1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Equality Predicates and Hash Functions</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../unordered.html" title="Chapter 44. Boost.Unordered"> 10<link rel="prev" href="buckets.html" title="The Data Structure"> 11<link rel="next" href="comparison.html" title="Comparison with Associative Containers"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<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> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<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> 29</h2></div></div></div> 30<p> 31 While the associative containers use an ordering relation to specify how the 32 elements are stored, the unordered associative containers use an equality predicate 33 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> 34 is declared as: 35 </p> 36<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span> 37 <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> 38 <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> 39 <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> 40 <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> 41<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> 42</pre> 43<p> 44 The hash function comes first as you might want to change the hash function 45 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 46 write: 47 </p> 48<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> 49 <span class="identifier">dictionary</span><span class="special">;</span> 50</pre> 51<p> 52 There is an <a href="../../../libs/unordered/examples/fnv1.hpp" target="_top">implementation 53 of FNV-1</a> in the examples directory. 54 </p> 55<p> 56 If you wish to use a different equality function, you will also need to use 57 a matching hash function. For example, to implement a case insensitive dictionary 58 you need to define a case insensitive equality predicate and hash function: 59 </p> 60<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">iequal_to</span> 61<span class="special">{</span> 62 <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> 63 <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> 64 <span class="special">{</span> 65 <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> 66 <span class="special">}</span> 67<span class="special">};</span> 68 69<span class="keyword">struct</span> <span class="identifier">ihash</span> 70<span class="special">{</span> 71 <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> 72 <span class="special">{</span> 73 <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> 74 <span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span> <span class="identifier">locale</span><span class="special">;</span> 75 76 <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> 77 <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> 78 <span class="special">{</span> 79 <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> 80 <span class="special">}</span> 81 82 <span class="keyword">return</span> <span class="identifier">seed</span><span class="special">;</span> 83 <span class="special">}</span> 84<span class="special">};</span> 85</pre> 86<p> 87 Which you can then use in a case insensitive dictionary: 88 </p> 89<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> 90 <span class="identifier">idictionary</span><span class="special">;</span> 91</pre> 92<p> 93 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> 94 which supports other locales and string types. 95 </p> 96<div class="caution"><table border="0" summary="Caution"> 97<tr> 98<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../doc/src/images/caution.png"></td> 99<th align="left">Caution</th> 100</tr> 101<tr><td align="left" valign="top"><p> 102 Be careful when using the equality (<code class="computeroutput"><span class="special">==</span></code>) 103 operator with custom equality predicates, especially if you're using a function 104 pointer. If you compare two containers with different equality predicates 105 then the result is undefined. For most stateless function objects this is 106 impossible - since you can only compare objects with the same equality predicate 107 you know the equality predicates must be equal. But if you're using function 108 pointers or a stateful equality predicate (e.g. boost::function) then you 109 can get into trouble. 110 </p></td></tr> 111</table></div> 112<h3> 113<a name="unordered.hash_equality.h0"></a> 114 <span class="phrase"><a name="unordered.hash_equality.custom_types"></a></span><a class="link" href="hash_equality.html#unordered.hash_equality.custom_types">Custom 115 Types</a> 116 </h3> 117<p> 118 Similarly, a custom hash function can be used for custom types: 119 </p> 120<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">point</span> <span class="special">{</span> 121 <span class="keyword">int</span> <span class="identifier">x</span><span class="special">;</span> 122 <span class="keyword">int</span> <span class="identifier">y</span><span class="special">;</span> 123<span class="special">};</span> 124 125<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> 126<span class="special">{</span> 127 <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> 128<span class="special">}</span> 129 130<span class="keyword">struct</span> <span class="identifier">point_hash</span> 131<span class="special">{</span> 132 <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> 133 <span class="special">{</span> 134 <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> 135 <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> 136 <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> 137 <span class="keyword">return</span> <span class="identifier">seed</span><span class="special">;</span> 138 <span class="special">}</span> 139<span class="special">};</span> 140 141<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> 142</pre> 143<p> 144 Since the default hash function is <a class="link" href="../hash.html" title="Chapter 10. Boost.ContainerHash">Boost.Hash</a>, 145 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 146 that the hash function doesn't need to be explicitly given: 147 </p> 148<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">point</span> <span class="special">{</span> 149 <span class="keyword">int</span> <span class="identifier">x</span><span class="special">;</span> 150 <span class="keyword">int</span> <span class="identifier">y</span><span class="special">;</span> 151<span class="special">};</span> 152 153<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> 154<span class="special">{</span> 155 <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> 156<span class="special">}</span> 157 158<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> 159 <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> 160 <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> 161 <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> 162 <span class="keyword">return</span> <span class="identifier">seed</span><span class="special">;</span> 163<span class="special">}</span> 164 165<span class="comment">// Now the default function objects work.</span> 166<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> 167</pre> 168<p> 169 See the <a class="link" href="../hash/custom.html" title="Extending boost::hash for a custom data type">Boost.Hash documentation</a> for more 170 detail on how to do this. Remember that it relies on extensions to the draft 171 standard - so it won't work for other implementations of the unordered associative 172 containers, you'll need to explicitly use Boost.Hash. 173 </p> 174<div class="table"> 175<a name="unordered.hash_equality.access_methods"></a><p class="title"><b>Table 44.3. Methods for accessing the hash and equality functions.</b></p> 176<div class="table-contents"><table class="table" summary="Methods for accessing the hash and equality functions."> 177<colgroup> 178<col> 179<col> 180</colgroup> 181<thead><tr> 182<th> 183 <p> 184 Method 185 </p> 186 </th> 187<th> 188 <p> 189 Description 190 </p> 191 </th> 192</tr></thead> 193<tbody> 194<tr> 195<td> 196 <p> 197 <code class="computeroutput"><span class="identifier">hasher</span> <span class="identifier">hash_function</span><span class="special">()</span> <span class="keyword">const</span></code> 198 </p> 199 </td> 200<td> 201 <p> 202 Returns the container's hash function. 203 </p> 204 </td> 205</tr> 206<tr> 207<td> 208 <p> 209 <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> 210 </p> 211 </td> 212<td> 213 <p> 214 Returns the container's key equality function. 215 </p> 216 </td> 217</tr> 218</tbody> 219</table></div> 220</div> 221<br class="table-break"> 222</div> 223<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 224<td align="left"></td> 225<td align="right"><div class="copyright-footer">Copyright © 2003, 2004 Jeremy B. Maitin-Shepard<br>Copyright © 2005-2008 Daniel 226 James<p> 227 Distributed under the Boost Software License, Version 1.0. (See accompanying 228 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>) 229 </p> 230</div></td> 231</tr></table> 232<hr> 233<div class="spirit-nav"> 234<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> 235</div> 236</body> 237</html> 238
