Question

I have this structure (from the Teem library):

struct NrrdEncoding {
  // ...
  int (*read)(FILE *file, void *data, size_t elementNum,
              Nrrd *nrrd, struct NrrdIoState_t *nio);
  int (*write)(FILE *file, const void *data, size_t elementNum,
               const Nrrd *nrrd, struct NrrdIoState_t *nio);
};

By default, Swig wants read and write to be userdata, which makes sense if they’re to be opaque and re-assignable. However, I want to be able to call these more than I want to be able to re-assign them, and I don’t know how. (Ideally I’d like to be able to do both.)

Answer 1

You need to give the object NrrdEncoding member functions via SWIG (ie: C functions that you bind as members of the object) that themselves will call the function pointers. It would work something like this:

struct NrrdEncoding
{
  ...
  //Don't expose the function pointers. The struct will still have them, but SWIG won't expose them.
};

%extend NrrdEncoding
{
  int read(FILE *file, void *data, size_t elementNum, Nrrd *nrrd, struct NrrdIoState_t *nio)
  {
    return $self->read(file, data, elementNum, nrrd, nio);
  }

  int write(FILE *file, const void *data, size_t elementNum, const Nrrd *nrrd, struct NrrdIoState_t *nio)
  {
    return $self->write(file, data, elementNum, nrrd, nio);
  }
}

In Lua, if you have an NrrdEncoding object, you can call read on it like this: object:read(...).