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4<title> Combining hash values</title>
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26<div class="titlepage"><div><div><h2 class="title" style="clear: both">
27<a name="hash.combine"></a> Combining hash values</h2></div></div></div>
28<p>
29      Say you have a point class, representing a two dimensional location:
30    </p>
31<pre class="programlisting">
32<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 href="../boost/hash_combine.html" title="Function template hash_combine">boost::hash_combine</a></code> is supplied for
51      this purpose:
52    </p>
53<pre class="programlisting">
54<span class="keyword">class</span> <span class="identifier">point</span>
55<span class="special">{</span>
56    <span class="special">...</span>
57
58    <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>
59    <span class="special">{</span>
60        <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>
61        <code class="computeroutput"><a href="../boost/hash_combine.html" title="Function template 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>
62        <code class="computeroutput"><a href="../boost/hash_combine.html" title="Function template 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>
63
64        <span class="keyword">return</span> <span class="identifier">seed</span><span class="special">;</span>
65    <span class="special">}</span>
66
67    <span class="special">...</span>
68<span class="special">};</span>
69</pre>
70<p>
71      Calls to hash_combine incrementally build the hash from the different members
72      of point, it can be repeatedly called for any number of elements. It calls
73      <code class="computeroutput"><a href="../boost/hash_value.html" title="Function hash_value">hash_value</a></code> on the supplied
74      element, and combines it with the seed.
75    </p>
76<p>
77      Full code for this example is at <a href="../../../libs/functional/hash/examples/point.cpp" target="_top">/libs/functional/hash/examples/point.cpp</a>.
78    </p>
79<div class="note"><table border="0" summary="Note">
80<tr>
81<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../images/note.png"></td>
82<th align="left">Note</th>
83</tr>
84<tr><td align="left" valign="top">
85<p>
86        When using <code class="computeroutput"><a href="../boost/hash_combine.html" title="Function template hash_combine">boost::hash_combine</a></code>
87        the order of the calls matters.
88</p>
89<pre class="programlisting">
90    std::size_t seed = 0;
91    boost::hash_combine(seed, 1);
92    boost::hash_combine(seed, 2);
93</pre>
94<p>
95results in a different seed to:
96</p>
97<pre class="programlisting">
98    std::size_t seed = 0;
99    boost::hash_combine(seed, 2);
100    boost::hash_combine(seed, 1);
101</pre>
102<p>
103
104        If you are calculating a hash value for data where the order of the data
105        doesn't matter in comparisons (e.g. a set) you will have to ensure that the
106        data is always supplied in the same order.
107      </p>
108</td></tr>
109</table></div>
110<p>
111      To calculate the hash of an iterator range you can use <code class="computeroutput"><a href="../boost/hash_range.html" title="Function hash_range">boost::hash_range</a></code>:
112    </p>
113<pre class="programlisting">
114<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>
115<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 href="../boost/hash_range.html" title="Function 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>
116</pre>
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