Override Prestashop 1.6.1.4

I am doing a module in Prestashop and I need override the method update in the class CartCore. I have created a class inside the folder override (I have tried in modules/name_of_module/override also) Cart extends CartCore (the file is Cart.php), and I have the next code.

public function update($null_values = false, $hook = true)
{
    if (isset(self::$_nbProducts[$this->id])) {
        unset(self::$_nbProducts[$this->id]);
    }

    if (isset(self::$_totalWeight[$this->id])) {
        unset(self::$_totalWeight[$this->id]);
    }

    $this->_products = null;
    $return = parent::update($null_values);
    if($hook) Hook::exec('actionCartSave');

    return $return;
}

I have deleted the file cache/class_index.php and I have activated the overrides in the back-office Performance but it does not work. Only works if I change the original Cart class, but I don´t want do this.

Thanks!