Create own library in the opencart. Problem with using DB

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Notice: Undefined property: ShippingDelivery::$db in J:\home\techno\www\system\library\shippingDelivery.php on line 44
Fatal error: Call to a member function query() on a non-object in J:\home\techno\www\system\library\shippingDelivery.php on line 44

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$mylibr = new ShippingDelivery();
$registry->set('mylibrary', $mylibr); // +

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  require_once(DIR_SYSTEM . 'library/shippingDelivery.php');
  $delivery_library = new ShippingDelivery($registry);
  $registry->set('shipping_delivery', $delivery_library);

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class shippingDelivery {
	private $db;
	private $config;
	private $cache;
	private $data = array();
	
	public function __construct($registry) {
		$this->db = $registry->get('db');
		$this->config = $registry->get('config');
		$this->cache = $registry->get('cache');
	}
	// Code
}