Then it would be the opposite.

The only portable solution which I see is to implement your own version of std::stoull for uint64_t. But in most practical cases, it is enough to add a static assertion: static_assert(sizeof(uint64_t) <= sizeof(unsigned long long)).

so what is ideal to use instead of std::stoull since u said it's not ideal

std::stoi returns int but the required return type is uint64_t. std::stoull is also not ideal because it could happen that sizeof(uint64_t) != sizeof(unsigned long long). But it works on Codewars.

I'm not sure what you mean...