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| 6 | <title>Boost.MultiIndex Documentation - Examples</title> |
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| 9 | |
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| 10 | <body> |
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| 11 | <h1><img src="../../../boost.png" alt="boost.png (6897 bytes)" align= |
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| 12 | "middle" width="277" height="86">Boost.MultiIndex Examples</h1> |
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| 13 | |
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| 14 | <div class="prev_link"><a href="performance.html"><img src="prev.gif" alt="performance" border="0"><br> |
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| 15 | Performance |
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| 16 | </a></div> |
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| 17 | <div class="up_link"><a href="index.html"><img src="up.gif" alt="index" border="0"><br> |
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| 18 | Index |
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| 19 | </a></div> |
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| 20 | <div class="next_link"><a href="tests.html"><img src="next.gif" alt="tests" border="0"><br> |
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| 21 | Tests |
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| 22 | </a></div><br clear="all" style="clear: all;"> |
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| 23 | |
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| 24 | <hr> |
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| 25 | |
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| 26 | <h2>Contents</h2> |
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| 27 | |
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| 28 | <ul> |
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| 29 | <li><a href="#example1">Example 1: basic usage</a></li> |
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| 30 | <li><a href="#example2">Example 2: using member functions as keys</a></li> |
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| 31 | <li><a href="#example3">Example 3: constructing <code>multi_index_container</code>s |
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| 32 | with <code>ctor_args_list</code></a></li> |
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| 33 | <li><a href="#example4">Example 4: bidirectional map</a></li> |
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| 34 | <li><a href="#example5">Example 5: sequenced indices</a></li> |
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| 35 | <li><a href="#example6">Example 6: complex searches and foreign keys</a></li> |
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| 36 | <li><a href="#example7">Example 7: composite keys</a></li> |
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| 37 | <li><a href="#example8">Example 8: hashed indices</a></li> |
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| 38 | <li><a href="#example9">Example 9: serialization and MRU lists</a></li> |
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| 39 | </ul> |
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| 40 | |
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| 41 | <h2><a name="example1">Example 1: basic usage</a></h2> |
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| 42 | |
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| 43 | <p> |
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| 44 | See <a href="../example/basic.cpp">source code</a>. |
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| 45 | </p> |
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| 46 | |
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| 47 | <p> |
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| 48 | Basic program showing the multi-indexing capabilities of Boost.MultiIndex |
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| 49 | with an admittedly boring set of <code>employee</code> records. |
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| 50 | </p> |
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| 51 | |
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| 52 | <h2><a name="example2">Example 2: using member functions as keys</a></h2> |
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| 53 | |
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| 54 | <p> |
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| 55 | See <a href="../example/memfun_key.cpp">source code</a>. |
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| 56 | </p> |
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| 57 | |
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| 58 | <p> |
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| 59 | Usually keys assigned to an index are based on a member variable of the |
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| 60 | element, but key extractors can be defined which take their value from |
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| 61 | a member function. This has some similarity with the concept of |
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| 62 | <i>calculated keys</i> supported by some relational database engines. |
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| 63 | The example shows how to use the predefined <code>const_mem_fun</code> |
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| 64 | key extractor to deal with this situation. |
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| 65 | </p> |
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| 66 | |
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| 67 | <p> |
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| 68 | Keys based on member functions usually will not be actual references, |
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| 69 | but rather the temporary values resulting from the invocation of the |
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| 70 | member function used. This implies that <code>modify_key</code> cannot be |
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| 71 | applied to this type of extractors, which is a perfectly logical |
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| 72 | constraint anyway. |
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| 73 | </p> |
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| 74 | |
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| 75 | <h2><a name="example3">Example 3: constructing <code>multi_index_container</code>s |
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| 76 | with <code>ctor_args_list</code></a></h2> |
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| 77 | |
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| 78 | <p> |
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| 79 | See <a href="../example/non_default_ctor.cpp">source code</a>. |
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| 80 | </p> |
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| 81 | |
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| 82 | <p> |
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| 83 | We show a practical example of usage of <code>multi_index_container::ctor_arg_list</code>, |
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| 84 | whose definition and purpose are explained in the |
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| 85 | <a href="advanced_topics.html#ctor_args_list">Advanced topics section</a>. The |
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| 86 | program groups a sorted collection of numbers based on identification through |
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| 87 | modulo arithmetics, by which <code>x</code> and <code>y</code> are equivalent |
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| 88 | if <code>(x%n)==(y%n)</code>, for some fixed <code>n</code>. |
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| 89 | </p> |
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| 90 | |
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| 91 | <h2><a name="example4">Example 4: bidirectional map</a></h2> |
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| 92 | |
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| 93 | <p> |
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| 94 | See <a href="../example/bimap.cpp">source code</a>. |
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| 95 | </p> |
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| 96 | |
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| 97 | <p> |
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| 98 | This example shows how to construct a bidirectional map with |
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| 99 | <code>multi_index_container</code>. By a <i>bidirectional map</i> we mean |
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| 100 | a container of elements of <code>std::pair<const FromType,const ToType></code> |
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| 101 | such that no two elements exists with the same <code>first</code> |
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| 102 | <i>or</i> <code>second</code> value (<code>std::map</code> only |
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| 103 | guarantees uniqueness of the first member). Fast lookup is provided |
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| 104 | for both keys. The program features a tiny Spanish-English |
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| 105 | dictionary with online query of words in both languages. |
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| 106 | </p> |
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| 107 | |
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| 108 | <h2><a name="example5">Example 5: sequenced indices</a></h2> |
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| 109 | |
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| 110 | <p> |
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| 111 | See <a href="../example/sequenced.cpp">source code</a>. |
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| 112 | </p> |
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| 113 | |
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| 114 | <p> |
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| 115 | The combination of a sequenced index with an index of type <code>ordered_non_unique</code> |
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| 116 | yields a <code>list</code>-like structure with fast lookup capabilities. The |
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| 117 | example performs some operations on a given text, like word counting and |
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| 118 | selective deletion of some words. |
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| 119 | </p> |
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| 120 | |
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| 121 | <h2><a name="example6">Example 6: complex searches and foreign keys</a></h2> |
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| 122 | |
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| 123 | <p> |
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| 124 | See <a href="../example/complex_structs.cpp">source code</a>. |
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| 125 | </p> |
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| 126 | |
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| 127 | <p> |
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| 128 | This program illustrates some advanced techniques that can be applied |
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| 129 | for complex data structures using <code>multi_index_container</code>. |
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| 130 | Consider a <code>car_model</code> class for storing information |
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| 131 | about automobiles. On a first approach, <code>car_model</code> can |
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| 132 | be defined as: |
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| 133 | </p> |
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| 134 | |
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| 135 | <blockquote><pre> |
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| 136 | <span class=keyword>struct</span> <span class=identifier>car_model</span> |
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| 137 | <span class=special>{</span> |
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| 138 | <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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| 139 | <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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| 140 | <span class=keyword>int</span> <span class=identifier>price</span><span class=special>;</span> |
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| 141 | <span class=special>};</span> |
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| 142 | </pre></blockquote> |
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| 143 | |
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| 144 | <p> |
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| 145 | This definition has a design flaw that any reader acquainted with |
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| 146 | relational databases can easily spot: The <code>manufacturer</code> |
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| 147 | member is duplicated among all cars having the same manufacturer. |
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| 148 | This is a waste of space and poses difficulties when, for instance, |
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| 149 | the name of a manufacturer has to be changed. Following the usual |
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| 150 | principles in relational database design, the appropriate design |
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| 151 | involves having the manufactures stored in a separate |
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| 152 | <code>multi_index_container</code> and store pointers to these in |
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| 153 | <code>car_model</code>: |
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| 154 | </p> |
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| 155 | |
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| 156 | <blockquote><pre> |
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| 157 | <span class=keyword>struct</span> <span class=identifier>car_manufacturer</span> |
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| 158 | <span class=special>{</span> |
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| 159 | <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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| 160 | <span class=special>};</span> |
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| 161 | |
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| 162 | <span class=keyword>struct</span> <span class=identifier>car_model</span> |
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| 163 | <span class=special>{</span> |
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| 164 | <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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| 165 | <span class=identifier>car_manufacturer</span><span class=special>*</span> <span class=identifier>manufacturer</span><span class=special>;</span> |
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| 166 | <span class=keyword>int</span> <span class=identifier>price</span><span class=special>;</span> |
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| 167 | <span class=special>};</span> |
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| 168 | </pre></blockquote> |
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| 169 | |
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| 170 | <p> |
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| 171 | Although predefined Boost.MultiIndex key extractors can handle many |
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| 172 | situations involving pointers (see |
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| 173 | <a href="advanced_topics.html#advanced_key_extractors">advanced features |
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| 174 | of Boost.MultiIndex key extractors</a> in the Advanced topics section), this case |
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| 175 | is complex enough that a suitable key extractor has to be defined. The following |
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| 176 | utility cascades two key extractors: |
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| 177 | </p> |
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| 178 | |
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| 179 | <blockquote><pre> |
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| 180 | <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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| 181 | <span class=keyword>struct</span> <span class=identifier>key_from_key</span> |
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| 182 | <span class=special>{</span> |
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| 183 | <span class=keyword>public</span><span class=special>:</span> |
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| 184 | <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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| 185 | |
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| 186 | <span class=identifier>key_from_key</span><span class=special>(</span> |
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| 187 | <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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| 188 | <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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| 189 | <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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| 190 | <span class=special>{}</span> |
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| 191 | |
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| 192 | <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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| 193 | <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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| 194 | <span class=special>{</span> |
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| 195 | <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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| 196 | <span class=special>}</span> |
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| 197 | |
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| 198 | <span class=keyword>private</span><span class=special>:</span> |
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| 199 | <span class=identifier>KeyExtractor1</span> <span class=identifier>key1</span><span class=special>;</span> |
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| 200 | <span class=identifier>KeyExtractor2</span> <span class=identifier>key2</span><span class=special>;</span> |
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| 201 | <span class=special>};</span> |
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| 202 | </pre></blockquote> |
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| 203 | |
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| 204 | <p> |
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| 205 | so that access from a <code>car_model</code> to the <code>name</code> field |
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| 206 | of its associated <code>car_manufacturer</code> can be accomplished with |
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| 207 | </p> |
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| 208 | |
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| 209 | <blockquote><pre> |
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| 210 | <span class=identifier>key_from_key</span><span class=special><</span> |
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| 211 | <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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| 212 | <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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| 213 | <span class=special>></span> |
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| 214 | </pre></blockquote> |
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| 215 | |
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| 216 | <p> |
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| 217 | The program asks the user for a car manufacturer and a range of prices |
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| 218 | and returns the car models satisfying these requirements. This is a complex |
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| 219 | search that cannot be performed on a single operation. Broadly sketched, |
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| 220 | one procedure for executing the selection is: |
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| 221 | <ol> |
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| 222 | <li>Select the elements with the given manufacturer by means |
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| 223 | of <code>equal_range</code>, |
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| 224 | <li>feed these elements into a <code>multi_index_container</code> sorted |
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| 225 | by price, |
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| 226 | <li>select by price using <code>lower_bound</code> and |
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| 227 | <code>upper_bound</code>; |
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| 228 | </ol> |
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| 229 | or alternatively: |
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| 230 | <ol> |
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| 231 | <li>Select the elements within the price range with |
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| 232 | <code>lower_bound</code> and <code>upper_bound</code>, |
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| 233 | <li>feed these elements into a <code>multi_index_container</code> sorted |
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| 234 | by manufacturer, |
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| 235 | <li>locate the elements with given manufacturer using |
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| 236 | <code>equal_range</code>. |
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| 237 | </ol> |
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| 238 | An interesting technique developed in the example lies in |
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| 239 | the construction of the intermediate <code>multi_index_container</code>. |
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| 240 | In order to avoid object copying, appropriate <i>view</i> types |
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| 241 | are defined with <code>multi_index_container</code>s having as elements |
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| 242 | pointers to <code>car_model</code>s instead of actual objects. |
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| 243 | These views have to be supplemented with appropriate |
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| 244 | dereferencing key extractors. |
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| 245 | </p> |
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| 246 | |
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| 247 | <h2><a name="example7">Example 7: composite keys</a></h2> |
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| 248 | |
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| 249 | <p> |
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| 250 | See <a href="../example/composite_keys.cpp">source code</a>. |
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| 251 | </p> |
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| 252 | |
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| 253 | <p> |
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| 254 | Boost.MultiIndex <a href="advanced_topics.html#composite_keys"> |
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| 255 | <code>composite_key</code></a> construct provides a flexible tool for |
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| 256 | creating indices with non-trivial sorting criteria. |
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| 257 | The program features a rudimentary simulation of a file system |
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| 258 | along with an interactive Unix-like shell. A file entry is represented by |
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| 259 | the following structure: |
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| 260 | </p> |
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| 261 | |
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| 262 | <blockquote><pre> |
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| 263 | <span class=keyword>struct</span> <span class=identifier>file_entry</span> |
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| 264 | <span class=special>{</span> |
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| 265 | <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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| 266 | <span class=keyword>unsigned</span> <span class=identifier>size</span><span class=special>;</span> |
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| 267 | <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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| 268 | <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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| 269 | <span class=special>};</span> |
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| 270 | </pre></blockquote> |
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| 271 | |
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| 272 | <p> |
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| 273 | Entries are kept in a <code>multi_index_container</code> maintaining two indices |
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| 274 | with composite keys: |
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| 275 | <ul> |
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| 276 | <li>A primary index ordered by directory and name,</li> |
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| 277 | <li>a secondary index ordered by directory and size.</li> |
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| 278 | </ul> |
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| 279 | The reason that the order is made firstly by the directory in which |
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| 280 | the files are located obeys to the local nature of the shell commands, |
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| 281 | like for instance <code>ls</code>. The shell simulation only has three |
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| 282 | commands: |
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| 283 | <ul> |
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| 284 | <li><code>cd [.|..|<i><directory></i>]</code></li> |
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| 285 | <li><code>ls [-s]</code> (<code>-s</code> orders the output by size)</li> |
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| 286 | <li><code>mkdir <i><directory></i></code></li> |
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| 287 | </ul> |
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| 288 | The program exits when the user presses the Enter key at the command prompt. |
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| 289 | </p> |
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| 290 | |
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| 291 | <p> |
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| 292 | The reader is challenged to add more functionality to the program; for |
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| 293 | instance: |
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| 294 | <ul> |
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| 295 | <li>Implement additional commands, like <code>cp</code>.</li> |
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| 296 | <li>Add handling of absolute paths.</li> |
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| 297 | <li>Use <a href="advanced_topics.html#serialization">serialization</a> |
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| 298 | to store and retrieve the filesystem state between program runs.</li> |
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| 299 | </ul> |
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| 300 | </p> |
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| 301 | |
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| 302 | <h2><a name="example8">Example 8: hashed indices</a></h2> |
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| 303 | |
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| 304 | <p> |
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| 305 | See <a href="../example/hashed.cpp">source code</a>. |
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| 306 | </p> |
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| 307 | |
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| 308 | <p> |
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| 309 | Hashed indices can be used as an alternative to ordered indices when |
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| 310 | fast lookup is needed and sorting information is of no interest. The |
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| 311 | example features a word counter where duplicate entries are checked |
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| 312 | by means of a hashed index. Confront the word counting algorithm with |
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| 313 | that of <a href="#example5">example 5</a>. |
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| 314 | </p> |
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| 315 | |
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| 316 | <h2><a name="example9">Example 9: serialization and MRU lists</a></h2> |
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| 317 | |
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| 318 | <p> |
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| 319 | See <a href="../example/serialization.cpp">source code</a>. |
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| 320 | </p> |
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| 321 | |
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| 322 | <p> |
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| 323 | A typical application of serialization capabilities allows a program to |
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| 324 | restore the user context between executions. The example program asks |
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| 325 | the user for words and keeps a record of the ten most recently entered |
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| 326 | ones, in the current or in previous sessions. The serialized data structure, |
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| 327 | sometimes called an <i>MRU (most recently used) list</i>, has some interest |
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| 328 | on its own: an MRU list behaves as a regular FIFO queue, with the exception |
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| 329 | that, when inserting a preexistent entry, this does not appear twice, but |
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| 330 | instead the entry is moved to the front of the list. You can observe this |
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| 331 | behavior in many programs featuring a "Recent files" menu command. This |
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| 332 | data structure is implemented with <code>multi_index_container</code> by |
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| 333 | combining a sequenced index and an index of type <code>hashed_unique</code>. |
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| 334 | </p> |
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| 335 | |
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| 336 | <hr> |
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| 337 | |
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| 338 | <div class="prev_link"><a href="performance.html"><img src="prev.gif" alt="performance" border="0"><br> |
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| 339 | Performance |
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| 340 | </a></div> |
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| 341 | <div class="up_link"><a href="index.html"><img src="up.gif" alt="index" border="0"><br> |
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| 342 | Index |
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| 343 | </a></div> |
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| 344 | <div class="next_link"><a href="tests.html"><img src="next.gif" alt="tests" border="0"><br> |
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| 345 | Tests |
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| 346 | </a></div><br clear="all" style="clear: all;"> |
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| 347 | |
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| 348 | <br> |
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| 349 | |
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| 350 | <p>Revised August 22nd 2005</p> |
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| 351 | |
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| 352 | <p>© Copyright 2003-2005 Joaquín M López Muñoz. |
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| 353 | Distributed under the Boost Software |
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| 354 | License, Version 1.0. (See accompanying file <a href="../../../LICENSE_1_0.txt"> |
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| 355 | LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt"> |
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| 356 | http://www.boost.org/LICENSE_1_0.txt</a>) |
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| 357 | </p> |
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| 358 | |
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| 359 | </body> |
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| 360 | </html> |
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