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

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<h2>Contents</h2>

<ul>
  <li><a href="#example1">Example 1: basic usage</a></li>
  <li><a href="#example2">Example 2: key-value flyweights</a></li>
  <li><a href="#example3">Example 3: flyweights and the composite pattern</a></li>
  <li><a href="#example4">Example 4: formatted text processing</a></li>
  <li><a href="#example5">Example 5: flyweight-based memoization</a></li>
  <li><a href="#example6">Example 6: performance comparison</a></li>
  <li><a href="#example7">Example 7: custom factory</a></li>
</ul>

<h2><a name="example1">Example 1: basic usage</a></h2>

<p>
See <a href="../example/basic.cpp">source code</a>.
</p>

<p>
Dummy program showing the basic capabilities of <code>flyweight</code>
explained at the <a href="tutorial/basics.html">tutorial</a>.
</p>

<h2><a name="example2">Example 2: key-value flyweights</a></h2>

<p>
See <a href="../example/key_value.cpp">source code</a>.
</p>

<p>
The program simulates the scenario described at the tutorial section on
<a href="tutorial/key_value.html">key-value flyweights</a>: The class
<code>texture</code> manages some texture rendering data stored in
a file whose location is given at construction time. The program
handles large quantities of objects of this class by encapsulating
them into key-value flyweights keyed by filename. Observe how the
execution of the program results in no extra constructions or copies
of objects of type <code>texture</code> except those absolutely
necessary.
</p>

<h2><a name="example3">Example 3: flyweights and the composite pattern</a></h2>

<p>
See <a href="../example/composite.cpp">source code</a>.
</p>

<p>
The <a href="http://c2.com/cgi/wiki?CompositePattern"><i>composite
design pattern</i></a> revolves about the idea that a tree data structure
can be easily constructed and manipulated by defining the tree node type
polymorphically so that either is a leaf node or else contains a list of
pointers to their child nodes.
This way, a tree is the exact same entity as its root node, which allows
for very simple recursive tree-handling algorithms. Large composite trees
having a high degree of duplication of nodes and subtrees (as for instance
those generated when parsing a computer program) are a natural fit for the
flyweight idiom: simply turning the node type into a flyweight
automatically deals with duplication at the node and subtree level.
</p>

<p>
The example program parses Lisp-like lists of the form
<code>(a<sub>1</sub> ... a<sub><i>n</i></sub>)</code> where each
<code>a<sub>i</sub></code> is a terminal string or a list. The parsed
data structure is a composite type defined using Boost.Flyweight in conjunction
with the recursive facilities of
<a href="../../variant/index.html">Boost.Variant</a>. So, given the list
</p>

<blockquote><pre>
(= (tan (+ x y))(/ (+ (tan x)(tan y))(- 1 (* (tan x)(tan y)))))
</pre></blockquote>

<p>
the resulting data structure implicitly detects the duplicated
occurrences of <code>+</code>, <code>x</code>, <code>y</code>,
<code>tan</code>, <code>(tan x)</code> and <code>(tan y)</code>.
</p>

<h2><a name="example4">Example 4: formatted text processing</a></h2>

<p>
See <a href="../example/html.cpp">source code</a>.
</p>

<p>
A classic example of application of the flyweight pattern is that of a
text processor which handles characters with rich formatting information,
like font type, size, color and special options (boldness, italics, etc.)
Coding the formatting information of each character takes considerable
space, but, given the high degree of repetition typical in a document,
maintaining formatted characters as flyweight objects drastically reduces
memory consumption.
</p>

<p>
The example program parses, manipulates and stores HTML documents following
flyweight-based representation techniques. Given the hierarchical nature
of HTML markup, a crude approximation to the formatting options of a given
character is just to equate them with the stack of tag contexts to which
the character belongs, as the figure shows.
</p>

<p align="center">
<img src="html.png"
alt="formatting contexts of characters in an HTML document"
width="320" height="275"><br>
<b>Fig. 1: Formatting contexts of characters in an HTML document.</b>
</p>

<p>
HTML documents are then parsed as arrays of (character, format)
pairs, where the format is the tag context as described above. The very high
degree of redundancy in formatting information is taken care of by the
use of Boost.Flyweight. This character-based representation makes it
easy to manipulate the document: transposition and elimination of
portions of text are trivial operations. As an example, the program
reverses the text occupying the central portion of the document.
Saving the result in HTML reduces to traversing the array of formatted
characters and emitting opening/closing HTML tags as the context of adjacent
characters varies.
</p>

<p>
For the sake of brevity, the HTML parsing capabilities of this program
are coarse: for instance, elements without end-tag (like &lt;BR&gt;), character
enconding and HTML entities (e.g. "&amp;copy;" for &copy;) are not properly
handled. Improving the parsing code is left as an exercise to the reader.
</p>

<h2><a name="example5">Example 5: flyweight-based memoization</a></h2>

<p>
See <a href="../example/fibonacci.cpp">source code</a>.
</p>

<p>
<a href="http://en.wikipedia.org/wiki/Memoization">Memoization</a>
is an optimization technique consisting in caching
the results of a computation for later reuse; this can dramatically
improve performance when calculating recursive numerical functions,
for instance. <a href="tutorial/key_value.html">Key-value flyweights</a>
can be used to implement memoization for a numerical function <i>f</i>
by modeling a memoized invocation of the function as a value of
type <code>flyweight&lt;key_value&lt;int,compute_f&gt; &gt;</code>, where
<code>compute_f</code> is a type that does the computation of
<i>f</i>(<i>n</i>) at its <code>compute_f::compute_f(int)</code> constructor.
For instance, the <a href="http://mathworld.wolfram.com/FibonacciNumber.html">Fibonacci
numbers</a> can be computed with memoization like this:
</p>

<blockquote><pre>
<span class=keyword>typedef</span> <span class=identifier>flyweight</span><span class=special>&lt;</span><span class=identifier>key_value</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>,</span><span class=identifier>compute_fibonacci</span><span class=special>&gt;,</span><span class=identifier>no_tracking</span><span class=special>&gt;</span> <span class=identifier>fibonacci</span><span class=special>;</span>

<span class=keyword>struct</span> <span class=identifier>compute_fibonacci</span>
<span class=special>{</span>
  <span class=identifier>compute_fibonacci</span><span class=special>(</span><span class=keyword>int</span> <span class=identifier>n</span><span class=special>):</span>
    <span class=identifier>result</span><span class=special>(</span><span class=identifier>n</span><span class=special>==</span><span class=number>0</span><span class=special>?</span><span class=number>0</span><span class=special>:</span><span class=identifier>n</span><span class=special>==</span><span class=number>1</span><span class=special>?</span><span class=number>1</span><span class=special>:</span><span class=identifier>fibonacci</span><span class=special>(</span><span class=identifier>n</span><span class=special>-</span><span class=number>2</span><span class=special>).</span><span class=identifier>get</span><span class=special>()+</span><span class=identifier>fibonacci</span><span class=special>(</span><span class=identifier>n</span><span class=special>-</span><span class=number>1</span><span class=special>).</span><span class=identifier>get</span><span class=special>())</span>
  <span class=special>{}</span>

  <span class=keyword>operator</span> <span class=keyword>int</span><span class=special>()</span><span class=keyword>const</span><span class=special>{</span><span class=keyword>return</span> <span class=identifier>result</span><span class=special>;}</span>
  <span class=keyword>int</span> <span class=identifier>result</span><span class=special>;</span>
<span class=special>};</span>
</pre></blockquote>

<p>
The <a href="tutorial/configuration.html#no_tracking"><code>no_tracking</code></a>
policy is used so that the memoized computations persist for future
use throughout the program. The provided program develops this example in full.
</p>

<h2><a name="example6">Example 6: performance comparison</a></h2>

<p>
See <a href="../example/perf.cpp">source code</a>.
</p>

<p>
This program measures the time and space performances of a simple
string type against several differently configured <code>flyweight</code>
instantations as used in a conventional task involving parsing a file and
doing some manipulations on the parsed text.
Memory consumption is computed by instrumenting the relevant
components (the string type itself, flyweight factories, etc.) with custom
allocators that keep track of the allocations and deallocations requested.
The program has been used to produce the experimental results given
at the <a href="performance.html#results">performance section</a>.
</p>

<h2><a name="example7">Example 7: custom factory</a></h2>

<p>
See <a href="../example/custom_factory.cpp">source code</a>.
</p>

<p>
The example shows how to write and use a custom factory class. This
"verbose" factory outputs messages tracing the invocations of its public interface
by Boost.Flyweight, so helping the user visualize factory usage patterns.
</p>

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<p>Revised December 2nd 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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