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<title>Issue 2978: Hash support for pmr::string and friends</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++20">C++20</a> status.</em></p>
<h3 id="2978"><a href="lwg-defects.html#2978">2978</a>. Hash support for <code>pmr::string</code> and friends</h3>
<p><b>Section:</b> 27.4.6 <a href="https://wg21.link/basic.string.hash">[basic.string.hash]</a>, 27.4.2 <a href="https://wg21.link/string.syn">[string.syn]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2017-06-14 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#basic.string.hash">issues</a> in [basic.string.hash].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In most cases, <code>std::pmr::<i>meow</i></code> is a drop-in replacement for <code>std::<i>meow</i></code>. The exception is
<code>std::pmr::{,w,u16,u32}string</code>, because unlike their <code>std::</code> counterparts, they don't come with enabled 
<code>std::hash</code> specializations.
</p>
<p>
The P/R below simply adds <code>std::hash</code> specializations for those four typedefs. An alternative approach, for which wording can be
produced if desired, is to make the <code>hash</code> specializations for <code>basic_string</code> allocator-agnostic, similar to the partial
specialization of <code>hash</code> for <code>vector&lt;bool&gt;</code>.
</p>

<p><i>[2017-07 Toronto Monday issue prioritization]</i></p>

<p>Priority 0; move to Ready</p>


<p id="res-2978"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4659">N4659</a>.
</p>
<ol>
<li><p> Edit 27.4.2 <a href="https://wg21.link/string.syn">[string.syn]</a>, header <code>&lt;string&gt;</code> synopsis, as indicated:</p>
<blockquote>
<pre>
<ins>  namespace pmr {
    template &lt;class charT, class traits = char_traits&lt;charT&gt;&gt;
      using basic_string = std::basic_string&lt;charT, traits, polymorphic_allocator&lt;charT&gt;&gt;;

    using string    = basic_string&lt;char&gt;;
    using u16string = basic_string&lt;char16_t&gt;;
    using u32string = basic_string&lt;char32_t&gt;;
    using wstring   = basic_string&lt;wchar_t&gt;;
  }</ins>
  
  // 27.4.6 <a href="https://wg21.link/basic.string.hash">[basic.string.hash]</a>, hash support
  template&lt;class T&gt; struct hash;
  template&lt;&gt; struct hash&lt;string&gt;;
  template&lt;&gt; struct hash&lt;u16string&gt;;
  template&lt;&gt; struct hash&lt;u32string&gt;;
  template&lt;&gt; struct hash&lt;wstring&gt;;
<ins>  template&lt;&gt; struct hash&lt;pmr::string&gt;;
  template&lt;&gt; struct hash&lt;pmr::u16string&gt;;
  template&lt;&gt; struct hash&lt;pmr::u32string&gt;;
  template&lt;&gt; struct hash&lt;pmr::wstring&gt;;</ins>

<del>  namespace pmr {
    template &lt;class charT, class traits = char_traits&lt;charT&gt;&gt;
      using basic_string = std::basic_string&lt;charT, traits, polymorphic_allocator&lt;charT&gt;&gt;;

    using string    = basic_string&lt;char&gt;;
    using u16string = basic_string&lt;char16_t&gt;;
    using u32string = basic_string&lt;char32_t&gt;;
    using wstring   = basic_string&lt;wchar_t&gt;;
  }</del>
</pre>
</blockquote>
</li>
<li><p> Edit 27.4.6 <a href="https://wg21.link/basic.string.hash">[basic.string.hash]</a> as indicated:</p>
<blockquote>
<pre>
  template&lt;&gt; struct hash&lt;string&gt;;
  template&lt;&gt; struct hash&lt;u16string&gt;;
  template&lt;&gt; struct hash&lt;u32string&gt;;
  template&lt;&gt; struct hash&lt;wstring&gt;;
<ins>  template&lt;&gt; struct hash&lt;pmr::string&gt;;
  template&lt;&gt; struct hash&lt;pmr::u16string&gt;;
  template&lt;&gt; struct hash&lt;pmr::u32string&gt;;
  template&lt;&gt; struct hash&lt;pmr::wstring&gt;;</ins>
</pre>
<blockquote>
<p>
  -1- If <code>S</code> is one of these string types, <code>SV</code> is the corresponding string view type, and <code>s</code> is an object of type <code>S</code>, then <code>hash&lt;S&gt;()(s) == hash&lt;SV&gt;()(SV(s))</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>





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