RE: [PHP] OO and "Singleton" class

Maybe somebody would be interested in my solution as well: http://webreflection.blogspot.com/2009/09/php-53-singleton-fast-and-abstract.html Regards > Date: Wed, 2 Sep 2009 14:02:25 +0900 > From: koyama@hoge.org > To: php-general@lists.php.net > Subject: Re: [PHP] OO and "Singleton" class > > Hi, > > After PHP 5.3, there is better implements for Singleton. > > http://d.hatena.ne.jp/Yudoufu/20090811/1250021010 > > This entry was written by Japanese, but you may understand php code. :-) > > On Wed, Sep 2, 2009 at 07:06, Eddie Drapkin<oorza2k5@gmail.com> wrote: > > On Tue, Sep 1, 2009 at 5:16 PM, Paul M Foster<paulf@quillandmouse.com> wrote: > >> > >> I'm a little fuzzy on some of the PHP implementation details for some > >> stuff. In PHP (5 <= phpversion() < 5.3), I'd like a configuration class > >> which can only effectively be instantiated once. Will the following code > >> do this? Any other suggestions or corrections? (Code is untested. Feel > >> free to complain about syntax.) > >> > >> class config > >> { > >> private static $cfg; > >> > >> // Initializes the configuration from main config file > >> function __construct() > >> { > >> if (!is_array(self::$cfg)) { > >> if (defined('CFGFILE')) { > >> include(CFGFILE); > >> self::$cfg = $config; > >> } > >> else > >> self::$cfg = array(); > >> } > >> } > >> > >> // Returns a config item or null if not found > >> function item($index) > >> { > >> if (array_key_exists($index, self::$cfg)) > >> return self::$cfg[$index]; > >> else > >> return null; > >> } > >> > >> // Sets a config item, optionally overwriting. > >> // Returns true on success, false on failure. > >> function set($index, $value, $overwrite = true) > >> { > >> $write = false; > >> if (array_key_exists($index, self::$cfg) and $overwrite) > >> $write = true; > >> elseif (!array_key_exists($index, self::$cfg)) > >> $write = true; > >> if ($write) { > >> self::$cfg[$index] = $value; > >> return true; > >> } > >> else > >> return false; > >> } > >> > >> }; > >> > >> Paul > >> > >> -- > >> Paul M. Foster > >> > >> -- > >> PHP General Mailing List (http://www.php.net/) > >> To unsubscribe, visit: http://www.php.net/unsub.php > >> > >> > > > > The primary thing you need to do to make a class a singleton is > > disallow the instantiation of a class directly. Which means using > > visibility, which is conspicuously absent from your example above. > > > > So, to prevent normal instantiation, we make the constructor private. > > Also, we need a method to retrieve the instance. Static members serve > > this purpose: > > > > class Singleton { > > private static $instance; > > > > private function __construct() { } > > > > public static function getInstance() { > > if( ! self::$instance instanceof Singleton) //replace Singleton > > with class name > > self::$instance = new Singleton; //replace Singleton with class name > > > > return self::$instance; > > } > > > > public function __clone() { > > throw new Exception('Cannot clone a Singleton'); > > } > > } > > > > This class "shell" will prevent people from instantiating it ($foo = > > new Singleton is an error). Instead, deployment will look like `$foo > > = Singleton::getInstance()` which, as you can tell, always returns the > > same instance. Throwing an exception out of __clone() will prevent > > something like $foo = Singleton::getInstance(); $bar = clone > > $foo; > > which is something I see overlooked a lot. > > > > So, if you wanted to convert your full class to a singleton it would > > look something like: > > > > class config { > > private static $instance; > > private static $cfg; > > > > private function __construct() { > > if(!is_array(self::$cfg)) { > > if(defined('CFGFILE')) { > > include(CFGFILE); > > self::$cfg = $config; > > } else { > > self::$cfg = array(); > > } > > } > > } > > > > public function getInstance() { > > if(!(self::$instance instanceof config)) { > > self::$instance = new config(); > > } > > > > return self::$instance; > > } > > > > public function __clone() { > > throw new Exception("Cannot clone a Singleton."); > > } > > > > public function item($index) { > > if(array_key_exists($index, self::$cfg)) > > return self::$cfg[$index]; > > else > > return null; > > } > > > > // Sets a config item, optionally overwriting. > > // Returns true on success, false on failure. > > function set($index, $value, $overwrite = true) { > > $write = false; > > if (array_key_exists($index, self::$cfg) and $overwrite) { > > $write = true; > > } elseif (!array_key_exists($index, self::$cfg)) { > > $write = true; > > } > > > > if ($write) { > > self::$cfg[$index] = $value; > > return true; > > } else { > > return false; > > } > > } > > > > > > } > > > > Note that this is also completely untested or anything of the sort. > > > > -- > > PHP General Mailing List (http://www.php.net/) > > To unsubscribe, visit: http://www.php.net/unsub.php > > > > > > > > -- > 小山哲志＠テックスタイル > koyama@techstyle.jp : http://techstyle.jp/ > koyama@hoge.org : http://d.hatena.ne.jp/koyhoge/ > > -- > PHP General Mailing List (http://www.php.net/) > To unsubscribe, visit: http://www.php.net/unsub.php > _________________________________________________________________ Share your memories online with anyone you want. http://www.microsoft.com/middleeast/windows/windowslive/products/photos-share.aspx?tab=1