Singleton Design Pattern

Issue no. 3 on design patterns: The singleton pattern.

Whenever we need a place to save and retrieve data throughout a script without using globals the singleton pattern comes in handy. It allows only one instance of itself. What seems like a drawback is quite useful: Wherever in your source you instantiate such an object: You can be sure that you’ll always get the instance of the same object. Remember the parameterized factory from the last chapter? Every time we wanted to create a new database-access-object we had to pass “mysql” or “pgsql” to the create-method. Wouldn’t it be easier if we just saved the database type to a singleton and retrieved it everytime we need it?

//------------------------------------------------
// Singleton
//------------------------------------------------
// Definition
class Singleton
{
	// "This is a handle to myself"
	private static $instance;
 
	// database type
	public $dbtype = "";
 
	// Private clone-function and private constructor 
	// make sure, that nobody can clone the object or
	// instantiate it using the standard constructor.
	private function __construct() {}
	private function __clone() {}
 
	// This one returns the instance handle to itself.
	// It's static, because it has to be accessible
	// before any object is available
	public static function GetInstance()
	{
		// "Am I already existing?"
		if (!isset(self::$instance))
		{
			// "No? Create myself and save the handle
			// to myself."
			$class = __CLASS__;
			self::$instance = new $class;
		}
 
		// "Return handle to mysqlf"
		return self::$instance;
	}
}
 
// ...
// somewhere in our code
function SetupScript()
{
	// Get handle to our singleton object
	$GlobalInfos = Singleton::GetInstance();
	// Set the $content variable to 42 
	$GlobalInfos->dbtype = "mysql";
}
 
// ...
// somewhere else in our code
SetupScript();
 
// ... deep down in the code
// Get handle to our singleton object
$Registry = Singleton::GetInstance();
// Remeber that from the last chapter?
$myDbObj= MyClassFactory::Create($Registry->dbtype);

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