Unfortunately, it's worse than that. My O(N^2), base-10^8 school-method multiplication stores multiple carries in its rows of the multiplication algorithm. These can can potentially overflow 64-bit.
Also, since cpp_dec_float does not climb the digit scale elegantly switching from school O(N^2) multiply to Karatsuba to FFT, the N^2 crushes efficiency for more than a few hundred digits. I had originally set an upper limit of 300 decimal digits.
It's so critical that I would maybe go so far as to use a static_assert on it.
For sure, looking at the code, this is the Coomba algorithm yes? Assuming that's correct I'm adding: // // There is a limit on how many limbs this algorithm can handle without dropping digits // due to overflow in the carry, it is: // // FLOOR( (2^64 - 1) / (10^8 * 10^8) ) == 1844 // BOOST_STATIC_ASSERT_MSG(mp_elem_number < 1800, "Too many limbs in the data type for the multiplication algorithm - unsupported precision in cpp_dec_float."); That still allows 14000 digits which would be grindingly slow at O(N^2), but at least the basic algorithm should work ;-) Thanks for the heads up! John.