how to produce a random string in c++ code example

Example: random string c++

#include <string>

string  gen_random(int len) {
    string s;
    static const char alphanum[] =
        "0123456789"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz";

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

    return s;
}