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<title>Issue 3427: operator&lt;=&gt;(const shared_ptr&lt;T&gt;&amp;, nullptr_t) definition ill-formed</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="3427"><a href="lwg-defects.html#3427">3427</a>. <code>operator&lt;=&gt;(const shared_ptr&lt;T&gt;&amp;, nullptr_t)</code> definition ill-formed</h3>
<p><b>Section:</b> 20.3.2.2.8 <a href="https://wg21.link/util.smartptr.shared.cmp">[util.smartptr.shared.cmp]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2020-04-11 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#util.smartptr.shared.cmp">issues</a> in [util.smartptr.shared.cmp].</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>
This is similar to LWG <a href="lwg-defects.html#3426" title="operator&lt;=&gt;(const unique_ptr&lt;T, D&gt;&amp;, nullptr_t) can't get no satisfaction (Status: C++23)">3426</a><sup><a href="https://cplusplus.github.io/LWG/issue3426" title="Latest snapshot">(i)</a></sup>: This time the definition of 
<code>operator&lt;=&gt;(const shared_ptr&lt;T&gt;&amp;, nullptr_t)</code> is ill-formed, whose effects are essentially 
specified as calling:
</p>
<blockquote><pre>
compare_three_way()(a.get(), nullptr)
</pre></blockquote>
<p>
This call will be ill-formed by constraint-violation, because both <code>nullptr &lt;=&gt; nullptr</code> as well as 
<code>((T*) 0) &lt;=&gt; nullptr</code> are ill-formed.
<p/>
As a short-term solution we can make it work as intended &mdash; equivalent to LWG <a href="lwg-defects.html#3426" title="operator&lt;=&gt;(const unique_ptr&lt;T, D&gt;&amp;, nullptr_t) can't get no satisfaction (Status: C++23)">3426</a><sup><a href="https://cplusplus.github.io/LWG/issue3426" title="Latest snapshot">(i)</a></sup> &mdash; by 
comparing <code>a.get()</code> to <code>(element_type*) nullptr</code> instead of to <code>nullptr</code> directly.
<p/>
As a long-term solution we should at least consider to deprecate the mixed relational operators as well as the 
mixed three-way comparison operator of all our smart-pointers with <code>std::nullptr_t</code> since the core language 
has eliminated relational comparisons of pointers with <code>std::nullptr_t</code> with 
<a href="https://wg21.link/n3624">N3624</a> four years after they had been originally accepted by 
<a href="https://wg21.link/cwg879">CWG 879</a>. Consequently, for C++20, the mixed three-way comparison between 
pointers and <code>nullptr</code> is not supported either. For this long-term solution I'm suggesting to handle this
via a proposal.
</p>

<p><i>[2020-05-16 Reflector discussions]</i></p>

<p>Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.</p>
<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3427"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

<ol>
<li><p>Modify 20.3.2.2.8 <a href="https://wg21.link/util.smartptr.shared.cmp">[util.smartptr.shared.cmp]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class T&gt;
  strong_ordering operator&lt;=&gt;(const shared_ptr&lt;T&gt;&amp; a, nullptr_t) noexcept;
</pre>

<blockquote>
<p>
-5- <i>Returns:</i> <code>compare_three_way()(a.get(), <ins>static_cast&lt;typename shared_ptr&lt;T&gt;::element_type*&gt;(</ins>nullptr<ins>)</ins>)</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>




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