boost_mpl/doc/refmanual/trivial-metafunction.html

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<div class="section" id="trivial-metafunction">
<h1><a class="toc-backref" href="./metafunctions-concepts.html#id1518">Trivial Metafunction</a></h1>
<div class="section" id="id853">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>A <a class="reference internal" href="./trivial-metafunction.html">Trivial Metafunction</a> accepts a single argument of a class type <tt class="literal"><span class="pre">x</span></tt> and
returns the <tt class="literal"><span class="pre">x</span></tt>'s nested type member <tt class="literal"><span class="pre">x::name</span></tt>, where <tt class="literal"><span class="pre">name</span></tt> is
a placeholder token for the actual member's name accessed by a specific
metafunction's instance. By convention, all <a class="reference internal" href="./trivial-metafunctions-summary.html">trivial metafunctions</a> in MPL
are named after the members they provide assess to. For instance, a <a class="reference internal" href="./trivial-metafunction.html">Trivial Metafunction</a> named <tt class="literal"><span class="pre">first</span></tt> reaches for the <tt class="literal"><span class="pre">x</span></tt>'s nested member
<tt class="literal"><span class="pre">::first</span></tt>.</p>
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<div class="section" id="id855">
<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">name</span></tt> is placeholder token for the names of
the <a class="reference internal" href="./trivial-metafunction.html">Trivial Metafunction</a> itself and the accessed member, and <tt class="literal"><span class="pre">x</span></tt> is
a class type such that <tt class="literal"><span class="pre">x::name</span></tt> is a valid <em>type-name</em>.</p>
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<tr><th class="head">Expression</th>
<th class="head">Type</th>
<th class="head">Complexity</th>
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<tr><td><tt class="literal"><span class="pre">name&lt;x&gt;::type</span></tt></td>
<td>Any type</td>
<td>Constant time.</td>
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<div class="section" id="id856">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<pre class="literal-block">
typedef name&lt;x&gt;::type r;
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<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><tt class="literal"><span class="pre">x::name</span></tt> is a valid <em>type-name</em>.</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">is_same&lt;r,x::name&gt;::value</span> <span class="pre">==</span> <span class="pre">true</span></tt>.</td>
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<div class="section" id="id857">
<h3><a class="subsection-title" href="#models" name="models">Models</a></h3>
<ul class="simple">
<li><a class="reference internal" href="./trivial-metafunctions-summary.html#first">first</a></li>
<li><a class="reference internal" href="./trivial-metafunctions-summary.html#second">second</a></li>
<li><a class="reference internal" href="./trivial-metafunctions-summary.html#base">base</a></li>
</ul>
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<div class="section" id="id858">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./metafunctions.html">Metafunctions</a>, <a class="reference internal" href="./trivial.html#label-metafunctions-trivial">Trivial Metafunctions</a>, <a class="reference internal" href="./identity.html">identity</a></p>
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