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<title>Issue 3650: Are std::basic_string's iterator and const_iterator constexpr iterators?</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="3650"><a href="lwg-defects.html#3650">3650</a>. Are <code>std::basic_string</code>'s <code>iterator</code> and <code>const_iterator</code> constexpr iterators?</h3>
<p><b>Section:</b> 27.4.3.1 <a href="https://wg21.link/basic.string.general">[basic.string.general]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2021-12-04 <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.general">issues</a> in [basic.string.general].</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>
<code>std::vector</code>'s <code>iterator</code> and <code>const_iterator</code> are required to meet constexpr iterator 
requirements in C++20 per <a href="https://wg21.link/P1004R2" title=" Making std::vector constexpr">P1004R2</a>, but it seems that the similar wording is missing for 
<code>std::basic_string</code> in both <a href="https://wg21.link/P0980R1" title=" Making std::string constexpr">P0980R1</a> and the current working draft.
<p/>
I think we should add a bullet "The types <code>iterator</code> and <code>const_iterator</code> meet the constexpr 
iterator requirements (24.3.1 <a href="https://wg21.link/iterator.requirements.general">[iterator.requirements.general]</a>)." to 27.4.3.1 <a href="https://wg21.link/basic.string.general">[basic.string.general]</a>.
</p>

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

<p>
Set status to Tentatively Ready after six 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-3650"><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 27.4.3.1 <a href="https://wg21.link/basic.string.general">[basic.string.general]</a>, as indicated:</p>

<blockquote>
<p>
-3- In all cases, <code>[data(), data() + size()]</code> is a valid range, <code>data() + size()</code> 
points at an object with value <code>charT()</code> (a "null terminator"), and <code>size() &lt;= capacity()</code>
is <code>true</code>.
<p/>
-4- A <code>size_type</code> parameter type in a <code>basic_string</code> deduction guide refers to the 
<code>size_type</code> member type of the type deduced by the deduction guide.
<p/>
<ins>-?- The types <code>iterator</code> and <code>const_iterator</code> meet the constexpr iterator requirements 
(24.3.1 <a href="https://wg21.link/iterator.requirements.general">[iterator.requirements.general]</a>).</ins>
</p>
</blockquote>
</li>

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