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  <meta name="dcterms.date" content="2024-02-14" />
  <title>views::cache_last</title>
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<h1 class="title" style="text-align:center"><code class="sourceCode cpp">views<span class="op">::</span>cache_last</code></h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P3138R0</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2024-02-14</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>
      SG9<br>
      SG1<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Tim Song<br>&lt;<a href="mailto:t.canens.cpp@gmail.com" class="email">t.canens.cpp@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="abstract"><span class="header-section-number">1</span> Abstract<a href="#abstract" class="self-link"></a></h1>
<p>This paper proposes the <code class="sourceCode cpp">views<span class="op">::</span>cache_last</code> adaptor that caches the result of the last dereference of the underlying iterator. To support this adaptor, it also proposes a relaxation of <span>16.4.6.10 <a href="https://wg21.link/res.on.data.races">[res.on.data.races]</a></span> for operations on input-only iterators.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="motivation"><span class="header-section-number">2</span> Motivation<a href="#motivation" class="self-link"></a></h1>
<p>The motivation for this view is given in <span class="citation" data-cites="P2760R1">[<a href="#ref-P2760R1" role="doc-biblioref">P2760R1</a>]</span> and quoted below for convenience:</p>
<blockquote>
<p>One of the adaptors that we considered for C++23 but ended up not pursuing was what range-v3 calls <code class="sourceCode cpp">cache1</code> and what we’d instead like to call something like <code class="sourceCode cpp">cache_last</code>. This is an adaptor which, as the name suggests, caches the last element. The reason for this is efficiency - specifically avoiding extra work that has to be done by iterator dereferencing.</p>
<p>The canonical example of this is <code class="sourceCode cpp">transform<span class="op">(</span>f<span class="op">)</span> <span class="op">|</span> filter<span class="op">(</span>g<span class="op">)</span></code>, where if you then iterate over the subsequent range, <code class="sourceCode cpp">f</code> will be invoked twice for every element that satisfies <code class="sourceCode cpp">g</code>:</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="dt">int</span> main<span class="op">()</span></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="op">{</span></span>
<span id="cb1-3"><a href="#cb1-3"></a>    std<span class="op">::</span>vector<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;</span> v <span class="op">=</span> <span class="op">{</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span>, <span class="dv">4</span>, <span class="dv">5</span><span class="op">}</span>;</span>
<span id="cb1-4"><a href="#cb1-4"></a></span>
<span id="cb1-5"><a href="#cb1-5"></a>    <span class="kw">auto</span> even_squares <span class="op">=</span> v</span>
<span id="cb1-6"><a href="#cb1-6"></a>        <span class="op">|</span> std<span class="op">::</span>views<span class="op">::</span>transform<span class="op">([](</span><span class="dt">int</span> i<span class="op">){</span></span>
<span id="cb1-7"><a href="#cb1-7"></a>                std<span class="op">::</span>print<span class="op">(</span><span class="st">&quot;transform: {}</span><span class="sc">\n</span><span class="st">&quot;</span>, i<span class="op">)</span>;</span>
<span id="cb1-8"><a href="#cb1-8"></a>                <span class="cf">return</span> i <span class="op">*</span> i;</span>
<span id="cb1-9"><a href="#cb1-9"></a>            <span class="op">})</span></span>
<span id="cb1-10"><a href="#cb1-10"></a>        <span class="op">|</span> std<span class="op">::</span>views<span class="op">::</span>filter<span class="op">([](</span><span class="dt">int</span> i<span class="op">){</span></span>
<span id="cb1-11"><a href="#cb1-11"></a>                std<span class="op">::</span>print<span class="op">(</span><span class="st">&quot;filter: {}</span><span class="sc">\n</span><span class="st">&quot;</span>, i<span class="op">)</span>;</span>
<span id="cb1-12"><a href="#cb1-12"></a>                <span class="cf">return</span> i <span class="op">%</span> <span class="dv">2</span> <span class="op">==</span> <span class="dv">0</span>;</span>
<span id="cb1-13"><a href="#cb1-13"></a>            <span class="op">})</span>;</span>
<span id="cb1-14"><a href="#cb1-14"></a></span>
<span id="cb1-15"><a href="#cb1-15"></a>    <span class="cf">for</span> <span class="op">(</span><span class="dt">int</span> i <span class="op">:</span> even_squares<span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-16"><a href="#cb1-16"></a>        std<span class="op">::</span>print<span class="op">(</span><span class="st">&quot;Got: {}</span><span class="sc">\n</span><span class="st">&quot;</span>, i<span class="op">)</span>;</span>
<span id="cb1-17"><a href="#cb1-17"></a>    <span class="op">}</span></span>
<span id="cb1-18"><a href="#cb1-18"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>prints the following (note that there are 7 invocations of <code class="sourceCode cpp">transform</code>):</p>
<blockquote>
<div class="sourceCode" id="cb2"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb2-1"><a href="#cb2-1"></a>transform: 1</span>
<span id="cb2-2"><a href="#cb2-2"></a>filter: 1</span>
<span id="cb2-3"><a href="#cb2-3"></a>transform: 2</span>
<span id="cb2-4"><a href="#cb2-4"></a>filter: 4</span>
<span id="cb2-5"><a href="#cb2-5"></a>transform: 2</span>
<span id="cb2-6"><a href="#cb2-6"></a>Got: 4</span>
<span id="cb2-7"><a href="#cb2-7"></a>transform: 3</span>
<span id="cb2-8"><a href="#cb2-8"></a>filter: 9</span>
<span id="cb2-9"><a href="#cb2-9"></a>transform: 4</span>
<span id="cb2-10"><a href="#cb2-10"></a>filter: 16</span>
<span id="cb2-11"><a href="#cb2-11"></a>transform: 4</span>
<span id="cb2-12"><a href="#cb2-12"></a>Got: 16</span>
<span id="cb2-13"><a href="#cb2-13"></a>transform: 5</span>
<span id="cb2-14"><a href="#cb2-14"></a>filter: 25</span></code></pre></div>
</blockquote>
<p>The solution here is to add a layer of caching:</p>
<blockquote>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">auto</span> even_squares <span class="op">=</span> v</span>
<span id="cb3-2"><a href="#cb3-2"></a>    <span class="op">|</span> views<span class="op">::</span>transform<span class="op">(</span>square<span class="op">)</span></span>
<span id="cb3-3"><a href="#cb3-3"></a>    <span class="op">|</span> views<span class="op">::</span>cache_last</span>
<span id="cb3-4"><a href="#cb3-4"></a>    <span class="op">|</span> views<span class="op">::</span>filter<span class="op">(</span>is_even<span class="op">)</span>;</span></code></pre></div>
</blockquote>
<p>Which will ensure that <code class="sourceCode cpp">square</code> will only be called once per element.</p>
</blockquote>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="design"><span class="header-section-number">3</span> Design<a href="#design" class="self-link"></a></h1>
<h2 data-number="3.1" id="caching-in-operator"><span class="header-section-number">3.1</span> Caching in <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">*</span></code><a href="#caching-in-operator" class="self-link"></a></h2>
<p>This is the only reasonable place for this caching to happen. Caching in <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">++</span></code> would require unnecessary overhead on every traversal if the user does not need to dereference every iterator (e.g., a striding view).</p>
<h2 data-number="3.2" id="relaxing-res.on.data.races"><span class="header-section-number">3.2</span> Relaxing [res.on.data.races]<a href="#relaxing-res.on.data.races" class="self-link"></a></h2>
<p>Because <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">*</span></code> is required to be a <code class="sourceCode cpp"><span class="kw">const</span></code> member function (the cost of relaxing that requirement would be prohibitive as it affects every iterator and range adaptor), yet we want to have it perform a potentially-modifying operation, our options are basically:</p>
<ul>
<li>Relax <span>16.4.6.10 <a href="https://wg21.link/res.on.data.races">[res.on.data.races]</a></span> p3</li>
<li>Require synchronization.</li>
</ul>
<p>This paper proposes the former. Input-only iterators, in general, are poor candidates for sharing across threads because, even when copyable (and they are not required to be in C++20), they are subject to “spooky action at a distance” where incrementing one iterator invalidates all copies thereof. This is why parallel algorithms require at least forward iterators. Sharing references to the same input iterator across threads seems like a fairly contrived scenario.</p>
<p>Moreover, <code class="sourceCode cpp">std<span class="op">::</span>istreambuf_iterator</code> already doesn’t meet this requirement in <a href="https://lists.isocpp.org/lib/2023/03/25724.php">every major standard library implementation</a> yet it does not seem to have appeared on any standard library maintainer’s radar in the decade since the publication of C++11. This further suggests that this guarantee is not relied upon in practice for input-only iterators.</p>
<p>It is true that we could require synchronization on <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">*()</span> <span class="kw">const</span></code>. This probably isn’t terrible expensive in this context (we only need to protect against <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">*</span></code> calls racing with each other, not them racing with <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">++</span></code>), but adding synchronization to an adaptor whose primary purpose is to improve performance seems particularly dissatisfying when that synchronization will almost never be actually necessary.</p>
<h2 data-number="3.3" id="whats-the-reference-type"><span class="header-section-number">3.3</span> What’s the reference type?<a href="#whats-the-reference-type" class="self-link"></a></h2>
<p>range-v3 uses <code class="sourceCode cpp">range_value_t<span class="op">&lt;</span>V<span class="op">&gt;&amp;&amp;</span></code>, but this somewhat defeats the purpose of caching if you can so easily invalidate it. Moreover, there doesn’t seem to be a reason to force an independent copy of the <code class="sourceCode cpp">value_type</code>. So long as the underlying iterator is not modified, the reference obtained from <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">*</span></code> should remain valid.</p>
<p>This paper therefore proposes <code class="sourceCode cpp">range_reference_t<span class="op">&lt;</span>V<span class="op">&gt;&amp;</span></code>. Note that even if the reference type is a real reference, it can be an <em>expensive-to-compute</em> reference, so caching could still make sense.</p>
<h2 data-number="3.4" id="properties"><span class="header-section-number">3.4</span> Properties<a href="#properties" class="self-link"></a></h2>
<p><code class="sourceCode cpp">cache_last</code> is never borrowed, input-only, never common, and not const-iterable.</p>
<h2 data-number="3.5" id="iter_move-and-iter_swap"><span class="header-section-number">3.5</span> <code class="sourceCode cpp">iter_move</code> and <code class="sourceCode cpp">iter_swap</code><a href="#iter_move-and-iter_swap" class="self-link"></a></h2>
<p><code class="sourceCode cpp">indirectly_readable</code> and <code class="sourceCode cpp">indirectly_swappable</code> requires <code class="sourceCode cpp">iter_move</code> and <code class="sourceCode cpp">iter_swap</code> to respectively not modify <code class="sourceCode cpp">i</code> (in the <span>18.2 <a href="https://wg21.link/concepts.equality">[concepts.equality]</a></span> sense). Moreover, <code class="sourceCode cpp">indirectly_readable</code> requires <code class="sourceCode cpp"><span class="op">*</span>i</code> to be equality-preserving. So the cache should not be invalidated by either operation. (The underlying element might be modified, but the reference itself, obtained from dereferencing the iterator, cannot.)</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="wording"><span class="header-section-number">4</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>This wording is relative to <span class="citation" data-cites="N4971">[<a href="#ref-N4971" role="doc-biblioref">N4971</a>]</span>.</p>
<h2 data-number="4.1" id="carve-out-for-input-iterators-from-res.on.data.races"><span class="header-section-number">4.1</span> Carve-out for input iterators from [res.on.data.races]<a href="#carve-out-for-input-iterators-from-res.on.data.races" class="self-link"></a></h2>
<p>Edit <span>16.4.6.10 <a href="https://wg21.link/res.on.data.races">[res.on.data.races]</a></span> p3 as indicated:</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">3</a></span> A C++ standard library function <span class="diffins">other than a member or friend function of a non-forward input iterator (<span>25.3.4 <a href="https://wg21.link/iterator.concepts">[iterator.concepts]</a></span>, <span>25.3.5 <a href="https://wg21.link/iterator.cpp17">[iterator.cpp17]</a></span>)</span> shall not directly or indirectly modify objects (<span>6.9.2 <a href="https://wg21.link/intro.multithread">[intro.multithread]</a></span>) accessible by threads other than the current thread unless the objects are accessed directly or indirectly via the function’s non-const arguments, including <code class="sourceCode cpp"><span class="kw">this</span></code>.</p>
<h2 data-number="4.2" id="addition-to-ranges"><span class="header-section-number">4.2</span> Addition to <code class="sourceCode cpp"><span class="op">&lt;</span>ranges<span class="op">&gt;</span></code><a href="#addition-to-ranges" class="self-link"></a></h2>
<p>Add the following to <span>26.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a></span>, header <code class="sourceCode cpp"><span class="op">&lt;</span>ranges<span class="op">&gt;</span></code> synopsis:</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a><span class="co">// [...]</span></span>
<span id="cb4-2"><a href="#cb4-2"></a><span class="kw">namespace</span> std<span class="op">::</span>ranges <span class="op">{</span></span>
<span id="cb4-3"><a href="#cb4-3"></a>  <span class="co">// [...]</span></span>
<span id="cb4-4"><a href="#cb4-4"></a></span>
<span id="cb4-5"><a href="#cb4-5"></a>  <span class="co">// [range.cache.last], to input view</span></span>
<span id="cb4-6"><a href="#cb4-6"></a>  <span class="kw">template</span><span class="op">&lt;</span>input_range V<span class="op">&gt;</span></span>
<span id="cb4-7"><a href="#cb4-7"></a>    <span class="kw">requires</span> view<span class="op">&lt;</span>V<span class="op">&gt;</span></span>
<span id="cb4-8"><a href="#cb4-8"></a>  <span class="kw">class</span> cache_last_view;</span>
<span id="cb4-9"><a href="#cb4-9"></a></span>
<span id="cb4-10"><a href="#cb4-10"></a>  <span class="kw">namespace</span> views <span class="op">{</span></span>
<span id="cb4-11"><a href="#cb4-11"></a>    <span class="kw">inline</span> <span class="kw">constexpr</span> <em>unspecified</em> cache_last <span class="op">=</span> <em>unspecified</em>;</span>
<span id="cb4-12"><a href="#cb4-12"></a>  <span class="op">}</span></span>
<span id="cb4-13"><a href="#cb4-13"></a></span>
<span id="cb4-14"><a href="#cb4-14"></a><span class="op">}</span></span></code></pre></div>
<h2 data-number="4.3" id="cache_last"><span class="header-section-number">4.3</span> <code class="sourceCode cpp">cache_last</code><a href="#cache_last" class="self-link"></a></h2>
<p>Add the following subclause to <span>26.7 <a href="https://wg21.link/range.adaptors">[range.adaptors]</a></span>:</p>
<h3 class="unnumbered" data-number id="cache-last-view-range.cache.last">26.7.? Cache last view [range.cache.last]<a href="#cache-last-view-range.cache.last" class="self-link"></a></h3>
<h4 class="unnumbered" data-number id="overview-range.cache.last.overview">26.7.?.1 Overview [range.cache.last.overview]<a href="#overview-range.cache.last.overview" class="self-link"></a></h4>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">1</a></span> <code class="sourceCode cpp">cache_last_view</code> caches the last element of its underlying sequence so that the element does not have to be recomputed on repeated access. <span class="note"><span>[ <em>Note 1:</em> </span>This is useful if computation of the element to produce is expensive.<span> — <em>end note</em> ]</span></span></p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">2</a></span> The name <code class="sourceCode cpp">views<span class="op">::</span>cache_last</code> denotes a range adaptor object (<span>26.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a></span>). Let <code class="sourceCode cpp">E</code> be an expression. The expression <code class="sourceCode cpp">views​<span class="op">::</span>cache_last<span class="op">(</span>E<span class="op">)</span></code> is expression-equivalent to <code class="sourceCode cpp">cache_last_view<span class="op">(</span>E<span class="op">)</span></code>. <span class="draftnote" style="color: #01796F">[ Drafting note: Intentional CTAD to avoid double wrapping if <code class="sourceCode cpp">E</code> is already a <code class="sourceCode cpp">cache_last_view</code>. ]</span></p>
<h4 class="unnumbered" data-number id="class-template-cache_last_view-range.cache.last.view">26.7.?.2 Class template <code class="sourceCode cpp">cache_last_view</code> [range.cache.last.view]<a href="#class-template-cache_last_view-range.cache.last.view" class="self-link"></a></h4>
<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">template</span><span class="op">&lt;</span>input_range V<span class="op">&gt;</span></span>
<span id="cb5-2"><a href="#cb5-2"></a>  <span class="kw">requires</span> view<span class="op">&lt;</span>V<span class="op">&gt;</span></span>
<span id="cb5-3"><a href="#cb5-3"></a><span class="kw">class</span> cache_last_view <span class="op">:</span> <span class="kw">public</span> view_interface<span class="op">&lt;</span>cache_last_view<span class="op">&lt;</span>V<span class="op">&gt;&gt;{</span></span>
<span id="cb5-4"><a href="#cb5-4"></a>  V <em>base_</em> <span class="op">=</span> V<span class="op">()</span>;                                                       <span class="co">// exposition only</span></span>
<span id="cb5-5"><a href="#cb5-5"></a>  <span class="kw">using</span> <em>cache_t</em> <span class="op">=</span> conditional_t<span class="op">&lt;</span>is_reference_v<span class="op">&lt;</span>range_reference_t<span class="op">&lt;</span>V<span class="op">&gt;&gt;</span>,  <span class="co">// exposition only</span></span>
<span id="cb5-6"><a href="#cb5-6"></a>                                add_pointer_t<span class="op">&lt;</span>range_reference_t<span class="op">&lt;</span>V<span class="op">&gt;&gt;</span>,</span>
<span id="cb5-7"><a href="#cb5-7"></a>                                range_reference_t<span class="op">&lt;</span>V<span class="op">&gt;&gt;</span>;</span>
<span id="cb5-8"><a href="#cb5-8"></a></span>
<span id="cb5-9"><a href="#cb5-9"></a>  <em>non-propagating-cache</em><span class="op">&lt;</span><em>cache_t</em><span class="op">&gt;</span> <em>cache_</em>;                               <span class="co">// exposition only</span></span>
<span id="cb5-10"><a href="#cb5-10"></a></span>
<span id="cb5-11"><a href="#cb5-11"></a>  <span class="kw">class</span> <em>iterator</em>;                                                      <span class="co">// exposition only</span></span>
<span id="cb5-12"><a href="#cb5-12"></a>  <span class="kw">class</span> <em>sentinel</em>;                                                      <span class="co">// exposition only</span></span>
<span id="cb5-13"><a href="#cb5-13"></a></span>
<span id="cb5-14"><a href="#cb5-14"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb5-15"><a href="#cb5-15"></a>  cache_last_view<span class="op">()</span> <span class="kw">requires</span> default_initializable<span class="op">&lt;</span>V<span class="op">&gt;</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb5-16"><a href="#cb5-16"></a>  <span class="kw">constexpr</span> <span class="kw">explicit</span> cache_last_view<span class="op">(</span>V base<span class="op">)</span>;</span>
<span id="cb5-17"><a href="#cb5-17"></a></span>
<span id="cb5-18"><a href="#cb5-18"></a>  <span class="kw">constexpr</span> V base<span class="op">()</span> <span class="kw">const</span> <span class="op">&amp;</span> <span class="kw">requires</span> copy_constructible<span class="op">&lt;</span>V<span class="op">&gt;</span> <span class="op">{</span> <span class="cf">return</span> <em>base_</em>; <span class="op">}</span></span>
<span id="cb5-19"><a href="#cb5-19"></a>  <span class="kw">constexpr</span> V base<span class="op">()</span> <span class="op">&amp;&amp;</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="cb5-20"><a href="#cb5-20"></a></span>
<span id="cb5-21"><a href="#cb5-21"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> begin<span class="op">()</span>;</span>
<span id="cb5-22"><a href="#cb5-22"></a>  <span class="kw">constexpr</span> <span class="kw">auto</span> end<span class="op">()</span>;</span>
<span id="cb5-23"><a href="#cb5-23"></a></span>
<span id="cb5-24"><a href="#cb5-24"></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">&lt;</span>V<span class="op">&gt;</span>;</span>
<span id="cb5-25"><a href="#cb5-25"></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">&lt;</span><span class="kw">const</span> V<span class="op">&gt;</span>;</span>
<span id="cb5-26"><a href="#cb5-26"></a><span class="op">}</span>;</span>
<span id="cb5-27"><a href="#cb5-27"></a></span>
<span id="cb5-28"><a href="#cb5-28"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> R<span class="op">&gt;</span></span>
<span id="cb5-29"><a href="#cb5-29"></a>  cache_last_view<span class="op">(</span>R<span class="op">&amp;&amp;)</span> <span class="op">-&gt;</span> cache_last_view<span class="op">&lt;</span>views<span class="op">::</span>all_t<span class="op">&lt;</span>R<span class="op">&gt;&gt;</span>;</span></code></pre></div>
<div>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">constexpr</span> <span class="kw">explicit</span> cache_last_view<span class="op">(</span>V base<span class="op">)</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">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>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1"></a><span class="kw">constexpr</span> <span class="kw">auto</span> begin<span class="op">()</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">2</a></span> <em>Effects</em>: Equivalent to:</p>
<blockquote>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1"></a><span class="cf">return</span> <em>iterator</em><span class="op">(*</span><span class="kw">this</span><span class="op">)</span>;</span></code></pre></div>
</blockquote>
</blockquote>
<div class="sourceCode" id="cb9"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb9-1"><a href="#cb9-1"></a><span class="kw">constexpr</span> <span class="kw">auto</span> end<span class="op">()</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_6" id="pnum_6">3</a></span> <em>Effects</em>: Equivalent to:</p>
<blockquote>
<div class="sourceCode" id="cb10"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb10-1"><a href="#cb10-1"></a><span class="cf">return</span> <em>sentinel</em><span class="op">(*</span><span class="kw">this</span><span class="op">)</span>;</span></code></pre></div>
</blockquote>
</blockquote>
<div class="sourceCode" id="cb11"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb11-1"><a href="#cb11-1"></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">&lt;</span>V<span class="op">&gt;</span>;</span>
<span id="cb11-2"><a href="#cb11-2"></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">&lt;</span><span class="kw">const</span> V<span class="op">&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_7" id="pnum_7">4</a></span> <em>Effects</em>: Equivalent to:</p>
<blockquote>
<div class="sourceCode" id="cb12"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb12-1"><a href="#cb12-1"></a><span class="cf">return</span> ranges<span class="op">::</span>size<span class="op">(</span><em>base_</em><span class="op">)</span>;</span></code></pre></div>
</blockquote>
</blockquote>
</div>
<h4 class="unnumbered" data-number id="class-cache_last_viewiterator-range.cache.last.iterator">26.7.?.3 Class <code class="sourceCode cpp">cache_last_view<span class="op">::</span><em>iterator</em></code> [range.cache.last.iterator]<a href="#class-cache_last_viewiterator-range.cache.last.iterator" class="self-link"></a></h4>
<div class="sourceCode" id="cb13"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb13-1"><a href="#cb13-1"></a><span class="kw">namespace</span> std<span class="op">::</span>ranges <span class="op">{</span></span>
<span id="cb13-2"><a href="#cb13-2"></a>  <span class="kw">template</span><span class="op">&lt;</span>input_range V<span class="op">&gt;</span></span>
<span id="cb13-3"><a href="#cb13-3"></a>    <span class="kw">requires</span> view<span class="op">&lt;</span>V<span class="op">&gt;</span></span>
<span id="cb13-4"><a href="#cb13-4"></a>  <span class="kw">class</span> cache_last_view<span class="op">&lt;</span>V<span class="op">&gt;::</span><em>iterator</em> <span class="op">{</span></span>
<span id="cb13-5"><a href="#cb13-5"></a>    cache_last_view<span class="op">*</span> <em>parent_</em>;                                 <span class="co">// exposition only</span></span>
<span id="cb13-6"><a href="#cb13-6"></a>    iterator_t<span class="op">&lt;</span>V<span class="op">&gt;</span> <em>current_</em>;                                   <span class="co">// exposition only</span></span>
<span id="cb13-7"><a href="#cb13-7"></a></span>
<span id="cb13-8"><a href="#cb13-8"></a>    <span class="kw">constexpr</span> <span class="kw">explicit</span> <em>iterator</em><span class="op">(</span>cache_last_view<span class="op">&amp;</span> parent<span class="op">)</span>;     <span class="co">// exposition only</span></span>
<span id="cb13-9"><a href="#cb13-9"></a></span>
<span id="cb13-10"><a href="#cb13-10"></a>  <span class="kw">public</span><span class="op">:</span></span>
<span id="cb13-11"><a href="#cb13-11"></a>    <span class="kw">using</span> difference_type <span class="op">=</span> range_difference_t<span class="op">&lt;</span>V<span class="op">&gt;</span>;</span>
<span id="cb13-12"><a href="#cb13-12"></a>    <span class="kw">using</span> value_type <span class="op">=</span> range_value_t<span class="op">&lt;</span>V<span class="op">&gt;</span>;</span>
<span id="cb13-13"><a href="#cb13-13"></a>    <span class="kw">using</span> iterator_concept <span class="op">=</span> input_iterator_tag;</span>
<span id="cb13-14"><a href="#cb13-14"></a></span>
<span id="cb13-15"><a href="#cb13-15"></a>    <em>iterator</em><span class="op">(</span><em>iterator</em><span class="op">&amp;&amp;)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb13-16"><a href="#cb13-16"></a>    <em>iterator</em><span class="op">&amp;</span> <span class="kw">operator</span><span class="op">=(</span><em>iterator</em><span class="op">&amp;&amp;)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb13-17"><a href="#cb13-17"></a></span>
<span id="cb13-18"><a href="#cb13-18"></a>    <span class="kw">constexpr</span> iterator_t<span class="op">&lt;</span>V<span class="op">&gt;</span> base<span class="op">()</span> <span class="op">&amp;&amp;</span>;</span>
<span id="cb13-19"><a href="#cb13-19"></a>    <span class="kw">constexpr</span> <span class="kw">const</span> iterator_t<span class="op">&lt;</span>V<span class="op">&gt;&amp;</span> base<span class="op">()</span> <span class="kw">const</span> <span class="op">&amp;</span> <span class="kw">noexcept</span>;</span>
<span id="cb13-20"><a href="#cb13-20"></a></span>
<span id="cb13-21"><a href="#cb13-21"></a>    <span class="kw">constexpr</span> range_reference_t<span class="op">&lt;</span>V<span class="op">&gt;&amp;</span> <span class="kw">operator</span><span class="op">*()</span> <span class="kw">const</span>;</span>
<span id="cb13-22"><a href="#cb13-22"></a></span>
<span id="cb13-23"><a href="#cb13-23"></a>    <span class="kw">constexpr</span> <em>iterator</em><span class="op">&amp;</span> <span class="kw">operator</span><span class="op">++()</span>;</span>
<span id="cb13-24"><a href="#cb13-24"></a>    <span class="kw">constexpr</span> <span class="dt">void</span> <span class="kw">operator</span><span class="op">++(</span><span class="dt">int</span><span class="op">)</span>;</span>
<span id="cb13-25"><a href="#cb13-25"></a></span>
<span id="cb13-26"><a href="#cb13-26"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> range_rvalue_reference_t<span class="op">&lt;</span>V<span class="op">&gt;</span> iter_move<span class="op">(</span><span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> i<span class="op">)</span></span>
<span id="cb13-27"><a href="#cb13-27"></a>      <span class="kw">noexcept</span><span class="op">(</span><span class="kw">noexcept</span><span class="op">(</span>ranges<span class="op">::</span>iter_move<span class="op">(</span>i<span class="op">.</span><em>current_</em><span class="op">)))</span>;</span>
<span id="cb13-28"><a href="#cb13-28"></a></span>
<span id="cb13-29"><a href="#cb13-29"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="dt">void</span> iter_swap<span class="op">(</span><span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> x, <span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> y<span class="op">)</span></span>
<span id="cb13-30"><a href="#cb13-30"></a>      <span class="kw">noexcept</span><span class="op">(</span><span class="kw">noexcept</span><span class="op">(</span>ranges<span class="op">::</span>iter_swap<span class="op">(</span>x<span class="op">.</span><em>current_</em>, y<span class="op">.</span><em>current_</em><span class="op">)))</span></span>
<span id="cb13-31"><a href="#cb13-31"></a>      <span class="kw">requires</span> indirectly_swappable<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span>V<span class="op">&gt;&gt;</span>;</span>
<span id="cb13-32"><a href="#cb13-32"></a>  <span class="op">}</span>;</span>
<span id="cb13-33"><a href="#cb13-33"></a><span class="op">}</span></span></code></pre></div>
<div>
<div class="sourceCode" id="cb14"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb14-1"><a href="#cb14-1"></a><span class="kw">constexpr</span> <span class="kw">explicit</span> <em>iterator</em><span class="op">(</span>cache_last_view<span class="op">&amp;</span> parent<span class="op">)</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_8" id="pnum_8">1</a></span> <em>Effects</em>: Initializes <code class="sourceCode cpp"><em>current_</em></code> with <code class="sourceCode cpp">ranges<span class="op">::</span>begin<span class="op">(</span>parent<span class="op">.</span><em>base_</em><span class="op">)</span></code> and <code class="sourceCode cpp"><em>parent_</em></code> with <code class="sourceCode cpp">addressof<span class="op">(</span>parent<span class="op">)</span></code>.</p>
</blockquote>
<div class="sourceCode" id="cb15"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb15-1"><a href="#cb15-1"></a><span class="kw">constexpr</span> iterator_t<span class="op">&lt;</span>V<span class="op">&gt;</span> base<span class="op">()</span> <span class="op">&amp;&amp;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_9" id="pnum_9">2</a></span> <em>Returns</em>: <code class="sourceCode cpp">std<span class="op">::</span>move<span class="op">(</span><em>current_</em><span class="op">)</span></code>.</p>
</blockquote>
<div class="sourceCode" id="cb16"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb16-1"><a href="#cb16-1"></a><span class="kw">constexpr</span> <span class="kw">const</span> iterator_t<span class="op">&lt;</span>V<span class="op">&gt;&amp;</span> base<span class="op">()</span> <span class="kw">const</span> <span class="op">&amp;</span> <span class="kw">noexcept</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_10" id="pnum_10">3</a></span> <em>Returns</em>: <code class="sourceCode cpp"><em>current_</em></code>.</p>
</blockquote>
<div class="sourceCode" id="cb17"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb17-1"><a href="#cb17-1"></a><span class="kw">constexpr</span> <em>iterator</em><span class="op">&amp;</span> <span class="kw">operator</span><span class="op">++()</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_11" id="pnum_11">4</a></span> <em>Effects:</em> Equivalent to:</p>
<blockquote>
<div class="sourceCode" id="cb18"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb18-1"><a href="#cb18-1"></a>   <span class="op">++</span><em>current_</em>;</span>
<span id="cb18-2"><a href="#cb18-2"></a>   <em>parent_</em><span class="op">-&gt;</span><em>cache_</em><span class="op">.</span>reset<span class="op">()</span>;</span>
<span id="cb18-3"><a href="#cb18-3"></a>   <span class="cf">return</span> <span class="op">*</span><span class="kw">this</span>;</span></code></pre></div>
</blockquote>
</blockquote>
<div class="sourceCode" id="cb19"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb19-1"><a href="#cb19-1"></a><span class="kw">constexpr</span> <span class="dt">void</span> <span class="kw">operator</span><span class="op">++(</span><span class="dt">int</span><span class="op">)</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_12" id="pnum_12">5</a></span> <em>Effects</em>: Equivalent to: <code class="sourceCode cpp"><span class="op">++*</span><span class="kw">this</span>;</code></p>
</blockquote>
<div class="sourceCode" id="cb20"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb20-1"><a href="#cb20-1"></a><span class="kw">constexpr</span> range_reference_t<span class="op">&lt;</span>V<span class="op">&gt;&amp;</span> <span class="kw">operator</span><span class="op">*()</span> <span class="kw">const</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_13" id="pnum_13">6</a></span> <em>Effects</em>: Equivalent to:</p>
<blockquote>
<div class="sourceCode" id="cb21"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb21-1"><a href="#cb21-1"></a>  <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>is_reference_v<span class="op">&lt;</span>range_reference_t<span class="op">&lt;</span>V<span class="op">&gt;&gt;)</span> <span class="op">{</span></span>
<span id="cb21-2"><a href="#cb21-2"></a>    <span class="cf">if</span> <span class="op">(!</span><em>parent_</em><span class="op">-&gt;</span><em>cache_</em><span class="op">)</span> <span class="op">{</span></span>
<span id="cb21-3"><a href="#cb21-3"></a>      <em>parent_</em><span class="op">-&gt;</span><em>cache_</em> <span class="op">=</span> addressof<span class="op">(</span><em>as-lvalue</em><span class="op">(*</span><em>current_</em><span class="op">))</span>;</span>
<span id="cb21-4"><a href="#cb21-4"></a>    <span class="op">}</span></span>
<span id="cb21-5"><a href="#cb21-5"></a>    <span class="cf">return</span> <span class="op">**</span><em>parent_</em><span class="op">-&gt;</span><em>cache_</em>;</span>
<span id="cb21-6"><a href="#cb21-6"></a>  <span class="op">}</span></span>
<span id="cb21-7"><a href="#cb21-7"></a>  <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb21-8"><a href="#cb21-8"></a>    <span class="cf">if</span> <span class="op">(!</span><em>parent_</em><span class="op">-&gt;</span><em>cache_</em><span class="op">)</span> <span class="op">{</span></span>
<span id="cb21-9"><a href="#cb21-9"></a>      <em>parent_</em><span class="op">-&gt;</span><em>cache_</em><span class="op">.</span><em>emplace-deref</em><span class="op">(</span><em>current_</em><span class="op">)</span>;</span>
<span id="cb21-10"><a href="#cb21-10"></a>    <span class="op">}</span></span>
<span id="cb21-11"><a href="#cb21-11"></a>    <span class="cf">return</span> <span class="op">*</span><em>parent_</em><span class="op">-&gt;</span><em>cache_</em>;</span>
<span id="cb21-12"><a href="#cb21-12"></a>  <span class="op">}</span></span></code></pre></div>
</blockquote>
</blockquote>
<div class="sourceCode" id="cb22"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb22-1"><a href="#cb22-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> range_rvalue_reference_t<span class="op">&lt;</span>V<span class="op">&gt;</span> iter_move<span class="op">(</span><span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> i<span class="op">)</span></span>
<span id="cb22-2"><a href="#cb22-2"></a>  <span class="kw">noexcept</span><span class="op">(</span><span class="kw">noexcept</span><span class="op">(</span>ranges<span class="op">::</span>iter_move<span class="op">(</span>i<span class="op">.</span><em>current_</em><span class="op">)))</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_14" id="pnum_14">7</a></span> <em>Effects</em>: Equivalent to: <code class="sourceCode cpp"><span class="cf">return</span> ranges<span class="op">::</span>iter_move<span class="op">(</span>i<span class="op">.</span><em>current_</em><span class="op">)</span>;</code></p>
</blockquote>
<div class="sourceCode" id="cb23"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb23-1"><a href="#cb23-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> <span class="dt">void</span> iter_swap<span class="op">(</span><span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> x, <span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> y<span class="op">)</span></span>
<span id="cb23-2"><a href="#cb23-2"></a>  <span class="kw">noexcept</span><span class="op">(</span><span class="kw">noexcept</span><span class="op">(</span>ranges<span class="op">::</span>iter_swap<span class="op">(</span>x<span class="op">.</span><em>current_</em>, y<span class="op">.</span><em>current_</em><span class="op">)))</span></span>
<span id="cb23-3"><a href="#cb23-3"></a>  <span class="kw">requires</span> indirectly_swappable<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span>V<span class="op">&gt;&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_15" id="pnum_15">8</a></span> <em>Effects</em>: Equivalent to: <code class="sourceCode cpp">ranges<span class="op">::</span>iter_swap<span class="op">(</span>x<span class="op">.</span><em>current_</em>, y<span class="op">.</span><em>current_</em><span class="op">)</span>;</code></p>
</blockquote>
</div>
<h4 class="unnumbered" data-number id="class-cache_last_viewsentinel-range.cache.last.sentinel">26.7.?.3 Class <code class="sourceCode cpp">cache_last_view<span class="op">::</span><em>sentinel</em></code> [range.cache.last.sentinel]<a href="#class-cache_last_viewsentinel-range.cache.last.sentinel" class="self-link"></a></h4>
<div class="sourceCode" id="cb24"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb24-1"><a href="#cb24-1"></a><span class="kw">namespace</span> std<span class="op">::</span>ranges <span class="op">{</span></span>
<span id="cb24-2"><a href="#cb24-2"></a>  <span class="kw">template</span><span class="op">&lt;</span>input_range V<span class="op">&gt;</span></span>
<span id="cb24-3"><a href="#cb24-3"></a>    <span class="kw">requires</span> view<span class="op">&lt;</span>V<span class="op">&gt;</span></span>
<span id="cb24-4"><a href="#cb24-4"></a>  <span class="kw">class</span> cache_last_view<span class="op">&lt;</span>V<span class="op">&gt;::</span><em>sentinel</em> <span class="op">{</span></span>
<span id="cb24-5"><a href="#cb24-5"></a>    sentinel_t<span class="op">&lt;</span>V<span class="op">&gt;</span> <em>end_</em> <span class="op">=</span> sentinel_t<span class="op">&lt;</span>V<span class="op">&gt;()</span>;               <span class="co">// exposition only</span></span>
<span id="cb24-6"><a href="#cb24-6"></a></span>
<span id="cb24-7"><a href="#cb24-7"></a>    <span class="kw">constexpr</span> <span class="kw">explicit</span> <em>sentinel</em><span class="op">(</span>cache_last_view<span class="op">&amp;</span> parent<span class="op">)</span>;   <span class="co">// exposition only</span></span>
<span id="cb24-8"><a href="#cb24-8"></a></span>
<span id="cb24-9"><a href="#cb24-9"></a>  <span class="kw">public</span><span class="op">:</span></span>
<span id="cb24-10"><a href="#cb24-10"></a>    <em>sentinel</em><span class="op">()</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb24-11"><a href="#cb24-11"></a></span>
<span id="cb24-12"><a href="#cb24-12"></a>    <span class="kw">constexpr</span> sentinel_t<span class="op">&lt;</span>V<span class="op">&gt;</span> base<span class="op">()</span> <span class="kw">const</span>;</span>
<span id="cb24-13"><a href="#cb24-13"></a></span>
<span id="cb24-14"><a href="#cb24-14"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span><span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> x, <span class="kw">const</span> <em>sentinel</em><span class="op">&amp;</span> y<span class="op">)</span>;</span>
<span id="cb24-15"><a href="#cb24-15"></a>  <span class="op">}</span>;</span>
<span id="cb24-16"><a href="#cb24-16"></a><span class="op">}</span></span></code></pre></div>
<div>
<div class="sourceCode" id="cb25"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb25-1"><a href="#cb25-1"></a><span class="kw">constexpr</span> <span class="kw">explicit</span> <em>sentinel</em><span class="op">(</span>cache_last_view<span class="op">&amp;</span> parent<span class="op">)</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_16" id="pnum_16">1</a></span> <em>Effects</em>: Initializes <code class="sourceCode cpp"><em>end_</em></code> with <code class="sourceCode cpp">ranges<span class="op">::</span>end<span class="op">(</span>parent<span class="op">.</span><em>base_</em><span class="op">)</span></code>.</p>
</blockquote>
<div class="sourceCode" id="cb26"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb26-1"><a href="#cb26-1"></a><span class="kw">constexpr</span> sentinel_t<span class="op">&lt;</span>V<span class="op">&gt;</span> base<span class="op">()</span> <span class="kw">const</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_17" id="pnum_17">2</a></span> <em>Returns:</em> <code class="sourceCode cpp"><em>end_</em></code>.</p>
</blockquote>
<div class="sourceCode" id="cb27"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb27-1"><a href="#cb27-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span><span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> x, <span class="kw">const</span> <em>sentinel</em><span class="op">&amp;</span> y<span class="op">)</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_18" id="pnum_18">3</a></span> <em>Returns:</em> <code class="sourceCode cpp">x<span class="op">.</span><em>current_</em> <span class="op">==</span> y<span class="op">.</span><em>end_</em>;</code></p>
</blockquote>
</div>
<h1 data-number="5" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">5</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-N4971">
<p>[N4971] Thomas Köppe. 2023-12-18. Working Draft, Programming Languages — C++. <br />
<a href="https://wg21.link/n4971">https://wg21.link/n4971</a></p>
</div>
<div id="ref-P2760R1">
<p>[P2760R1] Barry Revzin. 2023-12-15. A Plan for C++26 Ranges. <br />
<a href="https://wg21.link/p2760r1">https://wg21.link/p2760r1</a></p>
</div>
</div>
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