error while loading an abstract model with factory

Code:
class User extends CI_Controller {



    public function __construct() {

        parent::__construct();

        

        $this->load->library('factory');

        

    }



    function register(){

        $user = $this->factory->create('doctor'); // returns 'doctor_m' or 'patient_m'

        $this->load->model($user); 

        $this->$user->addUser();

    }

}



abstract class User_m extends CI_Model{

    

    function __construct() {

        parent::__construct();

    }

    

    abstract protected function addUser();

    abstract protected function getUser();

}



class Doctor_m extends User_m{



    function __construct() {

        parent::__construct();

    }

    

    function addUser() {

        echo "doctor";

    }

    

    function getUser() {

    }

}



class Patient_m extends User_m{

    

    function __construct() {

        parent::__construct();

    }

    

    function addUser() {

        echo 'patient';

    }

    

    function getUser() {

    }

}



class factory{

    function create($type){

        if($type == 'doctor') return 'doctor_m';

        else return 'patient_m';

    }

}