how to generate random charcters and append them to stringin c++ code example

Example: int random string generator c++

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;