下部这段另类的写法是为了啥

下面这段另类的写法是为了啥
下面这段写法中，问题一：构造函数里面竞然是空的，并且更另类的是他的下面竟然是实例化，如果构造函数是空的，下面如何实例化呢
道理是啥？ 
                                  

/**
 * 模板驱动
 *
 * 模板驱动，商城模板引擎
 *
 *
 * @package    tpl
 * @copyright  Copyright (c) 2007-2013 ShopNC Inc. (http://www.shopnc.net)
 * @license    http://www.shopnc.net
 * @link       http://www.shopnc.net
 * @author	   ShopNC Team
 * @since      File available since Release v1.1
 */
defined('InShopNC') or exit('Access Invalid!');
class Tpl{
	/**
	 * 单件对象
	 */
	private static $instance = null;
	/**
	 * 输出模板内容的数组，其他的变量不允许从程序中直接输出到模板
	 */
	private static $output_value = array();
	/**
	 * 模板路径设置
	 */
	private static $tpl_dir='';
	/**
	 * 默认layout
	 */
	private static $layout_file = 'layout';
	
	private function __construct(){}
	
	/**
	 * 实例化
	 *
	 * @return obj
	 */
	public static function getInstance(){
		if (self::$instance === null || !(self::$instance instanceof Tpl)){
			self::$instance = new Tpl();
		}
		return self::$instance;
	}
	
	/**
	 * 设置模板目录
	 *
	 * @param string $dir
	 * @return bool
	 */
	public static function setDir($dir){
		self::$tpl_dir = $dir;
		return true;
	}
	/**
	 * 设置布局
	 *
	 * @param string $layout
	 * @return bool
	 */
	public static function setLayout($layout){
		self::$layout_file = $layout;
		return true;
	}
	
	/**
	 * 抛出变量
	 *
	 * @param mixed $output
	 * @param  void
	 */
	public static function output($output,$input=''){
		self::getInstance();
		
		self::$output_value[$output] = $input;
	}
	
	/**
	 * 调用显示模板
	 *
	 * @param string $page_name
	 * @param string $layout
	 * @param int $time
	 */
	public static function showpage($page_name='',$layout='',$time=2000){
		if (!defined('TPL_NAME')) define('TPL_NAME','default');
		self::getInstance();
			if (!empty(self::$tpl_dir)){
				$tpl_dir = self::$tpl_dir.DS;
			}
			//默认是带有布局文件
			if (empty($layout)){
				$layout = 'layout'.DS.self::$layout_file.'.php';
			}else {
				$layout = 'layout'.DS.$layout.'.php';
			}
			$layout_file = BASE_PATH.'/templates/'.TPL_NAME.DS.$layout;
			$tpl_file = BASE_PATH.'/templates/'.TPL_NAME.DS.$tpl_dir.$page_name.'.php';
			if (file_exists($tpl_file)){
				//对模板变量进行赋值
				$output = self::$output_value;
				//页头
				$output['html_title'] = $output['html_title']!='' ? $output['html_title'] :$GLOBALS['setting_config']['site_name'];
				$output['seo_keywords'] = $output['seo_keywords']!='' ? $output['seo_keywords'] :$GLOBALS['setting_config']['site_name'];
				$output['seo_description'] = $output['seo_description']!='' ? $output['seo_description'] :$GLOBALS['setting_config']['site_name'];
				$output['ref_url'] = getReferer();

				Language::read('common');
				$lang = Language::getLangContent();

				@header("Content-type: text/html; charset=".CHARSET);
				//判断是否使用布局方式输出模板，如果是，那么包含布局文件，并且在布局文件中包含模板文件
				if ($layout != ''){
					if (file_exists($layout_file)){
						include_once($layout_file);
					}else {
						$error = 'Tpl ERROR:'.'templates'.DS.$layout.' is not exists';
						throw_exception($error);
					}
				}else {
					include_once($tpl_file);
				}
			}else {
				$error = 'Tpl ERROR:'.'templates'.DS.$tpl_dir.$page_name.'.php'.' is not exists';
				throw_exception($error);
			}
	}
	/**
	 * 显示页面Trace信息
	 *
	 * @return array
	 */
    public static function showTrace(){
    	$trace = array();
    	//当前页面
		$trace[Language::get('nc_debug_current_page')] =  $_SERVER['REQUEST_URI'].'
';
    	//请求时间
        $trace[Language::get('nc_debug_request_time')] =  date('Y-m-d H:i:s',$_SERVER['REQUEST_TIME']).'
';
        //系统运行时间
        $query_time = number_format((microtime(true)-StartTime),3).'s';
        $trace[Language::get('nc_debug_execution_time')] = $query_time.'
';
		//内存
		$trace[Language::get('nc_debug_memory_consumption')] = number_format(memory_get_usage()/1024/1024,2).'MB'.'
';
		//请求方法
        $trace[Language::get('nc_debug_request_method')] = $_SERVER['REQUEST_METHOD'].'
';
        //通信协议
        $trace[Language::get('nc_debug_communication_protocol')] = $_SERVER['SERVER_PROTOCOL'].'
';
        //用户代理
        $trace[Language::get('nc_debug_user_agent')] = $_SERVER['HTTP_USER_AGENT'].'
';
        //会话ID
        $trace[Language::get('nc_debug_session_id')] = session_id().'
';
        //执行日志
        $log    =   Log::read();
        $trace[Language::get('nc_debug_logging')]  = count($log)?count($log).Language::get('nc_debug_logging_1').'
'.implode('
',$log):Language::get('nc_debug_logging_2');
        $trace[Language::get('nc_debug_logging')] = $trace[Language::get('nc_debug_logging')].'
';
        //文件加载
		$files =  get_included_files();
		$trace[Language::get('nc_debug_load_files')] = count($files).str_replace("\n",'
',substr(substr(print_r($files,true),7),0,-2)).'
';
        return $trace;
    }
}