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<title>The MPL Reference Manual: Associative Sequence</title>
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<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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<div class="section" id="associative-sequence">
<h1><a class="toc-backref" href="./concepts.html#id1401">Associative Sequence</a></h1>
<div class="section" id="id29">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>An <a class="reference internal" href="./associative-sequence.html">Associative Sequence</a> is a <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a> that allows efficient retrieval of
elements based on keys. Unlike associative containers in the C++ Standard Library,
MPL associative sequences have no associated ordering relation. Instead,
<em>type identity</em> is used to impose an equivalence relation on keys, and the
order in which sequence elements are traversed during iteration is left
unspecified.</p>
</div>
<div class="section" id="id30">
<h3><a class="subsection-title" href="#definitions" name="definitions">Definitions</a></h3>
<ul class="simple" id="value-part">
<span id="key-part"></span><li>A <em>key</em> is a part of the element type used to identify and retrieve
the element within the sequence.</li>
<li>A <em>value</em> is a part of the element type retrievied from the sequence
by its key.</li>
</ul>
</div>
<div class="section" id="id31">
<h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3>
<p>In the following table and subsequent specifications, <tt class="literal"><span class="pre">s</span></tt> is an <a class="reference internal" href="./associative-sequence.html">Associative Sequence</a>,
<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>
<p>In addition to the requirements defined in <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>,
the following must be met:</p>
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<tr><th class="head">Expression</th>
<th class="head">Type</th>
<th class="head">Complexity</th>
</tr>
</thead>
<tbody valign="top">
<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>
<td>Boolean <a class="reference internal" href="./integral-constant.html">Integral Constant</a></td>
<td>Amortized constant time</td>
</tr>
<tr><td><tt class="literal"><span class="pre"><a href="./count.html" class="identifier">count</a>&lt;s,k&gt;::type</span></tt></td>
<td><a class="reference internal" href="./integral-constant.html">Integral Constant</a></td>
<td>Amortized constant time</td>
</tr>
<tr><td><tt class="literal"><span class="pre"><a href="./order.html" class="identifier">order</a>&lt;s,k&gt;::type</span></tt></td>
<td><a class="reference internal" 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>
<td>Amortized constant time</td>
</tr>
<tr><td><tt class="literal"><span class="pre"><a href="./at.html" class="identifier">at</a>&lt;s,k&gt;::type</span></tt></td>
<td>Any type</td>
<td>Amortized constant time</td>
</tr>
<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>
<td>Any type</td>
<td>Amortized constant time</td>
</tr>
<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>
<td>Any type</td>
<td>Amortized constant time</td>
</tr>
<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>
<td>Any type</td>
<td>Amortized constant time</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id32">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<p>The semantics of an expression are defined only
where they differ from, or are not defined in <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>.</p>
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<tr><th class="head">Expression</th>
<th class="head">Semantics</th>
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<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>
<td>A boolean <a class="reference internal" href="./integral-constant.html">Integral Constant</a> <tt class="literal"><span class="pre">c</span></tt> such that
<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
elements 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="reference internal" href="./has-key.html">has_key</a>.</td>
</tr>
<tr><td><tt class="literal"><span class="pre"><a href="./count.html" class="identifier">count</a>&lt;s,k&gt;::type</span></tt></td>
<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>;
see <a class="reference internal" href="./count.html">count</a>.</td>
</tr>
<tr><td><tt class="literal"><span class="pre"><a href="./order.html" class="identifier">order</a>&lt;s,k&gt;::type</span></tt></td>
<td>A unique unsigned <a class="reference internal" href="./integral-constant.html">Integral Constant</a> associated
with 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="reference internal" href="./order.html">order</a>.</td>
</tr>
<tr><td><pre class="first last literal-block">
<a href="./at.html" class="identifier">at</a>&lt;s,k&gt;::type
<a href="./at.html" class="identifier">at</a>&lt;s,k,def&gt;::type
</pre>
</td>
<td>The first element associated with 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="reference internal" href="./at.html">at</a>.</td>
</tr>
<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>
<td>The key part of the element <tt class="literal"><span class="pre">x</span></tt> that would be
used to identify <tt class="literal"><span class="pre">x</span></tt> in <tt class="literal"><span class="pre">s</span></tt>; see <a class="reference internal" href="./key-type.html">key_type</a>.</td>
</tr>
<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>
<td>The value part of the element <tt class="literal"><span class="pre">x</span></tt> that would be
used for <tt class="literal"><span class="pre">x</span></tt> in <tt class="literal"><span class="pre">s</span></tt>; see <a class="reference internal" href="./value-type.html">value_type</a>.</td>
</tr>
</tbody>
</table>
<!-- Invariants
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For any associative sequence ``s`` the following invariants always hold:

 * ??? -->
</div>
<div class="section" id="id33">
<h3><a class="subsection-title" href="#models" name="models">Models</a></h3>
<ul class="simple">
<li><a class="reference internal" href="./set.html">set</a></li>
<li><a class="reference internal" href="./map.html">map</a></li>
</ul>
<!-- * |multiset| -->
</div>
<div class="section" id="id34">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./sequences.html">Sequences</a>, <a class="reference internal" href="./extensible-associative-sequence.html">Extensible Associative Sequence</a>, <a class="reference internal" href="./has-key.html">has_key</a>, <a class="reference internal" href="./count.html">count</a>, <a class="reference internal" href="./order.html">order</a>, <a class="reference internal" href="./at.html">at</a>, <a class="reference internal" href="./key-type.html">key_type</a>, <a class="reference internal" href="./value-type.html">value_type</a></p>
<!-- Sequences/Concepts//Extensible Associative Sequence |80 -->
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