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- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_bimap.the_tutorial.additional_information"></a><a class="link" href="additional_information.html" title="Additional information">Additional
- information</a>
- </h3></div></div></div>
- <p>
- Bidirectional maps may have associated information about each relation. Suppose
- we want to represent a books and author bidirectional map.
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">bimap</span><span class="special"><</span>
- <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>
- <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>
- <span class="special">></span> <span class="identifier">bm_type</span><span class="special">;</span>
- <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>
- <span class="identifier">bm_type</span> <span class="identifier">bm</span><span class="special">;</span>
- <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>
- <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>
- <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>
- <span class="comment">// Print the author of Modern C++</span>
- <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>
- </pre>
- <p>
- </p>
- <p>
- Suppose now that we want to store abstract of each book. We have two options:
- </p>
- <div class="orderedlist"><ol class="orderedlist" type="1">
- <li class="listitem">
- 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>
- that relates books names with abstracts.
- </li>
- <li class="listitem">
- 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.
- </li>
- </ol></div>
- <p>
- Option 1 is the wrong approach, if we go this path we lost what bimap has
- won us. We now have to maintain the logic of two interdependent containers,
- there is an extra string stored for each book name, and the performance will
- be worse. This is far away from being a good solution.
- </p>
- <p>
- Option 2 is correct. We start thinking books as entries in a table. So it
- makes sense to start using Boost.MultiIndex. We can then add the year of
- publication, the price, etc... and we can index this new items too. So Boost.MultiIndex
- is a sound solution for our problem.
- </p>
- <p>
- The thing is that there are cases where we want to maintain bimap semantics
- (use <code class="computeroutput"><span class="identifier">at</span><span class="special">()</span></code>
- to find an author given a book name and the other way around) and add information
- about the relations that we are sure we will not want to index later (like
- the abstracts). Option 1 is not possible, option 2 neither.
- </p>
- <p>
- Boost.Bimap provides support for this kind of situations by means of an embedded
- information member. You can pass an extra parameter to a bimap: <code class="computeroutput"><span class="identifier">with_info</span><span class="special"><</span>
- <span class="identifier">InfoType</span> <span class="special">></span></code>
- and an <code class="computeroutput"><span class="identifier">info</span></code> member of type
- <code class="computeroutput"><span class="identifier">InfoType</span></code> will appear in the
- relation and bimap pairs.
- </p>
- <p>
- <span class="inlinemediaobject"><img src="../../images/bimap/relation.and.pair.with.info.png" alt="relation.and.pair.with.info"></span>
- </p>
- <p>
- Relations and bimap pairs constructors will take an extra argument. If only
- two arguments are used, the information will be initialized with their default
- constructor.
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">bimap</span><span class="special"><</span>
- <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>
- <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>
- <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>
- <span class="special">></span> <span class="identifier">bm_type</span><span class="special">;</span>
- <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>
- <span class="identifier">bm_type</span> <span class="identifier">bm</span><span class="special">;</span>
- <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="string">"For C++ old-timers, the first edition of this book is"</span>
- <span class="string">"the one that started it all—the font of our knowledge."</span> <span class="special">)</span>
- <span class="special">);</span>
- <span class="comment">// Print the author of the bible</span>
- <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>
- <span class="comment">// Print the abstract of this book</span>
- <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>
- <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>
- </pre>
- <p>
- </p>
- <p>
- Contrary to the two key types, the information will be mutable using iterators.
- </p>
- <p>
- </p>
- <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>
- </pre>
- <p>
- </p>
- <p>
- 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
- 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
- but returned the associated information instead of the data.
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="comment">// Print the new abstract</span>
- <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>
- </pre>
- <p>
- </p>
- <p>
- The info member can be tagged just as the left or the right member. The following
- is a rewrite of the above example using user defined names:
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">bimap</span><span class="special"><</span>
- <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>
- <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>
- <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>
- <span class="special">></span> <span class="identifier">bm_type</span><span class="special">;</span>
- <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>
- <span class="identifier">bm_type</span> <span class="identifier">bm</span><span class="special">;</span>
- <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="string">"For C++ old-timers, the first edition of this book is"</span>
- <span class="string">"the one that started it all—the font of our knowledge."</span> <span class="special">)</span>
- <span class="special">);</span>
- <span class="comment">// Print the author of the bible</span>
- <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>
- <span class="comment">// Print the abstract of this book</span>
- <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>
- <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>
- <span class="comment">// Contrary to the two key types, the information will be mutable</span>
- <span class="comment">// using iterators.</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> <span class="special">+=</span> <span class="string">"More details about this book"</span><span class="special">;</span>
- <span class="comment">// Print the new abstract</span>
- <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>
- </pre>
- <p>
- </p>
- <p>
- <a href="../../../../example/tutorial_info_hook.cpp" target="_top">Go to source code</a>
- </p>
- </div>
- <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
- <td align="left"></td>
- <td align="right"><div class="copyright-footer">Copyright © 2006-2012 Matias Capeletto<p>
- Distributed under the Boost Software License, Version 1.0. (See accompanying
- 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>)
- </p>
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