// if types conversion is valid (ie Tto(Tfrom) is a valid expression), then generate conversion function
template <class Tfrom, class Tto, class errmode>
struct convert_swap<Tfrom, Tto, errmode, true> {
    static void func(void* dptr_, const void* sptr_, ssize_t len) {
              Tto*   dptr =         (Tto*)dptr_;
        const Tfrom* sptr = (const Tfrom*)sptr_;

        for (ssize_t ii=0; ii < len; ii++) {
            dptr[ii] = Tto(byteswap(sptr[ii]));
        }
    };
};