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<h1 class="title" style="text-align:center"><code class="sourceCode cpp">views<span class="op">::</span>as_rvalue</code></h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2446R2</td>
</tr>
<tr>
<td>Date:</td>
<td>2022-02-14</td>
</tr>
<tr>
<td style="vertical-align:top">Project:</td>
<td>Programming Language C++</td>
</tr>
<tr>
<td style="vertical-align:top">Audience:</td>
<td>
LEWG<br>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to:</td>
<td>
Barry Revzin<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
</td>
</tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#revision-history"><span class="toc-section-number">1</span> Revision History<span></span></a></li>
<li><a href="#introduction"><span class="toc-section-number">2</span> Introduction<span></span></a>
<ul>
<li><a href="#but-why-now"><span class="toc-section-number">2.1</span> But why now?<span></span></a></li>
<li><a href="#naming"><span class="toc-section-number">2.2</span> Naming<span></span></a></li>
</ul></li>
<li><a href="#wording"><span class="toc-section-number">3</span> Wording<span></span></a>
<ul>
<li><a href="#as-rvalue-view-range.as.rvalue">24.7.? As rvalue view [range.as.rvalue]<span></span></a></li>
<li><a href="#feature-test-macro"><span class="toc-section-number">3.1</span> Feature-test macro<span></span></a></li>
</ul></li>
<li><a href="#bibliography"><span class="toc-section-number">4</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p>Since <span class="citation" data-cites="P2446R1">[<a href="#ref-P2446R1" role="doc-biblioref">P2446R1</a>]</span>, renamed to <code class="sourceCode cpp">views<span class="op">::</span>as_rvalue</code> and updated wording.</p>
<p>Since <span class="citation" data-cites="P2446R0">[<a href="#ref-P2446R0" role="doc-biblioref">P2446R0</a>]</span>, renamed to <code class="sourceCode cpp">views<span class="op">::</span>all_move</code> and added a feature-test macro.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">2</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>In <span class="citation" data-cites="P2214R1">[<a href="#ref-P2214R1" role="doc-biblioref">P2214R1</a>]</span>, I wrote:</p>
<div class="quote">
<p>Several of the above views that are labeled “not proposed” are variations on a common theme: <code class="sourceCode cpp">addressof</code>, <code class="sourceCode cpp">indirect</code>, and <code class="sourceCode cpp">move</code> are all basically wrappers around <code class="sourceCode cpp">transform</code> that take <code class="sourceCode cpp">std<span class="op">::</span>addressof</code>, <code class="sourceCode cpp">std<span class="op">::</span>dereference</code> (a function object we do not have at the moment), and <code class="sourceCode cpp">std<span class="op">::</span>move</code>, respectively. Basically, but not exactly, since one of those functions doesn’t exist yet and the other three we can’t pass as an argument anyway.</p>
<p>But some sort of broader ability to pass functions into functions would mostly alleviate the need for these. <code class="sourceCode cpp">views<span class="op">::</span>addressof</code> is shorter than <code class="sourceCode cpp">views<span class="op">::</span>transform<span class="op">(</span>LIFT<span class="op">(</span>std<span class="op">::</span>addressof<span class="op">))</span></code> (assuming a LIFT macro that wraps a name and emits a lambda), but we’re not sure that we necessarily need to add special cases of transform for every useful function.</p>
</div>
<p>While this is true for <code class="sourceCode cpp">views<span class="op">::</span>addressof</code> and <code class="sourceCode cpp">views<span class="op">::</span>indirect</code>, it’s actually <em>not</em> correct for <code class="sourceCode cpp">views<span class="op">::</span>move</code>. There is actually a lot more involvement here.</p>
<p>To start with, while if we had a range of lvalues, we would just want to <code class="sourceCode cpp">std<span class="op">::</span>move<span class="op">()</span></code> each element of the range, that’s not true if we had a range of rvalues. Those… we wouldn’t really have to do anything with (indeed, if we had a range of prvalues, the extra <code class="sourceCode cpp">std<span class="op">::</span>move<span class="op">()</span></code> would just add unnecessary overhead by materializing those objects earlier). Except in some cases, we <em>do</em> still want to do something with the prvalues - if we were <code class="sourceCode cpp">zip<span class="op">()</span></code>ing <span class="citation" data-cites="P2321R2">[<a href="#ref-P2321R2" role="doc-biblioref">P2321R2</a>]</span> one range of lvalues, we would get back a range of <code class="sourceCode cpp">tuple<span class="op"><</span>T<span class="op">&></span></code>. But the result of piping that into <code class="sourceCode cpp">views<span class="op">::</span>move</code> shouldn’t be <code class="sourceCode cpp">tuple<span class="op"><</span>T<span class="op">&>&&</span></code> (as a naive <code class="sourceCode cpp">views<span class="op">::</span>transform<span class="op">(</span>std<span class="op">::</span>move<span class="op">)</span></code> would do) and it shouldn’t be <code class="sourceCode cpp">tuple<span class="op"><</span>T<span class="op">&></span></code> (as a slightly less naive implementation that avoids transforming rvalues) - we should get back a range of <code class="sourceCode cpp">tuple<span class="op"><</span>T<span class="op">&&></span></code>.</p>
<p>Indeed, we already have a customization point that does this for us: <code class="sourceCode cpp">ranges<span class="op">::</span>iter_move</code> <span>23.3.3.1 <a href="https://wg21.link/iterator.cust.move">[iterator.cust.move]</a></span>. But we can’t pass that into <code class="sourceCode cpp">views<span class="op">::</span>transform</code>, because <code class="sourceCode cpp">ranges<span class="op">::</span>iter_move</code> operators on the iterator (as the name suggest) and not the underlying element. We would need to spell this as <code class="sourceCode cpp">views<span class="op">::</span>iter_transform<span class="op">(</span>ranges<span class="op">::</span>iter_move<span class="op">)</span></code>, but we don’t yet have a <code class="sourceCode cpp">views<span class="op">::</span>iter_transform</code> (although range-v3 does).</p>
<p>However, even if we could define <code class="sourceCode cpp">views<span class="op">::</span>move</code> as simply <code class="sourceCode cpp">views<span class="op">::</span>iter_transform<span class="op">(</span>ranges<span class="op">::</span>iter_move<span class="op">)</span></code>, that’s still a poor definition, for a very important reason: <code class="sourceCode cpp">ranges<span class="op">::</span>iter_move</code> (just like <code class="sourceCode cpp">std<span class="op">::</span>move</code>) isn’t just <em>some</em> arbitrary transformation. We know a lot about this particular one, and specifically we know that it consumes the source element. And as a result, that <em>should</em> limit the algorithms that you can perform on <code class="sourceCode cpp">views<span class="op">::</span>move<span class="op">(</span>e<span class="op">)</span></code> - such an adapted range needs to be an <em>input</em> range. <code class="sourceCode cpp">views<span class="op">::</span>transform<span class="op">(</span>e, std<span class="op">::</span>move<span class="op">)</span></code> and <code class="sourceCode cpp">views<span class="op">::</span>iter_transform<span class="op">(</span>e, ranges<span class="op">::</span>iter_move<span class="op">)</span></code> would both be potentially up to random-access, but we need to set a much much lower ceiling here.</p>
<p>As such, <code class="sourceCode cpp">views<span class="op">::</span>move</code> needs to be a first-class range adaptor.</p>
<h2 data-number="2.1" id="but-why-now"><span class="header-section-number">2.1</span> But why now?<a href="#but-why-now" class="self-link"></a></h2>
<p>The above argues why <code class="sourceCode cpp">views<span class="op">::</span>move</code> can’t just be a <code class="sourceCode cpp">views<span class="op">::</span>transform</code> or a <code class="sourceCode cpp">views<span class="op">::</span>iter_transform</code>, but why do we need it now? Well, there are two answers to this.</p>
<p>First, with the imminent adoption of <code class="sourceCode cpp">ranges<span class="op">::</span>to</code> <span class="citation" data-cites="P1206R6">[<a href="#ref-P1206R6" role="doc-biblioref">P1206R6</a>]</span>, we spent a lot of time on that paper trying to make collecting of elements as efficient as possible. As efficient as possible certainly includes <em>moving</em> elements instead of <em>copying</em> elements, where appropriate. However, right now, it’s really up to users to figure out how to do that. A <code class="sourceCode cpp">views<span class="op">::</span>move</code> would go a long way in making this as effortless as possible (and mirrors how users would move a single element).</p>
<p>Second, unlike <code class="sourceCode cpp">views<span class="op">::</span>as_const</code> <span class="citation" data-cites="P2278R1">[<a href="#ref-P2278R1" role="doc-biblioref">P2278R1</a>]</span>, where a lot of design effort had to be spent figuring out how to implement a constant iterator, that work has already been done for move iterators. <code class="sourceCode cpp">std<span class="op">::</span>move_iterator<span class="op"><</span>I<span class="op">></span></code> <span>23.5.3 <a href="https://wg21.link/move.iterators">[move.iterators]</a></span> exists and already does the right thing, and there’s already a <code class="sourceCode cpp">std<span class="op">::</span>move_sentinel<span class="op"><</span>S<span class="op">></span></code> <span>23.5.3.10 <a href="https://wg21.link/move.sentinel">[move.sentinel]</a></span> to handle non-common ranges. All the work is done, we just have to put it together. As you can see below, the wording is pretty small and very straightforward.</p>
<p>This is a tiny paper, and while this should be considered the lowest priority of all the range adaptors (as the C++23 Ranges Plan doesn’t even propose it at all), we may as well at least try to squeeze it in.</p>
<h2 data-number="2.2" id="naming"><span class="header-section-number">2.2</span> Naming<a href="#naming" class="self-link"></a></h2>
<p>In range-v3, the adaptors to move all of the elements and make all of the elements const were named <code class="sourceCode cpp">views<span class="op">::</span>move</code> and <code class="sourceCode cpp">views<span class="op">::</span>const_</code>, respectively. <span class="citation" data-cites="P2446R0">[<a href="#ref-P2446R0" role="doc-biblioref">P2446R0</a>]</span> and <span class="citation" data-cites="P2278R0">[<a href="#ref-P2278R0" role="doc-biblioref">P2278R0</a>]</span> simply used range-v3’s names. <span class="citation" data-cites="P2278R1">[<a href="#ref-P2278R1" role="doc-biblioref">P2278R1</a>]</span> changed from <code class="sourceCode cpp">views<span class="op">::</span>const_</code> to <code class="sourceCode cpp">views<span class="op">::</span>as_const</code> (as an analogue for <code class="sourceCode cpp">std<span class="op">::</span>as_const</code>). But during LEWG telecon discussion of these papers <span class="citation" data-cites="p2278-minutes">[<a href="#ref-p2278-minutes" role="doc-biblioref">p2278-minutes</a>]</span> <span class="citation" data-cites="p2446-minutes">[<a href="#ref-p2446-minutes" role="doc-biblioref">p2446-minutes</a>]</span>, it was suggested that having such names is confusing because of the ambiguity between whether these adaptors operate on the <em>range</em> or the <em>elements</em> thereof, and so we end up with the names <code class="sourceCode cpp">views<span class="op">::</span>all_move</code> and <code class="sourceCode cpp">views<span class="op">::</span>all_const</code> to avoid conflict with the two <code class="sourceCode cpp">std<span class="op">::</span>move</code>s we already have and the <code class="sourceCode cpp">std<span class="op">::</span>as_const</code>, that do something else.</p>
<p>I think these names are pretty bad (it’s not named <code class="sourceCode cpp">views<span class="op">::</span>all_transform</code>?), and there was a follow-up paper to suggest reverting this rename <span class="citation" data-cites="P2501R0">[<a href="#ref-P2501R0" role="doc-biblioref">P2501R0</a>]</span>. Ensuing discussion pointed out a potential issue that because <code class="sourceCode cpp">std<span class="op">::</span>move</code> and <code class="sourceCode cpp">std<span class="op">::</span>views<span class="op">::</span>move</code> can both be used unary and unqualified, there’s potential for code silently changing meaning if <code class="sourceCode cpp">move</code> were previously used unqualified. Example from Nicolai Josuttis:</p>
<blockquote>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">using</span> <span class="kw">namespace</span> std<span class="op">::</span>views;</span>
<span id="cb1-2"><a href="#cb1-2"></a></span>
<span id="cb1-3"><a href="#cb1-3"></a>std<span class="op">::</span>vector<span class="op"><</span>std<span class="op">::</span>string<span class="op">></span> v1<span class="op">{</span><span class="st">"hello"</span>, <span class="st">"world"</span><span class="op">}</span>;</span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="kw">auto</span> v2 <span class="op">=</span> move<span class="op">(</span>v1<span class="op">)</span>; <span class="co">// OOPS: initializes a view to v1</span></span></code></pre></div>
</blockquote>
<p>If the user had done a <code class="sourceCode cpp"><span class="kw">using</span> <span class="kw">namespace</span> std<span class="op">::</span>views</code> (to do some ranges pipeline work without having to qualify <code class="sourceCode cpp">std<span class="op">::</span>views<span class="op">::</span>meow</code>) but then had previously used <code class="sourceCode cpp">move<span class="op">(</span>x<span class="op">)</span></code> on some object with <code class="sourceCode cpp">std</code> as an associated namespace, in C++20 this would invoke <code class="sourceCode cpp">std<span class="op">::</span>move</code> whereas in C++23, if we add this range adaptor under the name <code class="sourceCode cpp">std<span class="op">::</span>views<span class="op">::</span>move</code>, this would invoke the adaptor instead. The moral of the story here is probably to just always qualify <code class="sourceCode cpp">std<span class="op">::</span>move</code> (indeed, the library specification always uses it qualified), but there was sufficient concern about this example that there was no consensus in LEWG to revert and go back to <code class="sourceCode cpp">views<span class="op">::</span>move</code>, preferring <code class="sourceCode cpp">views<span class="op">::</span>all_move</code>.</p>
<p>Nevertheless, <code class="sourceCode cpp">all_move</code> is just not a great name, and it’s also the wrong tense. A subsequent suggestion from Ville Voutilainen is <code class="sourceCode cpp">views<span class="op">::</span>as_rvalue</code>. This has the advantage of being the correct tense and not conflicting with anything else in the standard, and the result of this adaptor is that you do end up with a range of rvalues (whether the adaptor actually does something or not. Indeed that’s what the first sentence of the introductory wording has always said). An alternative name I would entertain would be <code class="sourceCode cpp">views<span class="op">::</span>move_each</code> (but not <code class="sourceCode cpp">views<span class="op">::</span>move_elements</code>, since <code class="sourceCode cpp">views<span class="op">::</span>elements<span class="op"><</span>N<span class="op">></span></code> exists and these don’t quite line up).</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="wording"><span class="header-section-number">3</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Add to <span>24.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a></span>:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb2"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb2-1"><a href="#cb2-1"></a>#include <compare> // see [compare.syn]</span>
<span id="cb2-2"><a href="#cb2-2"></a>#include <initializer_list> // see [initializer.list.syn]</span>
<span id="cb2-3"><a href="#cb2-3"></a>#include <iterator> // see [iterator.synopsis]</span>
<span id="cb2-4"><a href="#cb2-4"></a></span>
<span id="cb2-5"><a href="#cb2-5"></a>namespace std::ranges {</span>
<span id="cb2-6"><a href="#cb2-6"></a> // ...</span>
<span id="cb2-7"><a href="#cb2-7"></a><span class="va">+</span></span>
<span id="cb2-8"><a href="#cb2-8"></a><span class="va">+ template<view V></span></span>
<span id="cb2-9"><a href="#cb2-9"></a><span class="va">+ requires input_range<V></span></span>
<span id="cb2-10"><a href="#cb2-10"></a><span class="va">+ class as_rvalue_view;</span></span>
<span id="cb2-11"><a href="#cb2-11"></a><span class="va">+</span></span>
<span id="cb2-12"><a href="#cb2-12"></a><span class="va">+ template<class T></span></span>
<span id="cb2-13"><a href="#cb2-13"></a><span class="va">+ inline constexpr bool enable_borrowed_range<as_rvalue_view<T>> = enable_borrowed_range<T>;</span></span>
<span id="cb2-14"><a href="#cb2-14"></a><span class="va">+</span></span>
<span id="cb2-15"><a href="#cb2-15"></a><span class="va">+ namespace views { inline constexpr <em>unspecified</em> as_rvalue = <em>unspecified</em>; }</span></span>
<span id="cb2-16"><a href="#cb2-16"></a></span>
<span id="cb2-17"><a href="#cb2-17"></a> // ...</span>
<span id="cb2-18"><a href="#cb2-18"></a>}</span></code></pre></div>
</div>
</blockquote>
<h3 class="unnumbered" data-number id="as-rvalue-view-range.as.rvalue">24.7.? As rvalue view [range.as.rvalue]<a href="#as-rvalue-view-range.as.rvalue" class="self-link"></a></h3>
<h4 class="unnumbered" data-number id="overview-range.as.rvalue.overview">24.7.?.1 Overview [range.as.rvalue.overview]<a href="#overview-range.as.rvalue.overview" class="self-link"></a></h4>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">1</a></span> <code class="sourceCode cpp">as_rvalue_view</code> presents a <code class="sourceCode cpp">view</code> of an underlying sequence with the same behavior as the underlying sequence except that its elements are rvalues. Some generic algorithms can be called with a <code class="sourceCode cpp">as_rvalue_view</code> to replace copying with moving.</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">2</a></span> The name <code class="sourceCode cpp">views<span class="op">::</span>as_rvalue</code> denotes a range adaptor object ([range.adaptor.object]). Let <code class="sourceCode cpp">E</code> be an expression and let <code class="sourceCode cpp">T</code> be <code class="sourceCode cpp"><span class="kw">decltype</span><span class="op">((</span>E<span class="op">))</span></code>. The expression <code class="sourceCode cpp">views<span class="op">::</span>as_rvalue<span class="op">(</span>E<span class="op">)</span></code> is expression-equivalent to:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">(2.1)</a></span> <code class="sourceCode cpp">views<span class="op">::</span>all<span class="op">(</span>E<span class="op">)</span></code> if <code class="sourceCode cpp">same_as<span class="op"><</span>range_rvalue_reference_t<span class="op"><</span>T<span class="op">></span>, range_reference_t<span class="op"><</span>T<span class="op">>></span></code> is <code class="sourceCode cpp"><span class="kw">true</span></code></li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">(2.2)</a></span> Otherwise, <code class="sourceCode cpp">ranges<span class="op">::</span>as_rvalue_view<span class="op">(</span>E<span class="op">)</span></code>.</li>
</ul>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">3</a></span> [<em>Example</em>:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a>vector<span class="op"><</span>string<span class="op">></span> words <span class="op">=</span> <span class="op">{</span><span class="st">"the"</span>, <span class="st">"quick"</span>, <span class="st">"brown"</span>, <span class="st">"fox"</span>, <span class="st">"ate"</span>, <span class="st">"a"</span>, <span class="st">"pterodactyl"</span><span class="op">}</span>;</span>
<span id="cb3-2"><a href="#cb3-2"></a>vector<span class="op"><</span>string<span class="op">></span> new_words;</span>
<span id="cb3-3"><a href="#cb3-3"></a>ranges<span class="op">::</span>copy<span class="op">(</span>words <span class="op">|</span> views<span class="op">::</span>as_rvalue, back_inserter<span class="op">(</span>new_words<span class="op">))</span>; <span class="co">// moves each string from words into new_words</span></span></code></pre></div>
<p>-<em>end example</em>]</p>
</blockquote>
<h4 class="unnumbered" data-number id="class-template-as_rvalue_view-range.as.rvalue.view">24.7.?.2 Class template <code class="sourceCode cpp">as_rvalue_view</code> [range.as.rvalue.view]<a href="#class-template-as_rvalue_view-range.as.rvalue.view" class="self-link"></a></h4>
<blockquote>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a><span class="kw">namespace</span> std<span class="op">::</span>ranges <span class="op">{</span></span>
<span id="cb4-2"><a href="#cb4-2"></a> <span class="kw">template</span><span class="op"><</span>input_range V<span class="op">></span></span>
<span id="cb4-3"><a href="#cb4-3"></a> <span class="kw">requires</span> view<span class="op"><</span>V<span class="op">></span></span>
<span id="cb4-4"><a href="#cb4-4"></a> <span class="kw">class</span> as_rvalue_view <span class="op">:</span> <span class="kw">public</span> view_interface<span class="op"><</span>as_rvalue_view<span class="op"><</span>V<span class="op">>></span></span>
<span id="cb4-5"><a href="#cb4-5"></a> <span class="op">{</span></span>
<span id="cb4-6"><a href="#cb4-6"></a> V <em>base_</em> <span class="op">=</span> V<span class="op">()</span>; <span class="co">// exposition only</span></span>
<span id="cb4-7"><a href="#cb4-7"></a></span>
<span id="cb4-8"><a href="#cb4-8"></a> <span class="kw">public</span><span class="op">:</span></span>
<span id="cb4-9"><a href="#cb4-9"></a> as_rvalue_view<span class="op">()</span> <span class="kw">requires</span> default_initializable<span class="op"><</span>V<span class="op">></span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb4-10"><a href="#cb4-10"></a> <span class="kw">constexpr</span> <span class="kw">explicit</span> as_rvalue_view<span class="op">(</span>V base<span class="op">)</span>;</span>
<span id="cb4-11"><a href="#cb4-11"></a></span>
<span id="cb4-12"><a href="#cb4-12"></a> <span class="kw">constexpr</span> V base<span class="op">()</span> <span class="kw">const</span><span class="op">&</span> <span class="kw">requires</span> copy_constructible<span class="op"><</span>V<span class="op">></span> <span class="op">{</span> <span class="cf">return</span> <em>base_</em>; <span class="op">}</span></span>
<span id="cb4-13"><a href="#cb4-13"></a> <span class="kw">constexpr</span> V base<span class="op">()</span> <span class="op">&&</span> <span class="op">{</span> <span class="cf">return</span> std<span class="op">::</span>move<span class="op">(</span><em>base_</em><span class="op">)</span>; <span class="op">}</span></span>
<span id="cb4-14"><a href="#cb4-14"></a></span>
<span id="cb4-15"><a href="#cb4-15"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> begin<span class="op">()</span> <span class="kw">requires</span> <span class="op">(!</span><em>simple-view</em><span class="op"><</span>V<span class="op">>)</span> <span class="op">{</span> <span class="cf">return</span> move_iterator<span class="op">(</span>ranges<span class="op">::</span>begin<span class="op">(</span><em>base_</em><span class="op">))</span>; <span class="op">}</span></span>
<span id="cb4-16"><a href="#cb4-16"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> begin<span class="op">()</span> <span class="kw">const</span> <span class="kw">requires</span> range<span class="op"><</span><span class="kw">const</span> V<span class="op">></span> <span class="op">{</span> <span class="cf">return</span> move_iterator<span class="op">(</span>ranges<span class="op">::</span>begin<span class="op">(</span><em>base_</em><span class="op">))</span>; <span class="op">}</span></span>
<span id="cb4-17"><a href="#cb4-17"></a></span>
<span id="cb4-18"><a href="#cb4-18"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> end<span class="op">()</span> <span class="kw">requires</span> <span class="op">(!</span><em>simple-view</em><span class="op"><</span>V<span class="op">>)</span> <span class="op">{</span></span>
<span id="cb4-19"><a href="#cb4-19"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>common_range<span class="op"><</span>V<span class="op">>)</span> <span class="op">{</span></span>
<span id="cb4-20"><a href="#cb4-20"></a> <span class="cf">return</span> move_iterator<span class="op">(</span>ranges<span class="op">::</span>end<span class="op">(</span><em>base_</em><span class="op">))</span>;</span>
<span id="cb4-21"><a href="#cb4-21"></a> <span class="op">}</span> <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb4-22"><a href="#cb4-22"></a> <span class="cf">return</span> move_sentinel<span class="op">(</span>ranges<span class="op">::</span>end<span class="op">(</span><em>base_</em><span class="op">))</span>;</span>
<span id="cb4-23"><a href="#cb4-23"></a> <span class="op">}</span></span>
<span id="cb4-24"><a href="#cb4-24"></a> <span class="op">}</span></span>
<span id="cb4-25"><a href="#cb4-25"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> end<span class="op">()</span> <span class="kw">const</span> <span class="kw">requires</span> range<span class="op"><</span><span class="kw">const</span> V<span class="op">></span> <span class="op">{</span></span>
<span id="cb4-26"><a href="#cb4-26"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>common_range<span class="op"><</span><span class="kw">const</span> V<span class="op">>)</span> <span class="op">{</span></span>
<span id="cb4-27"><a href="#cb4-27"></a> <span class="cf">return</span> move_iterator<span class="op">(</span>ranges<span class="op">::</span>end<span class="op">(</span><em>base_</em><span class="op">))</span>;</span>
<span id="cb4-28"><a href="#cb4-28"></a> <span class="op">}</span> <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb4-29"><a href="#cb4-29"></a> <span class="cf">return</span> move_sentinel<span class="op">(</span>ranges<span class="op">::</span>end<span class="op">(</span><em>base_</em><span class="op">))</span>;</span>
<span id="cb4-30"><a href="#cb4-30"></a> <span class="op">}</span></span>
<span id="cb4-31"><a href="#cb4-31"></a> <span class="op">}</span></span>
<span id="cb4-32"><a href="#cb4-32"></a></span>
<span id="cb4-33"><a href="#cb4-33"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> size<span class="op">()</span> <span class="kw">requires</span> sized_range<span class="op"><</span>V<span class="op">></span> <span class="op">{</span> <span class="cf">return</span> ranges<span class="op">::</span>size<span class="op">(</span><em>base_</em><span class="op">)</span>; <span class="op">}</span></span>
<span id="cb4-34"><a href="#cb4-34"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> size<span class="op">()</span> <span class="kw">const</span> <span class="kw">requires</span> sized_range<span class="op"><</span><span class="kw">const</span> V<span class="op">></span> <span class="op">{</span> <span class="cf">return</span> ranges<span class="op">::</span>size<span class="op">(</span><em>base_</em><span class="op">)</span>; <span class="op">}</span></span>
<span id="cb4-35"><a href="#cb4-35"></a> <span class="op">}</span>;</span>
<span id="cb4-36"><a href="#cb4-36"></a></span>
<span id="cb4-37"><a href="#cb4-37"></a> <span class="kw">template</span><span class="op"><</span><span class="kw">class</span> R<span class="op">></span></span>
<span id="cb4-38"><a href="#cb4-38"></a> as_rvalue_view<span class="op">(</span>R<span class="op">&&)</span> <span class="op">-></span> as_rvalue_view<span class="op"><</span>views<span class="op">::</span>all_t<span class="op"><</span>R<span class="op">>></span>;</span>
<span id="cb4-39"><a href="#cb4-39"></a><span class="op">}</span></span></code></pre></div>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="kw">constexpr</span> <span class="kw">explicit</span> as_rvalue_view<span class="op">(</span>V base<span class="op">)</span>;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_6" id="pnum_6">1</a></span> <em>Effects</em>: Initializes <code class="sourceCode cpp"><em>base_</em></code> with <code class="sourceCode cpp">std<span class="op">::</span>move<span class="op">(</span>base<span class="op">)</span></code>.</p>
</blockquote>
<h2 data-number="3.1" id="feature-test-macro"><span class="header-section-number">3.1</span> Feature-test macro<a href="#feature-test-macro" class="self-link"></a></h2>
<p>Add the following macro definition to <span>17.3.2 <a href="https://wg21.link/version.syn">[version.syn]</a></span>, with the value selected by the editor to reflect the date of adoption of this paper:</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1"></a><span class="pp">#define __cpp_lib_ranges_as_rvalue </span><span class="dv">20</span><span class="er">XXXXL</span><span class="pp"> </span><span class="co">// also in <ranges></span></span></code></pre></div>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">4</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-P1206R6">
<p>[P1206R6] Corentin Jabot, Eric Niebler, Casey Carter. 2021-08-03. Conversions from ranges to containers. <br />
<a href="https://wg21.link/p1206r6">https://wg21.link/p1206r6</a></p>
</div>
<div id="ref-P2214R1">
<p>[P2214R1] Barry Revzin, Conor Hoekstra, Tim Song. 2021-09-14. A Plan for C++23 Ranges. <br />
<a href="https://wg21.link/p2214r1">https://wg21.link/p2214r1</a></p>
</div>
<div id="ref-p2278-minutes">
<p>[p2278-minutes] LEWG. 2021. LEWG minutes for P2278. <br />
<a href="https://wiki.edg.com/bin/view/Wg21telecons2021/P2278#Library-Evolution-2021-11-09">https://wiki.edg.com/bin/view/Wg21telecons2021/P2278#Library-Evolution-2021-11-09</a></p>
</div>
<div id="ref-P2278R0">
<p>[P2278R0] Barry Revzin. 2021-01-10. cbegin should always return a constant iterator. <br />
<a href="https://wg21.link/p2278r0">https://wg21.link/p2278r0</a></p>
</div>
<div id="ref-P2278R1">
<p>[P2278R1] Barry Revzin. 2021-09-15. cbegin should always return a constant iterator. <br />
<a href="https://wg21.link/p2278r1">https://wg21.link/p2278r1</a></p>
</div>
<div id="ref-P2321R2">
<p>[P2321R2] Tim Song. 2021-06-11. zip. <br />
<a href="https://wg21.link/p2321r2">https://wg21.link/p2321r2</a></p>
</div>
<div id="ref-p2446-minutes">
<p>[p2446-minutes] LEWG. 2021. LEWG minutes for P2446. <br />
<a href="https://wiki.edg.com/bin/view/Wg21telecons2021/P2446#Library-Evolution-2021-11-09">https://wiki.edg.com/bin/view/Wg21telecons2021/P2446#Library-Evolution-2021-11-09</a></p>
</div>
<div id="ref-P2446R0">
<p>[P2446R0] Barry Revzin. 2021-09-18. views::move. <br />
<a href="https://wg21.link/p2446r0">https://wg21.link/p2446r0</a></p>
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<div id="ref-P2446R1">
<p>[P2446R1] Barry Revzin. 2021-11-17. views::all_move. <br />
<a href="https://wg21.link/p2446r1">https://wg21.link/p2446r1</a></p>
</div>
<div id="ref-P2501R0">
<p>[P2501R0] Ville Voutilainen. 2021-12-14. Undo the rename of views::move and views::as_const. <br />
<a href="https://wg21.link/p2501r0">https://wg21.link/p2501r0</a></p>
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