#40968 [Opn->Bgs]: Multiple intances of a singleton class when stored in session

ID: 40968 Updated by: tony2001@php.net Reported By: oriol dot gual at gmail dot com -Status: Open +Status: Bogus Bug Type: Session related Operating System: Windows XP SP2 PHP Version: 5.2.1 New Comment: Sorry, but your problem does not imply a bug in PHP itself. For a list of more appropriate places to ask for help using PHP, please visit http://www.php.net/support.php as this bug system is not the appropriate forum for asking support questions. Due to the volume of reports we can not explain in detail here why your report is not a bug. The support channels will be able to provide an explanation for you. Thank you for your interest in PHP. Previous Comments: ------------------------------------------------------------------------ [2007-04-02 13:24:44] smlerman at gmail dot com The only thing that can guarantee only one instance of your class existing is that Singleton::$instance is the only place you store an instance and getInstance() is the only way to retrieve an instance. Once you create a second place that an instance can be stored, such as storing an instance somewhere else (in the session data) and destroying the instance in the static property (which happens when the script ends), you create a second way to retrieve an entirely different instance. In general, there's no 100% guaranteed way to ensure that only one instance of a given class can ever exist. ------------------------------------------------------------------------ [2007-04-01 01:42:08] b dot fore at mail dot com That's a gross abuse of ext/session dude. ------------------------------------------------------------------------ [2007-03-31 16:54:52] oriol dot gual at gmail dot com Description: ------------ When storing an object of a singleton class in the session, you can have more than one instance of that class in other subsequent executions. Reproduce code: --------------- class Singleton { private static $instance; private function __construct() {} final public static function getInstance() { if (!isset(Singleton::$instance)) { Singleton::$instance = &new Singleton; } return Singleton::$instance; } } session_start(); if(!isset($_SESSION['singleton'])) $_SESSION['singleton'] = Singleton::getInstance(); $test = Singleton::getInstance(); $anotherTest = Singleton::getInstance(); var_dump(Singleton::getInstance() === $test); var_dump(Singleton::getInstance() === $anotherTest); var_dump($test === $anotherTest); var_dump(Singleton::getInstance() === $_SESSION['singleton']); var_dump($test === $_SESSION['singleton']); var_dump($anotherTest === $_SESSION['singleton']); session_write_close(); Expected result: ---------------- Session not started (first execution) bool(true) bool(true) bool(true) bool(true) bool(true) bool(true) Subsequent executions bool(true) bool(true) bool(true) bool(true) bool(true) bool(true) Actual result: -------------- Session not started (first execution it's OK) bool(true) bool(true) bool(true) bool(true) bool(true) bool(true) Subsequent executions (fails, multiple instances) bool(true) bool(true) bool(true) bool(false) bool(false) bool(false) ------------------------------------------------------------------------ -- Edit this bug report at http://bugs.php.net/?id=40968&edit=1