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<title>Issue 3259: The definition of constexpr iterators should be adjusted</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="3259"><a href="lwg-defects.html#3259">3259</a>. The definition of <i>constexpr iterators</i> should be adjusted</h3>
<p><b>Section:</b> 24.3.1 <a href="https://wg21.link/iterator.requirements.general">[iterator.requirements.general]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2019-08-18 <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#iterator.requirements.general">issues</a> in [iterator.requirements.general].</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>
The current definition of <i>constexpr iterators</i> is specified in 
24.3.1 <a href="https://wg21.link/iterator.requirements.general">[iterator.requirements.general]</a> p16 as follows:
</p>
<blockquote>
<p>
Iterators are called <i>constexpr iterators</i> if all operations provided to 
meet iterator category requirements are constexpr functions, except for
</p>
<ol style="list-style-type: none">
<li><p>(16.1) &mdash; a pseudo-destructor call (7.5.5.5 <a href="https://wg21.link/expr.prim.id.dtor">[expr.prim.id.dtor]</a>), and</p></li>
<li><p>(16.2) &mdash; the construction of an iterator with a singular value.</p></li>
</ol>
</blockquote>
<p>
With the acceptance of some proposals during the Cologne 2019 meeting, these additional 
requirements become mostly obsolete, as it had already been pointed out during that meeting:
<p/>
With the acceptance of <a href="https://wg21.link/p0784r7">P0784R7</a>, destructors can be 
declared <code>constexpr</code> and it is possible to perform a pseudo-destructor call within a 
constant expression, so bullet (16.1) is no longer a necessary requirement.
<p/>
With the acceptance of <a href="https://wg21.link/p1331r2">P1331R2</a>, trivial default 
initialization in constexpr contexts is now possible, and there is no longer a requirement
to initialize all sub-objects of a class object within a constant expression.
<p/>
It seems to me that we should simply strike the above two constraining requirements of the
definition of <i>constexpr iterators</i> for C++20.
</p>

<p><i>[2019-09-14 Issue Prioritization]</i></p>

<p>Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.</p>


<p id="res-3259"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 24.3.1 <a href="https://wg21.link/iterator.requirements.general">[iterator.requirements.general]</a> as indicated:</p>

<blockquote>
<p>
-16- Iterators are called <i>constexpr iterators</i> if all operations provided to 
meet iterator category requirements are constexpr functions<ins>.</ins><del>, except for</del>
</p>
<ol style="list-style-type: none">
<li><p><del>(16.1) &mdash; a pseudo-destructor call (7.5.5.5 <a href="https://wg21.link/expr.prim.id.dtor">[expr.prim.id.dtor]</a>), and</del></p></li>
<li><p><del>(16.2) &mdash; the construction of an iterator with a singular value.</del></p></li>
</ol>
</blockquote>
</li>

</ol>




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