Persiste an object

[eluser]Unknown[/eluser]

Code:

//file name myclass.php
class Myclass extends CI_Model {
   private $id;
   private $title;
   private $content;
   private $date;

   function __construct() {
       parent::__construct();
   }
  //publics getters and setters
}

class Contro extends CI_Controller {

   function __construct() {
        parent::__construct();
    }

   function persistObj(Myclass $object) {
     $this->load->model("myclass");
     //all properties in $object are setted, less id == null;
     $this->db->insert('myclasstable', $object);
   }

   function persistArr() {
     $data = array(
      'title' => 'My title' ,
      'content' => 'My con' ,
      'date' => 'My date');
     $this->load->model("myclass");
     $this->db->insert('myclasstable', $data);

   }
}

Why persistObj() doesn't works? and persistArr() does it..
I tried changing the visibility of properties to public.