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  <meta name="dcterms.date" content="2020-02-12" />
  <title>elements_view needs its own sentinel</title>
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code span.at { } /* Attribute */
code span.bn { color: #9f6807; } /* BaseN */
code span.bu { color: #9f6807; } /* BuiltIn */
code span.cf { color: #00607c; } /* ControlFlow */
code span.ch { color: #9f6807; } /* Char */
code span.cn { } /* Constant */
code span.co { color: #008000; font-style: italic; } /* Comment */
code span.cv { color: #008000; font-style: italic; } /* CommentVar */
code span.do { color: #008000; } /* Documentation */
code span.dt { color: #00607c; } /* DataType */
code span.dv { color: #9f6807; } /* DecVal */
code span.er { color: #ff0000; font-weight: bold; } /* Error */
code span.ex { } /* Extension */
code span.fl { color: #9f6807; } /* Float */
code span.fu { } /* Function */
code span.im { } /* Import */
code span.in { color: #008000; } /* Information */
code span.kw { color: #00607c; } /* Keyword */
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span.tcode { font-family: monospace; font-style: normal; }
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margin-top: 0.8em;
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vertical-align: top;
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td.empty {
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td.left {
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td.right {
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<h1 class="title" style="text-align:center"><code class="sourceCode cpp">elements_view</code> needs its own <code class="sourceCode cpp">sentinel</code></h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #: </td>
    <td>P1994R1</td>
  </tr>
  <tr>
    <td>Date: </td>
    <td>2020-02-12</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project: </td>
    <td>Programming Language C++<br>
      LWG<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>
      Christopher Di Bella<br>&lt;<a href="mailto:cjdb.ns@gmail.com" class="email">cjdb.ns@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<h1 id="abstract" style="border-bottom:1px solid #cccccc"><span class="header-section-number">1</span> Abstract<a href="#abstract" class="self-link"></a></h1>
<p><code class="sourceCode cpp">elements_view</code> needs its own <code class="sourceCode cpp">sentinel</code> type. This paper resolves <span class="citation" data-cites="LWG3386">[<a href="#ref-LWG3386" role="doc-biblioref">LWG3386</a>]</span>.</p>
<h1 id="revision-history" style="border-bottom:1px solid #cccccc"><span class="header-section-number">2</span> Revision history<a href="#revision-history" class="self-link"></a></h1>
<ul>
<li>R1: Incorporate LWG review feedback in Prague. Rebased onto <span class="citation" data-cites="N4849">[<a href="#ref-N4849" role="doc-biblioref">N4849</a>]</span>.</li>
</ul>
<h1 id="discussion" style="border-bottom:1px solid #cccccc"><span class="header-section-number">3</span> Discussion<a href="#discussion" class="self-link"></a></h1>
<p><code class="sourceCode cpp">elements_view</code> is effectively a specialized version of <code class="sourceCode cpp">transform_view</code>; the latter has a custom <code class="sourceCode cpp">sentinel</code> type, and so should <code class="sourceCode cpp">elements_view</code>.</p>
<p>More generally, if a range adaptor has a custom iterator, it should probably also have a custom sentinel. The rationale is that the underlying range’s sentinel could encode a generic predicate that is equally meaningful for the adapted range, leading to an ambiguity.</p>
<p>For example, consider:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="kw">struct</span> S <span class="op">{</span> <span class="co">// sentinel that checks if the second element is zero</span></span>
<span id="cb1-3"><a href="#cb1-3"></a>  <span class="kw">friend</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span>input_iterator <span class="kw">auto</span> <span class="kw">const</span><span class="op">&amp;</span> i, S<span class="op">)</span> <span class="kw">requires</span> <span class="co">/* ... */</span></span>
<span id="cb1-4"><a href="#cb1-4"></a>  <span class="op">{</span> <span class="cf">return</span> get<span class="op">&lt;</span><span class="dv">1</span><span class="op">&gt;(*</span>i<span class="op">)</span> <span class="op">==</span> <span class="dv">0</span>; <span class="op">}</span></span>
<span id="cb1-5"><a href="#cb1-5"></a><span class="op">}</span>;</span>
<span id="cb1-6"><a href="#cb1-6"></a></span>
<span id="cb1-7"><a href="#cb1-7"></a><span class="dt">void</span> algo<span class="op">(</span>input_range <span class="kw">auto</span><span class="op">&amp;&amp;</span> r<span class="op">)</span> <span class="kw">requires</span> <span class="co">/* ... */</span> <span class="op">{</span></span>
<span id="cb1-8"><a href="#cb1-8"></a>  <span class="cf">for</span> <span class="op">(</span><span class="kw">auto</span><span class="op">&amp;&amp;</span> something <span class="op">:</span> subrange<span class="op">{</span>ranges<span class="op">::</span>begin<span class="op">(</span>r<span class="op">)</span>, S<span class="op">{}})</span></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="co">/* do something */</span></span>
<span id="cb1-11"><a href="#cb1-11"></a>  <span class="op">}</span></span>
<span id="cb1-12"><a href="#cb1-12"></a><span class="op">}</span></span>
<span id="cb1-13"><a href="#cb1-13"></a></span>
<span id="cb1-14"><a href="#cb1-14"></a><span class="kw">using</span> P <span class="op">=</span> pair<span class="op">&lt;</span>pair<span class="op">&lt;</span><span class="dt">char</span>, <span class="dt">int</span><span class="op">&gt;</span>, <span class="dt">long</span><span class="op">&gt;</span>;</span>
<span id="cb1-15"><a href="#cb1-15"></a>vector<span class="op">&lt;</span>P<span class="op">&gt;</span> something <span class="op">=</span> <span class="co">/* ... */</span>;</span>
<span id="cb1-16"><a href="#cb1-16"></a></span>
<span id="cb1-17"><a href="#cb1-17"></a>subrange r<span class="op">{</span>something<span class="op">.</span>begin<span class="op">()</span>, S<span class="op">{}}</span>;</span>
<span id="cb1-18"><a href="#cb1-18"></a></span>
<span id="cb1-19"><a href="#cb1-19"></a>algo<span class="op">(</span>r <span class="op">|</span> view<span class="op">::</span>keys<span class="op">)</span>;                 <span class="co">// checks the long, effectively iterating over r completely</span></span>
<span id="cb1-20"><a href="#cb1-20"></a>algo<span class="op">(</span>r <span class="op">|</span> view<span class="op">::</span>transform<span class="op">(&amp;</span>P<span class="op">::</span>first<span class="op">))</span>; <span class="co">// checks the int and stops at the first zero</span></span></code></pre></div>
<p><code class="sourceCode cpp">algo</code> is trying to use the sentinel <code class="sourceCode cpp">S</code> to stop at the first element <code class="sourceCode cpp">e</code> for which <code class="sourceCode cpp">get<span class="op">&lt;</span><span class="dv">1</span><span class="op">&gt;(</span>e<span class="op">)</span></code> is zero, and it works correctly for all ranges of <code class="sourceCode cpp">pair<span class="op">&lt;</span><span class="dt">char</span>, <span class="dt">int</span><span class="op">&gt;</span></code> … except things like <code class="sourceCode cpp">r <span class="op">|</span> view<span class="op">::</span>keys</code>. In the latter case, <code class="sourceCode cpp">ranges<span class="op">::</span>begin<span class="op">(</span>r <span class="op">|</span> view<span class="op">::</span>keys<span class="op">)</span> <span class="op">==</span> S<span class="op">{}</span></code> calls <code class="sourceCode cpp">elements_view<span class="op">::</span>iterator</code>’s <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">==</span></code> instead, which compares the <em>underlying range’s</em> iterator against the sentinel. In the above example, this means that we check the <code class="sourceCode cpp"><span class="dt">long</span></code> instead of the <code class="sourceCode cpp"><span class="dt">int</span></code>, and effectively iterate over the entire range.</p>
<p>A custom sentinel is needed to avoid this problem. <code class="sourceCode cpp">elements_view</code> is the only adaptor in the current WP that has a custom iterator but not a custom sentinel.</p>
<h1 id="wording" style="border-bottom:1px solid #cccccc"><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="N4849">[<a href="#ref-N4849" role="doc-biblioref">N4849</a>]</span>.</p>
<p><span class="draftnote" style="color: #01796F">[ Drafting note: These changes, including the overload set for <code class="sourceCode cpp">end</code>, match the specification of <code class="sourceCode cpp">transform_view</code>. This seems appropriate since <code class="sourceCode cpp">elements_view</code> is just a special kind of transform. ]</span></p>
<p>Edit <span>24.7.15.2 <a href="https://wg21.link/range.elements.view">[range.elements.view]</a></span>, class template <code class="sourceCode cpp">elements_view</code> synopsis, as indicated:</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> namespace std::ranges {</span>
<span id="cb2-2"><a href="#cb2-2"></a>    <em>[…]</em></span>
<span id="cb2-3"><a href="#cb2-3"></a></span>
<span id="cb2-4"><a href="#cb2-4"></a>   template&lt;input_range V, size_t N&gt;</span>
<span id="cb2-5"><a href="#cb2-5"></a>     requires view&lt;V&gt; &amp;&amp; <em>has-tuple-element</em>&lt;range_value_t&lt;V&gt;, N&gt; &amp;&amp;</span>
<span id="cb2-6"><a href="#cb2-6"></a>       <em>has-tuple-element</em>&lt;remove_reference_t&lt;range_reference_t&lt;V&gt;&gt;, N&gt;</span>
<span id="cb2-7"><a href="#cb2-7"></a>   class elements_view : public view_interface&lt;elements_view&lt;V, N&gt;&gt; {</span>
<span id="cb2-8"><a href="#cb2-8"></a>   public:</span>
<span id="cb2-9"><a href="#cb2-9"></a>     <em>[…]</em></span>
<span id="cb2-10"><a href="#cb2-10"></a></span>
<span id="cb2-11"><a href="#cb2-11"></a><span class="va">+    constexpr auto end()</span></span>
<span id="cb2-12"><a href="#cb2-12"></a><span class="va">+    { return <em>sentinel</em>&lt;false&gt;{ranges::end(<em>base_</em>)}; }</span></span>
<span id="cb2-13"><a href="#cb2-13"></a><span class="va">+</span></span>
<span id="cb2-14"><a href="#cb2-14"></a><span class="va">+    constexpr auto end() requires common_range&lt;V&gt;</span></span>
<span id="cb2-15"><a href="#cb2-15"></a><span class="va">+    { return <em>iterator</em>&lt;false&gt;{ranges::end(<em>base_</em>)}; }</span></span>
<span id="cb2-16"><a href="#cb2-16"></a><span class="va">+</span></span>
<span id="cb2-17"><a href="#cb2-17"></a><span class="va">+    constexpr auto end() const</span></span>
<span id="cb2-18"><a href="#cb2-18"></a><span class="va">+      requires range&lt;const V&gt;</span></span>
<span id="cb2-19"><a href="#cb2-19"></a><span class="va">+    { return <em>sentinel</em>&lt;true&gt;{ranges::end(<em>base_</em>)}; }</span></span>
<span id="cb2-20"><a href="#cb2-20"></a><span class="va">+</span></span>
<span id="cb2-21"><a href="#cb2-21"></a><span class="va">+    constexpr auto end() const</span></span>
<span id="cb2-22"><a href="#cb2-22"></a><span class="va">+      requires common_range&lt;const V&gt;</span></span>
<span id="cb2-23"><a href="#cb2-23"></a><span class="va">+    { return <em>iterator</em>&lt;true&gt;{ranges::end(<em>base_</em>)}; }</span></span>
<span id="cb2-24"><a href="#cb2-24"></a></span>
<span id="cb2-25"><a href="#cb2-25"></a><span class="st">-    constexpr auto end() requires (!<em>simple-view</em>&lt;V&gt;)</span></span>
<span id="cb2-26"><a href="#cb2-26"></a><span class="st">-    { return ranges::end(<em>base_</em>); }</span></span>
<span id="cb2-27"><a href="#cb2-27"></a></span>
<span id="cb2-28"><a href="#cb2-28"></a><span class="st">-    constexpr auto end() const requires <em>simple-view</em>&lt;V&gt;</span></span>
<span id="cb2-29"><a href="#cb2-29"></a><span class="st">-    { return ranges::end(<em>base_</em>); }</span></span>
<span id="cb2-30"><a href="#cb2-30"></a></span>
<span id="cb2-31"><a href="#cb2-31"></a>     <em>[…]</em></span>
<span id="cb2-32"><a href="#cb2-32"></a></span>
<span id="cb2-33"><a href="#cb2-33"></a>   private:</span>
<span id="cb2-34"><a href="#cb2-34"></a>     template&lt;bool&gt; struct <em>iterator</em>;                     // <em>exposition only</em></span>
<span id="cb2-35"><a href="#cb2-35"></a><span class="va">+    template&lt;bool&gt; struct <em>sentinel</em>;                     // <em>exposition only</em></span></span>
<span id="cb2-36"><a href="#cb2-36"></a>     V <em>base_</em> = V();                                      // <em>exposition only</em></span>
<span id="cb2-37"><a href="#cb2-37"></a>   };</span>
<span id="cb2-38"><a href="#cb2-38"></a> }</span></code></pre></div>
</div>
</blockquote>
<p>Edit <span>24.7.15.3 <a href="https://wg21.link/range.elements.iterator">[range.elements.iterator]</a></span>, class template <code>elements_view&lt;V, N&gt;::<i>iterator</i></code> synopsis, as indicated:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb3"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb3-1"><a href="#cb3-1"></a> namespace std::ranges {</span>
<span id="cb3-2"><a href="#cb3-2"></a>   template&lt;class V, size_t N&gt;</span>
<span id="cb3-3"><a href="#cb3-3"></a>   template&lt;bool Const&gt;</span>
<span id="cb3-4"><a href="#cb3-4"></a>   class elements_view&lt;V, N&gt;::<em>iterator</em> { // <em>exposition only</em></span>
<span id="cb3-5"><a href="#cb3-5"></a>   <em>[…]</em></span>
<span id="cb3-6"><a href="#cb3-6"></a></span>
<span id="cb3-7"><a href="#cb3-7"></a>   public:</span>
<span id="cb3-8"><a href="#cb3-8"></a>     <em>[…]</em></span>
<span id="cb3-9"><a href="#cb3-9"></a></span>
<span id="cb3-10"><a href="#cb3-10"></a><span class="st">-    friend constexpr bool operator==(const <em>iterator</em>&amp; x, const sentinel_t&lt;<em>base-t</em>&gt;&amp; y);</span></span>
<span id="cb3-11"><a href="#cb3-11"></a></span>
<span id="cb3-12"><a href="#cb3-12"></a>     <em>[…]</em></span>
<span id="cb3-13"><a href="#cb3-13"></a></span>
<span id="cb3-14"><a href="#cb3-14"></a><span class="st">-    friend constexpr difference_type</span></span>
<span id="cb3-15"><a href="#cb3-15"></a><span class="st">-      operator-(const <em>iterator</em>&lt;Const&gt;&amp; x, const sentinel_t&lt;<em>base-t</em>&gt;&amp; y)</span></span>
<span id="cb3-16"><a href="#cb3-16"></a><span class="st">-        requires sized_sentinel_for&lt;sentinel_t&lt;<em>base-t</em>&gt;, iterator_t&lt;<em>base-t</em>&gt;&gt;;</span></span>
<span id="cb3-17"><a href="#cb3-17"></a><span class="st">-    friend constexpr difference_type</span></span>
<span id="cb3-18"><a href="#cb3-18"></a><span class="st">-      operator-(const <em>iterator</em>&lt;<em>base-t</em>&gt;&amp; x, const iterator&lt;Const&gt;&amp; y)</span></span>
<span id="cb3-19"><a href="#cb3-19"></a><span class="st">-        requires sized_sentinel_for&lt;sentinel_t&lt;<em>base-t</em>&gt;, iterator_t&lt;<em>base-t</em>&gt;&gt;;</span></span>
<span id="cb3-20"><a href="#cb3-20"></a>   };</span>
<span id="cb3-21"><a href="#cb3-21"></a> }</span></code></pre></div>
</div>
</blockquote>
<p>Delete the specification of these operators in <span>24.7.15.3 <a href="https://wg21.link/range.elements.iterator">[range.elements.iterator]</a></span> (p13, 23 and 24):</p>
<div class="rm" style="color: #bf0303">

<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">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> sentinel_t<span class="op">&lt;</span><em>base-t</em><span class="op">&gt;&amp;</span> y<span class="op">)</span>;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized">13</a></span> <em>Effects:</em> Equivalent to: <code class="sourceCode cpp"><span class="cf">return</span> x<span class="op">.</span>current_ <span class="op">==</span> y;</code></p>
</blockquote>

</div>
<blockquote>
<p>…</p>
</blockquote>
<div class="rm" style="color: #bf0303">

<blockquote>
<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">friend</span> <span class="kw">constexpr</span> difference_type</span>
<span id="cb5-2"><a href="#cb5-2"></a>  <span class="kw">operator</span><span class="op">-(</span><span class="kw">const</span> <em>iterator</em><span class="op">&lt;</span>Const<span class="op">&gt;&amp;</span> x, <span class="kw">const</span> sentinel_t<span class="op">&lt;</span><em>base-t</em><span class="op">&gt;&amp;</span> y<span class="op">)</span></span>
<span id="cb5-3"><a href="#cb5-3"></a>    <span class="kw">requires</span> sized_sentinel_for<span class="op">&lt;</span>sentinel_t<span class="op">&lt;</span><em>base-t</em><span class="op">&gt;</span>, iterator_t<span class="op">&lt;</span><em>base-t</em><span class="op">&gt;&gt;</span>;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized">23</a></span> <em>Effects</em>: Equivalent to: <code class="sourceCode cpp"><span class="cf">return</span> x<span class="op">.</span>current_­ <span class="op">-</span> y;</code></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="kw">friend</span> <span class="kw">constexpr</span> difference_type</span>
<span id="cb6-2"><a href="#cb6-2"></a>  <span class="kw">operator</span><span class="op">-(</span><span class="kw">const</span> sentinel_t<span class="op">&lt;</span><em>base-t</em><span class="op">&gt;&amp;</span> x, <span class="kw">const</span> <em>iterator</em><span class="op">&lt;</span>Const<span class="op">&gt;&amp;</span> y<span class="op">)</span></span>
<span id="cb6-3"><a href="#cb6-3"></a>    <span class="kw">requires</span> sized_sentinel_for<span class="op">&lt;</span>sentinel_t<span class="op">&lt;</span><em>base-t</em><span class="op">&gt;</span>, iterator_t<span class="op">&lt;</span><em>base-t</em><span class="op">&gt;&gt;</span>;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized">24</a></span> <em>Effects</em>: Equivalent to: <code class="sourceCode cpp"><span class="cf">return</span> <span class="op">-(</span>y <span class="op">-</span> x<span class="op">)</span>;</code></p>
</blockquote>

</div>
<p>Add a new subclause after <span>24.7.15.3 <a href="https://wg21.link/range.elements.iterator">[range.elements.iterator]</a></span>:</p>
<div class="add" style="color: #006e28">

<h4 id="class-template-elements_viewsentinel-ranges.elements.sentinel" class="unnumbered">?.?.?.? Class template <code>elements_view::<i>sentinel</i></code> [ranges.elements.sentinel]<a href="#class-template-elements_viewsentinel-ranges.elements.sentinel" class="self-link"></a></h4>
<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">namespace</span> std<span class="op">::</span>ranges <span class="op">{</span></span>
<span id="cb7-2"><a href="#cb7-2"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> V, <span class="dt">size_t</span> N<span class="op">&gt;</span></span>
<span id="cb7-3"><a href="#cb7-3"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="dt">bool</span> Const<span class="op">&gt;</span></span>
<span id="cb7-4"><a href="#cb7-4"></a>  <span class="kw">class</span> elements_view<span class="op">&lt;</span>V, N<span class="op">&gt;::</span><em>sentinel</em> <span class="op">{</span>               <span class="co">// <em>exposition only</em></span></span>
<span id="cb7-5"><a href="#cb7-5"></a>  <span class="kw">private</span><span class="op">:</span></span>
<span id="cb7-6"><a href="#cb7-6"></a>    <span class="kw">using</span> <em>Base</em> <span class="op">=</span> conditional_t<span class="op">&lt;</span>Const, <span class="kw">const</span> V, V<span class="op">&gt;</span>;    <span class="co">// <em>exposition only</em></span></span>
<span id="cb7-7"><a href="#cb7-7"></a>    sentinel_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> <em>end_</em> <span class="op">=</span> sentinel_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;()</span>;       <span class="co">// <em>exposition only</em></span></span>
<span id="cb7-8"><a href="#cb7-8"></a>  <span class="kw">public</span><span class="op">:</span></span>
<span id="cb7-9"><a href="#cb7-9"></a>    <em>sentinel</em><span class="op">()</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb7-10"><a href="#cb7-10"></a>    <span class="kw">constexpr</span> <span class="kw">explicit</span> <em>sentinel</em><span class="op">(</span>sentinel_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> end<span class="op">)</span>;</span>
<span id="cb7-11"><a href="#cb7-11"></a>    <span class="kw">constexpr</span> <em>sentinel</em><span class="op">(</span><em>sentinel</em><span class="op">&lt;!</span>Const<span class="op">&gt;</span> other<span class="op">)</span></span>
<span id="cb7-12"><a href="#cb7-12"></a>      <span class="kw">requires</span> Const <span class="op">&amp;&amp;</span> convertible_to<span class="op">&lt;</span>sentinel_t<span class="op">&lt;</span>V<span class="op">&gt;</span>, sentinel_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span>;</span>
<span id="cb7-13"><a href="#cb7-13"></a></span>
<span id="cb7-14"><a href="#cb7-14"></a>    <span class="kw">constexpr</span> sentinel_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> base<span class="op">()</span> <span class="kw">const</span>;</span>
<span id="cb7-15"><a href="#cb7-15"></a></span>
<span id="cb7-16"><a href="#cb7-16"></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">&lt;</span>Const<span class="op">&gt;&amp;</span> x, <span class="kw">const</span> <em>sentinel</em><span class="op">&amp;</span> y<span class="op">)</span>;</span>
<span id="cb7-17"><a href="#cb7-17"></a></span>
<span id="cb7-18"><a href="#cb7-18"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> range_difference_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span></span>
<span id="cb7-19"><a href="#cb7-19"></a>      <span class="kw">operator</span><span class="op">-(</span><span class="kw">const</span> <em>iterator</em><span class="op">&lt;</span>Const<span class="op">&gt;&amp;</span> x, <span class="kw">const</span> <em>sentinel</em><span class="op">&amp;</span> y<span class="op">)</span></span>
<span id="cb7-20"><a href="#cb7-20"></a>        <span class="kw">requires</span> sized_sentinel_for<span class="op">&lt;</span>sentinel_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>, iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span>;</span>
<span id="cb7-21"><a href="#cb7-21"></a></span>
<span id="cb7-22"><a href="#cb7-22"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> range_difference_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span></span>
<span id="cb7-23"><a href="#cb7-23"></a>      <span class="kw">operator</span><span class="op">-(</span><span class="kw">const</span> <em>sentinel</em><span class="op">&amp;</span> x, <span class="kw">const</span> <em>iterator</em><span class="op">&lt;</span>Const<span class="op">&gt;&amp;</span> y<span class="op">)</span></span>
<span id="cb7-24"><a href="#cb7-24"></a>        <span class="kw">requires</span> sized_sentinel_for<span class="op">&lt;</span>sentinel_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>, iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span>;</span>
<span id="cb7-25"><a href="#cb7-25"></a>  <span class="op">}</span>;</span>
<span id="cb7-26"><a href="#cb7-26"></a><span class="op">}</span></span></code></pre></div>
<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="kw">constexpr</span> <span class="kw">explicit</span> <em>sentinel</em><span class="op">(</span>sentinel_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> end<span class="op">)</span>;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> <em>Effects:</em> Initializes <em><code class="sourceCode cpp">end_</code></em> with <code class="sourceCode cpp">end</code>.</p>
</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> <em>sentinel</em><span class="op">(</span><em>sentinel</em><span class="op">&lt;!</span>Const<span class="op">&gt;</span> other<span class="op">)</span></span>
<span id="cb9-2"><a href="#cb9-2"></a>  <span class="kw">requires</span> Const <span class="op">&amp;&amp;</span> convertible_to<span class="op">&lt;</span>sentinel_t<span class="op">&lt;</span>V<span class="op">&gt;</span>, sentinel_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span>;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> <em>Effects:</em> Initializes <em><code class="sourceCode cpp">end_</code></em> with <code>std::move(other.<i>end_</i>)</code>.</p>
<div class="sourceCode" id="cb10"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb10-1"><a href="#cb10-1"></a><span class="kw">constexpr</span> sentinel_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> base<span class="op">()</span> <span class="kw">const</span>;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized">3</a></span> <em>Effects:</em> Equivalent to: <code>return <i>end_</i>;</code></p>
<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">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">&lt;</span>Const<span class="op">&gt;&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>
<p><span class="marginalizedparent"><a class="marginalized">4</a></span> <em>Effects:</em> Equivalent to: <code>return x.<i>current_</i> == y.<i>end_</i>;</code></p>
<div class="sourceCode" id="cb12"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb12-1"><a href="#cb12-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> range_difference_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span></span>
<span id="cb12-2"><a href="#cb12-2"></a>  <span class="kw">operator</span><span class="op">-(</span><span class="kw">const</span> <em>iterator</em><span class="op">&lt;</span>Const<span class="op">&gt;&amp;</span> x, <span class="kw">const</span> <em>sentinel</em><span class="op">&amp;</span> y<span class="op">)</span></span>
<span id="cb12-3"><a href="#cb12-3"></a>    <span class="kw">requires</span> sized_sentinel_for<span class="op">&lt;</span>sentinel_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>, iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span>;</span></code></pre></div>
</blockquote>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">5</a></span> <em>Effects:</em> Equivalent to: <code>return x.<i>current_</i> - y.<i>end_</i>;</code></p>
<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">friend</span> <span class="kw">constexpr</span> range_difference_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span></span>
<span id="cb13-2"><a href="#cb13-2"></a>  <span class="kw">operator</span><span class="op">-(</span><span class="kw">const</span> <em>sentinel</em><span class="op">&amp;</span> x, <span class="kw">const</span> <em>iterator</em><span class="op">&lt;</span>Const<span class="op">&gt;&amp;</span> y<span class="op">)</span></span>
<span id="cb13-3"><a href="#cb13-3"></a>    <span class="kw">requires</span> sized_sentinel_for<span class="op">&lt;</span>sentinel_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>, iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span>;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized">6</a></span> <em>Effects:</em> Equivalent to: <code>return x.<i>end_</i> - y.<i>current_</i>;</code></p>
</blockquote>

</div>
<h1 id="references" style="border-bottom:1px solid #cccccc"><span class="header-section-number">5</span> References<a href="#references" class="self-link"></a></h1>

<div id="refs" role="doc-bibliography">
<div id="ref-LWG3386">
<p>[LWG3386] Tim Song. elements_view needs its own sentinel type. <br />
<a href="https://wg21.link/lwg3386">https://wg21.link/lwg3386</a></p>
</div>
<div id="ref-N4849">
<p>[N4849] Richard Smith. 2020. Working Draft, Standard for Programming Language C++. <br />
<a href="https://wg21.link/n4849">https://wg21.link/n4849</a></p>
</div>
</div>
</div>
</div>
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