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24 | </div> | |
25 | <div class="section boost_bimap_rationale"> | |
26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> | |
27 | <a name="boost_bimap.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a> | |
28 | </h2></div></div></div> | |
29 | <div class="toc"><dl> | |
30 | <dt><span class="section"><a href="rationale.html#boost_bimap.rationale.general_design">General Design</a></span></dt> | |
31 | <dt><span class="section"><a href="rationale/additional_features.html">Additional | |
32 | Features</a></span></dt> | |
33 | <dt><span class="section"><a href="rationale/code.html">Code</a></span></dt> | |
34 | <dt><span class="section"><a href="rationale/the_student_and_the_mentor.html">The | |
35 | student and the mentor</a></span></dt> | |
36 | </dl></div> | |
37 | <p> | |
38 | This section assumes that you have read all other sections, the most of important | |
39 | of which being <span class="emphasis"><em>tutorial</em></span>, <span class="emphasis"><em>std::set theory</em></span> | |
40 | and the <span class="emphasis"><em>reference</em></span>, and that you have tested the library. | |
41 | A lot of effort was invested in making the interface as intuitive and clean | |
42 | as possible. If you understand, and hopefully like, the interface of this library, | |
43 | it will be a lot easier to read this rationale. The following section is little | |
44 | more than a rationale. This library was coded in the context of the Google | |
45 | SoC 2006 and the student and mentor were in different continents. A great deal | |
46 | of email flowed between Joaquin and Matias. The juiciest parts of the conversations | |
47 | where extracted and rearranged here. | |
48 | </p> | |
49 | <div class="note"><table border="0" summary="Note"> | |
50 | <tr> | |
51 | <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td> | |
52 | <th align="left">Note</th> | |
53 | </tr> | |
54 | <tr><td align="left" valign="top"><p> | |
55 | To browse the code, you can use the <a href="../doxydoc/index.html" target="_top"><span class="emphasis"><em>Bimap | |
56 | Complete Reference</em></span></a>, a doxygen-powered document targeted | |
57 | at developers. | |
58 | </p></td></tr> | |
59 | </table></div> | |
60 | <div class="section boost_bimap_rationale_general_design"> | |
61 | <div class="titlepage"><div><div><h3 class="title"> | |
62 | <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> | |
63 | </h3></div></div></div> | |
64 | <p> | |
65 | The initial explanation includes few features. This section aims to describe | |
66 | the general design of the library and excludes details of those features | |
67 | that are of lesser importance; these features will be introduced later. | |
68 | </p> | |
69 | <p> | |
70 | The design of the library is divided into two parts. The first is the construction | |
71 | of a <code class="literal">relation</code> class. This will be the object stored and | |
72 | managed by the <code class="literal">multi_index_container</code> core. The idea is | |
73 | to make this class as easy as possible to use, while making it efficient | |
74 | in terms of memory and access time. This is a cornerstone in the design of | |
75 | <span class="bold"><strong>Boost.Bimap</strong></span> and, as you will see in this | |
76 | rationale, the rest of the design follows easily. | |
77 | </p> | |
78 | <p> | |
79 | The following interface is necessary for the <code class="literal">relation</code> | |
80 | class: | |
81 | </p> | |
82 | <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> | |
83 | ||
84 | <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> | |
85 | ||
86 | <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> | |
87 | <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> | |
88 | <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> | |
89 | ||
90 | <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> | |
91 | <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> | |
92 | ||
93 | <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> | |
94 | <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> | |
95 | ||
96 | <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> | |
97 | <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> | |
98 | <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> | |
99 | ||
100 | <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> | |
101 | <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> | |
102 | <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> | |
103 | ||
104 | <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> | |
105 | <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> | |
106 | ||
107 | <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> | |
108 | <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> | |
109 | </pre> | |
110 | <p> | |
111 | <span class="inlinemediaobject"><img src="../images/bimap/relation.png" alt="relation"></span> | |
112 | </p> | |
113 | <p> | |
114 | Although this seems straightforward, as will be seen later, it is the most | |
115 | difficult code hack of the library. It is indeed very easy if we relax some | |
116 | of the efficiency constraints. For example, it is trivial if we allow a relation | |
117 | to have greater size than the the sum of those of its components. It is equally | |
118 | simple if access speed is not important. One of the first decisions made | |
119 | about <span class="bold"><strong>Boost.Bimap</strong></span> was, however, that, in | |
120 | 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 | |
121 | constraint that can be relaxed: conformance to C++ standards, but this is | |
122 | quite unacceptable. Let us now suppose that we have coded this class, and | |
123 | it conforms to what was required. | |
124 | </p> | |
125 | <p> | |
126 | The second part is based on this <code class="literal">relation</code> class. We can | |
127 | 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>, | |
128 | <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>. | |
129 | Suppose that our bimap supports only one-to-one relations. (Other relation | |
130 | types are considered additional features in this design.) The proposed interface | |
131 | is very simple, and it is based heavily on the concepts of the STL. Given | |
132 | 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>: | |
133 | </p> | |
134 | <div class="orderedlist"><ol class="orderedlist" type="1"> | |
135 | <li class="listitem"> | |
136 | <code class="computeroutput"><span class="identifier">bm</span><span class="special">.</span><span class="identifier">left</span></code> is signature-compatible with a | |
137 | <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> | |
138 | </li> | |
139 | <li class="listitem"> | |
140 | <code class="computeroutput"><span class="identifier">bm</span><span class="special">.</span><span class="identifier">right</span></code> is signature-compatible with | |
141 | 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> | |
142 | </li> | |
143 | <li class="listitem"> | |
144 | <code class="computeroutput"><span class="identifier">bm</span></code> is signature-compatible | |
145 | 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> | |
146 | </li> | |
147 | </ol></div> | |
148 | <p> | |
149 | <span class="inlinemediaobject"><img src="../images/bimap/simple.bimap.png" alt="simple.bimap"></span> | |
150 | </p> | |
151 | <p> | |
152 | This interface is easily learned by users who have a STL background, as well | |
153 | as being simple and powerful. This is the general design. | |
154 | </p> | |
155 | <h5> | |
156 | <a name="boost_bimap.rationale.general_design.h0"></a> | |
157 | <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 | |
158 | Implementation</a> | |
159 | </h5> | |
160 | <p> | |
161 | This section explains the details of the actual <code class="literal">relation</code> | |
162 | class implementation. | |
163 | </p> | |
164 | <p> | |
165 | The first thing that we can imagine is the use of an <code class="literal">union</code>. | |
166 | Regrettably, the current C++ standard only allows unions of POD types. For | |
167 | 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 | |
168 | has two references named first and second that bind to <code class="computeroutput"><span class="identifier">A</span></code> | |
169 | 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>. | |
170 | </p> | |
171 | <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> | |
172 | ||
173 | <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> | |
174 | <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> | |
175 | </pre> | |
176 | <p> | |
177 | It is not difficult to code the relation class using this, but two references | |
178 | 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> | |
179 | 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 | |
180 | . There is another hidden drawback of using this scheme: it is not iterator-friendly, | |
181 | since the map views iterators must be degraded to <span class="emphasis"><em>Read Write</em></span> | |
182 | instead of <span class="emphasis"><em>LValue</em></span>. This will be explained later. | |
183 | </p> | |
184 | <p> | |
185 | At first, this seems to be the best we can do with the current C++ standard. | |
186 | However there is a solution to this problem that does not conform very well | |
187 | to C++ standards but does achieve zero overhead in terms of access time and | |
188 | memory, and additionally allows the view iterators to be upgraded to <span class="emphasis"><em>LValue</em></span> | |
189 | again. | |
190 | </p> | |
191 | <p> | |
192 | In order to use this, the compiler must conform to a layout-compatibility | |
193 | clause that is not currently in the standard but is very natural. The additional | |
194 | clause imposes that if we have two classes: | |
195 | </p> | |
196 | <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">class_a_b</span> | |
197 | <span class="special">{</span> | |
198 | <span class="identifier">Type1</span> <span class="identifier">name_a</span><span class="special">;</span> | |
199 | <span class="identifier">Type2</span> <span class="identifier">name_b</span><span class="special">;</span> | |
200 | <span class="special">};</span> | |
201 | ||
202 | <span class="keyword">struct</span> <span class="identifier">class_b_a</span> | |
203 | <span class="special">{</span> | |
204 | <span class="identifier">Type1</span> <span class="identifier">name_b</span><span class="special">;</span> | |
205 | <span class="identifier">Type2</span> <span class="identifier">name_a</span><span class="special">;</span> | |
206 | <span class="special">};</span> | |
207 | </pre> | |
208 | <p> | |
209 | then the storage layout of <code class="literal">class_a_b</code> is equal to the storage | |
210 | layout of <code class="literal">class_b_a</code>. If you are surprised to learn that | |
211 | this does not hold in a standards-compliant C++ compiler, welcome to the | |
212 | club. It is the natural way to implement it from the point of view of the | |
213 | compiler's vendor and is very useful for the developer. Maybe it will be | |
214 | included in the standard some day. Every current compiler conforms to this. | |
215 | </p> | |
216 | <p> | |
217 | If we are able to count on this, then we can implement an idiom called <code class="literal">mutant</code>. | |
218 | The idea is to provide a secure wrapper around <code class="literal">reinterpret_cast</code>. | |
219 | A class can declare that it can be viewed using different view classes that | |
220 | are storage-compatible with it. Then we use the free function <code class="literal">mutate<view>(mutant)</code> | |
221 | to get the view. The <code class="computeroutput"><span class="identifier">mutate</span></code> | |
222 | function checks at compile time that the requested view is declared in the | |
223 | mutant views list. We implement a class name <code class="computeroutput"><span class="identifier">structured_pair</span></code> | |
224 | that is signature-compatible with a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>, | |
225 | while the storage layout is configured with a third template parameter. Two | |
226 | instances of this template class will provide the views of the relation. | |
227 | </p> | |
228 | <p> | |
229 | The thing is that if we want to be standards-compliant, we cannot use this | |
230 | approach. It is very annoying not to be able to use something that we know | |
231 | will work with every compiler and that is far better than alternatives. So | |
232 | -- and this is an important decision -- we have to find a way to use it and | |
233 | still make the library standards-compliant. | |
234 | </p> | |
235 | <p> | |
236 | The idea is very simple. We code both approaches: the const_reference_pair-based | |
237 | and the mutant-based, and use the mutant approach if the compiler is compliant | |
238 | with our new layout-compatible clause. If the compiler really messes things | |
239 | up, we degrade the performance of the bimap a little. The only drawback here | |
240 | is that, while the mutant approach allows to make <span class="emphasis"><em>LValue</em></span> | |
241 | iterators, we have to degrade them to <span class="emphasis"><em>Read Write</em></span> in | |
242 | both cases, because we require that the same code be compilable by any standards-compliant | |
243 | compiler. | |
244 | </p> | |
245 | <div class="note"><table border="0" summary="Note"> | |
246 | <tr> | |
247 | <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td> | |
248 | <th align="left">Note</th> | |
249 | </tr> | |
250 | <tr><td align="left" valign="top"><p> | |
251 | Testing this approach in all the supported compilers indicated that the | |
252 | mutant idiom was always supported. The strictly compliant version was removed | |
253 | from the code because it was never used. | |
254 | </p></td></tr> | |
255 | </table></div> | |
256 | <h5> | |
257 | <a name="boost_bimap.rationale.general_design.h1"></a> | |
258 | <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 | |
259 | Implementation</a> | |
260 | </h5> | |
261 | <p> | |
262 | The core of bimap will be obviously a <code class="computeroutput"><span class="identifier">multi_index_container</span></code>. | |
263 | The basic idea to tackle the implementation of the bimap class is to use | |
264 | <code class="literal">iterator_adaptor</code> to convert the iterators from Boost.MultiIndex | |
265 | 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. | |
266 | 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 | |
267 | this new transformed iterators and a wrapper around each index of the core | |
268 | container. However, there is a hidden idiom here, that, once coded, will | |
269 | be very useful for other parts of this library and for Boost.MRU library. | |
270 | Following the ideas from <code class="computeroutput"><span class="identifier">iterator_adaptor</span></code>, | |
271 | Boost.Bimap views are implemented using a <code class="literal">container_adaptor</code>. | |
272 | There are several template classes (for example <code class="computeroutput"><span class="identifier">map_adaptor</span></code> | |
273 | and <code class="computeroutput"><span class="identifier">set_adaptor</span></code>) that take | |
274 | a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code> signature-conformant class and new | |
275 | iterators, and adapt the container so it now uses this iterators instead | |
276 | 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>, | |
277 | 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 | |
278 | <code class="computeroutput"><span class="identifier">set_adaptor</span></code> and <code class="literal">iterator_adaptor</code>. | |
279 | The combined use of this two tools is very powerful. A <code class="literal">container_adaptor</code> | |
280 | can take classes that do not fulfil all the requirements of the adapted container. | |
281 | The new container must define these missing functions. | |
282 | </p> | |
283 | </div> | |
284 | </div> | |
285 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
286 | <td align="left"></td> | |
287 | <td align="right"><div class="copyright-footer">Copyright © 2006-2012 Matias Capeletto<p> | |
288 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
289 | 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>) | |
290 | </p> | |
291 | </div></td> | |
292 | </tr></table> | |
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