note 80469 deleted from language.oop5.patterns by danbrown

Note Submitter: me at chrisstockton dot org ---- For anyone who reads: tims57 at yahoo dot com Please let me show everyone a example of how a global variable is nothing like a singleton pattern. Take into considering the following. A global variable, cares not the state of its contents, it is only a container. If it contains a instance, a boolean, or a integer, it is all the same to it as PHP is dynamically typed it would be very easy for a global variable to be overridden by some dynamically loaded module in a complicated architecture, which could in turn cause a instance that maintains account or billing data to destruct before values are written to it. The singleton is a means of protecting the instance to a single instantiation. A global variable offers no such protection, it is only a container for a value. <?php $unreliableInstanceNotProtectedAtAll = new StdClass; class Singleton { static private $_instance; private $_foo = 'foo'; private function __construct() {} static public function getInstance() { return isset(self::$_instance) ? self::$_instance : self::$_instance = new self(); } public function updateFoo($foo) { $this->_foo = $foo; } public function getFoo() { return $this->_foo; } } /** Singleton offers protection from being instantiated multiple times */ // file_2481_r4.php, the early foo module not used except under certain circumstances $foo = Singleton::getInstance(); var_dump($foo->getFoo()); // foo $foo->updateFoo('bar'); var_dump($foo->getFoo()); // bar // file_1444114_r1.php (author has no idea that file_2481_r4.php was loaded by // the "foo" module", doesn't matter because the class will be instantiated only once $foo = Singleton::getInstance(); var_dump($foo->getFoo()); // bar /** Global variable just contains a instance, has no care for what it contains */ // file_2481_r4.php, the early foo module not used except under certain circumstances $unreliableInstanceNotProtectedAtAll->foo = 'foo'; // file_1444114_r1.php (author has no idea that file_2481_r4.php was loaded by // the "foo" module", doesn't matter because the class will be instantiated only once $unreliableInstanceNotProtectedAtAll = new StdClass; $unreliableInstanceNotProtectedAtAll->foo = 'bar'; ?>