void countdown(int time)
{
    int h = (time / 3600);
    int m = (time / 60) - (h * 60);
    int s = time % 60;

    std::stringstream ss;
    ss << h << ":" << m << ":" << s;
    std::string string = ss.str();

    cout << "r" << string << endl;
}

cout << "r" << string << endl;

23:59:59

0:0:0

std::cout << 'r'
          << std::setw(2) << std::setfill('0') << h << ':'
          << std::setw(2) << m << ':'
          << std::setw(2) << s << std::flush;

cout << "r" << string ; works