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<title>Issue 3893: LWG 3661 broke atomic&lt;shared_ptr&lt;T&gt;&gt; a; a = nullptr;</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#WP">WP</a> status.</em></p>
<h3 id="3893"><a href="lwg-defects.html#3893">3893</a>. LWG 3661 broke <code>atomic&lt;shared_ptr&lt;T&gt;&gt; a; a = nullptr;</code></h3>
<p><b>Section:</b> 32.5.8.7.2 <a href="https://wg21.link/util.smartptr.atomic.shared">[util.smartptr.atomic.shared]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Zachary Wassall <b>Opened:</b> 2023-02-22 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#util.smartptr.atomic.shared">issues</a> in [util.smartptr.atomic.shared].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
LWG <a href="lwg-defects.html#3661" title="constinit atomic&lt;shared_ptr&lt;T&gt;&gt; a(nullptr); should work (Status: C++23)">3661</a><sup><a href="https://cplusplus.github.io/LWG/issue3661" title="Latest snapshot">(i)</a></sup>, "<code>constinit atomic&lt;shared_ptr&lt;T&gt;&gt; a(nullptr);</code> should work" 
added the following to <code>atomic&lt;shared_ptr&lt;T&gt;&gt;</code>:
</p>
<blockquote><pre>
constexpr atomic(nullptr_t) noexcept : atomic() { }
</pre></blockquote>
<p>
I believe that doing so broke the following example:
</p>
<blockquote><pre>
atomic&lt;shared_ptr&lt;T&gt;&gt; a;
a = nullptr;
</pre></blockquote>
<p>
For reference, <code>atomic&lt;shared_ptr&lt;T&gt;&gt;</code> provides two assignment operator overloads:
</p>
<blockquote><pre>
void operator=(const atomic&amp;) = delete;          // #1
void operator=(shared_ptr&lt;T&gt; desired) noexcept;  // #2
</pre></blockquote>
<p>
Prior to LWG <a href="lwg-defects.html#3661" title="constinit atomic&lt;shared_ptr&lt;T&gt;&gt; a(nullptr); should work (Status: C++23)">3661</a><sup><a href="https://cplusplus.github.io/LWG/issue3661" title="Latest snapshot">(i)</a></sup>, the assignment in the example unambiguously matches #2. #1 is not viable because 
<code>nullptr_t</code> is not convertible to <code>atomic&lt;shared_ptr&lt;T&gt;&gt;</code>. After LWG 3611, #1 is viable 
and the assignment is ambiguous between #1 and #2.
<p/>
I believe this could be remedied easily enough by adding an assignment operator to match the added 
<code>nullptr_t</code> constructor:
</p>
<blockquote><pre>
void operator=(nullptr_t) noexcept;
</pre></blockquote>

<p><i>[2023-02-25; Daniel comments and provides wording]</i></p>

<p>
The suggested delegation below to <code>store(nullptr)</code> is not ambiguous and calls the constructor
<code>shared_ptr(nullptr_t)</code>, which is guaranteed to be no-throwing.
</p>

<p><i>[2023-03-22; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after six votes in favour during reflector poll.
</p>

<p><i>[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3893"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</a>.
</p>

<ol>

<li><p>Modify 32.5.8.7.2 <a href="https://wg21.link/util.smartptr.atomic.shared">[util.smartptr.atomic.shared]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std {
  template&lt;class T&gt; struct atomic&lt;shared_ptr&lt;T&gt;&gt; {
    [&hellip;]
    constexpr atomic() noexcept;
    constexpr atomic(nullptr_t) noexcept : atomic() { }
    atomic(shared_ptr&lt;T&gt; desired) noexcept;
    atomic(const atomic&amp;) = delete;
    void operator=(const atomic&amp;) = delete;
    [&hellip;]
    void operator=(shared_ptr&lt;T&gt; desired) noexcept;
    <ins>void operator=(nullptr_t) noexcept;</ins>
    [&hellip;]
  private:
    shared_ptr&lt;T&gt; p; // <i>exposition only</i>
  };
}
</pre>
</blockquote>
[&hellip;]
<pre>
void operator=(shared_ptr&lt;T&gt; desired) noexcept;
</pre>
<blockquote>
<p>
-5- <i>Effects</i>: Equivalent to <code>store(desired)</code>.
</p>
</blockquote>
<pre>
<ins>void operator=(nullptr_t) noexcept;</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Equivalent to <code>store(nullptr)</code>.</ins>
</p>
</blockquote>
</blockquote>


</li>


</ol>





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