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<title>Issue 3633: Atomics are copy constructible and copy assignable from volatile atomics</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
<h3 id="3633"><a href="lwg-active.html#3633">3633</a>. Atomics are copy constructible and copy assignable from <code>volatile</code> atomics</h3>
<p><b>Section:</b> 32.5 <a href="https://wg21.link/atomics">[atomics]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Wesley Maxey <b>Opened:</b> 2021-11-05 <b>Last modified:</b> 2024-01-29</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#atomics">issues</a> in [atomics].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The specification of <code>atomic</code> and <code>atomic&lt;T*&gt;</code> (in 32.5.8.1 <a href="https://wg21.link/atomics.types.generic.general">[atomics.types.generic.general]</a> 
and 32.5.8.5 <a href="https://wg21.link/atomics.types.pointer">[atomics.types.pointer]</a>) explicitly deletes the following functions:
</p>
<blockquote>
<pre>
atomic(const atomic&amp;) = delete;
atomic&amp; operator=(const atomic&amp;) = delete;
atomic&amp; operator=(const atomic&amp;) volatile = delete;
</pre>
</blockquote>
<p>
The intent is to make atomic objects not copyable, so that initializing an atomic object 
from another atomic, or assigning an atomic object with a value from another atomic, 
must be an explicit operation.
<p/>
We also explicitly support <code>volatile</code> objects of types that are specializations of <code>std::atomic</code>; 
some of the functions that are vital for the support of volatile atomics are the following conversion operators:
</p>
<blockquote>
<pre>
operator T() const volatile noexcept; // for non-pointers
operator T*() const volatile noexcept; // for pointers
</pre>
</blockquote>
<p>
The presence of this conversion operator means that all the statements in the following piece of code 
compile successfully today, despite the deleted functions mentioned earlier:
</p>
<blockquote>
<pre>
volatile std::atomic&lt;int&gt; a;
volatile std::atomic&lt;int&gt; b(a);
std::atomic&lt;int&gt; c(a);
b = a;
c = a;
</pre>
</blockquote>
<p>
However, if <code>a</code> is not a volatile object, none of the last four lines compile.
</p>

<p><i>[2022-01-29; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
<br/>
This PR would allow
<pre>
atomic&lt;int&gt; x, y = std::move(x);
</pre>
because const volatile&amp; doesn't bind to rvalues. It sounds like we'll
need to delete both const volatile&amp; and const volatile&amp;&amp;.
</p>



<p id="res-3633"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>.
</p>

<ol>
<li><p>Modify 32.5.8.1 <a href="https://wg21.link/atomics.types.generic.general">[atomics.types.generic.general]</a>, class template <code>atomic</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
atomic(const <ins>volatile</ins> atomic&amp;) = delete;
atomic&amp; operator=(const <ins>volatile</ins> atomic&amp;) = delete;
atomic&amp; operator=(const <ins>volatile</ins> atomic&amp;) volatile = delete;
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 32.5.8.3 <a href="https://wg21.link/atomics.types.int">[atomics.types.int]</a>, class template <code>atomic&lt;<i>integral</i>&gt;</code> specialization 
synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
atomic(const <ins>volatile</ins> atomic&amp;) = delete;
atomic&amp; operator=(const <ins>volatile</ins> atomic&amp;) = delete;
atomic&amp; operator=(const <ins>volatile</ins> atomic&amp;) volatile = delete;
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 32.5.8.4 <a href="https://wg21.link/atomics.types.float">[atomics.types.float]</a>, class template <code>atomic&lt;<i>floating-point</i>&gt;</code> specialization 
synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
atomic(const <ins>volatile</ins> atomic&amp;) = delete;
atomic&amp; operator=(const <ins>volatile</ins> atomic&amp;) = delete;
atomic&amp; operator=(const <ins>volatile</ins> atomic&amp;) volatile = delete;
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 32.5.8.5 <a href="https://wg21.link/atomics.types.pointer">[atomics.types.pointer]</a>, class template <code>atomic&lt;T*&gt;</code> partial specialization 
synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
atomic(const <ins>volatile</ins> atomic&amp;) = delete;
atomic&amp; operator=(const <ins>volatile</ins> atomic&amp;) = delete;
atomic&amp; operator=(const <ins>volatile</ins> atomic&amp;) volatile = delete;
[&hellip;]
</pre>
</blockquote>
</li>

</ol>





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