ceph/src/boost/libs/iterator/doc/indirect_iterator.html

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   7 <title>Indirect Iterator</title>
   8 <meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" />
   9 <meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction" />
  10 <meta name="date" content="2006-09-11" />
  11 <meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003." />
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  15 <div class="document" id="indirect-iterator">
  16 <h1 class="title">Indirect Iterator</h1>
  17 <table class="docinfo" frame="void" rules="none">
  18 <col class="docinfo-name" />
  19 <col class="docinfo-content" />
  20 <tbody valign="top">
  21 <tr><th class="docinfo-name">Author:</th>
  22 <td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr>
  23 <tr><th class="docinfo-name">Contact:</th>
  24 <td><a class="first reference external" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a>, <a class="reference external" href="mailto:jsiek&#64;osl.iu.edu">jsiek&#64;osl.iu.edu</a>, <a class="last reference external" href="mailto:witt&#64;ive.uni-hannover.de">witt&#64;ive.uni-hannover.de</a></td></tr>
  25 <tr><th class="docinfo-name">Organization:</th>
  26 <td><a class="first reference external" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University <a class="reference external" href="http://www.osl.iu.edu">Open Systems
  27 Lab</a>, University of Hanover <a class="last reference external" href="http://www.ive.uni-hannover.de">Institute for Transport
  28 Railway Operation and Construction</a></td></tr>
  29 <tr><th class="docinfo-name">Date:</th>
  30 <td>2006-09-11</td></tr>
  31 <tr><th class="docinfo-name">Copyright:</th>
  32 <td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td></tr>
  33 </tbody>
  34 </table>
  35 <!-- Distributed under the Boost -->
  36 <!-- Software License, Version 1.0. (See accompanying -->
  37 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
  38 <table class="docutils field-list" frame="void" rules="none">
  39 <col class="field-name" />
  40 <col class="field-body" />
  41 <tbody valign="top">
  42 <tr class="field"><th class="field-name">abstract:</th><td class="field-body"><!-- Copyright David Abrahams 2006. Distributed under the Boost -->
  43 <!-- Software License, Version 1.0. (See accompanying -->
  44 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
  45 <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> adapts an iterator by applying an
  46 <em>extra</em> dereference inside of <tt class="docutils literal"><span class="pre">operator*()</span></tt>. For example, this
  47 iterator adaptor makes it possible to view a container of pointers
  48 (e.g. <tt class="docutils literal"><span class="pre">list&lt;foo*&gt;</span></tt>) as if it were a container of the pointed-to type
  49 (e.g. <tt class="docutils literal"><span class="pre">list&lt;foo&gt;</span></tt>).  <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> depends on two
  50 auxiliary traits, <tt class="docutils literal"><span class="pre">pointee</span></tt> and <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>, to
  51 provide support for underlying iterators whose <tt class="docutils literal"><span class="pre">value_type</span></tt> is
  52 not an iterator.</td>
  53 </tr>
  54 </tbody>
  55 </table>
  56 <div class="contents topic" id="table-of-contents">
  57 <p class="topic-title first">Table of Contents</p>
  58 <ul class="simple">
  59 <li><a class="reference internal" href="#indirect-iterator-synopsis" id="id2"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> synopsis</a></li>
  60 <li><a class="reference internal" href="#indirect-iterator-requirements" id="id3"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> requirements</a></li>
  61 <li><a class="reference internal" href="#indirect-iterator-models" id="id4"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models</a></li>
  62 <li><a class="reference internal" href="#indirect-iterator-operations" id="id5"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> operations</a></li>
  63 <li><a class="reference internal" href="#example" id="id6">Example</a></li>
  64 </ul>
  65 </div>
  66 <div class="section" id="indirect-iterator-synopsis">
  67 <h1><a class="toc-backref" href="#id2"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> synopsis</a></h1>
  68 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
  69 <!-- Software License, Version 1.0. (See accompanying -->
  70 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
  71 <pre class="literal-block">
  72 template &lt;
  73     class Iterator
  74   , class Value = use_default
  75   , class CategoryOrTraversal = use_default
  76   , class Reference = use_default
  77   , class Difference = use_default
  78 &gt;
  79 class indirect_iterator
  80 {
  81  public:
  82     typedef /* see below */ value_type;
  83     typedef /* see below */ reference;
  84     typedef /* see below */ pointer;
  85     typedef /* see below */ difference_type;
  86     typedef /* see below */ iterator_category;
  87
  88     indirect_iterator();
  89     indirect_iterator(Iterator x);
  90
  91     template &lt;
  92         class Iterator2, class Value2, class Category2
  93       , class Reference2, class Difference2
  94     &gt;
  95     indirect_iterator(
  96         indirect_iterator&lt;
  97              Iterator2, Value2, Category2, Reference2, Difference2
  98         &gt; const&amp; y
  99       , typename enable_if_convertible&lt;Iterator2, Iterator&gt;::type* = 0 // exposition
 100     );
 101
 102     Iterator const&amp; base() const;
 103     reference operator*() const;
 104     indirect_iterator&amp; operator++();
 105     indirect_iterator&amp; operator--();
 106 private:
 107    Iterator m_iterator; // exposition
 108 };
 109 </pre>
 110 <p>The member types of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> are defined according to
 111 the following pseudo-code, where <tt class="docutils literal"><span class="pre">V</span></tt> is
 112 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt></p>
 113 <pre class="literal-block">
 114 if (Value is use_default) then
 115     typedef remove_const&lt;pointee&lt;V&gt;::type&gt;::type value_type;
 116 else
 117     typedef remove_const&lt;Value&gt;::type value_type;
 118
 119 if (Reference is use_default) then
 120     if (Value is use_default) then
 121         typedef indirect_reference&lt;V&gt;::type reference;
 122     else
 123         typedef Value&amp; reference;
 124 else
 125     typedef Reference reference;
 126
 127 if (Value is use_default) then
 128     typedef pointee&lt;V&gt;::type* pointer;
 129 else
 130     typedef Value* pointer;
 131
 132 if (Difference is use_default)
 133     typedef iterator_traits&lt;Iterator&gt;::difference_type difference_type;
 134 else
 135     typedef Difference difference_type;
 136
 137 if (CategoryOrTraversal is use_default)
 138     typedef <em>iterator-category</em> (
 139         iterator_traversal&lt;Iterator&gt;::type,``reference``,``value_type``
 140     ) iterator_category;
 141 else
 142     typedef <em>iterator-category</em> (
 143         CategoryOrTraversal,``reference``,``value_type``
 144     ) iterator_category;
 145 </pre>
 146 </div>
 147 <div class="section" id="indirect-iterator-requirements">
 148 <h1><a class="toc-backref" href="#id3"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> requirements</a></h1>
 149 <p>The expression <tt class="docutils literal"><span class="pre">*v</span></tt>, where <tt class="docutils literal"><span class="pre">v</span></tt> is an object of
 150 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt>, shall be valid
 151 expression and convertible to <tt class="docutils literal"><span class="pre">reference</span></tt>.  <tt class="docutils literal"><span class="pre">Iterator</span></tt> shall
 152 model the traversal concept indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>.
 153 <tt class="docutils literal"><span class="pre">Value</span></tt>, <tt class="docutils literal"><span class="pre">Reference</span></tt>, and <tt class="docutils literal"><span class="pre">Difference</span></tt> shall be chosen so
 154 that <tt class="docutils literal"><span class="pre">value_type</span></tt>, <tt class="docutils literal"><span class="pre">reference</span></tt>, and <tt class="docutils literal"><span class="pre">difference_type</span></tt> meet
 155 the requirements indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>.</p>
 156 <p>[Note: there are further requirements on the
 157 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt> if the <tt class="docutils literal"><span class="pre">Value</span></tt>
 158 parameter is not <tt class="docutils literal"><span class="pre">use_default</span></tt>, as implied by the algorithm for
 159 deducing the default for the <tt class="docutils literal"><span class="pre">value_type</span></tt> member.]</p>
 160 </div>
 161 <div class="section" id="indirect-iterator-models">
 162 <h1><a class="toc-backref" href="#id4"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models</a></h1>
 163 <p>In addition to the concepts indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>
 164 and by <tt class="docutils literal"><span class="pre">iterator_traversal&lt;indirect_iterator&gt;::type</span></tt>, a
 165 specialization of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models the following
 166 concepts, Where <tt class="docutils literal"><span class="pre">v</span></tt> is an object of
 167 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt>:</p>
 168 <blockquote>
 169 <ul class="simple">
 170 <li>Readable Iterator if <tt class="docutils literal"><span class="pre">reference(*v)</span></tt> is convertible to
 171 <tt class="docutils literal"><span class="pre">value_type</span></tt>.</li>
 172 <li>Writable Iterator if <tt class="docutils literal"><span class="pre">reference(*v)</span> <span class="pre">=</span> <span class="pre">t</span></tt> is a valid
 173 expression (where <tt class="docutils literal"><span class="pre">t</span></tt> is an object of type
 174 <tt class="docutils literal"><span class="pre">indirect_iterator::value_type</span></tt>)</li>
 175 <li>Lvalue Iterator if <tt class="docutils literal"><span class="pre">reference</span></tt> is a reference type.</li>
 176 </ul>
 177 </blockquote>
 178 <p><tt class="docutils literal"><span class="pre">indirect_iterator&lt;X,V1,C1,R1,D1&gt;</span></tt> is interoperable with
 179 <tt class="docutils literal"><span class="pre">indirect_iterator&lt;Y,V2,C2,R2,D2&gt;</span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is
 180 interoperable with <tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
 181 </div>
 182 <div class="section" id="indirect-iterator-operations">
 183 <h1><a class="toc-backref" href="#id5"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> operations</a></h1>
 184 <p>In addition to the operations required by the concepts described
 185 above, specializations of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> provide the
 186 following operations.</p>
 187 <p><tt class="docutils literal"><span class="pre">indirect_iterator();</span></tt></p>
 188 <table class="docutils field-list" frame="void" rules="none">
 189 <col class="field-name" />
 190 <col class="field-body" />
 191 <tbody valign="top">
 192 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Iterator</span></tt> must be Default Constructible.</td>
 193 </tr>
 194 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> with
 195 a default-constructed <tt class="docutils literal"><span class="pre">m_iterator</span></tt>.</td>
 196 </tr>
 197 </tbody>
 198 </table>
 199 <p><tt class="docutils literal"><span class="pre">indirect_iterator(Iterator</span> <span class="pre">x);</span></tt></p>
 200 <table class="docutils field-list" frame="void" rules="none">
 201 <col class="field-name" />
 202 <col class="field-body" />
 203 <tbody valign="top">
 204 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> with
 205 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
 206 </tr>
 207 </tbody>
 208 </table>
 209 <pre class="literal-block">
 210 template &lt;
 211     class Iterator2, class Value2, unsigned Access, class Traversal
 212   , class Reference2, class Difference2
 213 &gt;
 214 indirect_iterator(
 215     indirect_iterator&lt;
 216          Iterator2, Value2, Access, Traversal, Reference2, Difference2
 217     &gt; const&amp; y
 218   , typename enable_if_convertible&lt;Iterator2, Iterator&gt;::type* = 0 // exposition
 219 );
 220 </pre>
 221 <table class="docutils field-list" frame="void" rules="none">
 222 <col class="field-name" />
 223 <col class="field-body" />
 224 <tbody valign="top">
 225 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Iterator2</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td>
 226 </tr>
 227 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> whose
 228 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> subobject is constructed from <tt class="docutils literal"><span class="pre">y.base()</span></tt>.</td>
 229 </tr>
 230 </tbody>
 231 </table>
 232 <p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
 233 <table class="docutils field-list" frame="void" rules="none">
 234 <col class="field-name" />
 235 <col class="field-body" />
 236 <tbody valign="top">
 237 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
 238 </tr>
 239 </tbody>
 240 </table>
 241 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
 242 <table class="docutils field-list" frame="void" rules="none">
 243 <col class="field-name" />
 244 <col class="field-body" />
 245 <tbody valign="top">
 246 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">**m_iterator</span></tt></td>
 247 </tr>
 248 </tbody>
 249 </table>
 250 <p><tt class="docutils literal"><span class="pre">indirect_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
 251 <table class="docutils field-list" frame="void" rules="none">
 252 <col class="field-name" />
 253 <col class="field-body" />
 254 <tbody valign="top">
 255 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_iterator</span></tt></td>
 256 </tr>
 257 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 258 </tr>
 259 </tbody>
 260 </table>
 261 <p><tt class="docutils literal"><span class="pre">indirect_iterator&amp;</span> <span class="pre">operator--();</span></tt></p>
 262 <table class="docutils field-list" frame="void" rules="none">
 263 <col class="field-name" />
 264 <col class="field-body" />
 265 <tbody valign="top">
 266 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_iterator</span></tt></td>
 267 </tr>
 268 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 269 </tr>
 270 </tbody>
 271 </table>
 272 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 273 <!-- Software License, Version 1.0. (See accompanying -->
 274 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 275 </div>
 276 <div class="section" id="example">
 277 <h1><a class="toc-backref" href="#id6">Example</a></h1>
 278 <p>This example prints an array of characters, using
 279 <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> to access the array of characters through an
 280 array of pointers. Next <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> is used with the
 281 <tt class="docutils literal"><span class="pre">transform</span></tt> algorithm to copy the characters (incremented by one) to
 282 another array. A constant indirect iterator is used for the source and
 283 a mutable indirect iterator is used for the destination. The last part
 284 of the example prints the original array of characters, but this time
 285 using the <tt class="docutils literal"><span class="pre">make_indirect_iterator</span></tt> helper function.</p>
 286 <pre class="literal-block">
 287 char characters[] = &quot;abcdefg&quot;;
 288 const int N = sizeof(characters)/sizeof(char) - 1; // -1 since characters has a null char
 289 char* pointers_to_chars[N];                        // at the end.
 290 for (int i = 0; i &lt; N; ++i)
 291   pointers_to_chars[i] = &amp;characters[i];
 292
 293 // Example of using indirect_iterator
 294
 295 boost::indirect_iterator&lt;char**, char&gt;
 296   indirect_first(pointers_to_chars), indirect_last(pointers_to_chars + N);
 297
 298 std::copy(indirect_first, indirect_last, std::ostream_iterator&lt;char&gt;(std::cout, &quot;,&quot;));
 299 std::cout &lt;&lt; std::endl;
 300
 301
 302 // Example of making mutable and constant indirect iterators
 303
 304 char mutable_characters[N];
 305 char* pointers_to_mutable_chars[N];
 306 for (int j = 0; j &lt; N; ++j)
 307   pointers_to_mutable_chars[j] = &amp;mutable_characters[j];
 308
 309 boost::indirect_iterator&lt;char* const*&gt; mutable_indirect_first(pointers_to_mutable_chars),
 310   mutable_indirect_last(pointers_to_mutable_chars + N);
 311 boost::indirect_iterator&lt;char* const*, char const&gt; const_indirect_first(pointers_to_chars),
 312   const_indirect_last(pointers_to_chars + N);
 313
 314 std::transform(const_indirect_first, const_indirect_last,
 315                mutable_indirect_first, std::bind1st(std::plus&lt;char&gt;(), 1));
 316
 317 std::copy(mutable_indirect_first, mutable_indirect_last,
 318           std::ostream_iterator&lt;char&gt;(std::cout, &quot;,&quot;));
 319 std::cout &lt;&lt; std::endl;
 320
 321
 322 // Example of using make_indirect_iterator()
 323
 324 std::copy(boost::make_indirect_iterator(pointers_to_chars),
 325           boost::make_indirect_iterator(pointers_to_chars + N),
 326           std::ostream_iterator&lt;char&gt;(std::cout, &quot;,&quot;));
 327 std::cout &lt;&lt; std::endl;
 328 </pre>
 329 <p>The output is:</p>
 330 <pre class="literal-block">
 331 a,b,c,d,e,f,g,
 332 b,c,d,e,f,g,h,
 333 a,b,c,d,e,f,g,
 334 </pre>
 335 <p>The source code for this example can be found <a class="reference external" href="../example/indirect_iterator_example.cpp">here</a>.</p>
 336 </div>
 337 </div>
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