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  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="hash.combine"></a><a class="link" href="combine.html" title="Combining hash values">Combining hash values</a>
  28 </h2></div></div></div>
  29 <p>
  30       Say you have a point class, representing a two dimensional location:
  31     </p>
  32 <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">point</span>
  33 <span class="special">{</span>
  34     <span class="keyword">int</span> <span class="identifier">x</span><span class="special">;</span>
  35     <span class="keyword">int</span> <span class="identifier">y</span><span class="special">;</span>
  36 <span class="keyword">public</span><span class="special">:</span>
  37     <span class="identifier">point</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">x</span><span class="special">(</span><span class="number">0</span><span class="special">),</span> <span class="identifier">y</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> <span class="special">{}</span>
  38     <span class="identifier">point</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">x</span><span class="special">),</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">y</span><span class="special">)</span> <span class="special">{}</span>
  39
  40     <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">==(</span><span class="identifier">point</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">other</span><span class="special">)</span> <span class="keyword">const</span>
  41     <span class="special">{</span>
  42         <span class="keyword">return</span> <span class="identifier">x</span> <span class="special">==</span> <span class="identifier">other</span><span class="special">.</span><span class="identifier">x</span> <span class="special">&amp;&amp;</span> <span class="identifier">y</span> <span class="special">==</span> <span class="identifier">other</span><span class="special">.</span><span class="identifier">y</span><span class="special">;</span>
  43     <span class="special">}</span>
  44 <span class="special">};</span>
  45 </pre>
  46 <p>
  47       and you wish to use it as the key for an <code class="computeroutput"><span class="identifier">unordered_map</span></code>.
  48       You need to customise the hash for this structure. To do this we need to combine
  49       the hash values for <code class="computeroutput"><span class="identifier">x</span></code> and
  50       <code class="computeroutput"><span class="identifier">y</span></code>. The function <code class="computeroutput"><a class="link" href="reference.html#boost.hash_combine">boost::hash_combine</a></code> is supplied for
  51       this purpose:
  52     </p>
  53 <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">point</span>
  54 <span class="special">{</span>
  55     <span class="special">...</span>
  56
  57     <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">hash_value</span><span class="special">(</span><span class="identifier">point</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">p</span><span class="special">)</span>
  58     <span class="special">{</span>
  59         <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">seed</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
  60         <code class="computeroutput"><a class="link" href="reference.html#boost.hash_combine">boost::hash_combine</a></code><span class="special">(</span><span class="identifier">seed</span><span class="special">,</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">x</span><span class="special">);</span>
  61         <code class="computeroutput"><a class="link" href="reference.html#boost.hash_combine">boost::hash_combine</a></code><span class="special">(</span><span class="identifier">seed</span><span class="special">,</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">y</span><span class="special">);</span>
  62
  63         <span class="keyword">return</span> <span class="identifier">seed</span><span class="special">;</span>
  64     <span class="special">}</span>
  65
  66     <span class="special">...</span>
  67 <span class="special">};</span>
  68 </pre>
  69 <p>
  70       Calls to hash_combine incrementally build the hash from the different members
  71       of point, it can be repeatedly called for any number of elements. It calls
  72       <code class="computeroutput"><a class="link" href="reference.html#boost.hash_value_id746661">hash_value</a></code> on the supplied
  73       element, and combines it with the seed.
  74     </p>
  75 <p>
  76       Full code for this example is at <a href="../../../libs/functional/hash/examples/point.cpp" target="_top">/libs/functional/hash/examples/point.cpp</a>.
  77     </p>
  78 <div class="note"><table border="0" summary="Note">
  79 <tr>
  80 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
  81 <th align="left">Note</th>
  82 </tr>
  83 <tr><td align="left" valign="top">
  84 <p>
  85         When using <code class="computeroutput"><a class="link" href="reference.html#boost.hash_combine">boost::hash_combine</a></code>
  86         the order of the calls matters. </p>
  87 <pre class="programlisting">
  88     std::size_t seed = 0;
  89     boost::hash_combine(seed, 1);
  90     boost::hash_combine(seed, 2);
  91 </pre>
  92 <p>
  93 results in a different seed to:
  94 </p>
  95 <pre class="programlisting">
  96     std::size_t seed = 0;
  97     boost::hash_combine(seed, 2);
  98     boost::hash_combine(seed, 1);
  99 </pre>
 100 <p>
 101 If you are calculating a hash value for data
 102         where the order of the data doesn't matter in comparisons (e.g. a set) you
 103         will have to ensure that the data is always supplied in the same order.
 104       </p>
 105 </td></tr>
 106 </table></div>
 107 <p>
 108       To calculate the hash of an iterator range you can use <code class="computeroutput"><a class="link" href="reference.html#boost.hash_range">boost::hash_range</a></code>:
 109     </p>
 110 <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">some_strings</span><span class="special">;</span>
 111 <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">hash</span> <span class="special">=</span> <code class="computeroutput"><a class="link" href="reference.html#boost.hash_range">boost::hash_range</a></code><span class="special">(</span><span class="identifier">some_strings</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">some_strings</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
 112 </pre>
 113 <p>
 114       Note that when writing template classes, you might not want to include the
 115       main hash header as it's quite an expensive include that brings in a lot of
 116       other headers, so instead you can include the <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">functional</span><span class="special">/</span><span class="identifier">hash_fwd</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
 117       header which forward declares <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code>,
 118       <code class="computeroutput"><a class="link" href="reference.html#boost.hash_range">boost::hash_range</a></code> and
 119       <code class="computeroutput"><a class="link" href="reference.html#boost.hash_combine">boost::hash_combine</a></code>.
 120       You'll need to include the main header before instantiating <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code>.
 121       When using a container that uses <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code>
 122       it should do that for you, so your type will work fine with the boost hash
 123       containers. There's an example of this in <a href="../../../libs/functional/hash/examples/template.hpp" target="_top">template.hpp</a>
 124       and <a href="../../../libs/functional/hash/examples/template.cpp" target="_top">template.cpp</a>.
 125     </p>
 126 </div>
 127 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 128 <td align="left"></td>
 129 <td align="right"><div class="copyright-footer">Copyright &#169; 2005-2008 Daniel
 130       James<p>
 131         Distributed under the Boost Software License, Version 1.0. (See accompanying
 132         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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