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<title>Issue 2817: std::hash for nullptr_t</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#Resolved">Resolved</a> status.</em></p>
<h3 id="2817"><a href="lwg-defects.html#2817">2817</a>. <code>std::hash</code> for <code>nullptr_t</code></h3>
<p><b>Section:</b> 22.10.19 <a href="https://wg21.link/unord.hash">[unord.hash]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Marshall Clow <b>Opened:</b> 2016-11-10 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#unord.hash">issues</a> in [unord.hash].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In the list of types that the library provides <code>std::hash</code> specializations, there is an entry for 
<code>hash&lt;T*&gt;</code>, but none entry for <code>hash&lt;nullptr_t&gt;</code>.
</p>

<p><i>[2016-11-13, Daniel provides wording]</i></p>


<p><i>[2016-11-28]</i></p>

<p>Resolved by P0513R0</p>


<p id="res-2817"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4606">N4606</a>.</p>

<ol>
<li><p>Change 22.10 <a href="https://wg21.link/function.objects">[function.objects]</a>, header <code>&lt;functional&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]

<i>// Hash function specializations</i>
template &lt;&gt; struct hash&lt;bool&gt;;
[&hellip;]
template &lt;&gt; struct hash&lt;long double&gt;;
<ins>template &lt;&gt; struct hash&lt;nullptr_t&gt;;</ins>

template&lt;class T&gt; struct hash&lt;T*&gt;;
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Change 22.10.19 <a href="https://wg21.link/unord.hash">[unord.hash]</a> before p2, as indicated:</p>

<blockquote>
<pre>
template &lt;&gt; struct hash&lt;bool&gt;;
[&hellip;]
template &lt;&gt; struct hash&lt;long double&gt;;
<ins>template &lt;&gt; struct hash&lt;nullptr_t&gt;;</ins>
template&lt;class T&gt; struct hash&lt;T*&gt;;
</pre>
</blockquote>
</li>
</ol>





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