ff::fmtln(stm, "i={0}", ff::to_x(123456)); // i=1e240
ff::fmtln(stm, "i={0}", ff::to_x(123456, 0, 10)); // i=000001e240
ff::fmtln(stm, "i={0}", ff::to_x(123456, 0, 10)); // i=000001e240
and, if you want it uppercase:
ff::fmtln(stm, "i={0}", ff::to_X(123456)); // i=1E240
ff::fmtln(stm, "i={0}", ff::to_X(123456, 0, 10)); // i=000001E240