1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<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 V1.79.1">
<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset">
<link rel="up" href="../unordered.html" title="Chapter 41. 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>
<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>
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>
<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>
Which you can then use in a case insensitive dictionary:
</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>
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>
<h3>
<a name="unordered.hash_equality.h0"></a>
<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
Types</a>
</h3>
<p>
Similarly, a custom hash function can be used for custom types:
</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>
Since the default hash function is <a class="link" href="../hash.html" title="Chapter 14. 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>
<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>
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 for other implementations of the unordered associative
containers, you'll need to explicitly use Boost.Hash.
</p>
<div class="table">
<a name="unordered.hash_equality.access_methods"></a><p class="title"><b>Table 41.3. 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>
|
