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  <title>noncopyable and nonmovable utility classes</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">noncopyable and nonmovable
utility classes</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P2895</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2023-05-17</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      LEWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Sebastian Theophil<br>&lt;<a href="mailto:stheophil@think-cell.com" class="email">stheophil@think-cell.com</a>&gt;<br>
      Jonathan Müller<br>&lt;<a href="mailto:jmueller@think-cell.com" class="email">jmueller@think-cell.com</a>&gt;<br>
    </td>
  </tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#abstract" id="toc-abstract"><span class="toc-section-number">1</span> Abstract</a></li>
<li><a href="#motivation" id="toc-motivation"><span class="toc-section-number">2</span> Motivation</a></li>
<li><a href="#naming" id="toc-naming"><span class="toc-section-number">3</span> Naming</a></li>
<li><a href="#proposed-implementation" id="toc-proposed-implementation"><span class="toc-section-number">4</span> Proposed Implementation</a></li>
<li><a href="#acknowledgements" id="toc-acknowledgements"><span class="toc-section-number">5</span> Acknowledgements</a></li>
<li><a href="#wording" id="toc-wording"><span class="toc-section-number">6</span> Wording</a></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">7</span> References</a></li>
</ul>
</div>
<h1 data-number="1" id="abstract"><span class="header-section-number">1</span> Abstract<a href="#abstract" class="self-link"></a></h1>
<p>We propose the addition of
<code class="sourceCode default">std::noncopyable</code> and
<code class="sourceCode default">std::nonmovable</code>, utility classes
you can inherit from to make your class non-copyable and/or
non-movable.</p>
<h1 data-number="2" id="motivation"><span class="header-section-number">2</span> Motivation<a href="#motivation" class="self-link"></a></h1>
<p>Developers may want to declare that a class is a move-only type,
i.e., that it is movable but non-copyable. This is often useful for RAII
classes that must not duplicate the resource they manage. Similarly,
developers may want to declare that a class is neither movable nor
copyable. Consider the following example that deals with a legacy
API:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Before</strong>
</div></th>
<th><div style="text-align:center">
<strong>After</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div>

<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a>```cpp</span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="kw">extern</span> <span class="st">&quot;C&quot;</span> RegisterCallbackInLegacyAPI<span class="op">(</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>      <span class="dt">void</span> <span class="op">(*</span> callback<span class="op">)(</span><span class="dt">void</span> <span class="kw">const</span><span class="op">*)</span>, <span class="dt">void</span> <span class="kw">const</span><span class="op">*</span> context<span class="op">)</span>;</span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a><span class="kw">static</span> <span class="dt">void</span> callback<span class="op">(</span><span class="dt">void</span> <span class="kw">const</span><span class="op">*</span> context<span class="op">)</span> <span class="kw">noexcept</span>;</span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a><span class="kw">class</span> legacy_callback <span class="op">{</span></span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a>    legacy_callback<span class="op">(</span>legacy_callback <span class="kw">const</span><span class="op">&amp;)</span> <span class="op">=</span> <span class="kw">delete</span>;</span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a>    legacy_callback<span class="op">&amp;</span> <span class="kw">operator</span><span class="op">=(</span>legacy_callback <span class="kw">const</span><span class="op">&amp;)</span> <span class="op">=</span> <span class="kw">delete</span>;</span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a>    <span class="dt">void</span> register_callback<span class="op">()</span> <span class="kw">noexcept</span> <span class="op">{</span></span>
<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a>        RegisterCallbackInLegacyAPI<span class="op">(</span>callback, <span class="kw">this</span><span class="op">)</span>;</span>
<span id="cb1-13"><a href="#cb1-13" aria-hidden="true" tabindex="-1"></a>    <span class="op">}</span></span>
<span id="cb1-14"><a href="#cb1-14" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>

</div></td>
<td><div>

<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="kw">extern</span> <span class="st">&quot;C&quot;</span> RegisterCallbackInLegacyAPI<span class="op">(</span></span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a>      <span class="dt">void</span> <span class="op">(*</span> callback<span class="op">)(</span><span class="dt">void</span> <span class="kw">const</span><span class="op">*)</span>, <span class="dt">void</span> <span class="kw">const</span><span class="op">*</span> context<span class="op">)</span>;</span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a><span class="kw">static</span> <span class="dt">void</span> callback<span class="op">(</span><span class="dt">void</span> <span class="kw">const</span><span class="op">*</span> context<span class="op">)</span> <span class="kw">noexcept</span>;</span>
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-6"><a href="#cb2-6" aria-hidden="true" tabindex="-1"></a><span class="kw">class</span> legacy_callback <span class="op">:</span> std<span class="op">::</span>nonmovable <span class="op">{</span></span>
<span id="cb2-7"><a href="#cb2-7" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-8"><a href="#cb2-8" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-9"><a href="#cb2-9" aria-hidden="true" tabindex="-1"></a>    <span class="dt">void</span> register_callback<span class="op">()</span> <span class="kw">noexcept</span> <span class="op">{</span></span>
<span id="cb2-10"><a href="#cb2-10" aria-hidden="true" tabindex="-1"></a>        RegisterCallbackInLegacyAPI<span class="op">(</span>callback, <span class="kw">this</span><span class="op">)</span>;</span>
<span id="cb2-11"><a href="#cb2-11" aria-hidden="true" tabindex="-1"></a>    <span class="op">}</span></span>
<span id="cb2-12"><a href="#cb2-12" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>

</div></td>
</tr>
</tbody>
</table>
<p>Passing the <code class="sourceCode default">this</code> pointer as
context to a legacy callback requires that
<code class="sourceCode default">this</code> remains constant.
<code class="sourceCode default">std::mutex</code> is also an object
that is neither copyable nor movable.</p>
<p>By declaring the copy-constructor and the copy-assignment operator of
a class as deleted, developers can make their own classes non-copyable
and non-movable. This seems straight-forward. Yet, users get this wrong.
Indeed, two very popular StackOverflow answers on how to make an object
non-copyable and non-movable were initially wrong, as the comments show.
<a href="https://stackoverflow.com/questions/7823990/what-are-the-advantages-of-boostnoncopyable">1</a>
<a href="https://stackoverflow.com/questions/31940886/is-there-a-stdnoncopyable-or-equivalent">2</a>.</p>
<p>Many libraries such as <a href="https://www.boost.org/doc/libs/1_82_0/libs/core/doc/html/core/noncopyable.html">boost</a>
have introduced a type called
<code class="sourceCode default">noncopyable</code> (or similar) that
user-defined classes can derive from and that deletes both
copy-construction and copy-assignment. Deriving from
<code class="sourceCode default">noncopyable</code> lets users declare
their <em>intent</em> clearly and succinctly. <a href="https://os.mbed.com/docs/mbed-os/v6.16/apis/noncopyable.html">3</a>
<a href="https://www.sfml-dev.org/documentation/2.5.1/classsf_1_1NonCopyable.php">4</a>
<a href="https://www.nsnam.org/docs/release/3.29/doxygen/classns3_1_1_non_copyable.html">5</a>
<a href="https://github.com/adobe/webkit/blob/master/Source/WTF/wtf/Noncopyable.h">6</a>
<a href="https://docs.nvidia.com/jetson/archives/l4t-multimedia-archived/l4t-multimedia-281/classArgus_1_1NonCopyable.html">7</a>
<a href="https://github.com/think-cell/think-cell-library/blob/main/tc/base/noncopyable.h">8</a></p>
<p><a href="https://codesearch.isocpp.org/cgi-bin/cgi_ppsearch?q=noncopyable&amp;search=Search">A
code search for “noncopyable” yields almost 5000 hits.</a> It has thus
clearly become a common idiom that is frequently used and reimplemented
and therefore deserves to be included in the standard library.</p>
<h1 data-number="3" id="naming"><span class="header-section-number">3</span> Naming<a href="#naming" class="self-link"></a></h1>
<p>Unfortunately,
<code class="sourceCode default">boost::noncopyable</code> (and all
other referenced implementations except <a href="https://github.com/think-cell/think-cell-library/blob/main/tc/base/noncopyable.h">8</a>)
makes a class non-copyable <strong>and</strong> non-movable because the
implementation of
<code class="sourceCode default">boost::noncopyable</code> precedes the
introduction of move semantics. This collides with the names of already
standardized concepts
<code class="sourceCode default">std::copyable</code> and
<code class="sourceCode default">std::movable</code>. A class may not
satisfy <code class="sourceCode default">std::copyable</code> yet may be
<code class="sourceCode default">std::movable</code>. We propose to
introduce types <code class="sourceCode default">noncopyable</code> and
<code class="sourceCode default">nonmovable</code> that match the names
of the already standardized concepts. Thus, a class deriving from
<code class="sourceCode default">noncopyable</code> cannot satisfy
<code class="sourceCode default">std::copyable</code> but may satisfy
<code class="sourceCode default">std::movable</code>. A class deriving
from <code class="sourceCode default">nonmovable</code> cannot satisfy
<code class="sourceCode default">std::movable</code> and thus cannot
satisfy <code class="sourceCode default">std::copyable</code>
either.</p>
<p>Type <code class="sourceCode default">noncopyable</code> could also
be called <code class="sourceCode default">moveonly</code> or
<code class="sourceCode default">move_only</code>.</p>
<p>According to <a href="https://codesearch.isocpp.org">codesearch.isocpp.org</a>, the name
<code class="sourceCode default">nonmovable</code> has only been used in
the <a href="https://codesearch.isocpp.org/actcd19/main/r/range-v3/range-v3_0.4.0-1/test/utility/concepts.cpp">test
framework for the range v3 library</a> together with a type
<code class="sourceCode default">moveonly</code>. The alternative
spelling <code class="sourceCode default">move_only</code> has also
often been used in test frameworks for libcxx, libstdc++, boost.hana and
gcc.</p>
<h1 data-number="4" id="proposed-implementation"><span class="header-section-number">4</span> Proposed Implementation<a href="#proposed-implementation" class="self-link"></a></h1>
<p>We propose the following definitions for both classes:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> noncopyable <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a>    noncopyable<span class="op">()</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a>    noncopyable<span class="op">(</span>noncopyable<span class="op">&amp;&amp;)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a>    noncopyable<span class="op">&amp;</span> <span class="kw">operator</span><span class="op">=(</span>noncopyable<span class="op">&amp;&amp;)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span>
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb3-7"><a href="#cb3-7" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> nonmovable <span class="op">{</span></span>
<span id="cb3-8"><a href="#cb3-8" aria-hidden="true" tabindex="-1"></a>    nonmovable<span class="op">()</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb3-9"><a href="#cb3-9" aria-hidden="true" tabindex="-1"></a>    nonmovable<span class="op">(</span>nonmovable <span class="kw">const</span><span class="op">&amp;)</span> <span class="op">=</span> <span class="kw">delete</span>;</span>
<span id="cb3-10"><a href="#cb3-10" aria-hidden="true" tabindex="-1"></a>    nonmovable<span class="op">&amp;</span> <span class="kw">operator</span><span class="op">=(</span>nonmovable <span class="kw">const</span><span class="op">&amp;)</span> <span class="op">=</span> <span class="kw">delete</span>;</span>
<span id="cb3-11"><a href="#cb3-11" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>
<p>In both classes, the default constructor is declared as default and
is thus trivial. <code class="sourceCode default">noncopyable</code> and
<code class="sourceCode default">nonmovable</code> are therefore empty
standard layout classes and “empty base optimization” is required when
deriving from either. Deriving from
<code class="sourceCode default">noncopyable</code> or
<code class="sourceCode default">nonmovable</code> therefore incurs no
space or runtime overhead.</p>
<h1 data-number="5" id="acknowledgements"><span class="header-section-number">5</span> Acknowledgements<a href="#acknowledgements" class="self-link"></a></h1>
<p>We thank Alisdair Meredith for the original paper proposing to
standardize the behavior of boost::noncopyable. <span class="citation" data-cites="N2675">[<a href="#ref-N2675" role="doc-biblioref">N2675</a>]</span>.</p>
<h1 data-number="6" id="wording"><span class="header-section-number">6</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Add to header <code class="sourceCode default">&lt;utility&gt;</code>
synopsis in <span>22.2.1
<a href="https://wg21.link/utility.syn">[utility.syn]</a></span></p>
<div class="add" style="color: #006e28">

<div class="sourceCode" id="cb4"><pre class="sourceCode default cpp"><code class="sourceCode default"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a>// [utility.noncopyable], class noncopyable</span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a>namespace noncopyable-adl-namespace {</span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a>   struct noncopyable;</span>
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a>}</span>
<span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a>using noncopyable-nonmovable-adl-namespace::noncopyable;</span>
<span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a>// [utility.nonmovable], class nonmovable</span>
<span id="cb4-8"><a href="#cb4-8" aria-hidden="true" tabindex="-1"></a>namespace nonmovable-adl-namespace {</span>
<span id="cb4-9"><a href="#cb4-9" aria-hidden="true" tabindex="-1"></a>   struct nonmovable;</span>
<span id="cb4-10"><a href="#cb4-10" aria-hidden="true" tabindex="-1"></a>}</span>
<span id="cb4-11"><a href="#cb4-11" aria-hidden="true" tabindex="-1"></a>using nonmovable-adl-namespace::nonmovable;</span></code></pre></div>

</div>
<p>Append a new section to <span>22.2
<a href="https://wg21.link/utility">[utility]</a></span></p>
<div class="add" style="color: #006e28">

<h2 class="unnumbered" id="x-support-classes-utility.support">22.2.x
Support Classes [utility.support]<a href="#x-support-classes-utility.support" class="self-link"></a></h2>
<p>The following classes are provided to simplify the implementation of
common idioms.</p>
<h3 class="unnumbered" id="x.1-class-noncopyable-utility.noncopyable">22.2.x.1 Class
<code class="sourceCode default">noncopyable</code>
[utility.noncopyable]<a href="#x.1-class-noncopyable-utility.noncopyable" class="self-link"></a></h3>
<div class="sourceCode" id="cb5"><pre class="sourceCode default cpp"><code class="sourceCode default"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a>namespace noncopyable-adl-namespace {</span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a>    struct noncopyable {</span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a>        noncopyable() = default;</span>
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a>        noncopyable(noncopyable&amp;&amp;) = default;</span>
<span id="cb5-5"><a href="#cb5-5" aria-hidden="true" tabindex="-1"></a>        noncopyable&amp; operator=(noncopyable&amp;&amp;) = default;</span>
<span id="cb5-6"><a href="#cb5-6" aria-hidden="true" tabindex="-1"></a>    };</span>
<span id="cb5-7"><a href="#cb5-7" aria-hidden="true" tabindex="-1"></a>}</span>
<span id="cb5-8"><a href="#cb5-8" aria-hidden="true" tabindex="-1"></a>using noncopyable-adl-namespace::noncopyable;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span>
<code class="sourceCode default">noncopyable</code> is provided to
simplify creation of classes that have move-only semantics, i.e. they
are movable but not copyable.</p>
<p>[<em>Note</em>: <code class="sourceCode default">noncopyable</code>
is provided in an unspecified nested namespace to limit argument
dependent lookup <span>6.5.4
<a href="https://wg21.link/basic.lookup.argdep">[basic.lookup.argdep]</a></span>;
no other names should be declared in this namespace. — <em>end
note</em>]</p>
<p>[<em>Example</em>:</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode default cpp"><code class="sourceCode default"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a>class file : std::noncopyable {</span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a>public:</span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a>    file(std::string const&amp; strPath)</span>
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a>     : fp(std::fopen(strPath.c_str(), &quot;w&quot;))</span>
<span id="cb6-5"><a href="#cb6-5" aria-hidden="true" tabindex="-1"></a>    {}</span>
<span id="cb6-6"><a href="#cb6-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb6-7"><a href="#cb6-7" aria-hidden="true" tabindex="-1"></a>    file(file&amp;&amp; f) : fp(std::exchange(f.fp, nullptr)) {}</span>
<span id="cb6-8"><a href="#cb6-8" aria-hidden="true" tabindex="-1"></a>    file&amp; operator=(file&amp;&amp; f) { ... }</span>
<span id="cb6-9"><a href="#cb6-9" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb6-10"><a href="#cb6-10" aria-hidden="true" tabindex="-1"></a>    ~file() { if(fp) std::fclose(fp); }</span>
<span id="cb6-11"><a href="#cb6-11" aria-hidden="true" tabindex="-1"></a>private:</span>
<span id="cb6-12"><a href="#cb6-12" aria-hidden="true" tabindex="-1"></a>    std::FILE* fp;</span>
<span id="cb6-13"><a href="#cb6-13" aria-hidden="true" tabindex="-1"></a>};</span></code></pre></div>
<p>— <em>end example</em>]</p>
<h3 class="unnumbered" id="x.2-class-nonmovable-utility.nonmovable">22.2.x.2 Class
<code class="sourceCode default">nonmovable</code>
[utility.nonmovable]<a href="#x.2-class-nonmovable-utility.nonmovable" class="self-link"></a></h3>
<div class="sourceCode" id="cb7"><pre class="sourceCode default cpp"><code class="sourceCode default"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a>namespace nonmovable-adl-namespace {</span>
<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a>    struct nonmovable {</span>
<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a>        nonmovable() = default;</span>
<span id="cb7-4"><a href="#cb7-4" aria-hidden="true" tabindex="-1"></a>        nonmovable(nonmovable const&amp;) = delete;</span>
<span id="cb7-5"><a href="#cb7-5" aria-hidden="true" tabindex="-1"></a>        nonmovable&amp; operator=(nonmovable const&amp;) = delete;</span>
<span id="cb7-6"><a href="#cb7-6" aria-hidden="true" tabindex="-1"></a>    };</span>
<span id="cb7-7"><a href="#cb7-7" aria-hidden="true" tabindex="-1"></a>}</span>
<span id="cb7-8"><a href="#cb7-8" aria-hidden="true" tabindex="-1"></a>using nonmovable-adl-namespace::nonmovable;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span>
<code class="sourceCode default">nonmovable</code> is provided to
simplify creation of classes that inhibit move and copy semantics.</p>
<p>[<em>Note</em>: <code class="sourceCode default">nonmovable</code> is
provided in an unspecified nested namespace to limit argument dependent
lookup <span>6.5.4
<a href="https://wg21.link/basic.lookup.argdep">[basic.lookup.argdep]</a></span>;
no other names should be declared in this namespace. — <em>end
note</em>]</p>
<p>[<em>Example</em>:</p>
<div class="sourceCode" id="cb8"><pre class="sourceCode default cpp"><code class="sourceCode default"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a>class mutex : std::nonmovable {</span>
<span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a>public:</span>
<span id="cb8-3"><a href="#cb8-3" aria-hidden="true" tabindex="-1"></a>    mutex();</span>
<span id="cb8-4"><a href="#cb8-4" aria-hidden="true" tabindex="-1"></a>    ~mutex();</span>
<span id="cb8-5"><a href="#cb8-5" aria-hidden="true" tabindex="-1"></a>};</span></code></pre></div>
<p>— <em>end example</em>]</p>
</div>
<h1 data-number="7" id="bibliography"><span class="header-section-number">7</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references csl-bib-body hanging-indent" role="list">
<div id="ref-N2675" class="csl-entry" role="listitem">
[N2675] Alisdair Meredith. 2008-06-13. noncopyable utility class
(revision 1). <a href="https://wg21.link/n2675"><div class="csl-block">https://wg21.link/n2675</div></a>
</div>
</div>
</div>
</div>
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