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<title>Issue 3657: std::hash&lt;std::filesystem::path&gt; is not enabled</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="3657"><a href="lwg-defects.html#3657">3657</a>. <code>std::hash&lt;std::filesystem::path&gt;</code> is not enabled</h3>
<p><b>Section:</b> 31.12.6 <a href="https://wg21.link/fs.class.path">[fs.class.path]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2022-01-02 <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#fs.class.path">issues</a> in [fs.class.path].</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>
The hash support of <code>std::filesystem::path</code> is provided by <code>std::filesystem::hash_value</code>, 
but the specialization <code>std::hash&lt;std::filesystem::path&gt;</code> is currently disabled. IMO the 
specialization should be enabled, and its <code>operator()</code> should return the same value as <code>hash_value</code>.
</p>

<p><i>[2022-01-15; Daniel provides wording]</i></p>


<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<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 31.12.4 <a href="https://wg21.link/fs.filesystem.syn">[fs.filesystem.syn]</a>, header <code>&lt;filesystem&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
#include &lt;compare&gt; <i>// see 17.12.1 <a href="https://wg21.link/compare.syn">[compare.syn]</a></i>

namespace std::filesystem {
  <i>// 31.12.6 <a href="https://wg21.link/fs.class.path">[fs.class.path]</a>, paths</i>
  class path;

  <i>// 31.12.6.8 <a href="https://wg21.link/fs.path.nonmember">[fs.path.nonmember]</a>, path non-member functions</i>
  void swap(path&amp; lhs, path&amp; rhs) noexcept;
  size_t hash_value(const path&amp; p) noexcept;

  [&hellip;]
}

<ins><i>// [fs.path.hash], hash support</i>
namespace std {
  template&lt;class T&gt; struct hash;
  template&lt;&gt; struct hash&lt;filesystem::path&gt;;
}
</ins>
</pre>
</blockquote>
</li>

<li><p>Following subclause 31.12.6.8 <a href="https://wg21.link/fs.path.nonmember">[fs.path.nonmember]</a>, introduce a new subclause [fs.path.hash], as indicated:</p>

<blockquote>
<p>
<ins><b>29.12.6.? Hash support [fs.path.hash]</b></ins>
</p>
<pre>
<ins>template&lt;&gt; struct hash&lt;filesystem::path&gt;;</ins>
</pre>
<blockquote>
<p>
<ins>-?- For an object <code>p</code> of type <code>filesystem::path</code>, <code>hash&lt;filesystem::path&gt;()(p)</code> 
shall evaluate to the same result as <code>hash_value(p)</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2022-01-18; Daniel improves wording based on reflector discussion feedback]</i></p>


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

<p>
Set status to Tentatively Ready after five 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-3657"><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 31.12.4 <a href="https://wg21.link/fs.filesystem.syn">[fs.filesystem.syn]</a>, header <code>&lt;filesystem&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
#include &lt;compare&gt; <i>// see 17.12.1 <a href="https://wg21.link/compare.syn">[compare.syn]</a></i>

namespace std::filesystem {
  <i>// 31.12.6 <a href="https://wg21.link/fs.class.path">[fs.class.path]</a>, paths</i>
  class path;

  <i>// 31.12.6.8 <a href="https://wg21.link/fs.path.nonmember">[fs.path.nonmember]</a>, path non-member functions</i>
  void swap(path&amp; lhs, path&amp; rhs) noexcept;
  size_t hash_value(const path&amp; p) noexcept;

  [&hellip;]
}

<ins><i>// [fs.path.hash], hash support</i>
namespace std {
  template&lt;class T&gt; struct hash;
  template&lt;&gt; struct hash&lt;filesystem::path&gt;;
}
</ins>
</pre>
</blockquote>
</li>

<li><p>Following subclause 31.12.6.8 <a href="https://wg21.link/fs.path.nonmember">[fs.path.nonmember]</a>, introduce a new subclause [fs.path.hash], as indicated:</p>

<blockquote>
<p>
<ins><b>29.12.6.? Hash support [fs.path.hash]</b></ins>
</p>
<pre>
<ins>template&lt;&gt; struct hash&lt;filesystem::path&gt;;</ins>
</pre>
<blockquote>
<p>
<ins>-?- For an object <code>p</code> of type <code>filesystem::path</code>, <code>hash&lt;filesystem::path&gt;()(p)</code> 
evaluates to the same result as <code>filesystem::hash_value(p)</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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