<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Combining hash values</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../hash.html" title="Chapter 8. Boost.Functional/Hash"> <link rel="prev" href="custom.html" title="Extending boost::hash for a custom data type"> <link rel="next" href="portability.html" title="Portability"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="custom.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../hash.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="portability.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="hash.combine"></a><a class="link" href="combine.html" title="Combining hash values"> Combining hash values</a> </h2></div></div></div> <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"> <tr> <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> <th align="left">Note</th> </tr> <tr><td align="left" valign="top"> <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> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <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>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="custom.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../hash.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="portability.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>