[Cython] Automatic conversion with fixed-size C arrays

[Kurt Smith]

On Thu, Jul 24, 2014 at 2:07 PM, Robert Bradshaw <robertwb at gmail.com> wrote:

> On Fri, Jul 18, 2014 at 12:44 PM, Robert Bradshaw <robertwb at gmail.com>
> wrote:
> > On Fri, Jul 18, 2014 at 12:08 PM, Kurt Smith <kwmsmith at gmail.com> wrote:
> >> On Wed, Jul 16, 2014 at 1:02 PM, Robert Bradshaw <robertwb at gmail.com>
> wrote:
> >>>
> >>>
> >>> Yes, this'd be nice to have. One difficulty with arrays is that they
> >>> can't be returned by value, and so the ordinary from_py_function
> >>> mechanism (which gets called recursively) would need to be adapted.
> >>> (Allowing to_py_function to be optinally be called by reference
> >>> instead of by value could be nice as well from a performance
> >>> standpoint.)
> >>
> >>
> >> OK, thanks for the pointers.  I'll put some time on this over the
> weekend.
> >> Should I just make a PR when things are ready to review, or should I
> put up
> >> an issue first?
> >
> > I think this thread is sufficient; looking forward to a pull request.
>
> Don't know if you had time to look at this yet,


Yes, I've put in some time.  Initial focus is getting

    cdef int a[10] = obj

working, and then using that for structs.

Took a little while to re-orient myself with the codebase.

I'm working on getting the `to_py_function` and `from_py_function`
infrastructure to take arguments by reference; right now I'm getting
something hacked into place, and I'd appreciate your review to point out
the right way (or at least a better way) to do it.

Will likely have a PR for you by this weekend.


> but another possible
> hack would be to use the fact that struct { int x[N]; } is bit
> compatible with int[N] and can be passed/assigned by value. E.g.
>

Yes, this is a good point, although if I can get the pass-by-reference
working for fixed-size arrays, ideally we can do the same for structs with
array members.


>
> struct wrapped_array {
>     int x[3];
> };
>
> int main(int argc, char** argv) {
>     int src[3];
>     int dest[3];
>     *((struct wrapped_array *) &dest) = *((struct wrapped_array *) &src);
> }
>
> OTOH, it might make sense to simply add array members as a special
> case in struct conversion.
>
> - Robert
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