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  25 <div class="section">
  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="intrusive.recursive"></a><a class="link" href="recursive.html" title="Recursive Boost.Intrusive containers">Recursive Boost.Intrusive containers</a>
  28 </h2></div></div></div>
  29 <p>
  30       <span class="bold"><strong>Boost.Intrusive</strong></span> containers can be used to
  31       define recursive structures very easily, allowing complex data structures with
  32       very low overhead. Let's see an example:
  33     </p>
  34 <p>
  35 </p>
  36 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
  37 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cassert</span><span class="special">&gt;</span>
  38
  39 <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">;</span>
  40
  41 <span class="keyword">typedef</span> <span class="identifier">list_base_hook</span><span class="special">&lt;&gt;</span> <span class="identifier">BaseHook</span><span class="special">;</span>
  42
  43 <span class="comment">//A recursive class</span>
  44 <span class="keyword">class</span> <span class="identifier">Recursive</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">BaseHook</span>
  45 <span class="special">{</span>
  46    <span class="keyword">private</span><span class="special">:</span>
  47    <span class="identifier">Recursive</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Recursive</span><span class="special">&amp;);</span>
  48    <span class="identifier">Recursive</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">Recursive</span><span class="special">&amp;);</span>
  49
  50    <span class="keyword">public</span><span class="special">:</span>
  51    <span class="identifier">Recursive</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">BaseHook</span><span class="special">(),</span> <span class="identifier">children</span><span class="special">(){}</span>
  52    <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">base_hook</span><span class="special">&lt;</span><span class="identifier">BaseHook</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">children</span><span class="special">;</span>
  53 <span class="special">};</span>
  54
  55 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
  56 <span class="special">{</span>
  57    <span class="identifier">Recursive</span> <span class="identifier">f</span><span class="special">,</span> <span class="identifier">f2</span><span class="special">;</span>
  58    <span class="comment">//A recursive list of Recursive</span>
  59    <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">base_hook</span><span class="special">&lt;</span><span class="identifier">BaseHook</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">l</span><span class="special">;</span>
  60
  61    <span class="comment">//Insert a node in parent list</span>
  62    <span class="identifier">l</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">f</span><span class="special">);</span>
  63
  64    <span class="comment">//Insert a node in child list</span>
  65    <span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">f2</span><span class="special">);</span>
  66
  67    <span class="comment">//Objects properly inserted</span>
  68    <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">size</span><span class="special">());</span>
  69    <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span>
  70
  71    <span class="comment">//Clear both lists</span>
  72    <span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
  73    <span class="identifier">l</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
  74    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
  75 <span class="special">}</span>
  76 </pre>
  77 <p>
  78     </p>
  79 <p>
  80       Recursive data structures using <span class="bold"><strong>Boost.Intrusive</strong></span>
  81       containers must avoid using hook deduction to avoid early type instantiation:
  82     </p>
  83 <pre class="programlisting"><span class="comment">//This leads to compilation error (Recursive is instantiated by</span>
  84 <span class="comment">//'list' to deduce hook properties (pointer type, tag, safe-mode...)</span>
  85 <span class="keyword">class</span> <span class="identifier">Recursive</span>
  86 <span class="special">{</span>  <span class="comment">//...</span>
  87
  88    <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span> <span class="special">&gt;</span> <span class="identifier">l</span><span class="special">;</span>
  89    <span class="comment">//...</span>
  90 <span class="special">};</span>
  91
  92 <span class="comment">//Ok, programmer must specify the hook type to avoid early Recursive instantiation</span>
  93 <span class="keyword">class</span> <span class="identifier">Recursive</span>
  94 <span class="special">{</span>  <span class="comment">//...</span>
  95    <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">base_hook</span><span class="special">&lt;</span><span class="identifier">BaseHook</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">l</span><span class="special">;</span>
  96    <span class="comment">//...</span>
  97 <span class="special">};</span>
  98 </pre>
  99 <p>
 100       Member hooks are not suitable for recursive structures:
 101     </p>
 102 <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Recursive</span>
 103 <span class="special">{</span>
 104    <span class="keyword">private</span><span class="special">:</span>
 105    <span class="identifier">Recursive</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Recursive</span><span class="special">&amp;);</span>
 106    <span class="identifier">Recursive</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">Recursive</span><span class="special">&amp;);</span>
 107
 108    <span class="keyword">public</span><span class="special">:</span>
 109    <span class="identifier">list_member_hook</span><span class="special">&lt;&gt;</span> <span class="identifier">memhook</span><span class="special">;</span>
 110    <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">member_hook</span><span class="special">&lt;</span><span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">list_member_hook</span><span class="special">&lt;&gt;,</span> <span class="special">&amp;</span><span class="identifier">Recursive</span><span class="special">::</span><span class="identifier">memhook</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">children</span><span class="special">;</span>
 111 <span class="special">};</span>
 112 </pre>
 113 <p>
 114       Specifying <code class="computeroutput"><span class="special">&amp;</span><span class="identifier">Recursive</span><span class="special">::</span><span class="identifier">memhook</span></code>
 115       (that is, the offset between memhook and Recursive) provokes an early instantiation
 116       of <code class="computeroutput"><span class="identifier">Recursive</span></code>. To define recursive
 117       structures using member hooks, a programmer should use <code class="computeroutput">function_hook</code>:
 118     </p>
 119 <p>
 120 </p>
 121 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 122 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">parent_from_member</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 123
 124 <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">;</span>
 125
 126 <span class="keyword">class</span> <span class="identifier">Recursive</span><span class="special">;</span>
 127
 128 <span class="comment">//Declaration of the functor that converts betwen the Recursive</span>
 129 <span class="comment">//class and the hook</span>
 130 <span class="keyword">struct</span> <span class="identifier">Functor</span>
 131 <span class="special">{</span>
 132    <span class="comment">//Required types</span>
 133    <span class="keyword">typedef</span> <span class="identifier">list_member_hook</span><span class="special">&lt;&gt;</span>    <span class="identifier">hook_type</span><span class="special">;</span>
 134    <span class="keyword">typedef</span> <span class="identifier">hook_type</span><span class="special">*</span>            <span class="identifier">hook_ptr</span><span class="special">;</span>
 135    <span class="keyword">typedef</span> <span class="keyword">const</span> <span class="identifier">hook_type</span><span class="special">*</span>      <span class="identifier">const_hook_ptr</span><span class="special">;</span>
 136    <span class="keyword">typedef</span> <span class="identifier">Recursive</span>             <span class="identifier">value_type</span><span class="special">;</span>
 137    <span class="keyword">typedef</span> <span class="identifier">value_type</span><span class="special">*</span>           <span class="identifier">pointer</span><span class="special">;</span>
 138    <span class="keyword">typedef</span> <span class="keyword">const</span> <span class="identifier">value_type</span><span class="special">*</span>     <span class="identifier">const_pointer</span><span class="special">;</span>
 139
 140    <span class="comment">//Required static functions</span>
 141    <span class="keyword">static</span> <span class="identifier">hook_ptr</span> <span class="identifier">to_hook_ptr</span> <span class="special">(</span><span class="identifier">value_type</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">);</span>
 142    <span class="keyword">static</span> <span class="identifier">const_hook_ptr</span> <span class="identifier">to_hook_ptr</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">value_type</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">);</span>
 143    <span class="keyword">static</span> <span class="identifier">pointer</span> <span class="identifier">to_value_ptr</span><span class="special">(</span><span class="identifier">hook_ptr</span> <span class="identifier">n</span><span class="special">);</span>
 144    <span class="keyword">static</span> <span class="identifier">const_pointer</span> <span class="identifier">to_value_ptr</span><span class="special">(</span><span class="identifier">const_hook_ptr</span> <span class="identifier">n</span><span class="special">);</span>
 145 <span class="special">};</span>
 146
 147 <span class="comment">//Define the recursive class</span>
 148 <span class="keyword">class</span> <span class="identifier">Recursive</span>
 149 <span class="special">{</span>
 150    <span class="keyword">private</span><span class="special">:</span>
 151    <span class="identifier">Recursive</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Recursive</span><span class="special">&amp;);</span>
 152    <span class="identifier">Recursive</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">Recursive</span><span class="special">&amp;);</span>
 153
 154    <span class="keyword">public</span><span class="special">:</span>
 155    <span class="identifier">Recursive</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">hook</span><span class="special">(),</span> <span class="identifier">children</span><span class="special">()</span> <span class="special">{}</span>
 156    <span class="identifier">list_member_hook</span><span class="special">&lt;&gt;</span> <span class="identifier">hook</span><span class="special">;</span>
 157    <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">function_hook</span><span class="special">&lt;</span> <span class="identifier">Functor</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">children</span><span class="special">;</span>
 158 <span class="special">};</span>
 159
 160 <span class="comment">//Definition of Functor functions</span>
 161 <span class="keyword">inline</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">hook_ptr</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">to_hook_ptr</span> <span class="special">(</span><span class="identifier">Functor</span><span class="special">::</span><span class="identifier">value_type</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">)</span>
 162    <span class="special">{</span>  <span class="keyword">return</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">.</span><span class="identifier">hook</span><span class="special">;</span> <span class="special">}</span>
 163 <span class="keyword">inline</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">const_hook_ptr</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">to_hook_ptr</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">value_type</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">)</span>
 164    <span class="special">{</span>  <span class="keyword">return</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">.</span><span class="identifier">hook</span><span class="special">;</span> <span class="special">}</span>
 165 <span class="keyword">inline</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">pointer</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">to_value_ptr</span><span class="special">(</span><span class="identifier">Functor</span><span class="special">::</span><span class="identifier">hook_ptr</span> <span class="identifier">n</span><span class="special">)</span>
 166    <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">get_parent_from_member</span><span class="special">&lt;</span><span class="identifier">Recursive</span><span class="special">&gt;(</span><span class="identifier">n</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Recursive</span><span class="special">::</span><span class="identifier">hook</span><span class="special">);</span>  <span class="special">}</span>
 167 <span class="keyword">inline</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">const_pointer</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">to_value_ptr</span><span class="special">(</span><span class="identifier">Functor</span><span class="special">::</span><span class="identifier">const_hook_ptr</span> <span class="identifier">n</span><span class="special">)</span>
 168    <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">get_parent_from_member</span><span class="special">&lt;</span><span class="identifier">Recursive</span><span class="special">&gt;(</span><span class="identifier">n</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Recursive</span><span class="special">::</span><span class="identifier">hook</span><span class="special">);</span>  <span class="special">}</span>
 169
 170 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 171 <span class="special">{</span>
 172    <span class="identifier">Recursive</span> <span class="identifier">f</span><span class="special">,</span> <span class="identifier">f2</span><span class="special">;</span>
 173    <span class="comment">//A recursive list of Recursive</span>
 174    <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">function_hook</span><span class="special">&lt;</span> <span class="identifier">Functor</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">l</span><span class="special">;</span>
 175
 176    <span class="comment">//Insert a node in parent list</span>
 177    <span class="identifier">l</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">f</span><span class="special">);</span>
 178
 179    <span class="comment">//Insert a node in child list</span>
 180    <span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">f2</span><span class="special">);</span>
 181
 182    <span class="comment">//Objects properly inserted</span>
 183    <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">size</span><span class="special">());</span>
 184    <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span>
 185
 186    <span class="comment">//Clear both lists</span>
 187    <span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
 188    <span class="identifier">l</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
 189    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 190 <span class="special">}</span>
 191 </pre>
 192 <p>
 193     </p>
 194 </div>
 195 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 196 <td align="left"></td>
 197 <td align="right"><div class="copyright-footer">Copyright &#169; 2005 Olaf Krzikalla<br>Copyright &#169; 2006-2013 Ion Gaztanaga<p>
 198         Distributed under the Boost Software License, Version 1.0. (See accompanying
 199         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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