171 lines
7.5 KiB
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171 lines
7.5 KiB
HTML
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>Boost.Flyweight Documentation - Tutorial</title>
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<h1><img src="../../../../boost.png" alt="Boost logo" align=
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"middle" width="277" height="86">Boost.Flyweight Tutorial</h1>
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Basics
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</a></div><br clear="all" style="clear: all;">
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<hr>
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<h2>Contents</h2>
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<ul>
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<li><a href="#rationale">Rationale</a></li>
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<li><a href="#namespace">Namespace</a></li>
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<li><a href="#guide">Guide to the reader</a></li>
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<li><a href="basics.html">Basics</a></li>
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<li><a href="key_value.html">Key-value flyweights</a></li>
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<li><a href="configuration.html">Configuring Boost.Flyweight</a></li>
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<li><a href="extension.html">Extending Boost.Flyweight</a></li>
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<li><a href="technical.html">Technical issues</a></li>
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<li><a href="lambda_expressions.html">Annex: MPL Lambda expressions</a></li>
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</ul>
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<h2><a name="rationale">Rationale</a></h2>
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<span style="float:left;margin-right:20px;margin-bottom:20px">
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<p align="center">
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<img src="flyweight_rep.png"
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alt="representation of a flyweight scenario"
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width="424" height="320"><br>
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<b>Fig. 1: Representation of a flyweight scenario.</b>
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</p>
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</span>
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<p>
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Consider an application that has to manage large quantities of objects of
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moderate size, potentially requiring more memory than reasonably available.
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When these objects are <i>immutable</i>, i.e. they do not modify its internal
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state except maybe for reattaching to a new set of state data, and some
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additional conditions are met, a very convenient optimization technique known
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as the <i>flyweight pattern</i> can be introduced.
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</p>
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<p>
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Let us say there are <i>N</i> different objects living at a given time
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inside the application, globally taking <i>M</i> different values. If <i>N</i>
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is much greater than <i>M</i>, that is, there are many equivalent objects,
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we can eliminate the implicit redundancy by replacing the original objects with
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handle classes which refer to a common repository of shared value objects,
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as depicted in the figure. The handle objects or flyweights, which act as
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proxies for the actual values, typically occupy the size of a mere pointer.
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The larger the value classes, and the greater the <i>N</i>/<i>M</i> ratio,
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the more significant the memory savings achieved by this tecnhique. The
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classical example of application of the flyweight idiom is that of a word
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processor: each letter in the document carries a large wealth of
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information, such as its Unicode identifier, font, size, typesetting effects,
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etc., but given that the degree of letter repetition in a document is extremely
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high, implementing those letters as flyweight classes allows us to easily
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handle documents ranging in the hundreds of thousands of characters.
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</p>
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<p>
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Most presentations of the design pattern found in the literature do make a
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distinction between the flyweight <i>intrinsic information</i> (the constant
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data factored out into the repository) and <i>extrinsic</i>, mutable
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information, which is stored along with the flyweight objects or passed
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externally. This separation analysis can have some merit from the point of
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view of application design, but when it comes to implementation extrinsic
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information has no impact on the overall flyweight scheme. So,
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Boost.Flyweight assumes that the type onto which the library operates
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entirely consists of intrinsic information: this allows for a particularly
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appealing realization of the idiom in C++ in which
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<code>flyweight<T></code> is an opaque type convertible to
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<code>const T&</code>.
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</p>
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<p>
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The central repository of shared value objects is known as the <i>flyweight
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factory</i>. This component is able to locate and return a reference to an
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object with a given value, or insert the value if no copy was previously
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stored. Boost.Flyweight controls the interaction of flyweights with
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their factory transparently to the programmer, so that a casual user of the
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library need not even be concerned about the presence of such factory.
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Boost.Flyweight uses by default a factory based on a hashed container which
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is expected to be suitable for most situations. When this is not the case, it
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is possible to customize the factory or even replace it with another one of
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a different type, either provided by Boost.Flyweight or defined by the user.
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Other aspects of the implementation are also customizable and extendable.
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</p>
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<h2 clear="all" style="clear: all;">
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<a name="namespace">Namespace</a>
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</h2>
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<p>
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All the public types of Boost.Flyweight reside in namespace <code>::boost::flyweights</code>.
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Additionaly, the main class template <code>flyweight</code> is lifted to namespace
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<code>::boost</code> by means of a <code>using</code> declaration. For brevity of
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exposition, the fragments of code in the documentation are written as if the following
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directives were in effect:
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</p>
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<blockquote><pre>
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<span class=keyword>using</span> <span class=keyword>namespace</span> <span class=special>::</span><span class=identifier>boost</span><span class=special>;</span>
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<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>
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</pre></blockquote>
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<h2><a name="guide">Guide to the reader</a></h2>
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<p>
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Although Boost.Flyweight features an extensive customization
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framework controlling many internal implementation aspects, the library is designed
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in such a way that most users need not be concerned about or
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even aware of the underlying complexity. Learning to use Boost.Flyweight
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as an off-the-shelf component can be acomplished merely by reading
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the <a href="basics.html">basics</a> section and skimming through the
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part on <a href="key_value.html">key-value flyweights</a>, the section on flyweight type
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<a href="configuration.html#tagging">tagging</a> and the discussion of some
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<a href="technical.html">technical issues</a>. The
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<a href="configuration.html">configuration</a> section teaches how to fine tune the
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different internal components of the library. Only very advanced usage
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scenarios will require implementing user-provided pluggable components:
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this is covered on the <a href="extension.html">extension</a> section.
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</p>
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<hr>
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Index
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Index
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<div class="next_link"><a href="basics.html"><img src="../next.gif" alt="basics" border="0"><br>
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Basics
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</a></div><br clear="all" style="clear: all;">
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<br>
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<p>Revised August 13th 2008</p>
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<p>© Copyright 2006-2008 Joaquín M López Muñoz.
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Distributed under the Boost Software
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License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt">
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LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
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http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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</body>
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