Extending a singleton

[MrAdam]

Hi guys,A little confused here. Basically I have an abstract Singleton class that I use as a base to provide the getInstance() method, and to track the instances within a static array:

abstract class Singleton{	// declare a static array to store instances	private static $instances = array();	// declare constructor protected	protected function __construct()	{	}	// get the single instance of the called class	public static function getInstance()	{		$class = get_called_class();		if (isset(self::$instances[$class]))		{			return self::$instances[$class];		}		self::$instances[$class] = new $class;		return self::$instances[$class];	}}

What I'm confused about is, say I have a DB class which extends MySQLi or PDO, how would I extend my DB class to that, but also extend from Singleton?Thanks for any help,Adam

[boen_robot]

Yhat you're trying to do is commonly reffered to "multiple inheritance". PHP doesn't have it... unfortunatly so IMHO.You could create a singleton interface instead, and create this abstract class as its base implementation. If you get into a situation where you need to inherit from both classes as with the example of MySQLi or PDO + singleton, you'll instead implement the interface and copy&paste (or call...) the abstract implementation.

[MrAdam]

Ah, I see. Ideally I wanted to keep the logic of the singleton class within the base class (so as to not have multiple copies of the getInstance() method). What about this as a work around then, just for the cases where I want to extend another class..?

abstract class Singleton{	// declare a static array to store instances	private static $_instances = array();	// declare constructor protected	protected function __construct()	{	}	// get the single instance of the called class	public static function getInstance($class = null)	{		if (is_null($class))		{			$class = get_called_class();		}		if (isset(self::$_instances[$class]))		{			return self::$_instances[$class];		}		self::$_instances[$class] = new $class;		return self::$_instances[$class];	}}class DB extends PDO{	public static function getInstance()	{		return Singleton::getInstance(get_class());	}}

Functionally it works, but is it a bad way to do it?

[MrAdam]

Improved it a little:

abstract class Singleton{	// declare a static array to store instances	private static $_instances = array();	// declare constructor protected by default	protected function __construct()	{	}	// get the single instance of the called class	public static function getInstance($class = null)	{		if (is_null($class))		{			$class = get_called_class();		}		if (!class_exists($class))		{			trigger_error('Unable to return instance of invalid class: ' . $class, E_USER_ERROR);		}				if (!isset(self::$_instances[$class]))		{			self::$_instances[$class] = new $class;		}		return self::$_instances[$class];	}	// prevent cloning	public function __clone()	{		trigger_error('Unable to clone classes extending Singleton', E_USER_ERROR);	}}