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 <h1><img src="../../../../boost.png" alt="Boost logo" align=
 "middle" width="277" height="86">Boost.Flyweight Tutorial</h1>

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 <h2>Contents</h2>

 <ul>
   <li><a href="#rationale">Rationale</a></li>
   <li><a href="#namespace">Namespace</a></li>
   <li><a href="#guide">Guide to the reader</a></li>
   <li><a href="basics.html">Basics</a></li>
   <li><a href="key_value.html">Key-value flyweights</a></li>
   <li><a href="configuration.html">Configuring Boost.Flyweight</a></li>
   <li><a href="extension.html">Extending Boost.Flyweight</a></li>
   <li><a href="technical.html">Technical issues</a></li>
   <li><a href="lambda_expressions.html">Annex: MPL Lambda expressions</a></li>
 </ul>

 <h2><a name="rationale">Rationale</a></h2>

 <span style="float:left;margin-right:20px;margin-bottom:20px">
 <p align="center">
 <img src="flyweight_rep.png"
 alt="representation of a flyweight scenario"
 width="424" height="320"><br>
 <b>Fig. 1: Representation of a flyweight scenario.</b>
 </p>
 </span>

 <p>
 Consider an application that has to manage large quantities of objects of
 moderate size, potentially requiring more memory than reasonably available.
 When these objects are <i>immutable</i>, i.e. they do not modify its internal
 state except maybe for reattaching to a new set of state data, and some
 additional conditions are met, a very convenient optimization technique known
 as the <i>flyweight pattern</i> can be introduced.
 </p>

 <p>
 Let us say there are <i>N</i> different objects living at a given time
 inside the application, globally taking <i>M</i> different values. If <i>N</i>
 is much greater than <i>M</i>, that is, there are many equivalent objects,
 we can eliminate the implicit redundancy by replacing the original objects with
 handle classes which refer to a common repository of shared value objects,
 as depicted in the figure. The handle objects or flyweights, which act as
 proxies for the actual values, typically occupy the size of a mere pointer.
 The larger the value classes, and the greater the <i>N</i>/<i>M</i> ratio,
 the more significant the memory savings achieved by this tecnhique. The
 classical example of application of the flyweight idiom is that of a word
 processor: each letter in the document carries a large wealth of
 information, such as its Unicode identifier, font, size, typesetting effects,
 etc., but given that the degree of letter repetition in a document is extremely
 high, implementing those letters as flyweight classes allows us to easily
 handle documents ranging in the hundreds of thousands of characters.
 </p>

 <p>
 Most presentations of the design pattern found in the literature do make a
 distinction between the flyweight <i>intrinsic information</i> (the constant
 data factored out into the repository) and <i>extrinsic</i>, mutable
 information, which is stored along with the flyweight objects or passed
 externally. This separation analysis can have some merit from the point of
 view of application design, but when it comes to implementation extrinsic
 information has no impact on the overall flyweight scheme. So,
 Boost.Flyweight assumes that the type onto which the library operates
 entirely consists of intrinsic information: this allows for a particularly
 appealing realization of the idiom in C++ in which
 <code>flyweight&lt;T&gt;</code> is an opaque type convertible to
 <code>const T&amp;</code>.
 </p>

 <p>
 The central repository of shared value objects is known as the <i>flyweight
 factory</i>. This component is able to locate and return a reference to an
 object with a given value, or insert the value if no copy was previously
 stored. Boost.Flyweight controls the interaction of flyweights with
 their factory transparently to the programmer, so that a casual user of the
 library need not even be concerned about the presence of such factory.
 Boost.Flyweight uses by default a factory based on a hashed container which
 is expected to be suitable for most situations. When this is not the case, it
 is possible to customize the factory or even replace it with another one of
 a different type, either provided by Boost.Flyweight or defined by the user.
 Other aspects of the implementation are also customizable and extendable.
 </p>

 <h2 clear="all" style="clear: all;">
 <a name="namespace">Namespace</a>
 </h2>

 <p>
 All the public types of Boost.Flyweight reside in namespace <code>::boost::flyweights</code>.
 Additionaly, the main class template <code>flyweight</code> is lifted to namespace
 <code>::boost</code> by means of a <code>using</code> declaration. For brevity of
 exposition, the fragments of code in the documentation are written as if the following
 directives were in effect:
 </p>

 <blockquote><pre>
 <span class=keyword>using</span> <span class=keyword>namespace</span> <span class=special>::</span><span class=identifier>boost</span><span class=special>;</span>
 <span class=keyword>using</span> <span class=keyword>namespace</span> <span class=special>::</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>flyweights</span><span class=special>;</span>
 </pre></blockquote>

 <h2><a name="guide">Guide to the reader</a></h2>

 <p>
 Although Boost.Flyweight features an extensive customization
 framework controlling many internal implementation aspects, the library is designed
 in such a way that most users need not be concerned about or
 even aware of the underlying complexity. Learning to use Boost.Flyweight
 as an off-the-shelf component can be acomplished merely by reading
 the <a href="basics.html">basics</a> section and skimming through the
 part on <a href="key_value.html">key-value flyweights</a>, the section on flyweight type
 <a href="configuration.html#tagging">tagging</a>  and the discussion of some
 <a href="technical.html">technical issues</a>. The
 <a href="configuration.html">configuration</a> section teaches how to fine tune the
 different internal components of the library. Only very advanced usage
 scenarios will require implementing user-provided pluggable components:
 this is covered on the <a href="extension.html">extension</a> section.
 </p>

 <hr>

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 <br>

 <p>Revised August 13th 2008</p>

 <p>&copy; Copyright 2006-2008 Joaqu&iacute;n M L&oacute;pez Mu&ntilde;oz.
 Distributed under the Boost Software
 License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt">
 LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
 http://www.boost.org/LICENSE_1_0.txt</a>)
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	"middle" width="277" height="86">Boost.Flyweight Tutorial</h1>

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	<hr>

	<h2>Contents</h2>

	<ul>
	<li><a href="#rationale">Rationale</a></li>
	<li><a href="#namespace">Namespace</a></li>
	<li><a href="#guide">Guide to the reader</a></li>
	<li><a href="basics.html">Basics</a></li>
	<li><a href="key_value.html">Key-value flyweights</a></li>
	<li><a href="configuration.html">Configuring Boost.Flyweight</a></li>
	<li><a href="extension.html">Extending Boost.Flyweight</a></li>
	<li><a href="technical.html">Technical issues</a></li>
	<li><a href="lambda_expressions.html">Annex: MPL Lambda expressions</a></li>
	</ul>

	<h2><a name="rationale">Rationale</a></h2>

	<span style="float:left;margin-right:20px;margin-bottom:20px">
	<p align="center">
	<img src="flyweight_rep.png"
	alt="representation of a flyweight scenario"
	width="424" height="320"><br>
	<b>Fig. 1: Representation of a flyweight scenario.</b>
	</p>
	</span>

	<p>
	Consider an application that has to manage large quantities of objects of
	moderate size, potentially requiring more memory than reasonably available.
	When these objects are <i>immutable</i>, i.e. they do not modify its internal
	state except maybe for reattaching to a new set of state data, and some
	additional conditions are met, a very convenient optimization technique known
	as the <i>flyweight pattern</i> can be introduced.
	</p>

	<p>
	Let us say there are <i>N</i> different objects living at a given time
	inside the application, globally taking <i>M</i> different values. If <i>N</i>
	is much greater than <i>M</i>, that is, there are many equivalent objects,
	we can eliminate the implicit redundancy by replacing the original objects with
	handle classes which refer to a common repository of shared value objects,
	as depicted in the figure. The handle objects or flyweights, which act as
	proxies for the actual values, typically occupy the size of a mere pointer.
	The larger the value classes, and the greater the <i>N</i>/<i>M</i> ratio,
	the more significant the memory savings achieved by this tecnhique. The
	classical example of application of the flyweight idiom is that of a word
	processor: each letter in the document carries a large wealth of
	information, such as its Unicode identifier, font, size, typesetting effects,
	etc., but given that the degree of letter repetition in a document is extremely
	high, implementing those letters as flyweight classes allows us to easily
	handle documents ranging in the hundreds of thousands of characters.
	</p>

	<p>
	Most presentations of the design pattern found in the literature do make a
	distinction between the flyweight <i>intrinsic information</i> (the constant
	data factored out into the repository) and <i>extrinsic</i>, mutable
	information, which is stored along with the flyweight objects or passed
	externally. This separation analysis can have some merit from the point of
	view of application design, but when it comes to implementation extrinsic
	information has no impact on the overall flyweight scheme. So,
	Boost.Flyweight assumes that the type onto which the library operates
	entirely consists of intrinsic information: this allows for a particularly
	appealing realization of the idiom in C++ in which
	<code>flyweight<T></code> is an opaque type convertible to
	<code>const T&</code>.
	</p>

	<p>
	The central repository of shared value objects is known as the <i>flyweight
	factory</i>. This component is able to locate and return a reference to an
	object with a given value, or insert the value if no copy was previously
	stored. Boost.Flyweight controls the interaction of flyweights with
	their factory transparently to the programmer, so that a casual user of the
	library need not even be concerned about the presence of such factory.
	Boost.Flyweight uses by default a factory based on a hashed container which
	is expected to be suitable for most situations. When this is not the case, it
	is possible to customize the factory or even replace it with another one of
	a different type, either provided by Boost.Flyweight or defined by the user.
	Other aspects of the implementation are also customizable and extendable.
	</p>

	<h2 clear="all" style="clear: all;">
	<a name="namespace">Namespace</a>
	</h2>

	<p>
	All the public types of Boost.Flyweight reside in namespace <code>::boost::flyweights</code>.
	Additionaly, the main class template <code>flyweight</code> is lifted to namespace
	<code>::boost</code> by means of a <code>using</code> declaration. For brevity of
	exposition, the fragments of code in the documentation are written as if the following
	directives were in effect:
	</p>

	<blockquote><pre>
	<span class=keyword>using</span> <span class=keyword>namespace</span> <span class=special>::</span><span class=identifier>boost</span><span class=special>;</span>
	<span class=keyword>using</span> <span class=keyword>namespace</span> <span class=special>::</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>flyweights</span><span class=special>;</span>
	</pre></blockquote>

	<h2><a name="guide">Guide to the reader</a></h2>

	<p>
	Although Boost.Flyweight features an extensive customization
	framework controlling many internal implementation aspects, the library is designed
	in such a way that most users need not be concerned about or
	even aware of the underlying complexity. Learning to use Boost.Flyweight
	as an off-the-shelf component can be acomplished merely by reading
	the <a href="basics.html">basics</a> section and skimming through the
	part on <a href="key_value.html">key-value flyweights</a>, the section on flyweight type
	<a href="configuration.html#tagging">tagging</a> and the discussion of some
	<a href="technical.html">technical issues</a>. The
	<a href="configuration.html">configuration</a> section teaches how to fine tune the
	different internal components of the library. Only very advanced usage
	scenarios will require implementing user-provided pluggable components:
	this is covered on the <a href="extension.html">extension</a> section.
	</p>

	<hr>

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	<br>

	<p>Revised August 13th 2008</p>

	<p>© Copyright 2006-2008 Joaquín M López Muñoz.
	Distributed under the Boost Software
	License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt">
	LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
	http://www.boost.org/LICENSE_1_0.txt</a>)
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