not really suprising.  You are treating the resulting pointer `a` as an untyped pointer.  When you do `(mem-aref a :float 2)` you are basically saying:

1. Assume a is a pointer to an array of floats 2. Give me the third element (element at index 2).

Unfortunately `a` is not a pointer to an array of floats, but a pointer to a struct DMatch.

You should lookup `defcstruct` in the CFFI documentation:

http://common-lisp.net/project/cffi/manual/html_node/defcstruct.html#defcstr...

Wim Oudshoorn.

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