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<title>Issue 3661: constinit atomic&lt;shared_ptr&lt;T&gt;&gt; a(nullptr); should work</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#C++23">C++23</a> status.</em></p>
<h3 id="3661"><a href="lwg-defects.html#3661">3661</a>. <code>constinit atomic&lt;shared_ptr&lt;T&gt;&gt; a(nullptr);</code> should work</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#C++23">C++23</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2022-01-21 <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#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
All the following are valid except for the last line:
</p>
<blockquote>
<pre>
constinit int i1{};
constinit std::atomic&lt;int&gt; a1{};
constinit int i2{0};
constinit std::atomic&lt;int&gt; a2{0};
constinit std::shared_ptr&lt;int&gt; i3{};
constinit std::atomic&lt;std::shared_ptr&lt;int&gt;&gt; a3{};
constinit std::shared_ptr&lt;int&gt; i4{nullptr};
constinit std::atomic&lt;std::shared_ptr&lt;int&gt;&gt; a4{nullptr}; <span style="color:red;font-weight:bolder">// error</span>
</pre>
</blockquote>
<p>
The initializer for <code>a4</code> will create a <code>shared_ptr&lt;int&gt;</code> temporary (using the same constructor 
as <code>i4</code>) but then try to use <code>atomic(shared_ptr&lt;int&gt;)</code> which is not <code>constexpr</code>.
<p/>
This is an unnecessary inconsistency in the API for <code>atomic&lt;shared_ptr&lt;T&gt;&gt;</code> that can 
easily be fixed. The proposed resolution has been implemented in libstdc++.
<p/>
There is no need to also change <code>atomic&lt;weak_ptr&lt;T&gt;&gt;</code> because <code>weak_ptr</code> doesn't have a 
constructor taking <code>nullptr</code>.
</p>

<p><i>[2022-01-30; Reflector poll]</i></p>

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

<p><i>[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3661"><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 32.5.8.7.2 <a href="https://wg21.link/util.smartptr.atomic.shared">[util.smartptr.atomic.shared]</a>, class template partial specialization 
<code>atomic&lt;shared_ptr&lt;T&gt;&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
constexpr atomic() noexcept;
<ins>constexpr atomic(nullptr_t) noexcept : atomic() { }</ins>
atomic(shared_ptr&lt;T&gt; desired) noexcept;
atomic(const atomic&amp;) = delete;
void operator=(const atomic&amp;) = delete;
[&hellip;]
</pre>
</blockquote>
</li>
</ol>





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