ngerakines · September 23, 2009 07:12
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 <ul> 
    <li>Getting Started
        <ul> 
            <li><a href="introduction.html">Introduction</a></li> 
            <li>Working with Patterns</li> 
        </ul> 
    </li> 
    <li>Design Patterns Revisited</li> 
        <ul> 
            <li>Decorator</li> 
            <li>Prototype</li> 
            <li><a href="singleton.html">Singleton</a></li> 
            <li>Flyweight</li> 
            <li>Command</li> 
            <li>Observer</li> 
            <li>State</li> 
            <li>Strategy</li> 
        </ul> 
    <li>Game Programming Patterns
        <ul> 
            <li>Sequencing Patterns
                <ul> 
                    <li>Game Loop</li> 
                    <li>Update Method</li> 
                    <li>Double Buffer</li> 
                    <li>Dead Pool</li> 
                </ul> 
            </li> 
            <li>Behaving Patterns
                <ul> 
                    <li>Split Representation</li> 
                    <li>Metaclass</li> 
                    <li>Sandbox Method</li> 
                    <li>Context Parameter</li> 
                    <li>Virtual Machine</li> 
                </ul> 
            </li> 
            <li>Communicating Patterns
                <ul> 
                    <li>Service</li> 
                    <li>Message Queue</li> 
                    <li>Blackboard</li> 
                    <li>Area Effect</li> 
                    <li>Collision</li> 
                </ul> 
            </li> 
            <li>Optimizing Patterns
                <ul> 
                    <li><a href="object-pool.html">Object Pool</a></li> 
                    <li>Structure of Arrays</li> 
                    <li>Dirty Bits</li> 
                </ul> 
            </li> 
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 </ul> 
 </div><div id="content"> 
 <h1 class="book">Game Programming Patterns</h1> 
 <h1>Singleton</h1> 
 <div class="feedback"> 
 
            <p>You're reading a work-in-progress. If you'd like to help
            make it better, please give me some feedback by emailing.
            The address is my name (robert) at stuffwithstuff.com.</p> 
            <p><a href="#" class="hide">hide</a></p> 
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            <p>This chapter is an anomaly. Every other chapter in this book shows
 you how to use a design pattern. This one shows you how <em>not</em> to use
 one.</p> 
 <p>Despite noble intentions, the <a class="gof-pattern"
 href="http://c2.com/cgi/wiki?SingletonPattern">Singleton</a> pattern
 described by the Gang of Four usually does more harm than good. While
 they stress that the pattern should be used sparingly, game
 programmers struggling with object-oriented programming took it and
 ran with it. Like any pattern, using it where it doesn&#x2019;t belong is
 about as helpful as treating a bullet wound with a splint. Since it&#x2019;s
 so over-used, most of this chapter will be about <em>avoiding</em> 
 Singletons, but first, let&#x2019;s go over the pattern itself.</p> 
 <h2>The Singleton Pattern</h2> 
 <p><em>Design Patterns</em> summarizes Singleton like this:</p> 
 <blockquote> 
 <p>Ensure a class has one instance, and provide a global point of 
 access to it.</p> 
 </blockquote> 
 <p>We&#x2019;ll split that at &ldquo;and&rdquo; and consider each half
 separately.</p> 
 <h3>Restricting a Class To One Instance</h3> 
 <p>There are times when a class cannot perform correctly if there is more
 than one instance of it. The common case is when the class interacts
 with an external system that maintains its own global state.</p> 
 <p>Consider a class that wraps an underlying file system API. Because
 file operations can take a while to complete, our class performs
 operations asynchronously. This means multiple operations can be
 running concurrently, so they must be coordinated with each other. If
 we start one call to create a file, and another one to delete that
 same file, our wrapper needs to be aware of both to make sure they
 don&#x2019;t interfere with each other.</p> 
 <p>To do this, a call into our wrapper needs to have access to every
 previous operation. If users could freely create instances of our
 class, one instance has no way of knowing about operations that other
 instances started. Enter the Singleton. It provides a way for a class
 to ensure at compile time that there is only a single instance of the
 class.</p> 
 <h3>Providing a Global Point of Access</h3> 
 <p>Several different systems in the game will use our file system
 wrapper: logging, content loading, game state saving, etc. If those
 systems can&#x2019;t create their own instances of our file system wrapper,
 how do they get ahold of one?</p> 
 <p>Singleton provides a solution to this too. In addition to creating the
 single instance, it also provides a globally-available method to get
 it. This way, anyone
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 a.outline:after %7
	<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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	<head>
	<meta http-equiv="Content-type" content="text/html;charset=UTF-8" />
	<title>Game Programming Patterns / Singleton</title>
	<link rel="stylesheet" type="text/css" href="styles.css" />
	<script src="jquery-1.3.2.min.js" type="text/javascript"></script>
	</head>
	<body>
	<div class="nav">
	<h1><a href="index.html">Navigation</a></h1>
	<ul>
	<li>Getting Started
	<ul>
	<li><a href="introduction.html">Introduction</a></li>
	<li>Working with Patterns</li>
	</ul>
	</li>
	<li>Design Patterns Revisited</li>
	<ul>
	<li>Decorator</li>
	<li>Prototype</li>
	<li><a href="singleton.html">Singleton</a></li>
	<li>Flyweight</li>
	<li>Command</li>
	<li>Observer</li>
	<li>State</li>
	<li>Strategy</li>
	</ul>
	<li>Game Programming Patterns
	<ul>
	<li>Sequencing Patterns
	<ul>
	<li>Game Loop</li>
	<li>Update Method</li>
	<li>Double Buffer</li>
	<li>Dead Pool</li>
	</ul>
	</li>
	<li>Behaving Patterns
	<ul>
	<li>Split Representation</li>
	<li>Metaclass</li>
	<li>Sandbox Method</li>
	<li>Context Parameter</li>
	<li>Virtual Machine</li>
	</ul>
	</li>
	<li>Communicating Patterns
	<ul>
	<li>Service</li>
	<li>Message Queue</li>
	<li>Blackboard</li>
	<li>Area Effect</li>
	<li>Collision</li>
	</ul>
	</li>
	<li>Optimizing Patterns
	<ul>
	<li><a href="object-pool.html">Object Pool</a></li>
	<li>Structure of Arrays</li>
	<li>Dirty Bits</li>
	</ul>
	</li>
	</ul>
	</li>
	</ul>
	</div><div id="content">
	<h1 class="book">Game Programming Patterns</h1>
	<h1>Singleton</h1>
	<div class="feedback">

	<p>You're reading a work-in-progress. If you'd like to help
	make it better, please give me some feedback by emailing.
	The address is my name (robert) at stuffwithstuff.com.</p>
	<p><a href="#" class="hide">hide</a></p>
	<p>Thank you!</p>
	</div>
	<script type="text/javascript">
	/* <![CDATA[ */
	$(document).ready(function()
	{
	$("a.hide").click(function(event) {
	$(this).parent().parent().hide();
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	<p>This chapter is an anomaly. Every other chapter in this book shows
	you how to use a design pattern. This one shows you how <em>not</em> to use
	one.</p>
	<p>Despite noble intentions, the <a class="gof-pattern"
	href="http://c2.com/cgi/wiki?SingletonPattern">Singleton</a> pattern
	described by the Gang of Four usually does more harm than good. While
	they stress that the pattern should be used sparingly, game
	programmers struggling with object-oriented programming took it and
	ran with it. Like any pattern, using it where it doesn’t belong is
	about as helpful as treating a bullet wound with a splint. Since it’s
	so over-used, most of this chapter will be about <em>avoiding</em>
	Singletons, but first, let’s go over the pattern itself.</p>
	<h2>The Singleton Pattern</h2>
	<p><em>Design Patterns</em> summarizes Singleton like this:</p>
	<blockquote>
	<p>Ensure a class has one instance, and provide a global point of
	access to it.</p>
	</blockquote>
	<p>We’ll split that at “and” and consider each half
	separately.</p>
	<h3>Restricting a Class To One Instance</h3>
	<p>There are times when a class cannot perform correctly if there is more
	than one instance of it. The common case is when the class interacts
	with an external system that maintains its own global state.</p>
	<p>Consider a class that wraps an underlying file system API. Because
	file operations can take a while to complete, our class performs
	operations asynchronously. This means multiple operations can be
	running concurrently, so they must be coordinated with each other. If
	we start one call to create a file, and another one to delete that
	same file, our wrapper needs to be aware of both to make sure they
	don’t interfere with each other.</p>
	<p>To do this, a call into our wrapper needs to have access to every
	previous operation. If users could freely create instances of our
	class, one instance has no way of knowing about operations that other
	instances started. Enter the Singleton. It provides a way for a class
	to ensure at compile time that there is only a single instance of the
	class.</p>
	<h3>Providing a Global Point of Access</h3>
	<p>Several different systems in the game will use our file system
	wrapper: logging, content loading, game state saving, etc. If those
	systems can’t create their own instances of our file system wrapper,
	how do they get ahold of one?</p>
	<p>Singleton provides a solution to this too. In addition to creating the
	single instance, it also provides a globally-available method to get
	it. This way, anyone
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