C++11 random code generator ONLY FOR C++ 11

string random(size_t len)
{
    static const string a = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    static mt19937 x(time(NULL));
    static uniform_int_distribution<size_t> d(0, a.size() - 1);
    string r;
    srand(time(NULL));
    for(size_t i = 0; i < len; i++) r += a.at[d(x)];
    return r;
}