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464 lines
21 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0.1 Transitional//EN">
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>Boost.MultiIndex Documentation - Examples</title>
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</head>
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<body>
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<h1><img src="../../../boost.png" alt="boost.png (6897 bytes)" align=
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"middle" width="277" height="86">Boost.MultiIndex Examples</h1>
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<div class="prev_link"><a href="performance.html"><img src="prev.gif" alt="performance" border="0"><br>
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Performance
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Index
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Tests
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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="#example1">Example 1: basic usage</a></li>
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<li><a href="#example2">Example 2: using functions as keys</a></li>
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<li><a href="#example3">Example 3: constructing <code>multi_index_container</code>s
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with <code>ctor_args_list</code></a></li>
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<li><a href="#example4">Example 4: bidirectional map</a></li>
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<li><a href="#example5">Example 5: sequenced indices</a></li>
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<li><a href="#example6">Example 6: complex searches and foreign keys</a></li>
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<li><a href="#example7">Example 7: composite keys</a></li>
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<li><a href="#example8">Example 8: hashed indices</a></li>
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<li><a href="#example9">Example 9: serialization and MRU lists</a></li>
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<li><a href="#example10">Example 10: random access indices</a></li>
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<li><a href="#example11">Example 11: index rearrangement</a></li>
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<li><a href="#example12">Example 12: using Boost.Interprocess allocators</a></li>
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</ul>
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<h2><a name="example1">Example 1: basic usage</a></h2>
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<p>
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See <a href="../example/basic.cpp">source code</a>.
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</p>
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<p>
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Basic program showing the multi-indexing capabilities of Boost.MultiIndex
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with an admittedly boring set of <code>employee</code> records.
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</p>
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<h2><a name="example2">Example 2: using functions as keys</a></h2>
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<p>
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See <a href="../example/fun_key.cpp">source code</a>.
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</p>
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<p>
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Usually keys assigned to an index are based on a member variable of the
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element, but key extractors can be defined which take their value from
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a member function or a global function. This has some similarity with the concept of
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<i>calculated keys</i> supported by some relational database engines.
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The example shows how to use the predefined <code>const_mem_fun</code>
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and <code>global_fun</code> key extractors to deal with this situation.
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</p>
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<p>
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Keys based on functions usually will not be actual references,
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but rather the temporary values resulting from the invocation of the
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member function used. This implies that <code>modify_key</code> cannot be
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applied to this type of extractors, which is a perfectly logical
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constraint anyway.
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</p>
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<h2><a name="example3">Example 3: constructing <code>multi_index_container</code>s
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with <code>ctor_args_list</code></a></h2>
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<p>
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See <a href="../example/non_default_ctor.cpp">source code</a>.
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</p>
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<p>
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We show a practical example of usage of <code>multi_index_container::ctor_arg_list</code>,
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whose definition and purpose are explained in the
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<a href="tutorial/creation.html#ctor_args_list">tutorial</a>. The
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program groups a sorted collection of numbers based on identification through
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modulo arithmetics, by which <code>x</code> and <code>y</code> are equivalent
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if <code>(x%n)==(y%n)</code>, for some fixed <code>n</code>.
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</p>
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<h2><a name="example4">Example 4: bidirectional map</a></h2>
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<p>
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See <a href="../example/bimap.cpp">source code</a>.
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</p>
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<p>
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This example shows how to construct a bidirectional map with
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<code>multi_index_container</code>. By a <i>bidirectional map</i> we mean
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a container of <code>(const FromType,const ToType)</code> pairs
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such that no two elements exists with the same first
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<i>or</i> second component (<code>std::map</code> only
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guarantees uniqueness of the first component). Fast lookup is provided
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for both keys. The program features a tiny Spanish-English
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dictionary with online query of words in both languages.
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</p>
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<p>
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This bidirectional map can be considered as a primitive precursor
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to the full-fledged container provided by
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<a href="../../bimap/index.html">Boost.Bimap</a>.
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</p>
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<h2><a name="example5">Example 5: sequenced indices</a></h2>
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<p>
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See <a href="../example/sequenced.cpp">source code</a>.
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</p>
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<p>
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The combination of a sequenced index with an index of type <code>ordered_non_unique</code>
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yields a <code>list</code>-like structure with fast lookup capabilities. The
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example performs some operations on a given text, like word counting and
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selective deletion of some words.
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</p>
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<h2><a name="example6">Example 6: complex searches and foreign keys</a></h2>
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<p>
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See <a href="../example/complex_structs.cpp">source code</a>.
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</p>
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<p>
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This program illustrates some advanced techniques that can be applied
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for complex data structures using <code>multi_index_container</code>.
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Consider a <code>car_model</code> class for storing information
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about automobiles. On a first approach, <code>car_model</code> can
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be defined as:
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</p>
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<blockquote><pre>
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<span class=keyword>struct</span> <span class=identifier>car_model</span>
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<span class=special>{</span>
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<span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>model</span><span class=special>;</span>
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<span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>manufacturer</span><span class=special>;</span>
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<span class=keyword>int</span> <span class=identifier>price</span><span class=special>;</span>
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<span class=special>};</span>
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</pre></blockquote>
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<p>
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This definition has a design flaw that any reader acquainted with
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relational databases can easily spot: The <code>manufacturer</code>
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member is duplicated among all cars having the same manufacturer.
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This is a waste of space and poses difficulties when, for instance,
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the name of a manufacturer has to be changed. Following the usual
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principles in relational database design, the appropriate design
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involves having the manufactures stored in a separate
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<code>multi_index_container</code> and store pointers to these in
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<code>car_model</code>:
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</p>
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<blockquote><pre>
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<span class=keyword>struct</span> <span class=identifier>car_manufacturer</span>
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<span class=special>{</span>
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<span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>name</span><span class=special>;</span>
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<span class=special>};</span>
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<span class=keyword>struct</span> <span class=identifier>car_model</span>
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<span class=special>{</span>
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<span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>model</span><span class=special>;</span>
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<span class=identifier>car_manufacturer</span><span class=special>*</span> <span class=identifier>manufacturer</span><span class=special>;</span>
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<span class=keyword>int</span> <span class=identifier>price</span><span class=special>;</span>
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<span class=special>};</span>
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</pre></blockquote>
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<p>
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Although predefined Boost.MultiIndex key extractors can handle many
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situations involving pointers (see
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<a href="tutorial/key_extraction.html#advanced_key_extractors">advanced features
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of Boost.MultiIndex key extractors</a> in the tutorial), this case
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is complex enough that a suitable key extractor has to be defined. The following
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utility cascades two key extractors:
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</p>
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<blockquote><pre>
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<span class=keyword>template</span><span class=special><</span><span class=keyword>class</span> <span class=identifier>KeyExtractor1</span><span class=special>,</span><span class=keyword>class</span> <span class=identifier>KeyExtractor2</span><span class=special>></span>
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<span class=keyword>struct</span> <span class=identifier>key_from_key</span>
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<span class=special>{</span>
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<span class=keyword>public</span><span class=special>:</span>
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<span class=keyword>typedef</span> <span class=keyword>typename</span> <span class=identifier>KeyExtractor1</span><span class=special>::</span><span class=identifier>result_type</span> <span class=identifier>result_type</span><span class=special>;</span>
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<span class=identifier>key_from_key</span><span class=special>(</span>
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<span class=keyword>const</span> <span class=identifier>KeyExtractor1</span><span class=special>&</span> <span class=identifier>key1_</span><span class=special>=</span><span class=identifier>KeyExtractor1</span><span class=special>(),</span>
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<span class=keyword>const</span> <span class=identifier>KeyExtractor2</span><span class=special>&</span> <span class=identifier>key2_</span><span class=special>=</span><span class=identifier>KeyExtractor2</span><span class=special>()):</span>
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<span class=identifier>key1</span><span class=special>(</span><span class=identifier>key1_</span><span class=special>),</span><span class=identifier>key2</span><span class=special>(</span><span class=identifier>key2_</span><span class=special>)</span>
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<span class=special>{}</span>
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<span class=keyword>template</span><span class=special><</span><span class=keyword>typename</span> <span class=identifier>Arg</span><span class=special>></span>
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<span class=identifier>result_type</span> <span class=keyword>operator</span><span class=special>()(</span><span class=identifier>Arg</span><span class=special>&</span> <span class=identifier>arg</span><span class=special>)</span><span class=keyword>const</span>
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<span class=special>{</span>
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<span class=keyword>return</span> <span class=identifier>key1</span><span class=special>(</span><span class=identifier>key2</span><span class=special>(</span><span class=identifier>arg</span><span class=special>));</span>
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<span class=special>}</span>
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<span class=keyword>private</span><span class=special>:</span>
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<span class=identifier>KeyExtractor1</span> <span class=identifier>key1</span><span class=special>;</span>
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<span class=identifier>KeyExtractor2</span> <span class=identifier>key2</span><span class=special>;</span>
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<span class=special>};</span>
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</pre></blockquote>
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<p>
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so that access from a <code>car_model</code> to the <code>name</code> field
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of its associated <code>car_manufacturer</code> can be accomplished with
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</p>
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<blockquote><pre>
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<span class=identifier>key_from_key</span><span class=special><</span>
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<span class=identifier>member</span><span class=special><</span><span class=identifier>car_manufacturer</span><span class=special>,</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&</span><span class=identifier>car_manufacturer</span><span class=special>::</span><span class=identifier>name</span><span class=special>>,</span>
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<span class=identifier>member</span><span class=special><</span><span class=identifier>car_model</span><span class=special>,</span><span class=keyword>const</span> <span class=identifier>car_manufacturer</span> <span class=special>*,</span><span class=identifier>car_model</span><span class=special>::</span><span class=identifier>manufacturer</span><span class=special>></span>
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<span class=special>></span>
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</pre></blockquote>
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<p>
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The program asks the user for a car manufacturer and a range of prices
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and returns the car models satisfying these requirements. This is a complex
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search that cannot be performed on a single operation. Broadly sketched,
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one procedure for executing the selection is:
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<ol>
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<li>Select the elements with the given manufacturer by means
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of <code>equal_range</code>,
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<li>feed these elements into a <code>multi_index_container</code> sorted
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by price,
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<li>select by price using <code>lower_bound</code> and
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<code>upper_bound</code>;
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</ol>
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or alternatively:
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<ol>
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<li>Select the elements within the price range with
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<code>lower_bound</code> and <code>upper_bound</code>,
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<li>feed these elements into a <code>multi_index_container</code> sorted
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by manufacturer,
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<li>locate the elements with given manufacturer using
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<code>equal_range</code>.
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</ol>
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An interesting technique developed in the example lies in
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the construction of the intermediate <code>multi_index_container</code>.
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In order to avoid object copying, appropriate <i>view</i> types
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are defined with <code>multi_index_container</code>s having as elements
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pointers to <code>car_model</code>s instead of actual objects.
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These views have to be supplemented with appropriate
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dereferencing key extractors.
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</p>
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<h2><a name="example7">Example 7: composite keys</a></h2>
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<p>
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See <a href="../example/composite_keys.cpp">source code</a>.
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</p>
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<p>
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Boost.MultiIndex <a href="tutorial/key_extraction.html#composite_keys">
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<code>composite_key</code></a> construct provides a flexible tool for
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creating indices with non-trivial sorting criteria.
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The program features a rudimentary simulation of a file system
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along with an interactive Unix-like shell. A file entry is represented by
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the following structure:
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</p>
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<blockquote><pre>
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<span class=keyword>struct</span> <span class=identifier>file_entry</span>
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<span class=special>{</span>
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<span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>name</span><span class=special>;</span>
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<span class=keyword>unsigned</span> <span class=identifier>size</span><span class=special>;</span>
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<span class=keyword>bool</span> <span class=identifier>is_dir</span><span class=special>;</span> <span class=comment>// true if the entry is a directory</span>
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<span class=keyword>const</span> <span class=identifier>file_entry</span><span class=special>*</span> <span class=identifier>dir</span><span class=special>;</span> <span class=comment>// directory this entry belongs in</span>
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<span class=special>};</span>
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</pre></blockquote>
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<p>
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Entries are kept in a <code>multi_index_container</code> maintaining two indices
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with composite keys:
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<ul>
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<li>A primary index ordered by directory and name,</li>
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<li>a secondary index ordered by directory and size.</li>
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</ul>
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The reason that the order is made firstly by the directory in which
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the files are located obeys to the local nature of the shell commands,
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like for instance <code>ls</code>. The shell simulation only has three
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commands:
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<ul>
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<li><code>cd [.|..|<i><directory></i>]</code></li>
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<li><code>ls [-s]</code> (<code>-s</code> orders the output by size)</li>
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<li><code>mkdir <i><directory></i></code></li>
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</ul>
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The program exits when the user presses the Enter key at the command prompt.
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</p>
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<p>
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The reader is challenged to add more functionality to the program; for
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instance:
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<ul>
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<li>Implement additional commands, like <code>cp</code>.</li>
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<li>Add handling of absolute paths.</li>
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<li>Use <a href="tutorial/creation.html#serialization">serialization</a>
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to store and retrieve the filesystem state between program runs.</li>
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</ul>
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</p>
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<h2><a name="example8">Example 8: hashed indices</a></h2>
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<p>
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See <a href="../example/hashed.cpp">source code</a>.
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</p>
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<p>
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Hashed indices can be used as an alternative to ordered indices when
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fast lookup is needed and sorting information is of no interest. The
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example features a word counter where duplicate entries are checked
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by means of a hashed index. Confront the word counting algorithm with
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that of <a href="#example5">example 5</a>.
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</p>
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<h2><a name="example9">Example 9: serialization and MRU lists</a></h2>
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<p>
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See <a href="../example/serialization.cpp">source code</a>.
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</p>
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<p>
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A typical application of serialization capabilities allows a program to
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restore the user context between executions. The example program asks
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the user for words and keeps a record of the ten most recently entered
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ones, in the current or in previous sessions. The serialized data structure,
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sometimes called an <i>MRU (most recently used) list</i>, has some interest
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on its own: an MRU list behaves as a regular FIFO queue, with the exception
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that, when inserting a preexistent entry, this does not appear twice, but
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instead the entry is moved to the front of the list. You can observe this
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behavior in many programs featuring a "Recent files" menu command. This
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data structure is implemented with <code>multi_index_container</code> by
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combining a sequenced index and an index of type <code>hashed_unique</code>.
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</p>
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<h2><a name="example10">Example 10: random access indices</a></h2>
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<p>
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See <a href="../example/random_access.cpp">source code</a>.
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</p>
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<p>
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The example resumes the text container introduced in
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<a href="#example5">example 5</a> and shows how substituting a random
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access index for a sequenced index allows for extra capabilities like
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efficient access by position and calculation of the offset of a given
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element into the container.
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</p>
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<h2><a name="example11">Example 11: index rearrangement</a></h2>
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<p>
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See <a href="../example/rearrange.cpp">source code</a>.
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</p>
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<p>
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There is a relatively common piece of urban lore claiming that
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a deck of cards must be shuffled seven times in a row to be perfectly
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mixed. The statement derives from the works of mathematician Persi
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Diaconis on <i>riffle shuffling</i>: this shuffling
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technique involves splitting the deck in two packets roughly the same
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size and then dropping the cards from both packets so that they become
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interleaved. It has been shown that when repeating this procedure
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seven times the statistical distribution of cards is reasonably
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close to that associated with a truly random permutation. A measure
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of "randomness" can be estimated by counting <i>rising sequences</i>:
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consider a permutation of the sequence 1,2, ... , <i>n</i>, a rising sequence
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is a maximal chain of consecutive elements <i>m</i>, <i>m+1</i>, ... , <i>m+r</i>
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such that they are arranged in ascending order. For instance, the permutation
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125364789 is composed of the two rising sequences 1234 and 56789,
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as becomes obvious by displaying the sequence like this,
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<span style="vertical-align:sub">1</span><span style="vertical-align:sub">2</span><span style="vertical-align:super">5</span><span style="vertical-align:sub">3</span><span style="vertical-align:super">6</span><span style="vertical-align:sub">4</span><span style="vertical-align:super">7</span><span style="vertical-align:super">8</span><span style="vertical-align:super">9</span>.
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The average number of rising sequences in a random permutation of
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<i>n</i> elements is (<i>n</i>+1)/2: by contrast, after a single riffle
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shuffle of an initially sorted deck of cards, there cannot be more than
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two rising sequences. The average number of rising sequences approximates
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to (<i>n</i>+1)/2 as the number of consecutive riffle shuffles increases,
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with seven shuffles yielding a close result for a 52-card poker deck.
|
|
Brad Mann's paper
|
|
<a href="http://www.dartmouth.edu/~chance/teaching_aids/books_articles/Mann.pdf">"How
|
|
many times should you shuffle a deck of cards?"</a> provides a
|
|
rigorous yet very accessible treatment of this subject.
|
|
|
|
</p>
|
|
|
|
<p>
|
|
The example program estimates the average number of rising sequences
|
|
in a 52-card deck after repeated riffle shuffling as well as applying
|
|
a completely random permutation. The deck is modeled by the following
|
|
container:
|
|
<blockquote><pre>
|
|
<span class=identifier>multi_index_container</span><span class=special><</span>
|
|
<span class=keyword>int</span><span class=special>,</span>
|
|
<span class=identifier>indexed_by</span><span class=special><</span>
|
|
<span class=identifier>random_access</span><span class=special><>,</span>
|
|
<span class=identifier>random_access</span><span class=special><></span>
|
|
<span class=special>></span>
|
|
<span class=special>></span>
|
|
</pre></blockquote>
|
|
where the first index stores the current arrangement of the deck, while
|
|
the second index is used to remember the start position. This representation
|
|
allows for an efficient implementation of a rising sequences counting
|
|
algorithm in linear time.
|
|
<a href="reference/rnd_indices.html#rearrange"><code>rearrange</code></a>
|
|
is used to apply to the deck a shuffle performed externally on an
|
|
auxiliary data structure.
|
|
</p>
|
|
|
|
<h2><a name="example12">Example 12: using Boost.Interprocess allocators</a></h2>
|
|
|
|
<p>
|
|
See <a href="../example/ip_allocator.cpp">source code</a>.
|
|
</p>
|
|
|
|
<p>
|
|
Boost.MultiIndex supports special allocators such as those provided by
|
|
<a href="../../interprocess/index.html">Boost.Interprocess</a>,
|
|
which allows for <code>multi_index_container</code>s to be placed in shared
|
|
memory. The example features a front-end to a small book database
|
|
implemented by means of a <code>multi_index_container</code> stored
|
|
in a Boost.Interprocess memory mapped file. The reader can verify that several
|
|
instances of the program correctly work simultaneously and immediately see
|
|
the changes to the database performed by any other instance.
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<div class="prev_link"><a href="performance.html"><img src="prev.gif" alt="performance" border="0"><br>
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Tests
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</a></div><br clear="all" style="clear: all;">
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<br>
|
|
|
|
<p>Revised May 26th 2009</p>
|
|
|
|
<p>© Copyright 2003-2009 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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</p>
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