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  25 <div class="section">
  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="intrusive.function_hooks"></a><a class="link" href="function_hooks.html" title="Using function hooks">Using function hooks</a>
  28 </h2></div></div></div>
  29 <p>
  30       A programmer might find that base or member hooks are not flexible enough in
  31       some situations. In some applications it would be optimal to put a hook deep
  32       inside a member of a class or just outside the class. <span class="bold"><strong>Boost.Intrusive</strong></span>
  33       has an easy option to allow such cases: <code class="computeroutput"><a class="link" href="../boost/intrusive/function_hook.html" title="Struct template function_hook">function_hook</a></code>.
  34     </p>
  35 <p>
  36       This option is similar to <code class="computeroutput"><a class="link" href="../boost/intrusive/member_hook.html" title="Struct template member_hook">member_hook</a></code>
  37       or <code class="computeroutput"><a class="link" href="../boost/intrusive/base_hook.html" title="Struct template base_hook">base_hook</a></code>, but
  38       the programmer can specify a function object that tells the container how to
  39       obtain a hook from a value and vice versa. The programmer just needs to define
  40       the following function object:
  41     </p>
  42 <pre class="programlisting"><span class="comment">//This functor converts between value_type and a hook_type</span>
  43 <span class="keyword">struct</span> <span class="identifier">Functor</span>
  44 <span class="special">{</span>
  45    <span class="comment">//Required types</span>
  46    <span class="keyword">typedef</span> <span class="comment">/*impl-defined*/</span>      <span class="identifier">hook_type</span><span class="special">;</span>
  47    <span class="keyword">typedef</span> <span class="comment">/*impl-defined*/</span>      <span class="identifier">hook_ptr</span><span class="special">;</span>
  48    <span class="keyword">typedef</span> <span class="comment">/*impl-defined*/</span>      <span class="identifier">const_hook_ptr</span><span class="special">;</span>
  49    <span class="keyword">typedef</span> <span class="comment">/*impl-defined*/</span>      <span class="identifier">value_type</span><span class="special">;</span>
  50    <span class="keyword">typedef</span> <span class="comment">/*impl-defined*/</span>      <span class="identifier">pointer</span><span class="special">;</span>
  51    <span class="keyword">typedef</span> <span class="comment">/*impl-defined*/</span>      <span class="identifier">const_pointer</span><span class="special">;</span>
  52    <span class="comment">//Required static functions</span>
  53    <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>
  54    <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>
  55    <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>
  56    <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>
  57 <span class="special">};</span>
  58 </pre>
  59 <p>
  60       Converting from values to hooks is generally easy, since most hooks are in
  61       practice members or base classes of class data members. The inverse operation
  62       is a bit more complicated, but <span class="bold"><strong>Boost.Intrusive</strong></span>
  63       offers a bit of help with the function <code class="computeroutput"><a class="link" href="../boost/intrusive/get_parent_fro_idp64070480.html" title="Function template get_parent_from_member">get_parent_from_member</a></code>,
  64       which allows easy conversions from the address of a data member to the address
  65       of the parent holding that member. Let's see a little example of <code class="computeroutput"><a class="link" href="../boost/intrusive/function_hook.html" title="Struct template function_hook">function_hook</a></code>:
  66     </p>
  67 <p>
  68 </p>
  69 <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>
  70 <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>
  71
  72 <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>
  73
  74 <span class="keyword">struct</span> <span class="identifier">MyClass</span>
  75 <span class="special">{</span>
  76    <span class="keyword">int</span> <span class="identifier">dummy</span><span class="special">;</span>
  77    <span class="comment">//This internal type has a member hook</span>
  78    <span class="keyword">struct</span> <span class="identifier">InnerNode</span>
  79    <span class="special">{</span>
  80       <span class="keyword">int</span> <span class="identifier">dummy</span><span class="special">;</span>
  81       <span class="identifier">list_member_hook</span><span class="special">&lt;&gt;</span> <span class="identifier">hook</span><span class="special">;</span>
  82    <span class="special">}</span> <span class="identifier">inner</span><span class="special">;</span>
  83 <span class="special">};</span>
  84
  85 <span class="comment">//This functor converts between MyClass and InnerNode's member hook</span>
  86 <span class="keyword">struct</span> <span class="identifier">Functor</span>
  87 <span class="special">{</span>
  88    <span class="comment">//Required types</span>
  89    <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>
  90    <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>
  91    <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>
  92    <span class="keyword">typedef</span> <span class="identifier">MyClass</span>               <span class="identifier">value_type</span><span class="special">;</span>
  93    <span class="keyword">typedef</span> <span class="identifier">value_type</span><span class="special">*</span>           <span class="identifier">pointer</span><span class="special">;</span>
  94    <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>
  95
  96    <span class="comment">//Required static functions</span>
  97    <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>
  98       <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">inner</span><span class="special">.</span><span class="identifier">hook</span><span class="special">;</span> <span class="special">}</span>
  99    <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>
 100       <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">inner</span><span class="special">.</span><span class="identifier">hook</span><span class="special">;</span> <span class="special">}</span>
 101    <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>
 102    <span class="special">{</span>
 103       <span class="keyword">return</span> <span class="identifier">get_parent_from_member</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;</span>
 104          <span class="special">(</span><span class="identifier">get_parent_from_member</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">::</span><span class="identifier">InnerNode</span><span class="special">&gt;(</span><span class="identifier">n</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">MyClass</span><span class="special">::</span><span class="identifier">InnerNode</span><span class="special">::</span><span class="identifier">hook</span><span class="special">)</span>
 105          <span class="special">,&amp;</span><span class="identifier">MyClass</span><span class="special">::</span><span class="identifier">inner</span>
 106       <span class="special">);</span>
 107    <span class="special">}</span>
 108    <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>
 109    <span class="special">{</span>
 110       <span class="keyword">return</span> <span class="identifier">get_parent_from_member</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;</span>
 111          <span class="special">(</span><span class="identifier">get_parent_from_member</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">::</span><span class="identifier">InnerNode</span><span class="special">&gt;(</span><span class="identifier">n</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">MyClass</span><span class="special">::</span><span class="identifier">InnerNode</span><span class="special">::</span><span class="identifier">hook</span><span class="special">)</span>
 112          <span class="special">,&amp;</span><span class="identifier">MyClass</span><span class="special">::</span><span class="identifier">inner</span>
 113       <span class="special">);</span>
 114    <span class="special">}</span>
 115 <span class="special">};</span>
 116
 117 <span class="comment">//Define a list that will use the hook accessed through the function object</span>
 118 <span class="keyword">typedef</span> <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">MyClass</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">List</span><span class="special">;</span>
 119
 120 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 121 <span class="special">{</span>
 122    <span class="identifier">MyClass</span> <span class="identifier">n</span><span class="special">;</span>
 123    <span class="identifier">List</span> <span class="identifier">l</span><span class="special">;</span>
 124    <span class="comment">//Insert the node in both lists</span>
 125    <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">n</span><span class="special">);</span>
 126    <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>
 127    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 128 <span class="special">}</span>
 129 </pre>
 130 <p>
 131     </p>
 132 </div>
 133 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 134 <td align="left"></td>
 135 <td align="right"><div class="copyright-footer">Copyright &#169; 2005 Olaf Krzikalla<br>Copyright &#169; 2006-2013 Ion Gaztanaga<p>
 136         Distributed under the Boost Software License, Version 1.0. (See accompanying
 137         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>)
 138       </p>
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