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<title>Issue 2861: basic_string should require that charT match traits::char_type</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++17">C++17</a> status.</em></p>
<h3 id="2861"><a href="lwg-defects.html#2861">2861</a>. <code>basic_string</code> should require that <code>charT</code> match <code>traits::char_type</code></h3>
<p><b>Section:</b> 27.4.3.2 <a href="https://wg21.link/string.require">[string.require]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> United States <b>Opened:</b> 2017-02-02 <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#string.require">issues</a> in [string.require].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses US 145</b></p>

<p>There is no requirement that <code>traits::char_type</code> is <code>charT</code>, although there is a requirement that 
<code>allocator::value_type</code> is <code>charT</code>. This means that it might be difficult to honour both methods returning reference 
(such as <code>operator[]</code>) and <code>charT&amp;</code> (like <code>front</code>/<code>back</code>) when traits has a surprising 
<code>char_type</code>. It seems that the allocator should <em>not</em> rebind in such cases, making the reference-returning 
signatures the problematic ones.</p>

<p>Suggested resolution: Add a requirement that <code>is_same_v&lt;typename traits::char_type, charT&gt;</code> is true, and simplify so that value_type is just an alias for <code>charT</code>.</p>

<p><i>[2017-02-02 Marshall adds]</i></p>

<p>In [string.require]/3, there's already a note that the types shall be the same. In [string.view.template]/1, it says "In every specialization <code>basic_string_view&lt;charT, traits, Allocator&gt;</code>, the type <code>traits</code> shall satisfy the character traits requirements (21.2), and the type <code>traits::char_type</code> shall name the same type as <code>charT</code>".</p>

<p><i>[Kona 2017-02-28]</i></p>

<p>Accepted as Immediate to resolve NB comment.</p>


<p id="res-2861"><b>Proposed resolution:</b></p>
<p>Changes are based off of <a href="https://wg21.link/n4618">N4618</a></p>

<ol>
<li><p>Modify [basic.string] as indicated (in the synopsis):</p>
<blockquote><pre>
class basic_string {
public:
// <i>types:</i>
  using traits_type      = traits;
  using value_type       = <ins>charT</ins><del>typename traits::char_type</del>;
  using allocator_type   = Allocator
</pre></blockquote>
</li>

<li>
<p>Change [string.require]/3 as indicated:</p>
<p>-3- In every specialization <code>basic_string&lt;charT, traits, Allocator&gt;</code>, the type <code>allocator_traits&lt;Allocator&gt;::value_type</code> shall name the same type as <code>charT</code>. Every object of type <code>basic_string&lt;charT, traits, Allocator&gt;</code> shall use an object of type <code>Allocator</code> to allocate and free storage for the contained <code>charT</code> objects as needed. The <code>Allocator</code> object used shall be obtained as described in 23.2.1. <del>[ Note: </del>In every specialization <code>basic_string&lt;charT, traits, Allocator&gt;</code>, <ins>the type <code>traits</code> shall satisfy the character traits requirements (21.2), and </ins> the type <code>traits::char_type</code> shall <ins>name</ins><del>be</del> the same type as <code>charT</code><del>; see 21.2. — end note ]</del>
</p>
</li>
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