<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3705: Hashability shouldn't depend on basic_string's allocator</title>
<meta property="og:title" content="Issue 3705: Hashability shouldn't depend on basic_string's allocator">
<meta property="og:description" content="C++ library issue. Status: C++23">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3705.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<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="3705"><a href="lwg-defects.html#3705">3705</a>. Hashability shouldn't depend on <code>basic_string</code>'s allocator</h3>
<p><b>Section:</b> 27.4.6 <a href="https://wg21.link/basic.string.hash">[basic.string.hash]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2022-05-26 <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#basic.string.hash">issues</a> in [basic.string.hash].</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>
27.4.6 <a href="https://wg21.link/basic.string.hash">[basic.string.hash]</a> says:
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
<pre>
template&lt;&gt; struct hash&lt;string&gt;;
template&lt;&gt; struct hash&lt;u8string&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;;
template&lt;&gt; struct hash&lt;pmr::string&gt;;
template&lt;&gt; struct hash&lt;pmr::u8string&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;;
</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>
<p>
Despite that the hash value of a <code>basic_string</code> object is equivalent to the hash value of a 
corresponding <code>basic_string_view</code> object, which has no allocator, the capability to hash a 
<code>basic_string</code> depends on its allocator. All of the enabled specializations have specific 
allocators, which fact becomes more clear if we expand the type aliases:
</p>
<blockquote><pre>
template&lt;&gt; struct hash&lt;basic_string&lt;char, char_traits&lt;char&gt;, allocator&lt;char&gt;&gt;;
template&lt;&gt; struct hash&lt;basic_string&lt;char8_t, char_traits&lt;char8_t&gt;, allocator&lt;char8_t&gt;&gt;;

template&lt;&gt; struct hash&lt;basic_string&lt;char16_t, char_traits&lt;char16_t&gt;, allocator&lt;char16_t&gt;&gt;;

template&lt;&gt; struct hash&lt;basic_string&lt;char32_t, char_traits&lt;char32_t&gt;, allocator&lt;char32_t&gt;&gt;;

template&lt;&gt; struct hash&lt;basic_string&lt;wchar_t, char_traits&lt;wchar_t&gt;, allocator&lt;wchar_t&gt;&gt;;

template&lt;&gt; struct hash&lt;basic_string&lt;char, char_traits&lt;char&gt;, pmr::polymorphic_allocator&lt;char&gt;&gt;;
template&lt;&gt; struct hash&lt;basic_string&lt;char8_t, char_traits&lt;char8_t&gt;, pmr::polymorphic_allocator&lt;char8_t&gt;&gt;;

template&lt;&gt; struct hash&lt;basic_string&lt;char16_t, char_traits&lt;char16_t&gt;, pmr::polymorphic_allocator&lt;char16_t&gt;&gt;;

template&lt;&gt; struct hash&lt;basic_string&lt;char32_t, char_traits&lt;char32_t&gt;, pmr::polymorphic_allocator&lt;char32_t&gt;&gt;;

template&lt;&gt; struct hash&lt;basic_string&lt;wchar_t, char_traits&lt;wchar_t&gt;, pmr::polymorphic_allocator&lt;wchar_t&gt;&gt;;
</pre></blockquote>
<p>
If the hash value doesn't depend on the allocator type, why should we care about the allocator type? 
I posit that we should not, and that these ten explicit specializations should be replaced by 5 partial 
specializations that enable hashing <code>basic_string</code> specializations using these combinations of 
character type and traits type with any allocator type.
</p>

<p><i>[2022-06-21; Reflector poll]</i></p>

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

<p><i>[2022-07-15; LWG telecon: move to Ready]</i></p>


<p><i>[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3705"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 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>
[&hellip;]

<i>// 27.4.6 <a href="https://wg21.link/basic.string.hash">[basic.string.hash]</a>, hash support</i>
template&lt;class T&gt; struct hash;
<del>template&lt;&gt; struct hash&lt;string&gt;;
template&lt;&gt; struct hash&lt;u8string&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;;
template&lt;&gt; struct hash&lt;pmr::string&gt;;
template&lt;&gt; struct hash&lt;pmr::u8string&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;;</del>
<ins>template&lt;class A&gt; struct hash&lt;basic_string&lt;char, char_traits&lt;char&gt;, A&gt;&gt;;
template&lt;class A&gt; struct hash&lt;basic_string&lt;char8_t, char_traits&lt;char8_t&gt;, A&gt;&gt;;
template&lt;class A&gt; struct hash&lt;basic_string&lt;char16_t, char_traits&lt;char16_t&gt;, A&gt;&gt;;
template&lt;class A&gt; struct hash&lt;basic_string&lt;char32_t, char_traits&lt;char32_t&gt;, A&gt;&gt;;
template&lt;class A&gt; struct hash&lt;basic_string&lt;wchar_t, char_traits&lt;wchar_t&gt;, A&gt;&gt;;</ins>

[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 27.4.6 <a href="https://wg21.link/basic.string.hash">[basic.string.hash]</a> as indicated:</p>

<blockquote>
<pre>
<del>template&lt;&gt; struct hash&lt;string&gt;;
template&lt;&gt; struct hash&lt;u8string&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;;
template&lt;&gt; struct hash&lt;pmr::string&gt;;
template&lt;&gt; struct hash&lt;pmr::u8string&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;;</del>
<ins>template&lt;class A&gt; struct hash&lt;basic_string&lt;char, char_traits&lt;char&gt;, A&gt;&gt;;
template&lt;class A&gt; struct hash&lt;basic_string&lt;char8_t, char_traits&lt;char8_t&gt;, A&gt;&gt;;
template&lt;class A&gt; struct hash&lt;basic_string&lt;char16_t, char_traits&lt;char16_t&gt;, A&gt;&gt;;
template&lt;class A&gt; struct hash&lt;basic_string&lt;char32_t, char_traits&lt;char32_t&gt;, A&gt;&gt;;
template&lt;class A&gt; struct hash&lt;basic_string&lt;wchar_t, char_traits&lt;wchar_t&gt;, A&gt;&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>





</body>
</html>
