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| <a name="hash.combine"></a><a class="link" href="combine.html" title="Combining hash values">Combining hash values</a> |
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| <p> |
| Say you have a point class, representing a two dimensional location: |
| </p> |
| <pre class="programlisting"><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> |
| </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 class="link" href="reference.html#boost.hash_combine">boost::hash_combine</a></code> is supplied for |
| this purpose: |
| </p> |
| <pre class="programlisting"><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 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> |
| <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> |
| |
| <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> |
| </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 class="link" href="reference.html#boost.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="note"><table border="0" summary="Note"> |
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| <th align="left">Note</th> |
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| <p> |
| When using <code class="computeroutput"><a class="link" href="reference.html#boost.hash_combine">boost::hash_combine</a></code> |
| the order of the calls matters. </p> |
| <pre class="programlisting"> |
| std::size_t seed = 0; |
| boost::hash_combine(seed, 1); |
| boost::hash_combine(seed, 2); |
| </pre> |
| <p> |
| results in a different seed to: |
| </p> |
| <pre class="programlisting"> |
| std::size_t seed = 0; |
| boost::hash_combine(seed, 2); |
| boost::hash_combine(seed, 1); |
| </pre> |
| <p> |
| 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. |
| </p> |
| </td></tr> |
| </table></div> |
| <p> |
| 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>: |
| </p> |
| <pre class="programlisting"><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 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> |
| </pre> |
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| <td align="right"><div class="copyright-footer">Copyright © 2005-2008 Daniel |
| James<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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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