How to print __int128 in g++?

Question

I am using the GCC built-in type __int128 for a few things in my C++ program, nothing really significant, at least not enough to justify to use BigInt library o

Answer 1

If it's not performance-critical, here's a simple, readable way to convert a non-negative int128 to a base-10 string (which can then be printed of course):

std::string toString(__int128 num) {
    std::string str;
    do {
        int digit = num % 10;
        str = std::to_string(digit) + str;
        num = (num - digit) / 10;
    } while (num != 0);
    return str;
}

We can make this several times faster by getting the digits in larger chunks instead of one at a time. But it requires us to check each chunk for any leading zeroes that have been lost and add them back in:

std::string toString(__int128 num) {
    auto tenPow18 = 1000000000000000000;
    std::string str;
    do {
        long long digits = num % tenPow18;
        auto digitsStr = std::to_string(digits);
        auto leading0s = (digits != num) ? std::string(18 - digitsStr.length(), '0') : "";
        str = leading0s + digitsStr + str;
        num = (num - digits) / tenPow18;
    } while (num != 0);
    return str;
}

Note: I've also posted a version of this answer for unsigned int128s here.