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<title>Issue 4120: move_iterator should provide iterator_category only when it models
forward_iterator</title>
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forward_iterator">
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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#New">New</a> status.</em></p>
<h3 id="4120"><a href="lwg-active.html#4120">4120</a>. <code>move_iterator</code> should provide <code>iterator_category</code> only when it models
<code>forward_iterator</code></h3>
<p><b>Section:</b> 24.5.4.2 <a href="https://wg21.link/move.iterator">[move.iterator]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-07-12 <b>Last modified:</b> 2024-08-31</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#move.iterator">active issues</a> in [move.iterator].</p>
<p><b>View all other</b> <a href="lwg-index.html#move.iterator">issues</a> in [move.iterator].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Following up LWG <a href="lwg-active.html#4116" title="enumerate_view::iterator and cartesian_product_view::iterator should not
    always provide iterator_category (Status: New)">4116</a><sup><a href="https://cplusplus.github.io/LWG/issue4116" title="Latest snapshot">(i)</a></sup>, when the underlying iterator does not satisfy <code>forward_iterator</code>,
<code>move_iterator::operator++(int)</code> will return <code>void</code>, which fails to meet the C++17 iterator
requirements, which should not provide <code>iterator_category</code>.
</p>

<p><i>[2024-08-02; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>
<p>
"NAD, would be a major breaking change to something present since C++11."
</p>
<p>
"We're knowingly breaking the rules here for backwards compatibility.
Should have a note here indicating that the rule is broken for good reason,
and that you should do as the Standard says and not as the Standard does."
</p>



<p id="res-4120"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4986" title=" Working Draft, Programming Languages — C++">N4986</a>.
</p>

<ol>
<li><p>Modify 24.5.4.2 <a href="https://wg21.link/move.iterator">[move.iterator]</a> as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> <code>Iterator</code> that models <code>forward_iterator</code> implies that
<code>iterator_traits&lt;Iterator&gt;::iterator_category</code> is always valid as it satisfies the
exposition-only concept <code><i>cpp17-iterator</i></code>.]
</p>
</blockquote>

<blockquote>
<p>
-2-
The member <i>typedef-name</i> <code>iterator_category</code> is defined if and only if <del>the <i>qualified-id</i>
  <code>iterator_traits&lt;Iterator&gt;::iterator_category</code> is valid and denotes a
  type</del><ins><code>Iterator</code> models <code>forward_iterator</code></ins>. In that case,
<code>iterator_category</code> denotes
</p>
<ol style="list-style-type: none">
<li>
<p>(2.1) &mdash; <code>random_access_iterator_tag</code> if the type
<code>iterator_traits&lt;Iterator&gt;::iterator_category</code> models
<code>derived_from&lt;random_access_iterator_tag&gt;</code>, and</p>
</li>
<li>
<p>(2.2) &mdash; <code>iterator_traits&lt;Iterator&gt;::iterator_category</code> otherwise.</p>
</li>
</ol>
</blockquote>
</li>
</ol>





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