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<title> Combining hash values</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="hash.combine"></a> Combining hash values</h3></div></div></div>
<p>
Say you have a point class, representing a two dimensional location:</p>
<pre class="programlisting"><code class="literal"><span class="keyword">class</span><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="keyword">
public</span><span class="special">:</span><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><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><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">&</span><span class="identifier"> other</span><span class="special">)</span><span class="keyword"> const</span><span class="special">
{</span><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"> &&</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><span class="special">
}</span><span class="special">
};</span></code></pre>
<p>
and you wish to use it as the key for an <code class="computeroutput"><span class="identifier">unordered_map</span></code>. You need to
customise the hash for this structure. To do this we need to combine
the hash values for <code class="computeroutput"><span class="identifier">x</span></code> and <code class="computeroutput"><span class="identifier">y</span></code>. The function
<code class="computeroutput"><a href="../hash_combine.html" title="Function template hash_combine">boost::hash_combine</a></code> is supplied for this purpose:</p>
<pre class="programlisting"><code class="literal"><span class="keyword">class</span><span class="identifier"> point</span><span class="special">
{</span><span class="special">
...</span><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">&</span><span class="identifier"> p</span><span class="special">)</span><span class="special">
{</span><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>
<code class="computeroutput"><a href="../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>
<code class="computeroutput"><a href="../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><span class="keyword">
return</span><span class="identifier"> seed</span><span class="special">;</span><span class="special">
}</span><span class="special">
...</span><span class="special">
};</span></code></pre>
<p>
Calls to hash_combine incrementally build the hash from the different members
of point, it can be repeatedly called for any number of elements. It calls
<code class="computeroutput"><a href="../id1042434.html" title="Function hash_value">hash_value</a></code> on the supplied element, and combines it with the seed.</p>
<p>
Full code for this example is at
<a href="../../../libs/functional/hash/examples/point.cpp" target="_top">/libs/functional/hash/examples/point.cpp</a>.</p>
<div class="informaltable"><table class="table">
<colgroup><col></colgroup>
<tbody><tr><td class="blurb">
When using __hash_combine the order of the
calls matters.
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
std::size_t seed = 0;
boost::hash_combine(seed, 1);
boost::hash_combine(seed, 2);
</pre>
results in a different seed to:
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
std::size_t seed = 0;
boost::hash_combine(seed, 2);
boost::hash_combine(seed, 1);
</pre>
If you are calculating a hash value for data where the order of the data
doesn't matter in comparisons (e.g. a set) you will have to ensure that the
data is always supplied in the same order.
</td></tr></tbody>
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<p>
To calculate the hash of an iterator range you can use <code class="computeroutput"><a href="../hash_range.html" title="Function hash_range">boost::hash_range</a></code>:</p>
<pre class="programlisting"><code class="literal"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span><span class="identifier"> some_strings</span><span class="special">;</span><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="../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></code></pre>
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