Can not extends core Exception.php

[eluser]xander9x[/eluser]
Hi, I'm trying to extend the Exception but always receive Fatal error: Call to undefined function. I tried to add function to both the system/core and application/core/MY_Exceptions.php. Both failed.

Code:

class MY_Exceptions extends CI_Exceptions
{

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

                        public function foo()
                        {
                                                echo 'foo';
                        }

Code:

class Users extends CI_Controller
{

                        public function index()
                        {
                                                foo(); // not work
                                                show_error('bar'); // works

Thanks!