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<title>Issue 4097: views::reverse should be specialized for some view 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#LEWG">LEWG</a> status.</em></p>
<h3 id="4097"><a href="lwg-active.html#4097">4097</a>. <code>views::reverse</code> should be specialized for some view types</h3>
<p><b>Section:</b> 25.7.21.1 <a href="https://wg21.link/range.reverse.overview">[range.reverse.overview]</a> <b>Status:</b> <a href="lwg-active.html#LEWG">LEWG</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-05-09 <b>Last modified:</b> 2024-06-24</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#LEWG">LEWG</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, when <code>views::reverse</code> is applied to <code>empty_view</code>, <code>single_view</code>, or 
<code>repeat_view</code>, a <code>reverse_view</code> with the iterator type <code>reverse_iterator</code> will 
be produced.
<p/>
Such an amount of instantiation seems less necessary for these three simple view types, since returning a copy 
of them is sufficient.
<p/>
Note that this also makes <code>views::repeat(0) | views::reverse | views::take(5)</code> no longer timeout, 
which seems to be an improvement.
</p>

<p><i>[2024-06-24; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll. Send to LEWG for review.
The <code class='backtick'>repeat</code> part is related to LWG <a href="lwg-active.html#4019" title="Reversing an infinite range leads to an infinite loop (Status: SG9)">4019</a><sup><a href="https://cplusplus.github.io/LWG/issue4019" title="Latest snapshot">(i)</a></sup>.
</p>



<p id="res-4097"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4981" title=" Working Draft, Programming Languages — C++">N4981</a>.
</p>

<ol>

<li><p>Modify 25.7.21.1 <a href="https://wg21.link/range.reverse.overview">[range.reverse.overview]</a> as indicated:</p>

<blockquote>
<p>
-1- <code>reverse_view</code> takes a bidirectional view and produces another view that 
iterates the same elements in reverse order.
<p/>
-2- The name <code>views::reverse</code> denotes a range adaptor object (25.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a>).
Given a subexpression <code>E</code>, the expression <code>views::reverse(E)</code>
is expression-equivalent to:</p>
<ol style="list-style-type: none">
<li>
<p>(2.1) &mdash; If the type of <code>E</code> is a (possibly cv-qualified) specialization of
<code>reverse_view</code>, equivalent to <code>E.base()</code>.</p></li>
<li><p><ins>(2.?) &mdash; Otherwise, if the type of <code>E</code> is a (possibly cv-qualified) specialization of 
<code>empty_view</code> (25.6.2.2 <a href="https://wg21.link/range.empty.view">[range.empty.view]</a>), <code>single_view</code> 
(25.6.3.2 <a href="https://wg21.link/range.single.view">[range.single.view]</a>), or <code>repeat_view</code> (25.6.5.2 <a href="https://wg21.link/range.repeat.view">[range.repeat.view]</a>),
equivalent to <code><i>decay-copy</i>(E)</code>.</ins></p></li>
<li><p>(2.2) &mdash; Otherwise, if the type of <code>E</code> is <i>cv</i> 
<code>subrange&lt;reverse_iterator&lt;I&gt;, reverse_iterator&lt;I&gt;, K&gt;</code> for some iterator 
type <code>I</code> and value <code>K</code> of type <code>subrange_kind</code>,</p>
<ol style="list-style-type: none">
<li>
<p>(2.2.1) &mdash; if <code>K</code> is <code>subrange_kind::sized</code>, equivalent to:</p>
<blockquote>
<pre>subrange&lt;I, I, K&gt;(E.end().base(), E.begin().base(), E.size())</pre>
</blockquote>
</li>
<li>
<p>(2.2.2) &mdash; otherwise, equivalent to:</p>
<blockquote>
<pre>subrange&lt;I, I, K&gt;(E.end().base(), E.begin().base())</pre>
</blockquote>
</li>
</ol>
<p>However, in either case <code>E</code> is evaluated only once.</p>
</li>
<li>
<p>(2.3) &mdash; Otherwise, equivalent to <code>reverse_view{E}</code>.</p>
</li>
</ol>
</blockquote>

</li>
</ol>






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