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Organization: Calligram Ltd
To: Wez Furlong <kingwez@gmail.com>
Date: Wed, 11 May 2005 10:14:59 +0100
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Message-ID: <200505111014.59892.duncan@calligram.co.uk>
Subject: Re: [PHP-DEV] make new return a reference to existent object
From: duncan@calligram.co.uk (Duncan McIntyre)

Wez,

I have similar problems. I think your suggested solution is the wrong approach 
because your problem is not creating objects, it is destroying them.

My solution would be to define a new magic static method __free() which is 
given an instance of the class every time a reference to that instance is 
deleted. This would be a hook into the code which calls __destruct() once an 
object's ref count reaches zero, except that it would call __free() every 
time an object's ref count was decremented.

class SomeClass {
	public static __free($instance) {
		/**
		* Do some static instance caching magic here
		**/
	}
}

This would be a neat addition to the language, but unless it is implemented 
you will have to use a bit of discipline in writing your code instead.

The example below is probably the best you can do:

/**
* This class provides the mechanism to load objects from the database
* and caches loaded instances
**/
class ObjectStore {

	static $instances=array();

	public static function loadObject($class, $id) {
		if(isset(ObjectStore::instances[$class][$id])) {
			ObjectStore::instances[$class][$id][1]++;
			return ObjectStore::instances[$class][$id][0];
		}

		$handle = SomeDatabaseClass::getHandle();
		$result = $handle->query("SELECT * FROM $class WHERE id=$id");
		if(!$result->num_rows()) {
			throw new Exception("Object $class $id does not exist in the database");
		}
		$r = $result->fetch_object();
		$obj = new $class();
		$obj->deserialize($r);
		ObjectStore::instances[$class][$id]=array($obj, 1);
		return $obj;
	}

	public static function free($obj) {
		ObjectStore::instances[$obj->class][$obj->id][1]--;
		if(!ObjectStore::instances[$obj->class][$obj->id][1]) {
			unset(ObjectStore::instances[$obj->class][$obj->id]);
		}
	}
}

/**
* All classes stored in the database must extend this class
**/
abstract class PersistentObject {
	public function free() {
		static $freed;
		if(!$freed) {
			ObjectStore::free($this);
			$freed=true;
		}
		return $this;
	}

	/**
	* Retrieve the object's properties from a database row
	**/
	abstract public function deserialize($db_object);
}

Then load objects by calling

$obj = ObjectStore::loadObject("SomeClass",27); 
And when you're done with them call unset($obj->free());

Any classes which consume PersistentObjects must have a method __destruct()
which calls free() on any PersistentObjects they have references to in order 
for this to work.

Duncan