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12<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./sequences.html" class="navigation-link">Sequences</a> / <a href="./concepts.html" class="navigation-link">Concepts</a> / <a href="./associative-sequence.html" class="navigation-link">Associative Sequence</a></td>
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14<div class="section" id="associative-sequence">
15<h1><a class="toc-backref" href="./concepts.html#id330" name="associative-sequence">Associative Sequence</a></h1>
16<div class="section" id="associative-description">
17<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
18<p>An <a class="reference" href="./associative-sequence.html">Associative Sequence</a> is a <a class="reference" href="./forward-sequence.html">Forward Sequence</a> that allows efficient retrieval of
19elements based on keys. Unlike associative containers in the C++ Standard Library,
20MPL associative sequences have no associated ordering relation. Instead,
21<em>type identity</em> is used to impose an equivalence relation on keys, and the
22order in which sequence elements are traversed during iteration is left
23unspecified.</p>
24</div>
25<div class="section" id="associative-definitions">
26<h3><a class="subsection-title" href="#definitions" name="definitions">Definitions</a></h3>
27<a class="target" id="key-part" name="key-part"></a><a class="target" id="value-part" name="value-part"></a><ul class="simple">
28<li>A <em>key</em> is a part of the element type used to identify and retrieve
29the element within the sequence.</li>
30<li>A <em>value</em> is a part of the element type retrievied from the sequence
31by its key.</li>
32</ul>
33</div>
34<div class="section" id="associative-expression-requirements">
35<h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3>
36<p>In the following table and subsequent specifications, <tt class="literal"><span class="pre">s</span></tt> is an <a class="reference" href="./associative-sequence.html">Associative Sequence</a>,
37<tt class="literal"><span class="pre">x</span></tt> is a sequence element, and <tt class="literal"><span class="pre">k</span></tt> and <tt class="literal"><span class="pre">def</span></tt> are arbitrary types.</p>
38<p>In addition to the requirements defined in <a class="reference" href="./forward-sequence.html">Forward Sequence</a>,
39the following must be met:</p>
40<table border="1" class="table">
41<colgroup>
42<col width="33%" />
43<col width="38%" />
44<col width="29%" />
45</colgroup>
46<thead valign="bottom">
47<tr><th>Expression</th>
48<th>Type</th>
49<th>Complexity</th>
50</tr>
51</thead>
52<tbody valign="top">
53<tr><td><tt class="literal"><span class="pre"><a href="./has-key.html" class="identifier">has_key</a>&lt;s,k&gt;::type</span></tt></td>
54<td>Boolean <a class="reference" href="./integral-constant.html">Integral Constant</a></td>
55<td>Amortized constant time</td>
56</tr>
57<tr><td><tt class="literal"><span class="pre"><a href="./count.html" class="identifier">count</a>&lt;s,k&gt;::type</span></tt></td>
58<td><a class="reference" href="./integral-constant.html">Integral Constant</a></td>
59<td>Amortized constant time</td>
60</tr>
61<tr><td><tt class="literal"><span class="pre"><a href="./order.html" class="identifier">order</a>&lt;s,k&gt;::type</span></tt></td>
62<td><a class="reference" href="./integral-constant.html">Integral Constant</a> or <tt class="literal"><span class="pre"><a href="./void.html" class="identifier">void_</a></span></tt></td>
63<td>Amortized constant time</td>
64</tr>
65<tr><td><tt class="literal"><span class="pre"><a href="./at.html" class="identifier">at</a>&lt;s,k&gt;::type</span></tt></td>
66<td>Any type</td>
67<td>Amortized constant time</td>
68</tr>
69<tr><td><tt class="literal"><span class="pre"><a href="./at.html" class="identifier">at</a>&lt;s,k,def&gt;::type</span></tt></td>
70<td>Any type</td>
71<td>Amortized constant time</td>
72</tr>
73<tr><td><tt class="literal"><span class="pre"><a href="./key-type.html" class="identifier">key_type</a>&lt;s,x&gt;::type</span></tt></td>
74<td>Any type</td>
75<td>Amortized constant time</td>
76</tr>
77<tr><td><tt class="literal"><span class="pre"><a href="./value-type.html" class="identifier">value_type</a>&lt;s,x&gt;::type</span></tt></td>
78<td>Any type</td>
79<td>Amortized constant time</td>
80</tr>
81</tbody>
82</table>
83</div>
84<div class="section" id="associative-expression-semantics">
85<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
86<p>The semantics of an expression are defined only
87where they differ from, or are not defined in <a class="reference" href="./forward-sequence.html">Forward Sequence</a>.</p>
88<table border="1" class="table">
89<colgroup>
90<col width="33%" />
91<col width="67%" />
92</colgroup>
93<thead valign="bottom">
94<tr><th>Expression</th>
95<th>Semantics</th>
96</tr>
97</thead>
98<tbody valign="top">
99<tr><td><tt class="literal"><span class="pre"><a href="./has-key.html" class="identifier">has_key</a>&lt;s,k&gt;::type</span></tt></td>
100<td>A boolean <a class="reference" href="./integral-constant.html">Integral Constant</a> <tt class="literal"><span class="pre">c</span></tt> such that
101<tt class="literal"><span class="pre">c::value</span> <span class="pre">==</span> <span class="pre">true</span></tt> if and only if there is one or more
102elements with the key <tt class="literal"><span class="pre">k</span></tt> in <tt class="literal"><span class="pre">s</span></tt>; see <a class="refentry reference" href="./has-key.html"><tt class="refentry literal"><span class="pre">has_key</span></tt></a>.</td>
103</tr>
104<tr><td><tt class="literal"><span class="pre"><a href="./count.html" class="identifier">count</a>&lt;s,k&gt;::type</span></tt></td>
105<td>The number of elements with the key <tt class="literal"><span class="pre">k</span></tt> in <tt class="literal"><span class="pre">s</span></tt>;
106see <a class="refentry reference" href="./count.html"><tt class="refentry literal"><span class="pre">count</span></tt></a>.</td>
107</tr>
108<tr><td><tt class="literal"><span class="pre"><a href="./order.html" class="identifier">order</a>&lt;s,k&gt;::type</span></tt></td>
109<td>A unique unsigned <a class="reference" href="./integral-constant.html">Integral Constant</a> associated
110with the key <tt class="literal"><span class="pre">k</span></tt> in the sequence <tt class="literal"><span class="pre">s</span></tt>; see <a class="refentry reference" href="./order.html"><tt class="refentry literal"><span class="pre">order</span></tt></a>.</td>
111</tr>
112<tr><td><pre class="first last literal-block">
113<a href="./at.html" class="identifier">at</a>&lt;s,k&gt;::type
114<a href="./at.html" class="identifier">at</a>&lt;s,k,def&gt;::type
115</pre>
116</td>
117<td>The first element associated with the key <tt class="literal"><span class="pre">k</span></tt>
118in the sequence <tt class="literal"><span class="pre">s</span></tt>; see <a class="refentry reference" href="./at.html"><tt class="refentry literal"><span class="pre">at</span></tt></a>.</td>
119</tr>
120<tr><td><tt class="literal"><span class="pre"><a href="./key-type.html" class="identifier">key_type</a>&lt;s,x&gt;::type</span></tt></td>
121<td>The key part of the element <tt class="literal"><span class="pre">x</span></tt> that would be
122used to identify <tt class="literal"><span class="pre">x</span></tt> in <tt class="literal"><span class="pre">s</span></tt>; see <a class="refentry reference" href="./key-type.html"><tt class="refentry literal"><span class="pre">key_type</span></tt></a>.</td>
123</tr>
124<tr><td><tt class="literal"><span class="pre"><a href="./value-type.html" class="identifier">value_type</a>&lt;s,x&gt;::type</span></tt></td>
125<td>The value part of the element <tt class="literal"><span class="pre">x</span></tt> that would be
126used for <tt class="literal"><span class="pre">x</span></tt> in <tt class="literal"><span class="pre">s</span></tt>; see <a class="refentry reference" href="./value-type.html"><tt class="refentry literal"><span class="pre">value_type</span></tt></a>.</td>
127</tr>
128</tbody>
129</table>
130<!-- Invariants
131- - - - - - - - - -
132
133For any associative sequence ``s`` the following invariants always hold:
134
135 * ??? -->
136</div>
137<div class="section" id="associative-models">
138<h3><a class="subsection-title" href="#models" name="models">Models</a></h3>
139<ul class="simple">
140<li><a class="refentry reference" href="./set.html"><tt class="refentry literal"><span class="pre">set</span></tt></a></li>
141<li><a class="refentry reference" href="./map.html"><tt class="refentry literal"><span class="pre">map</span></tt></a></li>
142</ul>
143<!-- * |multiset| -->
144</div>
145<div class="section" id="associative-see-also">
146<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
147<p><a class="reference" href="./sequences.html">Sequences</a>, <a class="reference" href="./extensible-associative.html">Extensible Associative Sequence</a>, <a class="refentry reference" href="./has-key.html"><tt class="refentry literal"><span class="pre">has_key</span></tt></a>, <a class="refentry reference" href="./count.html"><tt class="refentry literal"><span class="pre">count</span></tt></a>, <a class="refentry reference" href="./order.html"><tt class="refentry literal"><span class="pre">order</span></tt></a>, <a class="refentry reference" href="./at.html"><tt class="refentry literal"><span class="pre">at</span></tt></a>, <a class="refentry reference" href="./key-type.html"><tt class="refentry literal"><span class="pre">key_type</span></tt></a>, <a class="refentry reference" href="./value-type.html"><tt class="refentry literal"><span class="pre">value_type</span></tt></a></p>
148<!-- modtime: November 13, 2004 00:51:14 +0000 -->
149<!-- Sequences/Concepts//Extensible Associative Sequence |80 -->
150</div>
151</div>
152
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