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| <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> |
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| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2006 -2007 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>) |
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