public static function save($post_id)
 {
     $class_name = get_called_class();
     if (!isset($_POST[$class_name . '_nonce']) || !wp_verify_nonce($_POST[$class_name . '_nonce'], plugin_basename(__FILE__))) {
         return $post_id;
     }
     if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
         return $post_id;
     }
     custom_fields::save($post_id, $class_name::$fields);
     return $post_id;
 }