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<title>Issue 2897: array::iterator and array::const_iterator should be literal types</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#Resolved">Resolved</a> status.</em></p>
<h3 id="2897"><a href="lwg-defects.html#2897">2897</a>. <code>array::iterator</code> and <code>array::const_iterator</code> should be literal types</h3>
<p><b>Section:</b> 23.3.3.1 <a href="https://wg21.link/array.overview">[array.overview]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Russia <b>Opened:</b> 2017-02-03 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>2
</p>
<p><b>View other</b> <a href="lwg-index-open.html#array.overview">active issues</a> in [array.overview].</p>
<p><b>View all other</b> <a href="lwg-index.html#array.overview">issues</a> in [array.overview].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<b>Addresses RU 3</b>

<p>Force the literal type requirement for the <code>iterator</code> and <code>const_iterator</code> in the <code>std::array</code> 
so that iterators of <code>std::array</code> could be used in <code>constexpr</code> functions.</p>
<p>Proposed change: Add to the end of the [array.overview] section: <ins><code>iterator</code> and <code>const_iterator</code> 
shall be literal types.</ins></p>

<p><i>[2017-02-20 Marshall adds wording]</i></p>

<p>I used the formulation "are literal types", rather than "shall be", since that denotes a requirement on the user.</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4640">N4640</a>.</p>

<ol>
<li><p>Add a new paragraph at the end of 23.3.3.1 <a href="https://wg21.link/array.overview">[array.overview]</a>:</p>
<blockquote>
<ins>-?- <code>iterator</code> and <code>const_iterator</code> are literal types.</ins>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2017-03-01, Kona]</i></p>

<p>
Antony Polukhin provides revised wording.
</p>

<p><i>[2017-03-02, Kona]</i></p>

<p>
Wording tweaks suggested by LWG applied.
</p>

<p><i>[2017-03-02, Tim Song comments]</i></p>

<p>
I don't believe the blanket "all operations" wording is quite correct.
<p/>
First, <code>T t;</code> (required by <code>DefaultConstructible</code>) isn't usable in a constant expression if the iterator is a 
pointer, since it would leave it uninitialized. 
<p/>
Second, an explicit destructor call <code>u.~T()</code> (required by <code>Destructible</code>) isn't usable if the iterator is a 
class type because it explicitly invokes a non-<code>constexpr</code> function (the destructor); see [expr.const]/2.2.
</p>

<p><i>[2017-03-04, Kona]</i></p>

<p>Set priority to 2. Lisa and Alisdair to work with Antony to come up with better wording. The same wording can be used 
for <a href="lwg-defects.html#2938" title="basic_string_view::const_iterator should be literal types (Status: Resolved)">2938</a><sup><a href="https://cplusplus.github.io/LWG/issue2938" title="Latest snapshot">(i)</a></sup>.</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4640">N4640</a>.</p>

<ol>
<li><p>Add a new paragraph at the end of 23.3.3.1 <a href="https://wg21.link/array.overview">[array.overview]</a>:</p>
<blockquote>
<p>
-3- An <code>array</code> satisfies all of the requirements of a container [&hellip;]
<p/>
<ins>-?- All operations on <code>iterator</code> and <code>const_iterator</code> that are required to satisfy the 
random access iterator requirements are usable in constant expressions.</ins>
</p>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2017-04-22, Antony Polukhin provides improved wording]</i></p>


<p><i>[2017-11 Albuquerque Wednesday issue processing]</i></p>

<p>Status to Open; We don't want to do this yet; gated on Core issue 1581 (affects swap). See also <a href="lwg-defects.html#2800" title="constexpr swap (Status: Resolved)">2800</a><sup><a href="https://cplusplus.github.io/LWG/issue2800" title="Latest snapshot">(i)</a></sup>.</p>
<p>Thomas to talk to Anthony about writing a paper detailing what it takes to be a constexpr iterator.</p>

<p><i>[2017-11 Albuquerque Thursday]</i></p>

<p>It looks like 1581 is going to be resolved this week, so we should revisit soon.</p>

<p><i>[2017-11 Albuquerque Saturday issues processing]</i></p>

<p><a href="https://wg21.link/P0858R0">P0858R0</a> (adopted on Sat; to be moved in Jacksonville) will resolve this.</p>

<p><i>[2018-06 Rapperswil Wednesday issues processing]</i></p>

<p>This was resolved by <a href="https://wg21.link/P0858">P0858</a>, which was adopted in Jacksonville.</p>


<p id="res-2897"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4659">N4659</a>.</p>

<ol>
<li><p>Add a new paragraph at the end of 24.3.1 <a href="https://wg21.link/iterator.requirements.general">[iterator.requirements.general]</a> as indicated:</p>
<blockquote>
<p>
-12- An <i>invalid</i> iterator is an iterator that may be singular.(footnote: [&hellip;])
<p/>
<ins>Iterators are called <i>constexpr iterators</i> if all defined iterator category operations, except 
a pseudo-destructor call ( [expr.pseudo]) or the construction of an iterator with a singular value, 
are usable as constant expressions. 
[<i>Note:</i> For example, the types "pointer to int" and <code>reverse_iterator&lt;int*&gt;</code> are 
<i>constexpr random access iterators</i>. &mdash; <i>end note</i>]</ins>
<p/>
-13- In the following sections, <code>a</code> and <code>b</code> denote [&hellip;]
</p>
</blockquote>
</li>

<li><p>Add a new paragraph at the end of 23.3.3.1 <a href="https://wg21.link/array.overview">[array.overview]</a>:</p>
<blockquote>
<p>
-3- An <code>array</code> satisfies all of the requirements of a container [&hellip;]
<p/>
<ins>-?- <code>iterator</code> and <code>const_iterator</code> satisfy the constexpr iterator requirements 
(24.3.1 <a href="https://wg21.link/iterator.requirements.general">[iterator.requirements.general]</a>).</ins>
</p>
</blockquote>
</li>

</ol>





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