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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> | |
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