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Date: Mon, 5 Mar 2007 03:56:55 +1000
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Subject: RE: [PHP-DEV] suggestion SplFileInfo
From: scott.mcnaught@synergy8.com

Hello All,

I am new to the php internals mailing list.  I am a fairly experienced
programmer with a few ideas floating around.  I have come from a C++ games
development background and have now moved to primarily writing in php.

One thing that I used extensively in C++ was the singleton design pattern.
I assume that most of you know what that is, if not a quick google search
will let you know.  I have written an implementation of singleton in php,
but it's slow, and you have to use strings to reference the classes, or
duplicate a lot of code. (See methods 1 and 2 below)

I recently submitted an RFC to bugs.php.net about this, but there was never
feedback from the internals group, so I am going to try here.

http://bugs.php.net/bug.php?id=39946

In a few months time, I would like to start helping the development of php,
but I am too busy at the moment.

I was wondering if anyone has the time to implement this winner feature.  I
think it would encourage good programming practise and speed up a lot of php
apps.  Also I would like people's thoughts and feedback on the idea.

If no-one can implement this before me, I will have a go at it in a few
months time.

Thankyou,

Scott McNaught



Description:
------------
###
# Introduction
###
This document is an RFC for adding a small patch to the zend engine to
allow for native singleton classes.  The problem 
is that currently theres no clean way to implement singleton classes in
user-land over the entirety of a project.

Singleton classes are beneficial because:

- Removes the overhead of having multiple instances of the one object
when there is no need
- Allows you to keep the objects state rather than always starting from
an initial state. 
- They provide namespaces with the benefits of polymorphism (eg -
singleton classes can override / inherit from each other)

Throughout this document, I will use an example of a singleton class
"members" which acts as an interface to a database table.  
This class can save and load members from this database table simply by
calling the following methods in this class.

members::get($member_id) Loads a member from a member id and returns an
associative array with info about that member
members::save($member) Saves a member to the database from an array of
properties about that member

With the recent phase of tiered and service oriented architecture, the
need for Singleton has become more and more apparent.

###
# Singleton in php5
###
In the past, I have implemented Singleton two different ways.  Both of
these have problems.

# Method 1:
The first method involves having a public static getInstance method in
every singleton class.  This sucks because you
need to manually copy and paste it into every singleton class you make. 
Using a singleton class in this way is also confusing
for novice programmers. Eg:

<?php

class members
{
	static public function getInstance()
	{
		static $object = null;
		
		if($object)
		{
			return $object;
		}
		
		$object = new members();
		return $object;
	}
	
	/**
	 * Returns a member from the database based on their id
	 */
	function get($id)
	{
		// ...
	}
	
	// save method etc...
}

// Usage
$arrMember = members::getInstance()->get(49);
$arrMember['member_f_name'] = 'Scott';
members::getInstance()->save($arrMember);

?>

# Method 2:
This method involves an associative array of class names to their
instances, probably via a helper function similar to this.

<?php

class members
{
	/**
	 * Returns a member from the database based on their id
	 */
	function get($id)
	{
		// ...
	}
	
	// save method etc...
}

/**
 * Returns an instance of a singleton class from its class name
 */
function getInstance($strClass)
{
	static $objects = array();
	
	if(!isset($objects[$strClass]))
	{
		return $objects[$strClass];
	}
	
	$objects[$strClass] = new members();
	return $objects[$strClass];
}

// Usage
$arrMember = getInstance('members')->get(49);
$arrMember['member_f_name'] = 'Scott';
getInstance('members')->save($arrMember);

?>

This sucks because its slow, confusing for novices, and IDEs never pick
up the class for code hinting.

###
# Proposed new functionality
###

I propose that singleton classes become a native part of the php
language by adding the "singleton" class modifier T_SINGLETON.

I don't know if native singleton classes have been implemented in a
language before.  Most other languages eg - C++
you can use template classes or generics to implement a clean
singleton.

The zend engine could feature a hash table and store the instances of
objects.

An example of the new way of using singleton classes would be:

<?php

/**
 * A class for saving / retreiving members from a database
 */
singleton class members extends dataadapter
{
	/**
	 * Returns a member from the database based on their id
	 */
	function get($id)
	{
		// ...
	}
	
	// save method etc...
}

// Usage
$arrMember = members->get(49);
members->save($arrMember);

?>

###
# Edge cases / got-cha's
###
Some investigation will have to be performed as to how inheritence will
work with singletons.

Eg 
- Never have singleton abstract classes?

- What happens when $members = new members() is called
-- I say an exception being thrown and a suggestion to remove "new"

- Is it ok to go $members = members; ?
-- I'd say yes

- Singleton constructors should not have parameters

- Perhaps a new function singleton_getobjects needs to be added to
enable access to the internal hash table 

- Dereferencing a non-singleton class should produce the same error