How do I create a random alpha-numeric string in C++?

Mehrdad Afshari’s answer would do the trick, but I found it a bit too verbose for this simple task. Look-up tables can sometimes do wonders:

#include <ctime>
#include <iostream>
#include <unistd.h>

std::string gen_random(const int len) {
    static const char alphanum[] =
    std::string tmp_s;

    for (int i = 0; i < len; ++i) {
        tmp_s += alphanum[rand() % (sizeof(alphanum) - 1)];
    return tmp_s;

int main(int argc, char *argv[]) {
    srand((unsigned)time(NULL) * getpid());     
    std::cout << gen_random(12) << "\n";        
    return 0;

Note that rand generates poor-quality random numbers.

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