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

The answers so far are good, but I just wanted to add to the answer from James Kanze. Firstly note that because of the unsigned conversion, it will not work for the number -0x80000000000000000000000000000000. Secondly, you can advantage the fact that printing with 64-bit integers works, to optimize the function implementation as follows:

std::ostream& operator<<(std::ostream& os, __int128_t value) {
    if (value < 0) {
        os << '-';
        value = -value;
    }
    // save flags to restore them
    std::ios_base::fmtflags flags(os.flags());
    // set zero fill
    os << std::setfill('0') << std::setw(13);

    // 128-bit number has at most 39 digits,
    // so the below loop will run at most 3 times
    const int64_t modulus = 10000000000000; // 10**13
    do {
        int64_t val = value % modulus;
        value /= modulus;
        if (value == 0) {
            os.flags(flags);
            return os << val;
        }
        os << val;
    } while (1);
}