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27 <div class="titlepage"><div>
28 <div><h2 class="title">
29 <a name="unordered"></a>Chapter 44. Boost.Unordered</h2></div>
30 <div><div class="author"><h3 class="author">
31 <span class="firstname">Daniel</span> <span class="surname">James</span>
33 <div><p class="copyright">Copyright © 2003, 2004 Jeremy B. Maitin-Shepard</p></div>
34 <div><p class="copyright">Copyright © 2005-2008 Daniel
36 <div><div class="legalnotice">
37 <a name="unordered.legal"></a><p>
38 Distributed under the Boost Software License, Version 1.0. (See accompanying
39 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>)
44 <p><b>Table of Contents</b></p>
46 <dt><span class="section"><a href="unordered.html#unordered.intro">Introduction</a></span></dt>
47 <dt><span class="section"><a href="unordered/buckets.html">The Data Structure</a></span></dt>
48 <dt><span class="section"><a href="unordered/hash_equality.html">Equality Predicates and Hash Functions</a></span></dt>
49 <dt><span class="section"><a href="unordered/comparison.html">Comparison with Associative Containers</a></span></dt>
50 <dt><span class="section"><a href="unordered/compliance.html">C++11 Compliance</a></span></dt>
52 <dt><span class="section"><a href="unordered/compliance.html#unordered.compliance.move">Move emulation</a></span></dt>
53 <dt><span class="section"><a href="unordered/compliance.html#unordered.compliance.allocator_compliance">Use of allocators</a></span></dt>
54 <dt><span class="section"><a href="unordered/compliance.html#unordered.compliance.pairs">Pairs</a></span></dt>
55 <dt><span class="section"><a href="unordered/compliance.html#unordered.compliance.misc">Miscellaneous</a></span></dt>
57 <dt><span class="section"><a href="unordered/rationale.html">Implementation Rationale</a></span></dt>
58 <dt><span class="section"><a href="unordered/changes.html">Change Log</a></span></dt>
59 <dt><span class="section"><a href="unordered/reference.html">Reference</a></span></dt>
61 <dt><span class="section"><a href="unordered/reference.html#header.boost.unordered_set_hpp">Header <boost/unordered_set.hpp></a></span></dt>
62 <dt><span class="section"><a href="unordered/reference.html#header.boost.unordered_map_hpp">Header <boost/unordered_map.hpp></a></span></dt>
64 <dt><span class="section"><a href="unordered/bibliography.html">Bibliography</a></span></dt>
68 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
69 <a name="unordered.intro"></a><a class="link" href="unordered.html#unordered.intro" title="Introduction">Introduction</a>
70 </h2></div></div></div>
72 For accessing data based on key lookup, the C++ standard library offers <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">set</span></code>,
73 <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">multiset</span></code>
74 and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">multimap</span></code>. These are generally implemented
75 using balanced binary trees so that lookup time has logarithmic complexity.
76 That is generally okay, but in many cases a <a href="http://en.wikipedia.org/wiki/Hash_table" target="_top">hash
77 table</a> can perform better, as accessing data has constant complexity,
78 on average. The worst case complexity is linear, but that occurs rarely and
79 with some care, can be avoided.
82 Also, the existing containers require a 'less than' comparison object to order
83 their elements. For some data types this is impossible to implement or isn't
84 practical. In contrast, a hash table only needs an equality function and a
85 hash function for the key.
88 With this in mind, unordered associative containers were added to the C++ standard.
89 This is an implementation of the containers described in C++11, with some
90 <a class="link" href="unordered/compliance.html" title="C++11 Compliance">deviations from the standard</a> in
91 order to work with non-C++11 compilers and libraries.
94 <code class="computeroutput"><span class="identifier">unordered_set</span></code> and <code class="computeroutput"><span class="identifier">unordered_multiset</span></code> are defined in the header
95 <<code class="computeroutput"><a class="link" href="unordered/reference.html#header.boost.unordered_set_hpp" title="Header <boost/unordered_set.hpp>">boost/unordered_set.hpp</a></code>>
97 <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
98 <span class="keyword">template</span> <span class="special"><</span>
99 <span class="keyword">class</span> <span class="identifier">Key</span><span class="special">,</span>
100 <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>
101 <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>
102 <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">Key</span><span class="special">></span> <span class="special">></span>
103 <span class="keyword">class</span> <code class="computeroutput"><a class="link" href="boost/unordered_set.html" title="Class template unordered_set">unordered_set</a></code><span class="special">;</span>
105 <span class="keyword">template</span><span class="special"><</span>
106 <span class="keyword">class</span> <span class="identifier">Key</span><span class="special">,</span>
107 <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>
108 <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>
109 <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">Key</span><span class="special">></span> <span class="special">></span>
110 <span class="keyword">class</span> <code class="computeroutput"><a class="link" href="boost/unordered_multiset.html" title="Class template unordered_multiset">unordered_multiset</a></code><span class="special">;</span>
111 <span class="special">}</span>
114 <code class="computeroutput"><span class="identifier">unordered_map</span></code> and <code class="computeroutput"><span class="identifier">unordered_multimap</span></code> are defined in the header
115 <<code class="computeroutput"><a class="link" href="unordered/reference.html#header.boost.unordered_map_hpp" title="Header <boost/unordered_map.hpp>">boost/unordered_map.hpp</a></code>>
117 <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
118 <span class="keyword">template</span> <span class="special"><</span>
119 <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>
120 <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>
121 <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>
122 <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>
123 <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>
125 <span class="keyword">template</span><span class="special"><</span>
126 <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>
127 <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>
128 <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>
129 <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>
130 <span class="keyword">class</span> <code class="computeroutput"><a class="link" href="boost/unordered_multimap.html" title="Class template unordered_multimap">unordered_multimap</a></code><span class="special">;</span>
131 <span class="special">}</span>
134 When using Boost.TR1, these classes are included from <code class="computeroutput"><span class="special"><</span><span class="identifier">unordered_set</span><span class="special">></span></code>
135 and <code class="computeroutput"><span class="special"><</span><span class="identifier">unordered_map</span><span class="special">></span></code>, with the classes added to the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">tr1</span></code>
139 The containers are used in a similar manner to the normal associative containers:
143 <pre class="programlisting"><span class="keyword">typedef</span> <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">map</span><span class="special">;</span>
144 <span class="identifier">map</span> <span class="identifier">x</span><span class="special">;</span>
145 <span class="identifier">x</span><span class="special">[</span><span class="string">"one"</span><span class="special">]</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span>
146 <span class="identifier">x</span><span class="special">[</span><span class="string">"two"</span><span class="special">]</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
147 <span class="identifier">x</span><span class="special">[</span><span class="string">"three"</span><span class="special">]</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span>
149 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">at</span><span class="special">(</span><span class="string">"one"</span><span class="special">)</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span>
150 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="string">"missing"</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
155 But since the elements aren't ordered, the output of:
159 <pre class="programlisting"><span class="identifier">BOOST_FOREACH</span><span class="special">(</span><span class="identifier">map</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span>
160 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">i</span><span class="special">.</span><span class="identifier">first</span><span class="special"><<</span><span class="string">","</span><span class="special"><<</span><span class="identifier">i</span><span class="special">.</span><span class="identifier">second</span><span class="special"><<</span><span class="string">"\n"</span><span class="special">;</span>
161 <span class="special">}</span>
166 can be in any order. For example, it might be:
168 <pre class="programlisting"><span class="identifier">two</span><span class="special">,</span><span class="number">2</span>
169 <span class="identifier">one</span><span class="special">,</span><span class="number">1</span>
170 <span class="identifier">three</span><span class="special">,</span><span class="number">3</span>
173 To store an object in an unordered associative container requires both an key
174 equality function and a hash function. The default function objects in the
175 standard containers support a few basic types including integer types, floating
176 point types, pointer types, and the standard strings. Since Boost.Unordered
177 uses <code class="computeroutput"><a class="link" href="boost/hash.html" title="Struct template hash">boost::hash</a></code> it also supports
178 some other types, including standard containers. To use any types not supported
179 by these methods you have to <a class="link" href="hash/custom.html" title="Extending boost::hash for a custom data type">extend Boost.Hash
180 to support the type</a> or use your own custom equality predicates and hash
181 functions. See the <a class="link" href="unordered/hash_equality.html" title="Equality Predicates and Hash Functions">Equality Predicates
182 and Hash Functions</a> section for more details.
185 There are other differences, which are listed in the <a class="link" href="unordered/comparison.html" title="Comparison with Associative Containers">Comparison
186 with Associative Containers</a> section.
190 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
191 <td align="left"><p><small>Last revised: April 17, 2017 at 02:30:57 GMT</small></p></td>
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