Overriding core classes

I tryed to override the core controller following this page :
https://www.codeigniter.com/userguide3/g...asses.html

I added a new class in /application/core/ 

PHP Code:

<?php
class MY_Controller extends CI_Controller
{
    function __construct()
    {
        parent::__construct();
        echo 'Hello !';
        die();
    }
} 


I checked, line 374 of /system/core/CodeIgniter.php is called properly :

PHP Code:

require_once APPPATH.'core/'.$CFG->config['subclass_prefix'].'Controller.php'; 


But constructor is not called. What am I doing wrong ?