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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> |
| <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> |
| <a name="boost_bimap.rationale.general_design.relation_implementation"></a><h5> |
| <a name="id874221"></a> |
| <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> |
| <a name="boost_bimap.rationale.general_design.bimap_implementation"></a><h5> |
| <a name="id874795"></a> |
| <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> |
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| <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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