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| 24 | </div> | |
| 25 | <div class="section boost_bimap_the_tutorial_additional_information"> | |
| 26 | <div class="titlepage"><div><div><h3 class="title"> | |
| 27 | <a name="boost_bimap.the_tutorial.additional_information"></a><a class="link" href="additional_information.html" title="Additional information">Additional | |
| 28 | information</a> | |
| 29 | </h3></div></div></div> | |
| 30 | <p> | |
| 31 | Bidirectional maps may have associated information about each relation. Suppose | |
| 32 | we want to represent a books and author bidirectional map. | |
| 33 | </p> | |
| 34 | <p> | |
| 35 | </p> | |
| 36 | <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">bimap</span><span class="special"><</span> | |
| 37 | ||
| 38 | <span class="identifier">multiset_of</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">>,</span> <span class="comment">// author</span> | |
| 39 | <span class="identifier">set_of</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">></span> <span class="comment">// title</span> | |
| 40 | ||
| 41 | <span class="special">></span> <span class="identifier">bm_type</span><span class="special">;</span> | |
| 42 | <span class="keyword">typedef</span> <span class="identifier">bm_type</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">book</span><span class="special">;</span> | |
| 43 | ||
| 44 | <span class="identifier">bm_type</span> <span class="identifier">bm</span><span class="special">;</span> | |
| 45 | ||
| 46 | <span class="identifier">bm</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span> <span class="identifier">book</span><span class="special">(</span> <span class="string">"Bjarne Stroustrup"</span> <span class="special">,</span> <span class="string">"The C++ Programming Language"</span> <span class="special">)</span> <span class="special">);</span> | |
| 47 | <span class="identifier">bm</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span> <span class="identifier">book</span><span class="special">(</span> <span class="string">"Scott Meyers"</span> <span class="special">,</span> <span class="string">"Effective C++"</span> <span class="special">)</span> <span class="special">);</span> | |
| 48 | <span class="identifier">bm</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span> <span class="identifier">book</span><span class="special">(</span> <span class="string">"Andrei Alexandrescu"</span> <span class="special">,</span> <span class="string">"Modern C++ Design"</span> <span class="special">)</span> <span class="special">);</span> | |
| 49 | ||
| 50 | <span class="comment">// Print the author of Modern C++</span> | |
| 51 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">bm</span><span class="special">.</span><span class="identifier">right</span><span class="special">.</span><span class="identifier">at</span><span class="special">(</span> <span class="string">"Modern C++ Design"</span> <span class="special">);</span> | |
| 52 | </pre> | |
| 53 | <p> | |
| 54 | </p> | |
| 55 | <p> | |
| 56 | Suppose now that we want to store abstract of each book. We have two options: | |
| 57 | </p> | |
| 58 | <div class="orderedlist"><ol class="orderedlist" type="1"> | |
| 59 | <li class="listitem"> | |
| 60 | Books name are unique identifiers, so we can create a separate <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span> <span class="identifier">string</span><span class="special">,</span> <span class="identifier">string</span> <span class="special">></span></code> | |
| 61 | that relates books names with abstracts. | |
| 62 | </li> | |
| 63 | <li class="listitem"> | |
| 64 | We can use <a href="http://www.boost.org/libs/multi_index/doc/index.html" target="_top"><span class="bold"><strong>Boost.MultiIndex</strong></span></a> for the new beast. | |
| 65 | </li> | |
| 66 | </ol></div> | |
| 67 | <p> | |
| 68 | Option 1 is the wrong approach, if we go this path we lost what bimap has | |
| 69 | won us. We now have to maintain the logic of two interdependent containers, | |
| 70 | there is an extra string stored for each book name, and the performance will | |
| 71 | be worse. This is far away from being a good solution. | |
| 72 | </p> | |
| 73 | <p> | |
| 74 | Option 2 is correct. We start thinking books as entries in a table. So it | |
| 75 | makes sense to start using Boost.MultiIndex. We can then add the year of | |
| 76 | publication, the price, etc... and we can index this new items too. So Boost.MultiIndex | |
| 77 | is a sound solution for our problem. | |
| 78 | </p> | |
| 79 | <p> | |
| 80 | The thing is that there are cases where we want to maintain bimap semantics | |
| 81 | (use <code class="computeroutput"><span class="identifier">at</span><span class="special">()</span></code> | |
| 82 | to find an author given a book name and the other way around) and add information | |
| 83 | about the relations that we are sure we will not want to index later (like | |
| 84 | the abstracts). Option 1 is not possible, option 2 neither. | |
| 85 | </p> | |
| 86 | <p> | |
| 87 | Boost.Bimap provides support for this kind of situations by means of an embedded | |
| 88 | information member. You can pass an extra parameter to a bimap: <code class="computeroutput"><span class="identifier">with_info</span><span class="special"><</span> | |
| 89 | <span class="identifier">InfoType</span> <span class="special">></span></code> | |
| 90 | and an <code class="computeroutput"><span class="identifier">info</span></code> member of type | |
| 91 | <code class="computeroutput"><span class="identifier">InfoType</span></code> will appear in the | |
| 92 | relation and bimap pairs. | |
| 93 | </p> | |
| 94 | <p> | |
| 95 | <span class="inlinemediaobject"><img src="../../images/bimap/relation.and.pair.with.info.png" alt="relation.and.pair.with.info"></span> | |
| 96 | </p> | |
| 97 | <p> | |
| 98 | Relations and bimap pairs constructors will take an extra argument. If only | |
| 99 | two arguments are used, the information will be initialized with their default | |
| 100 | constructor. | |
| 101 | </p> | |
| 102 | <p> | |
| 103 | </p> | |
| 104 | <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">bimap</span><span class="special"><</span> | |
| 105 | ||
| 106 | <span class="identifier">multiset_of</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">>,</span> <span class="comment">// author</span> | |
| 107 | <span class="identifier">set_of</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">>,</span> <span class="comment">// title</span> | |
| 108 | ||
| 109 | <span class="identifier">with_info</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">></span> <span class="comment">// abstract</span> | |
| 110 | ||
| 111 | <span class="special">></span> <span class="identifier">bm_type</span><span class="special">;</span> | |
| 112 | <span class="keyword">typedef</span> <span class="identifier">bm_type</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">book</span><span class="special">;</span> | |
| 113 | ||
| 114 | <span class="identifier">bm_type</span> <span class="identifier">bm</span><span class="special">;</span> | |
| 115 | ||
| 116 | <span class="identifier">bm</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span> | |
| 117 | ||
| 118 | <span class="identifier">book</span><span class="special">(</span> <span class="string">"Bjarne Stroustrup"</span> <span class="special">,</span> <span class="string">"The C++ Programming Language"</span><span class="special">,</span> | |
| 119 | ||
| 120 | <span class="string">"For C++ old-timers, the first edition of this book is"</span> | |
| 121 | <span class="string">"the one that started it all—the font of our knowledge."</span> <span class="special">)</span> | |
| 122 | <span class="special">);</span> | |
| 123 | ||
| 124 | ||
| 125 | <span class="comment">// Print the author of the bible</span> | |
| 126 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">bm</span><span class="special">.</span><span class="identifier">right</span><span class="special">.</span><span class="identifier">at</span><span class="special">(</span><span class="string">"The C++ Programming Language"</span><span class="special">);</span> | |
| 127 | ||
| 128 | <span class="comment">// Print the abstract of this book</span> | |
| 129 | <span class="identifier">bm_type</span><span class="special">::</span><span class="identifier">left_iterator</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">bm</span><span class="special">.</span><span class="identifier">left</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="string">"Bjarne Stroustrup"</span><span class="special">);</span> | |
| 130 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">i</span><span class="special">-></span><span class="identifier">info</span><span class="special">;</span> | |
| 131 | </pre> | |
| 132 | <p> | |
| 133 | </p> | |
| 134 | <p> | |
| 135 | Contrary to the two key types, the information will be mutable using iterators. | |
| 136 | </p> | |
| 137 | <p> | |
| 138 | </p> | |
| 139 | <pre class="programlisting"><span class="identifier">i</span><span class="special">-></span><span class="identifier">info</span> <span class="special">+=</span> <span class="string">"More details about this book"</span><span class="special">;</span> | |
| 140 | </pre> | |
| 141 | <p> | |
| 142 | </p> | |
| 143 | <p> | |
| 144 | A new function is included in <span class="emphasis"><em>unique</em></span> map views: <code class="computeroutput"><span class="identifier">info_at</span><span class="special">(</span><span class="identifier">key</span><span class="special">)</span></code>, that | |
| 145 | mimics the standard <code class="computeroutput"><span class="identifier">at</span><span class="special">(</span><span class="identifier">key</span><span class="special">)</span></code> function | |
| 146 | but returned the associated information instead of the data. | |
| 147 | </p> | |
| 148 | <p> | |
| 149 | </p> | |
| 150 | <pre class="programlisting"><span class="comment">// Print the new abstract</span> | |
| 151 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">bm</span><span class="special">.</span><span class="identifier">right</span><span class="special">.</span><span class="identifier">info_at</span><span class="special">(</span><span class="string">"The C++ Programming Language"</span><span class="special">);</span> | |
| 152 | </pre> | |
| 153 | <p> | |
| 154 | </p> | |
| 155 | <p> | |
| 156 | The info member can be tagged just as the left or the right member. The following | |
| 157 | is a rewrite of the above example using user defined names: | |
| 158 | </p> | |
| 159 | <p> | |
| 160 | </p> | |
| 161 | <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">bimap</span><span class="special"><</span> | |
| 162 | ||
| 163 | <span class="identifier">multiset_of</span><span class="special"><</span> <span class="identifier">tagged</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">author</span> <span class="special">></span> <span class="special">>,</span> | |
| 164 | <span class="identifier">set_of</span><span class="special"><</span> <span class="identifier">tagged</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">title</span> <span class="special">></span> <span class="special">>,</span> | |
| 165 | ||
| 166 | <span class="identifier">with_info</span><span class="special"><</span> <span class="identifier">tagged</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">abstract</span> <span class="special">></span> <span class="special">></span> | |
| 167 | ||
| 168 | <span class="special">></span> <span class="identifier">bm_type</span><span class="special">;</span> | |
| 169 | <span class="keyword">typedef</span> <span class="identifier">bm_type</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">book</span><span class="special">;</span> | |
| 170 | ||
| 171 | <span class="identifier">bm_type</span> <span class="identifier">bm</span><span class="special">;</span> | |
| 172 | ||
| 173 | <span class="identifier">bm</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span> | |
| 174 | ||
| 175 | <span class="identifier">book</span><span class="special">(</span> <span class="string">"Bjarne Stroustrup"</span> <span class="special">,</span> <span class="string">"The C++ Programming Language"</span><span class="special">,</span> | |
| 176 | ||
| 177 | <span class="string">"For C++ old-timers, the first edition of this book is"</span> | |
| 178 | <span class="string">"the one that started it all—the font of our knowledge."</span> <span class="special">)</span> | |
| 179 | <span class="special">);</span> | |
| 180 | ||
| 181 | <span class="comment">// Print the author of the bible</span> | |
| 182 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">bm</span><span class="special">.</span><span class="identifier">by</span><span class="special"><</span><span class="identifier">title</span><span class="special">>().</span><span class="identifier">at</span><span class="special">(</span><span class="string">"The C++ Programming Language"</span><span class="special">);</span> | |
| 183 | ||
| 184 | <span class="comment">// Print the abstract of this book</span> | |
| 185 | <span class="identifier">bm_type</span><span class="special">::</span><span class="identifier">map_by</span><span class="special"><</span><span class="identifier">author</span><span class="special">>::</span><span class="identifier">iterator</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">bm</span><span class="special">.</span><span class="identifier">by</span><span class="special"><</span><span class="identifier">author</span><span class="special">>().</span><span class="identifier">find</span><span class="special">(</span><span class="string">"Bjarne Stroustrup"</span><span class="special">);</span> | |
| 186 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">i</span><span class="special">-></span><span class="identifier">get</span><span class="special"><</span><span class="identifier">abstract</span><span class="special">>();</span> | |
| 187 | ||
| 188 | <span class="comment">// Contrary to the two key types, the information will be mutable</span> | |
| 189 | <span class="comment">// using iterators.</span> | |
| 190 | ||
| 191 | <span class="identifier">i</span><span class="special">-></span><span class="identifier">get</span><span class="special"><</span><span class="identifier">abstract</span><span class="special">>()</span> <span class="special">+=</span> <span class="string">"More details about this book"</span><span class="special">;</span> | |
| 192 | ||
| 193 | <span class="comment">// Print the new abstract</span> | |
| 194 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">bm</span><span class="special">.</span><span class="identifier">by</span><span class="special"><</span><span class="identifier">title</span><span class="special">>().</span><span class="identifier">info_at</span><span class="special">(</span><span class="string">"The C++ Programming Language"</span><span class="special">);</span> | |
| 195 | </pre> | |
| 196 | <p> | |
| 197 | </p> | |
| 198 | <p> | |
| 199 | <a href="../../../../example/tutorial_info_hook.cpp" target="_top">Go to source code</a> | |
| 200 | </p> | |
| 201 | </div> | |
| 202 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 203 | <td align="left"></td> | |
| 204 | <td align="right"><div class="copyright-footer">Copyright © 2006-2012 Matias Capeletto<p> | |
| 205 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 206 | file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) | |
| 207 | </p> | |
| 208 | </div></td> | |
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