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- <div class="section">
- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <a name="boost_bimap.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
- </h2></div></div></div>
- <div class="toc"><dl class="toc">
- <dt><span class="section"><a href="rationale.html#boost_bimap.rationale.general_design">General Design</a></span></dt>
- <dt><span class="section"><a href="rationale/additional_features.html">Additional
- Features</a></span></dt>
- <dt><span class="section"><a href="rationale/code.html">Code</a></span></dt>
- <dt><span class="section"><a href="rationale/the_student_and_the_mentor.html">The
- student and the mentor</a></span></dt>
- </dl></div>
- <p>
- This section assumes that you have read all other sections, the most of important
- of which being <span class="emphasis"><em>tutorial</em></span>, <span class="emphasis"><em>std::set theory</em></span>
- and the <span class="emphasis"><em>reference</em></span>, and that you have tested the library.
- A lot of effort was invested in making the interface as intuitive and clean
- as possible. If you understand, and hopefully like, the interface of this library,
- it will be a lot easier to read this rationale. The following section is little
- more than a rationale. This library was coded in the context of the Google
- SoC 2006 and the student and mentor were in different continents. A great deal
- of email flowed between Joaquin and Matias. The juiciest parts of the conversations
- where extracted and rearranged here.
- </p>
- <div class="note"><table border="0" summary="Note">
- <tr>
- <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
- <th align="left">Note</th>
- </tr>
- <tr><td align="left" valign="top"><p>
- To browse the code, you can use the <a href="../doxydoc/index.html" target="_top"><span class="emphasis"><em>Bimap
- Complete Reference</em></span></a>, a doxygen-powered document targeted
- at developers.
- </p></td></tr>
- </table></div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_bimap.rationale.general_design"></a><a class="link" href="rationale.html#boost_bimap.rationale.general_design" title="General Design">General Design</a>
- </h3></div></div></div>
- <p>
- The initial explanation includes few features. This section aims to describe
- the general design of the library and excludes details of those features
- that are of lesser importance; these features will be introduced later.
- </p>
- <p>
- The design of the library is divided into two parts. The first is the construction
- of a <code class="literal">relation</code> class. This will be the object stored and
- managed by the <code class="literal">multi_index_container</code> core. The idea is
- to make this class as easy as possible to use, while making it efficient
- in terms of memory and access time. This is a cornerstone in the design of
- <span class="bold"><strong>Boost.Bimap</strong></span> and, as you will see in this
- rationale, the rest of the design follows easily.
- </p>
- <p>
- The following interface is necessary for the <code class="literal">relation</code>
- class:
- </p>
- <pre class="programlisting"><span class="keyword">typedef</span> <span class="special">-</span><span class="identifier">unspecified</span><span class="special">-</span> <span class="identifier">TA</span><span class="special">;</span> <span class="keyword">typedef</span> <span class="special">-</span><span class="identifier">unspecified</span><span class="special">-</span> <span class="identifier">TB</span><span class="special">;</span>
- <span class="identifier">TA</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">ai</span><span class="special">;</span> <span class="identifier">TB</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">bi</span><span class="special">;</span>
- <span class="keyword">typedef</span> <span class="identifier">relation</span><span class="special"><</span> <span class="identifier">TA</span><span class="special">,</span> <span class="identifier">TB</span> <span class="special">></span> <span class="identifier">rel</span><span class="special">;</span>
- <span class="identifier">STATIC_ASSERT</span><span class="special">(</span> <span class="identifier">is_same</span><span class="special"><</span> <span class="identifier">rel</span><span class="special">::</span><span class="identifier">left_type</span> <span class="special">,</span> <span class="identifier">TA</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">);</span>
- <span class="identifier">STATIC_ASSERT</span><span class="special">(</span> <span class="identifier">is_same</span><span class="special"><</span> <span class="identifier">rel</span><span class="special">::</span><span class="identifier">right_type</span><span class="special">,</span> <span class="identifier">TB</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">);</span>
- <span class="identifier">rel</span> <span class="identifier">r</span><span class="special">(</span><span class="identifier">ai</span><span class="special">,</span><span class="identifier">bi</span><span class="special">);</span>
- <span class="identifier">assert</span><span class="special">(</span> <span class="identifier">r</span><span class="special">.</span><span class="identifier">left</span> <span class="special">==</span> <span class="identifier">ai</span> <span class="special">&&</span> <span class="identifier">r</span><span class="special">.</span><span class="identifier">right</span> <span class="special">==</span> <span class="identifier">bi</span> <span class="special">);</span>
- <span class="identifier">r</span><span class="special">.</span><span class="identifier">left</span> <span class="special">=</span> <span class="identifier">a</span><span class="special">;</span> <span class="identifier">r</span><span class="special">.</span><span class="identifier">right</span> <span class="special">=</span> <span class="identifier">b</span><span class="special">;</span>
- <span class="identifier">assert</span><span class="special">(</span> <span class="identifier">r</span><span class="special">.</span><span class="identifier">left</span> <span class="special">==</span> <span class="identifier">a</span> <span class="special">&&</span> <span class="identifier">r</span><span class="special">.</span><span class="identifier">right</span> <span class="special">==</span> <span class="identifier">b</span> <span class="special">);</span>
- <span class="keyword">typedef</span> <span class="identifier">pair_type_by</span><span class="special"><</span> <span class="identifier">member_at</span><span class="special">::</span><span class="identifier">left</span> <span class="special">,</span> <span class="identifier">rel</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">pba_type</span><span class="special">;</span>
- <span class="identifier">STATIC_ASSERT</span><span class="special">(</span> <span class="identifier">is_same</span><span class="special"><</span> <span class="identifier">pba_type</span><span class="special">::</span><span class="identifier">first_type</span> <span class="special">,</span> <span class="identifier">TA</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">);</span>
- <span class="identifier">STATIC_ASSERT</span><span class="special">(</span> <span class="identifier">is_same</span><span class="special"><</span> <span class="identifier">pba_type</span><span class="special">::</span><span class="identifier">second_type</span><span class="special">,</span> <span class="identifier">TB</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">);</span>
- <span class="keyword">typedef</span> <span class="identifier">pair_type_by</span><span class="special"><</span> <span class="identifier">member_at</span><span class="special">::</span><span class="identifier">right</span><span class="special">,</span> <span class="identifier">rel</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">pbb_type</span><span class="special">;</span>
- <span class="identifier">STATIC_ASSERT</span><span class="special">(</span> <span class="identifier">is_same</span><span class="special"><</span> <span class="identifier">pbb_type</span><span class="special">::</span><span class="identifier">first_type</span> <span class="special">,</span> <span class="identifier">TB</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">);</span>
- <span class="identifier">STATIC_ASSERT</span><span class="special">(</span> <span class="identifier">is_same</span><span class="special"><</span> <span class="identifier">pbb_type</span><span class="special">::</span><span class="identifier">second_type</span><span class="special">,</span> <span class="identifier">TA</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">);</span>
- <span class="identifier">pba_type</span> <span class="identifier">pba</span> <span class="special">=</span> <span class="identifier">pair_by</span><span class="special"><</span> <span class="identifier">member_at</span><span class="special">::</span><span class="identifier">left</span> <span class="special">>(</span><span class="identifier">r</span><span class="special">);</span>
- <span class="identifier">assert</span><span class="special">(</span> <span class="identifier">pba</span><span class="special">.</span><span class="identifier">first</span> <span class="special">==</span> <span class="identifier">r</span><span class="special">.</span><span class="identifier">left</span> <span class="special">&&</span> <span class="identifier">pba</span><span class="special">.</span><span class="identifier">second</span> <span class="special">==</span> <span class="identifier">r</span><span class="special">.</span><span class="identifier">right</span> <span class="special">);</span>
- <span class="identifier">pbb_type</span> <span class="identifier">pbb</span> <span class="special">=</span> <span class="identifier">pair_by</span><span class="special"><</span> <span class="identifier">member_at</span><span class="special">::</span><span class="identifier">right</span> <span class="special">>(</span><span class="identifier">r</span><span class="special">);</span>
- <span class="identifier">assert</span><span class="special">(</span> <span class="identifier">pbb</span><span class="special">.</span><span class="identifier">first</span> <span class="special">==</span> <span class="identifier">r</span><span class="special">.</span><span class="identifier">right</span> <span class="special">&&</span> <span class="identifier">pbb</span><span class="special">.</span><span class="identifier">second</span> <span class="special">==</span> <span class="identifier">r</span><span class="special">.</span><span class="identifier">left</span> <span class="special">);</span>
- </pre>
- <p>
- <span class="inlinemediaobject"><img src="../images/bimap/relation.png" alt="relation"></span>
- </p>
- <p>
- Although this seems straightforward, as will be seen later, it is the most
- difficult code hack of the library. It is indeed very easy if we relax some
- of the efficiency constraints. For example, it is trivial if we allow a relation
- to have greater size than the the sum of those of its components. It is equally
- simple if access speed is not important. One of the first decisions made
- about <span class="bold"><strong>Boost.Bimap</strong></span> was, however, that, in
- order to be useful, it had to achieve zero overhead over the wrapped <span class="bold"><strong>Boost.MultiIndex</strong></span> container. Finally, there is another
- constraint that can be relaxed: conformance to C++ standards, but this is
- quite unacceptable. Let us now suppose that we have coded this class, and
- it conforms to what was required.
- </p>
- <p>
- The second part is based on this <code class="literal">relation</code> class. We can
- now view the data in any of three ways: <code class="computeroutput"><span class="identifier">pair</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">></span></code>,
- <code class="computeroutput"><span class="identifier">relation</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">></span></code> and <code class="computeroutput"><span class="identifier">pair</span><span class="special"><</span><span class="identifier">B</span><span class="special">,</span><span class="identifier">A</span><span class="special">></span></code>.
- Suppose that our bimap supports only one-to-one relations. (Other relation
- types are considered additional features in this design.) The proposed interface
- is very simple, and it is based heavily on the concepts of the STL. Given
- a <code class="computeroutput"><span class="identifier">bimap</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">></span> <span class="identifier">bm</span></code>:
- </p>
- <div class="orderedlist"><ol class="orderedlist" type="1">
- <li class="listitem">
- <code class="computeroutput"><span class="identifier">bm</span><span class="special">.</span><span class="identifier">left</span></code> is signature-compatible with a
- <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">></span></code>
- </li>
- <li class="listitem">
- <code class="computeroutput"><span class="identifier">bm</span><span class="special">.</span><span class="identifier">right</span></code> is signature-compatible with
- a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">B</span><span class="special">,</span><span class="identifier">A</span><span class="special">></span></code>
- </li>
- <li class="listitem">
- <code class="computeroutput"><span class="identifier">bm</span></code> is signature-compatible
- with a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">set</span><span class="special"><</span><span class="identifier">relation</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">></span> <span class="special">></span></code>
- </li>
- </ol></div>
- <p>
- <span class="inlinemediaobject"><img src="../images/bimap/simple.bimap.png" alt="simple.bimap"></span>
- </p>
- <p>
- This interface is easily learned by users who have a STL background, as well
- as being simple and powerful. This is the general design.
- </p>
- <h5>
- <a name="boost_bimap.rationale.general_design.h0"></a>
- <span class="phrase"><a name="boost_bimap.rationale.general_design.relation_implementation"></a></span><a class="link" href="rationale.html#boost_bimap.rationale.general_design.relation_implementation">Relation
- Implementation</a>
- </h5>
- <p>
- This section explains the details of the actual <code class="literal">relation</code>
- class implementation.
- </p>
- <p>
- The first thing that we can imagine is the use of an <code class="literal">union</code>.
- Regrettably, the current C++ standard only allows unions of POD types. For
- the views, we can try a wrapper around a <code class="computeroutput"><span class="identifier">relation</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">></span></code> that
- has two references named first and second that bind to <code class="computeroutput"><span class="identifier">A</span></code>
- and <code class="computeroutput"><span class="identifier">B</span></code>, or to <code class="computeroutput"><span class="identifier">B</span></code> and <code class="computeroutput"><span class="identifier">A</span></code>.
- </p>
- <pre class="programlisting"><span class="identifier">relation</span><span class="special"><</span><span class="identifier">TA</span><span class="special">,</span><span class="identifier">TB</span><span class="special">></span> <span class="identifier">r</span><span class="special">;</span>
- <span class="identifier">const_reference_pair</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">></span> <span class="identifier">pba</span><span class="special">(</span><span class="identifier">r</span><span class="special">);</span>
- <span class="identifier">const_reference_pair</span><span class="special"><</span><span class="identifier">B</span><span class="special">,</span><span class="identifier">A</span><span class="special">></span> <span class="identifier">pbb</span><span class="special">(</span><span class="identifier">r</span><span class="special">);</span>
- </pre>
- <p>
- It is not difficult to code the relation class using this, but two references
- are initialized at every access and using of <code class="computeroutput"><span class="identifier">pba</span><span class="special">.</span><span class="identifier">first</span></code>
- will be slower in most compilers than using <code class="computeroutput"><span class="identifier">r</span><span class="special">.</span><span class="identifier">left</span></code> directly
- . There is another hidden drawback of using this scheme: it is not iterator-friendly,
- since the map views iterators must be degraded to <span class="emphasis"><em>Read Write</em></span>
- instead of <span class="emphasis"><em>LValue</em></span>. This will be explained later.
- </p>
- <p>
- At first, this seems to be the best we can do with the current C++ standard.
- However there is a solution to this problem that does not conform very well
- to C++ standards but does achieve zero overhead in terms of access time and
- memory, and additionally allows the view iterators to be upgraded to <span class="emphasis"><em>LValue</em></span>
- again.
- </p>
- <p>
- In order to use this, the compiler must conform to a layout-compatibility
- clause that is not currently in the standard but is very natural. The additional
- clause imposes that if we have two classes:
- </p>
- <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">class_a_b</span>
- <span class="special">{</span>
- <span class="identifier">Type1</span> <span class="identifier">name_a</span><span class="special">;</span>
- <span class="identifier">Type2</span> <span class="identifier">name_b</span><span class="special">;</span>
- <span class="special">};</span>
- <span class="keyword">struct</span> <span class="identifier">class_b_a</span>
- <span class="special">{</span>
- <span class="identifier">Type1</span> <span class="identifier">name_b</span><span class="special">;</span>
- <span class="identifier">Type2</span> <span class="identifier">name_a</span><span class="special">;</span>
- <span class="special">};</span>
- </pre>
- <p>
- then the storage layout of <code class="literal">class_a_b</code> is equal to the storage
- layout of <code class="literal">class_b_a</code>. If you are surprised to learn that
- this does not hold in a standards-compliant C++ compiler, welcome to the
- club. It is the natural way to implement it from the point of view of the
- compiler's vendor and is very useful for the developer. Maybe it will be
- included in the standard some day. Every current compiler conforms to this.
- </p>
- <p>
- If we are able to count on this, then we can implement an idiom called <code class="literal">mutant</code>.
- The idea is to provide a secure wrapper around <code class="literal">reinterpret_cast</code>.
- A class can declare that it can be viewed using different view classes that
- are storage-compatible with it. Then we use the free function <code class="literal">mutate<view>(mutant)</code>
- to get the view. The <code class="computeroutput"><span class="identifier">mutate</span></code>
- function checks at compile time that the requested view is declared in the
- mutant views list. We implement a class name <code class="computeroutput"><span class="identifier">structured_pair</span></code>
- that is signature-compatible with a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>,
- while the storage layout is configured with a third template parameter. Two
- instances of this template class will provide the views of the relation.
- </p>
- <p>
- The thing is that if we want to be standards-compliant, we cannot use this
- approach. It is very annoying not to be able to use something that we know
- will work with every compiler and that is far better than alternatives. So
- -- and this is an important decision -- we have to find a way to use it and
- still make the library standards-compliant.
- </p>
- <p>
- The idea is very simple. We code both approaches: the const_reference_pair-based
- and the mutant-based, and use the mutant approach if the compiler is compliant
- with our new layout-compatible clause. If the compiler really messes things
- up, we degrade the performance of the bimap a little. The only drawback here
- is that, while the mutant approach allows to make <span class="emphasis"><em>LValue</em></span>
- iterators, we have to degrade them to <span class="emphasis"><em>Read Write</em></span> in
- both cases, because we require that the same code be compilable by any standards-compliant
- compiler.
- </p>
- <div class="note"><table border="0" summary="Note">
- <tr>
- <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
- <th align="left">Note</th>
- </tr>
- <tr><td align="left" valign="top"><p>
- Testing this approach in all the supported compilers indicated that the
- mutant idiom was always supported. The strictly compliant version was removed
- from the code because it was never used.
- </p></td></tr>
- </table></div>
- <h5>
- <a name="boost_bimap.rationale.general_design.h1"></a>
- <span class="phrase"><a name="boost_bimap.rationale.general_design.bimap_implementation"></a></span><a class="link" href="rationale.html#boost_bimap.rationale.general_design.bimap_implementation">Bimap
- Implementation</a>
- </h5>
- <p>
- The core of bimap will be obviously a <code class="computeroutput"><span class="identifier">multi_index_container</span></code>.
- The basic idea to tackle the implementation of the bimap class is to use
- <code class="literal">iterator_adaptor</code> to convert the iterators from Boost.MultiIndex
- to the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code> and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">set</span></code> behaviour.
- The <code class="computeroutput"><span class="identifier">map_view</span></code> and the <code class="computeroutput"><span class="identifier">set_view</span></code> can be implemented directly using
- this new transformed iterators and a wrapper around each index of the core
- container. However, there is a hidden idiom here, that, once coded, will
- be very useful for other parts of this library and for Boost.MRU library.
- Following the ideas from <code class="computeroutput"><span class="identifier">iterator_adaptor</span></code>,
- Boost.Bimap views are implemented using a <code class="literal">container_adaptor</code>.
- There are several template classes (for example <code class="computeroutput"><span class="identifier">map_adaptor</span></code>
- and <code class="computeroutput"><span class="identifier">set_adaptor</span></code>) that take
- a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code> signature-conformant class and new
- iterators, and adapt the container so it now uses this iterators instead
- of the originals. For example, if you have a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">set</span><span class="special"><</span><span class="keyword">int</span><span class="special">*></span></code>,
- you can build other container that behaves exactly as a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">set</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> using
- <code class="computeroutput"><span class="identifier">set_adaptor</span></code> and <code class="literal">iterator_adaptor</code>.
- The combined use of this two tools is very powerful. A <code class="literal">container_adaptor</code>
- can take classes that do not fulfil all the requirements of the adapted container.
- The new container must define these missing functions.
- </p>
- </div>
- </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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