<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Equality Predicates and Hash Functions</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="../unordered.html" title="Chapter 27. Boost.Unordered"> <link rel="prev" href="buckets.html" title="The Data Structure"> <link rel="next" href="comparison.html" title="Comparison with Associative Containers"> </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="buckets.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../unordered.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="comparison.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="unordered.hash_equality"></a><a class="link" href="hash_equality.html" title="Equality Predicates and Hash Functions"> Equality Predicates and Hash Functions</a> </h2></div></div></div> <p> While the associative containers use an ordering relation to specify how the elements are stored, the unordered associative containers use an equality predicate and a hash function. For example, <code class="computeroutput"><a class="link" href="../boost/unordered_map.html" title="Class template unordered_map">boost::unordered_map</a></code> is declared as: </p> <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Mapped</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Hash</span> <span class="special">=</span> <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special"><</span><span class="identifier">Key</span><span class="special">>,</span> <span class="keyword">class</span> <span class="identifier">Pred</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">equal_to</span><span class="special"><</span><span class="identifier">Key</span><span class="special">>,</span> <span class="keyword">class</span> <span class="identifier">Alloc</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">Key</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">Mapped</span><span class="special">></span> <span class="special">></span> <span class="special">></span> <span class="keyword">class</span> <code class="computeroutput"><a class="link" href="../boost/unordered_map.html" title="Class template unordered_map">unordered_map</a></code><span class="special">;</span> </pre> <p> The hash function comes first as you might want to change the hash function but not the equality predicate. For example, if you wanted to use the <a href="http://www.isthe.com/chongo/tech/comp/fnv/" target="_top">FNV-1 hash</a> you could write: </p> <p> </p> <p> </p> <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered_map</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="keyword">int</span><span class="special">,</span> <span class="identifier">hash</span><span class="special">::</span><span class="identifier">fnv_1</span><span class="special">></span> <span class="identifier">dictionary</span><span class="special">;</span> </pre> <p> </p> <p> </p> <p> There is an <a href="../../../libs/unordered/examples/fnv1.hpp" target="_top">implementation of FNV-1</a> in the examples directory. </p> <p> If you wish to use a different equality function, you will also need to use a matching hash function. For example, to implement a case insensitive dictionary you need to define a case insensitive equality predicate and hash function: </p> <p> </p> <p> </p> <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">iequal_to</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">binary_function</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">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">bool</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">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">algorithm</span><span class="special">::</span><span class="identifier">iequals</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">std</span><span class="special">::</span><span class="identifier">locale</span><span class="special">());</span> <span class="special">}</span> <span class="special">};</span> <span class="keyword">struct</span> <span class="identifier">ihash</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">unary_function</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">std</span><span class="special">::</span><span class="identifier">size_t</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="keyword">operator</span><span class="special">()(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">)</span> <span class="keyword">const</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> <span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span> <span class="identifier">locale</span><span class="special">;</span> <span class="keyword">for</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">const_iterator</span> <span class="identifier">it</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span> <span class="identifier">it</span> <span class="special">!=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">end</span><span class="special">();</span> <span class="special">++</span><span class="identifier">it</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash_combine</span><span class="special">(</span><span class="identifier">seed</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">toupper</span><span class="special">(*</span><span class="identifier">it</span><span class="special">,</span> <span class="identifier">locale</span><span class="special">));</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> </pre> <p> </p> <p> </p> <p> Which you can then use in a case insensitive dictionary: </p> <p> </p> <p> </p> <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered_map</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="keyword">int</span><span class="special">,</span> <span class="identifier">ihash</span><span class="special">,</span> <span class="identifier">iequal_to</span><span class="special">></span> <span class="identifier">idictionary</span><span class="special">;</span> </pre> <p> </p> <p> </p> <p> This is a simplified version of the example at <a href="../../../libs/unordered/examples/case_insensitive.hpp" target="_top">/libs/unordered/examples/case_insensitive.hpp</a> which supports other locales and string types. </p> <div class="caution"><table border="0" summary="Caution"> <tr> <td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../doc/src/images/caution.png"></td> <th align="left">Caution</th> </tr> <tr><td align="left" valign="top"><p> Be careful when using the equality (<code class="computeroutput"><span class="special">==</span></code>) operator with custom equality predicates, especially if you're using a function pointer. If you compare two containers with different equality predicates then the result is undefined. For most stateless function objects this is impossible - since you can only compare objects with the same equality predicate you know the equality predicates must be equal. But if you're using function pointers or a stateful equality predicate (e.g. boost::function) then you can get into trouble. </p></td></tr> </table></div> <a name="unordered.hash_equality.custom_types"></a><h3> <a name="id3027790"></a> <a class="link" href="hash_equality.html#unordered.hash_equality.custom_types">Custom Types</a> </h3> <p> Similarly, a custom hash function can be used for custom types: </p> <p> </p> <p> </p> <pre class="programlisting"><span class="keyword">struct</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="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">p1</span><span class="special">,</span> <span class="identifier">point</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">p2</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">p1</span><span class="special">.</span><span class="identifier">x</span> <span class="special">==</span> <span class="identifier">p2</span><span class="special">.</span><span class="identifier">x</span> <span class="special">&&</span> <span class="identifier">p1</span><span class="special">.</span><span class="identifier">y</span> <span class="special">==</span> <span class="identifier">p2</span><span class="special">.</span><span class="identifier">y</span><span class="special">;</span> <span class="special">}</span> <span class="keyword">struct</span> <span class="identifier">point_hash</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">unary_function</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</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="keyword">operator</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="keyword">const</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> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash_combine</span><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> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash_combine</span><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="identifier">boost</span><span class="special">::</span><span class="identifier">unordered_multiset</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span> <span class="identifier">point_hash</span><span class="special">></span> <span class="identifier">points</span><span class="special">;</span> </pre> <p> </p> <p> </p> <p> Since the default hash function is <a class="link" href="../hash.html" title="Chapter 8. Boost.Functional/Hash">Boost.Hash</a>, we can <a class="link" href="../hash/custom.html" title="Extending boost::hash for a custom data type">extend it to support the type</a> so that the hash function doesn't need to be explicitly given: </p> <p> </p> <p> </p> <pre class="programlisting"><span class="keyword">struct</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="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">p1</span><span class="special">,</span> <span class="identifier">point</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">p2</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">p1</span><span class="special">.</span><span class="identifier">x</span> <span class="special">==</span> <span class="identifier">p2</span><span class="special">.</span><span class="identifier">x</span> <span class="special">&&</span> <span class="identifier">p1</span><span class="special">.</span><span class="identifier">y</span> <span class="special">==</span> <span class="identifier">p2</span><span class="special">.</span><span class="identifier">y</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">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> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash_combine</span><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> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash_combine</span><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="comment">// Now the default function objects work. </span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered_multiset</span><span class="special"><</span><span class="identifier">point</span><span class="special">></span> <span class="identifier">points</span><span class="special">;</span> </pre> <p> </p> <p> </p> <p> See the <a class="link" href="../hash/custom.html" title="Extending boost::hash for a custom data type">Boost.Hash documentation</a> for more detail on how to do this. Remember that it relies on extensions to the draft standard - so it won't work on other implementations of the unordered associative containers. </p> <div class="table"> <a name="id3028852"></a><p class="title"><b>Table 27.3. Methods for accessing the hash and equality functions.</b></p> <div class="table-contents"><table class="table" summary="Methods for accessing the hash and equality functions."> <colgroup> <col> <col> </colgroup> <thead><tr> <th> <p> Method </p> </th> <th> <p> Description </p> </th> </tr></thead> <tbody> <tr> <td> <p> <code class="computeroutput"><span class="identifier">hasher</span> <span class="identifier">hash_function</span><span class="special">()</span> <span class="keyword">const</span></code> </p> </td> <td> <p> Returns the container's hash function. </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">key_equal</span> <span class="identifier">key_eq</span><span class="special">()</span> <span class="keyword">const</span></code> </p> </td> <td> <p> Returns the container's key equality function. </p> </td> </tr> </tbody> </table></div> </div> <br class="table-break"> </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 © 2003, 2004 Jeremy B. Maitin-Shepard<br>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="buckets.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../unordered.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="comparison.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>