The valgrind error I mentioned previously seems to be due to this code at cpp_int.hpp line 967: result.resize(x); typename cpp_int_backend<MinBits, Signed, Allocator>::const_limb_pointer pa = a.limbs(); typename cpp_int_backend<MinBits, Signed, Allocator>::const_limb_pointer pb = b.limbs(); typename cpp_int_backend<MinBits, Signed, Allocator>::limb_pointer pr = result.limbs(); bool swapped = false; int c = a.compare_unsigned(b); resize() doesn't zero-initialise the new storage. In this particular case, a == result because the expression being evaluated is something like result = result + other. So resizing result is also resizing a. Hence the compare_unsigned looks at the limbs of a that have just been added by the resize, which are undefined. It appears that this could be fixed by zeroing the new limbs in resize(), but that is unnecessary in other cases. Is it even correct that a == result is possible in this code? Regards, Phil.