void save_settings()
        {
            var x_doc = new XDocument();
            var root  = new XElement("WindowsBackup_App");

            x_doc.Add(root);

            // <BasicKeyManager>, <FileNameRegistration>
            root.Add(key_manager.to_xml());
            root.Add(file_name_reg.to_xml());

            // <cloud_backup_services>
            root.Add(new XElement("cloud_backup_services"));
            foreach (var cloud_backup in cloud_backup_services)
            {
                root.Element("cloud_backup_services").Add(cloud_backup.to_xml());
            }

            // <BackupManager>, <RestoreManager>
            root.Add(backup_manager.to_xml());
            root.Add(restore_manager.to_xml());

            x_doc.Save(settings_path);
        }