use of random alphanumeric generator in c++11

actuially i have been using srand and rand to generate random alphanumeric number but didn't get the desired output some strings were repeated at a regular time whihc is not what i want i want a totally unique string at every output of the program.Please do share a sample code which give the accurate solution to this problem.reference to a library may also help me a lot :) actually what i want to generate unique random alphanumeric string at every output of the program basically this random string wil be used as client id in mqtt CONNECT packet.hope i can explain the scenario now.

Thanks, Kushal