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Subject: Re: [PHP-DEV] Traits and static methods
From: sebastian@php.net (Sebastian Bergmann)

Am 16.11.2010 18:46, schrieb Simas Toleikis:
> p.s. a Singleton-Trait implementation could look like:
> 
> trait Singleton {
>   public static function getInstance() { ... }
> }
> 
> class Child extends Parent {
>   use Singleton;
> }
> 
> Child::getInstance();

 This is not really a good example as the getInstance() method alone is
 not sufficient for a reusable implementation of the Singleton pattern.

 A trait for a reusable implementation of the Singleton pattern should
 look more like

     trait Singleton
     {
         private static $uniqueInstance = NULL;

         protected function __construct() {}
         private final function __clone() {}

         public static function getInstance()
         {
             if (self::$uniqueInstance === NULL) {
                 self::$uniqueInstance = new Singleton;
             }

             return self::$uniqueInstance;
         }
     }

 which of course does not work as traits are stateless and

     class MySingleton
     {
         use Singleton;
     }

     MySingleton::getInstance();

 results in

     Access to undeclared static property: MySingleton::$uniqueInstance

 And rightfully so. Nobody should need a mechanism to make it as easy as
 pie to clutter the code base with singletons ;-)

-- 
Sebastian Bergmann                    Co-Founder and Principal Consultant
http://sebastian-bergmann.de/                           http://thePHP.cc/