Call to undefined method CI_Controller::CI_Controller()

[eluser]huangxiao[/eluser]
Hi i just want to ask why is it that i have this error

Call to undefined method CI_Controller::CI_Controller()

can yo help me please why

Code:

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class Success extends CI_Controller{

   function __construct(){
      parent::CI_Controller();
       $this->is_logged_in();

   }


    function members_area(){

        $this->load->view('members_area');
    }


    function is_logged_in(){
        $this->session->userdata('is_logged_in');

       if(is_logged_in != true){
          echo "Not allowed here";
       }
    }

}

Thank you in advance.