to_base

to_base.cc

#include <iostream>

template <typename T, typename U = typename std::make_unsigned<T>::type>
std::string to_base(T _n, int base) {
    static const char *base_vals = "0123456789"
                                   "abcdefghijklmnopqrstuvwxyz"
                                   "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    static char buf[8 * sizeof (U) + 1];

    U n = _n;

    int i = 8 * sizeof (U);
    do {
        buf[--i] = base_vals[n % base];
    } while (n /= base);

    return std::string(buf + i);
}

int main(int argc, char **argv) {
    if (argc != 3) {
        std::cerr << "Usage: " << argv[0] << " <NUMBER> <BASE>\n";
        return 1;
    }
    uint64_t n = std::stoul(argv[1]);
    int base = std::stoi(argv[2]);
    std::cout << n << ": " << to_base(n, base) << std::endl;
}