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  <meta name="dcterms.date" content="2021-12-20" />
  <title>stride_view</title>
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<h1 class="title" style="text-align:center"><code class="sourceCode cpp">stride_view</code></h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P1899R2</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2021-12-20</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>
      LEWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Christopher Di Bella<br>&lt;<a href="mailto:cjdb.ns@gmail.com" class="email">cjdb.ns@gmail.com</a>&gt;<br>
      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">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#abstract"><span class="toc-section-number">1</span> Abstract<span></span></a></li>
<li><a href="#revision-history"><span class="toc-section-number">2</span> Revision history<span></span></a>
<ul>
<li><a href="#r2"><span class="toc-section-number">2.1</span> R2<span></span></a></li>
<li><a href="#r1"><span class="toc-section-number">2.2</span> R1<span></span></a></li>
<li><a href="#r0"><span class="toc-section-number">2.3</span> R0<span></span></a></li>
</ul></li>
<li><a href="#motivation"><span class="toc-section-number">3</span> Motivation<span></span></a>
<ul>
<li><a href="#risk-of-not-having-stride_view"><span class="toc-section-number">3.1</span> Risk of not having <code class="sourceCode cpp">stride_view</code><span></span></a></li>
</ul></li>
<li><a href="#implementation-experience"><span class="toc-section-number">4</span> Implementation experience<span></span></a></li>
<li><a href="#design-notes"><span class="toc-section-number">5</span> Design notes<span></span></a>
<ul>
<li><a href="#preconditions"><span class="toc-section-number">5.1</span> Preconditions<span></span></a></li>
<li><a href="#complex-iteration-model"><span class="toc-section-number">5.2</span> Complex iteration model<span></span></a></li>
<li><a href="#properties"><span class="toc-section-number">5.3</span> Properties<span></span></a></li>
</ul></li>
<li><a href="#proposed-wording"><span class="toc-section-number">6</span> Proposed wording<span></span></a>
<ul>
<li><a href="#addition-to-ranges"><span class="toc-section-number">6.1</span> Addition to <code class="sourceCode cpp"><span class="op">&lt;</span>ranges<span class="op">&gt;</span></code><span></span></a></li>
<li><a href="#stride"><span class="toc-section-number">6.2</span> <code class="sourceCode cpp">stride</code><span></span></a>
<ul>
<li><a href="#stride-view-range.stride">24.7.? Stride view [range.stride]<span></span></a>
<ul>
<li><a href="#overview-range.stride.overview">24.7.?.1 Overview [range.stride.overview]<span></span></a></li>
<li><a href="#class-template-stride_view-range.stride.view">24.7.?.2 Class template <code class="sourceCode cpp">stride_view</code> [range.stride.view]<span></span></a></li>
<li><a href="#class-template-stride_viewiterator-range.stride.iterator">24.7.?.3 Class template <code class="sourceCode cpp">stride_view<span class="op">::</span><em>iterator</em></code> [range.stride.iterator]<span></span></a></li>
</ul></li>
</ul></li>
<li><a href="#feature-test-macro"><span class="toc-section-number">6.3</span> Feature-test macro<span></span></a></li>
</ul></li>
<li><a href="#acknowledgements"><span class="toc-section-number">7</span> Acknowledgements<span></span></a></li>
<li><a href="#bibliography"><span class="toc-section-number">8</span> References<span></span></a></li>
</ul>
</div>
<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>The ability to use algorithms over an evenly-spaced subset of a range has been missed in the STL for a quarter of a century. Given that there’s no way to compose a strided range adaptor in C++20, this should be adopted for C++23.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">2</span> Revision history<a href="#revision-history" class="self-link"></a></h1>
<h2 data-number="2.1" id="r2"><span class="header-section-number">2.1</span> R2<a href="#r2" class="self-link"></a></h2>
<ul>
<li>Rewrite wording to match <code class="sourceCode cpp">chunk</code> from <span class="citation" data-cites="P2442R1">[<a href="#ref-P2442R1" role="doc-biblioref">P2442R1</a>]</span>.</li>
</ul>
<h2 data-number="2.2" id="r1"><span class="header-section-number">2.2</span> R1<a href="#r1" class="self-link"></a></h2>
<ul>
<li>PDF -&gt; HTML.</li>
<li>Adds a section discussing the design.</li>
<li>Adds feature-test macro.</li>
<li>Cleans up some stuff that was ported in from the implementation by mistake.</li>
<li>Adds <code class="sourceCode cpp">iterator_concept</code>, and corrects <code class="sourceCode cpp">iterator_category</code> so it can’t be contiguous.</li>
<li>Fixes calls to <em><code class="sourceCode cpp">compute<span class="op">-</span>distace</code></em> so they pass in size of underlying range instead of themselves.</li>
<li>Adds precondition to ensure stride is positive.</li>
<li>Makes multi-arg constructors non-explicit.</li>
</ul>
<h2 data-number="2.3" id="r0"><span class="header-section-number">2.3</span> R0<a href="#r0" class="self-link"></a></h2>
<p>Initial revision.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="motivation"><span class="header-section-number">3</span> Motivation<a href="#motivation" class="self-link"></a></h1>
<p>The ability to use algorithms over an evenly-spaced subset of a range has been missed in the STL for a quarter of a century. This is, in part, due to the complexity required to use an iterator that can safely describe such a range. It also means that the following examples cannot be transformed from raw loops into algorithms, due to a lacking iterator.</p>
<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">namespace</span> stdr <span class="op">=</span> std<span class="op">::</span>ranges;</span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="kw">namespace</span> stdv <span class="op">=</span> std<span class="op">::</span>views;</span>
<span id="cb1-3"><a href="#cb1-3"></a></span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="cf">for</span> <span class="op">(</span><span class="kw">auto</span> i <span class="op">=</span> <span class="dv">0</span>; i <span class="op">&lt;</span> ssize<span class="op">(</span>v<span class="op">)</span>; i <span class="op">+=</span> <span class="dv">2</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-5"><a href="#cb1-5"></a>  v<span class="op">[</span>i<span class="op">]</span> <span class="op">=</span> <span class="dv">42</span>; <span class="co">// fill</span></span>
<span id="cb1-6"><a href="#cb1-6"></a><span class="op">}</span></span>
<span id="cb1-7"><a href="#cb1-7"></a></span>
<span id="cb1-8"><a href="#cb1-8"></a><span class="cf">for</span> <span class="op">(</span><span class="kw">auto</span> i <span class="op">=</span> <span class="dv">0</span>; i <span class="op">&lt;</span> ssize<span class="op">(</span>v<span class="op">)</span>; i <span class="op">+=</span> <span class="dv">3</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-9"><a href="#cb1-9"></a>  v<span class="op">[</span>i<span class="op">]</span> <span class="op">=</span> f<span class="op">()</span>; <span class="co">// transform</span></span>
<span id="cb1-10"><a href="#cb1-10"></a><span class="op">}</span></span>
<span id="cb1-11"><a href="#cb1-11"></a></span>
<span id="cb1-12"><a href="#cb1-12"></a><span class="cf">for</span> <span class="op">(</span><span class="kw">auto</span> i <span class="op">=</span> <span class="dv">0</span>; i <span class="op">&lt;</span> ssize<span class="op">(</span>v<span class="op">)</span>; i <span class="op">+=</span> <span class="dv">3</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-13"><a href="#cb1-13"></a>  <span class="cf">for</span> <span class="op">(</span><span class="kw">auto</span> j <span class="op">=</span> i; j <span class="op">&lt;</span> ssize<span class="op">(</span>v<span class="op">)</span>; i <span class="op">+=</span> <span class="dv">3</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-14"><a href="#cb1-14"></a>    <span class="cf">if</span> <span class="op">(</span>v<span class="op">[</span>j<span class="op">]</span> <span class="op">&lt;</span> v<span class="op">[</span>i<span class="op">])</span> <span class="op">{</span></span>
<span id="cb1-15"><a href="#cb1-15"></a>      stdr<span class="op">::</span>swap<span class="op">(</span>v<span class="op">[</span>i<span class="op">]</span>, v<span class="op">[</span>j<span class="op">])</span>; <span class="co">// selection sort, but hopefully the idea is conveyed</span></span>
<span id="cb1-16"><a href="#cb1-16"></a>    <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>
<p>Boost.Range 2.0 introduced a range adaptor called <code class="sourceCode cpp">strided</code>, and range-v3’s equivalent is <code class="sourceCode cpp">stride_view</code>, both of which make striding far easier than when using iterators:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a>stdr<span class="op">::</span>fill<span class="op">(</span>v <span class="op">|</span> stdv<span class="op">::</span>stride<span class="op">(</span><span class="dv">2</span><span class="op">)</span>, <span class="dv">42</span><span class="op">)</span>;</span>
<span id="cb2-2"><a href="#cb2-2"></a></span>
<span id="cb2-3"><a href="#cb2-3"></a><span class="kw">auto</span> strided_v <span class="op">=</span> v <span class="op">|</span> stdv<span class="op">::</span>stride<span class="op">(</span><span class="dv">3</span><span class="op">)</span>;</span>
<span id="cb2-4"><a href="#cb2-4"></a>stdr<span class="op">::</span>transform<span class="op">(</span>strided_v, stdr<span class="op">::</span>begin<span class="op">(</span>strided_v<span class="op">)</span> f<span class="op">)</span>;</span>
<span id="cb2-5"><a href="#cb2-5"></a></span>
<span id="cb2-6"><a href="#cb2-6"></a>stdr<span class="op">::</span>stable_sort<span class="op">(</span>strided_v<span class="op">)</span>; <span class="co">// order restored!</span></span></code></pre></div>
<p>Given that there’s no way to compose a strided range adaptor in C++20, this should be one of the earliest range adaptors put into C++23.</p>
<h2 data-number="3.1" id="risk-of-not-having-stride_view"><span class="header-section-number">3.1</span> Risk of not having <code class="sourceCode cpp">stride_view</code><a href="#risk-of-not-having-stride_view" class="self-link"></a></h2>
<p>Although it isn’t possible to compose <code class="sourceCode cpp">stride_view</code> in C++20, someone inexperienced with the ranges design space might mistake <code class="sourceCode cpp">filter_view</code> as a suitable way to “compose” <code class="sourceCode cpp">stride_view</code>:</p>
<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> bad_stride <span class="op">=</span> <span class="op">[](</span><span class="kw">auto</span> <span class="kw">const</span> step<span class="op">)</span> <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2"></a>  <span class="cf">return</span> views<span class="op">::</span>filter<span class="op">([</span>n <span class="op">=</span> <span class="dv">0</span>, step<span class="op">](</span><span class="kw">auto</span><span class="op">&amp;&amp;)</span> <span class="kw">mutable</span> <span class="op">{</span></span>
<span id="cb3-3"><a href="#cb3-3"></a>    <span class="cf">return</span> n<span class="op">++</span> <span class="op">%</span> step <span class="op">==</span> <span class="dv">0</span>;</span>
<span id="cb3-4"><a href="#cb3-4"></a>  <span class="op">})</span>;</span>
<span id="cb3-5"><a href="#cb3-5"></a><span class="op">}</span>;</span></code></pre></div>
<p>This implementation is broken for two reasons:</p>
<ol type="1">
<li><code class="sourceCode cpp">filter_view</code> expects a <code class="sourceCode cpp">predicate</code> as its input, but the lambda we have provided does not model <code class="sourceCode cpp">predicate</code> (a call to <code class="sourceCode cpp">invoke</code> on a <code class="sourceCode cpp">predicate</code> mustn’t modify the function object, yet we clearly are).</li>
<li>The lambda provided doesn’t account for moving backward, so despite <em>satisfying</em> <code class="sourceCode cpp">bidirectional_iterator</code>, it does not model the concept, thus rendering any program containing it ill-formed, with no diagnostic being required.</li>
</ol>
<p>For these reasons, the author regrets not proposing this in the C++20 design space.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="implementation-experience"><span class="header-section-number">4</span> Implementation experience<a href="#implementation-experience" class="self-link"></a></h1>
<p>Both Boost.Range 2.0 and range-v3 are popular ranges libraries that support a striding range adaptor. The proposed wording has mostly been implemented in cmcstl2 and in a CppCon main session.</p>
<h1 data-number="5" style="border-bottom:1px solid #cccccc" id="design-notes"><span class="header-section-number">5</span> Design notes<a href="#design-notes" class="self-link"></a></h1>
<h2 data-number="5.1" id="preconditions"><span class="header-section-number">5.1</span> Preconditions<a href="#preconditions" class="self-link"></a></h2>
<p>Boost.Range 2.0’s <code class="sourceCode cpp">strided</code> has a precondition that <code class="sourceCode cpp"><span class="dv">0</span> <span class="op">&lt;=</span> n</code>, but this isn’t strong enough: we need <code class="sourceCode cpp">n</code> to be <em>positive</em>.</p>
<p>The stride needs to be positive since a negative stride doesn’t really make sense, and a semantic requirement of <code class="sourceCode cpp">std<span class="op">::</span>weakly_incrementable</code> (<span>23.3.4.4 <a href="https://wg21.link/iterator.concept.winc">[iterator.concept.winc]</a></span>) is that incrementing actually moves the iterator to the next element: this means a zero-stride isn’t allowed either.</p>
<p>LEWG unanimously agreed that this was the correct decision in Prague.</p>
<h2 data-number="5.2" id="complex-iteration-model"><span class="header-section-number">5.2</span> Complex iteration model<a href="#complex-iteration-model" class="self-link"></a></h2>
<p>A simple implementation of <code class="sourceCode cpp">stride_view</code> would be similar to what’s in Boost.Range 2.0: a single-pass range adaptor. With some effort, we can go all the way to a random-access range adaptor, which is what this section mainly covers.</p>
<p>A naive random-access range adaptor would be implemented by simply moving the iterator forward or backward by <code class="sourceCode cpp">n</code> positions (where <code class="sourceCode cpp">n</code> is the stride length). While this produce a correct iterator when moving forward, its <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">--</span></code> will be incorrect whenever <code class="sourceCode cpp">n</code> doesn’t evenly divide the underlying range’s length. For example:</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="kw">auto</span> x <span class="op">=</span> std<span class="op">::</span>vector<span class="op">{</span><span class="dv">0</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="dv">6</span>, <span class="dv">7</span>, <span class="dv">8</span>, <span class="dv">9</span>, <span class="dv">10</span>, <span class="dv">11</span><span class="op">}</span>;</span>
<span id="cb4-2"><a href="#cb4-2"></a></span>
<span id="cb4-3"><a href="#cb4-3"></a><span class="co">// prints 0 3 6 9</span></span>
<span id="cb4-4"><a href="#cb4-4"></a>stdr<span class="op">::</span>copy<span class="op">(</span>stdv<span class="op">::</span>stride<span class="op">(</span>x, <span class="dv">3</span><span class="op">)</span>, std<span class="op">::</span>ostream_iterator<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;(</span>std<span class="op">::</span>cout, <span class="st">&quot; &quot;</span><span class="op">))</span>;</span>
<span id="cb4-5"><a href="#cb4-5"></a></span>
<span id="cb4-6"><a href="#cb4-6"></a><span class="co">// prints 9 6 3 0</span></span>
<span id="cb4-7"><a href="#cb4-7"></a>stdr<span class="op">::</span>copy<span class="op">(</span>stdv<span class="op">::</span>stride<span class="op">(</span>x, <span class="dv">3</span><span class="op">)</span> <span class="op">|</span> stdv<span class="op">::</span>reverse, std<span class="op">::</span>ostream_iterator<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;(</span>std<span class="op">::</span>cout, <span class="st">&quot; &quot;</span><span class="op">))</span>;</span>
<span id="cb4-8"><a href="#cb4-8"></a></span>
<span id="cb4-9"><a href="#cb4-9"></a><span class="kw">auto</span> y <span class="op">=</span> std<span class="op">::</span>vector<span class="op">{</span><span class="dv">0</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="dv">6</span>, <span class="dv">7</span>, <span class="dv">8</span>, <span class="dv">9</span>, <span class="dv">10</span><span class="op">}</span>;</span>
<span id="cb4-10"><a href="#cb4-10"></a></span>
<span id="cb4-11"><a href="#cb4-11"></a><span class="co">// prints 0 3 6 9</span></span>
<span id="cb4-12"><a href="#cb4-12"></a>stdr<span class="op">::</span>copy<span class="op">(</span>stdv<span class="op">::</span>stride<span class="op">(</span>y, <span class="dv">3</span><span class="op">)</span>, std<span class="op">::</span>ostream_iterator<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;(</span>std<span class="op">::</span>cout, <span class="st">&quot; &quot;</span><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="co">// prints 8 5 2: not the same range in reverse!?</span></span>
<span id="cb4-15"><a href="#cb4-15"></a>stdr<span class="op">::</span>copy<span class="op">(</span>stdv<span class="op">::</span>stride<span class="op">(</span>y, <span class="dv">3</span><span class="op">)</span> <span class="op">|</span> stdv<span class="op">::</span>reverse, std<span class="op">::</span>ostream_iterator<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;(</span>std<span class="op">::</span>cout, <span class="st">&quot; &quot;</span><span class="op">))</span>;</span></code></pre></div>
<p>The problem here is that going from the one-before-past-the-end iterator to the past-the-end iterator may involve iterating fewer than <code class="sourceCode cpp">stride</code> steps, so to correctly iterate backwards, we need to know that distance.</p>
<p>This is the same problem as <code class="sourceCode cpp">chunk</code> (<span class="citation" data-cites="P2442R1">[<a href="#ref-P2442R1" role="doc-biblioref">P2442R1</a>]</span>) and can be solved in the same way. After all, <code class="sourceCode cpp">stride<span class="op">(</span>n<span class="op">)</span></code> is basically a more efficient version of <code class="sourceCode cpp">chunk<span class="op">(</span>n<span class="op">)</span> <span class="op">|</span> transform<span class="op">(</span>ranges<span class="op">::</span>front<span class="op">)</span></code> (if we actually had a <code class="sourceCode cpp">ranges<span class="op">::</span>front</code>).</p>
<h2 data-number="5.3" id="properties"><span class="header-section-number">5.3</span> Properties<a href="#properties" class="self-link"></a></h2>
<p><code class="sourceCode cpp">stride_view</code> is borrowed if the underlying view is. It is a common range if the underlying range is common and either sized or non-bidirectional.</p>
<h1 data-number="6" style="border-bottom:1px solid #cccccc" id="proposed-wording"><span class="header-section-number">6</span> Proposed wording<a href="#proposed-wording" class="self-link"></a></h1>
<h2 data-number="6.1" id="addition-to-ranges"><span class="header-section-number">6.1</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>24.2 <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="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a></span>
<span id="cb5-2"><a href="#cb5-2"></a><span class="co">// [...]</span></span>
<span id="cb5-3"><a href="#cb5-3"></a><span class="kw">namespace</span> std<span class="op">::</span>ranges <span class="op">{</span></span>
<span id="cb5-4"><a href="#cb5-4"></a>  <span class="co">// [...]</span></span>
<span id="cb5-5"><a href="#cb5-5"></a></span>
<span id="cb5-6"><a href="#cb5-6"></a>  <span class="co">// [range.stride], stride view</span></span>
<span id="cb5-7"><a href="#cb5-7"></a>  <span class="kw">template</span><span class="op">&lt;</span>input_range V<span class="op">&gt;</span></span>
<span id="cb5-8"><a href="#cb5-8"></a>    <span class="kw">requires</span> view<span class="op">&lt;</span>V<span class="op">&gt;</span></span>
<span id="cb5-9"><a href="#cb5-9"></a>  <span class="kw">class</span> stride_view;</span>
<span id="cb5-10"><a href="#cb5-10"></a></span>
<span id="cb5-11"><a href="#cb5-11"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> V<span class="op">&gt;</span></span>
<span id="cb5-12"><a href="#cb5-12"></a>    <span class="kw">inline</span> <span class="kw">constexpr</span> <span class="dt">bool</span> enable_borrowed_range<span class="op">&lt;</span>stride_view<span class="op">&lt;</span>V<span class="op">&gt;&gt;</span> <span class="op">=</span></span>
<span id="cb5-13"><a href="#cb5-13"></a>      enable_borrowed_range<span class="op">&lt;</span>V<span class="op">&gt;</span>;</span>
<span id="cb5-14"><a href="#cb5-14"></a></span>
<span id="cb5-15"><a href="#cb5-15"></a>  <span class="kw">namespace</span> views <span class="op">{</span> <span class="kw">inline</span> <span class="kw">constexpr</span> <em>unspecified</em> stride <span class="op">=</span> <em>unspecified</em>; <span class="op">}</span></span>
<span id="cb5-16"><a href="#cb5-16"></a></span>
<span id="cb5-17"><a href="#cb5-17"></a>  <span class="co">// [...]</span></span>
<span id="cb5-18"><a href="#cb5-18"></a><span class="op">}</span></span></code></pre></div>
<h2 data-number="6.2" id="stride"><span class="header-section-number">6.2</span> <code class="sourceCode cpp">stride</code><a href="#stride" class="self-link"></a></h2>
<p>Add the following subclause to <span>24.7 <a href="https://wg21.link/range.adaptors">[range.adaptors]</a></span>.</p>
<p><span class="ednote" style="color: #0000ff">[ Editor&#39;s note: This wording assumes the exposition-only <code class="sourceCode default"><em>div-ceil</em></code> function template introduced in <span class="citation" data-cites="P2442R1">[<a href="#ref-P2442R1" role="doc-biblioref">P2442R1</a>]</span>. ]</span></p>
<h3 class="unnumbered" data-number id="stride-view-range.stride">24.7.? Stride view [range.stride]<a href="#stride-view-range.stride" class="self-link"></a></h3>
<h4 class="unnumbered" data-number id="overview-range.stride.overview">24.7.?.1 Overview [range.stride.overview]<a href="#overview-range.stride.overview" class="self-link"></a></h4>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">1</a></span> <code class="sourceCode cpp">stride_view</code> presents a view of an underlying sequence, advancing over <code class="sourceCode cpp">n</code> elements at a time, as opposed to the usual single-step succession.</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>stride</code> denotes a range adaptor object <span>24.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a></span>. Given subexpressions <code class="sourceCode cpp">E</code> and <code class="sourceCode cpp">N</code>, the expression <code class="sourceCode cpp">views<span class="op">::</span>stride<span class="op">(</span>E, N<span class="op">)</span></code> is expression-equivalent to <code class="sourceCode cpp">stride_view<span class="op">(</span>E, N<span class="op">)</span></code>.</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">3</a></span> [<em>Example</em>:</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">auto</span> input <span class="op">=</span> views<span class="op">::</span>iota<span class="op">(</span><span class="dv">0</span>, <span class="dv">12</span><span class="op">)</span> <span class="op">|</span> views<span class="op">::</span>stride<span class="op">(</span><span class="dv">3</span><span class="op">)</span>;</span>
<span id="cb6-2"><a href="#cb6-2"></a>ranges<span class="op">::</span>copy<span class="op">(</span>input, ostream_iterator<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;(</span>cout, <span class="st">&quot; &quot;</span><span class="op">))</span>; <span class="co">// prints 0 3 6 9</span></span>
<span id="cb6-3"><a href="#cb6-3"></a>ranges<span class="op">::</span>copy<span class="op">(</span>input <span class="op">|</span> views<span class="op">::</span>reverse, ostream_iterator<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;(</span>cout, <span class="st">&quot; &quot;</span><span class="op">))</span>; <span class="co">// prints 9 6 3 0</span></span></code></pre></div>
<p>— <em>end example</em>]</p>
<h4 class="unnumbered" data-number id="class-template-stride_view-range.stride.view">24.7.?.2 Class template <code class="sourceCode cpp">stride_view</code> [range.stride.view]<a href="#class-template-stride_view-range.stride.view" 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>input_range V<span class="op">&gt;</span></span>
<span id="cb7-3"><a href="#cb7-3"></a>    <span class="kw">requires</span> view<span class="op">&lt;</span>V<span class="op">&gt;</span></span>
<span id="cb7-4"><a href="#cb7-4"></a>  <span class="kw">class</span> stride_view <span class="op">:</span> <span class="kw">public</span> view_interface<span class="op">&lt;</span>stride_view<span class="op">&lt;</span>V<span class="op">&gt;&gt;</span> <span class="op">{</span></span>
<span id="cb7-5"><a href="#cb7-5"></a>    V <em>base_</em> <span class="op">=</span> V<span class="op">()</span>;                           <span class="co">// exposition only</span></span>
<span id="cb7-6"><a href="#cb7-6"></a>    range_difference_t<span class="op">&lt;</span>V<span class="op">&gt;</span> <em>stride_</em> <span class="op">=</span> <span class="dv">1</span>;       <span class="co">// exposition only</span></span>
<span id="cb7-7"><a href="#cb7-7"></a>    <span class="kw">template</span><span class="op">&lt;</span><span class="dt">bool</span><span class="op">&gt;</span> <span class="kw">class</span> <em>iterator</em>;           <span class="co">// exposition only</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>    stride_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="cb7-10"><a href="#cb7-10"></a>    <span class="kw">constexpr</span> <span class="kw">explicit</span> stride_view<span class="op">(</span>V base, range_difference_t<span class="op">&lt;</span>V<span class="op">&gt;</span> stride<span class="op">)</span>;</span>
<span id="cb7-11"><a href="#cb7-11"></a></span>
<span id="cb7-12"><a href="#cb7-12"></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="cb7-13"><a href="#cb7-13"></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="cb7-14"><a href="#cb7-14"></a></span>
<span id="cb7-15"><a href="#cb7-15"></a>    <span class="kw">constexpr</span> range_difference_t<span class="op">&lt;</span>V<span class="op">&gt;</span> stride<span class="op">()</span> <span class="kw">const</span> <span class="kw">noexcept</span>;</span>
<span id="cb7-16"><a href="#cb7-16"></a></span>
<span id="cb7-17"><a href="#cb7-17"></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">&lt;</span>V<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb7-18"><a href="#cb7-18"></a>      <span class="cf">return</span> <em>iterator</em><span class="op">&lt;</span><span class="kw">false</span><span class="op">&gt;(</span><span class="kw">this</span>, ranges<span class="op">::</span>begin<span class="op">(</span><em>base_</em><span class="op">))</span>;</span>
<span id="cb7-19"><a href="#cb7-19"></a>    <span class="op">}</span></span>
<span id="cb7-20"><a href="#cb7-20"></a></span>
<span id="cb7-21"><a href="#cb7-21"></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">&lt;</span><span class="kw">const</span> V<span class="op">&gt;</span> <span class="op">{</span></span>
<span id="cb7-22"><a href="#cb7-22"></a>      <span class="cf">return</span> <em>iterator</em><span class="op">&lt;</span><span class="kw">true</span><span class="op">&gt;(</span><span class="kw">this</span>, ranges<span class="op">::</span>begin<span class="op">(</span><em>base_</em><span class="op">))</span>;</span>
<span id="cb7-23"><a href="#cb7-23"></a>    <span class="op">}</span></span>
<span id="cb7-24"><a href="#cb7-24"></a></span>
<span id="cb7-25"><a href="#cb7-25"></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">&lt;</span>V<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb7-26"><a href="#cb7-26"></a>      <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>common_range<span class="op">&lt;</span>V<span class="op">&gt;</span> <span class="op">&amp;&amp;</span> sized_range<span class="op">&lt;</span>V<span class="op">&gt;</span> <span class="op">&amp;&amp;</span> forward_range<span class="op">&lt;</span>V<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb7-27"><a href="#cb7-27"></a>        <span class="kw">auto</span> missing <span class="op">=</span> <span class="op">(</span><em>stride_</em> <span class="op">-</span> ranges<span class="op">::</span>distance<span class="op">(</span><em>base_</em><span class="op">)</span> <span class="op">%</span> <em>stride_</em><span class="op">)</span> <span class="op">%</span> <em>stride_</em>;</span>
<span id="cb7-28"><a href="#cb7-28"></a>        <span class="cf">return</span> <em>iterator</em><span class="op">&lt;</span><span class="kw">false</span><span class="op">&gt;(</span><span class="kw">this</span>, ranges<span class="op">::</span>end<span class="op">(</span><em>base_</em><span class="op">)</span>, missing<span class="op">)</span>;</span>
<span id="cb7-29"><a href="#cb7-29"></a>      <span class="op">}</span></span>
<span id="cb7-30"><a href="#cb7-30"></a>      <span class="cf">else</span> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>common_range<span class="op">&lt;</span>V<span class="op">&gt;</span> <span class="op">&amp;&amp;</span> <span class="op">!</span>bidirectional_range<span class="op">&lt;</span>V<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb7-31"><a href="#cb7-31"></a>        <span class="cf">return</span> <em>iterator</em><span class="op">&lt;</span><span class="kw">false</span><span class="op">&gt;(</span><span class="kw">this</span>, ranges<span class="op">::</span>end<span class="op">(</span><em>base_</em><span class="op">))</span>;</span>
<span id="cb7-32"><a href="#cb7-32"></a>      <span class="op">}</span></span>
<span id="cb7-33"><a href="#cb7-33"></a>      <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb7-34"><a href="#cb7-34"></a>        <span class="cf">return</span> default_sentinel;</span>
<span id="cb7-35"><a href="#cb7-35"></a>      <span class="op">}</span></span>
<span id="cb7-36"><a href="#cb7-36"></a>    <span class="op">}</span></span>
<span id="cb7-37"><a href="#cb7-37"></a></span>
<span id="cb7-38"><a href="#cb7-38"></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">&lt;</span><span class="kw">const</span> V<span class="op">&gt;</span> <span class="op">{</span></span>
<span id="cb7-39"><a href="#cb7-39"></a>      <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>common_range<span class="op">&lt;</span><span class="kw">const</span> V<span class="op">&gt;</span> <span class="op">&amp;&amp;</span> sized_range<span class="op">&lt;</span><span class="kw">const</span> V<span class="op">&gt;</span> <span class="op">&amp;&amp;</span> forward_range<span class="op">&lt;</span><span class="kw">const</span> V<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb7-40"><a href="#cb7-40"></a>        <span class="kw">auto</span> missing <span class="op">=</span> <span class="op">(</span><em>stride_</em> <span class="op">-</span> ranges<span class="op">::</span>distance<span class="op">(</span><em>base_</em><span class="op">)</span> <span class="op">%</span> <em>stride_</em><span class="op">)</span> <span class="op">%</span> <em>stride_</em>;</span>
<span id="cb7-41"><a href="#cb7-41"></a>        <span class="cf">return</span> <em>iterator</em><span class="op">&lt;</span><span class="kw">true</span><span class="op">&gt;(</span><span class="kw">this</span>, ranges<span class="op">::</span>end<span class="op">(</span><em>base_</em><span class="op">)</span>, missing<span class="op">)</span>;</span>
<span id="cb7-42"><a href="#cb7-42"></a>      <span class="op">}</span></span>
<span id="cb7-43"><a href="#cb7-43"></a>      <span class="cf">else</span> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>common_range<span class="op">&lt;</span><span class="kw">const</span> V<span class="op">&gt;</span> <span class="op">&amp;&amp;</span> <span class="op">!</span>bidirectional_range<span class="op">&lt;</span><span class="kw">const</span> V<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb7-44"><a href="#cb7-44"></a>        <span class="cf">return</span> <em>iterator</em><span class="op">&lt;</span><span class="kw">true</span><span class="op">&gt;(</span><span class="kw">this</span>, ranges<span class="op">::</span>end<span class="op">(</span><em>base_</em><span class="op">))</span>;</span>
<span id="cb7-45"><a href="#cb7-45"></a>      <span class="op">}</span></span>
<span id="cb7-46"><a href="#cb7-46"></a>      <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb7-47"><a href="#cb7-47"></a>        <span class="cf">return</span> default_sentinel;</span>
<span id="cb7-48"><a href="#cb7-48"></a>      <span class="op">}</span></span>
<span id="cb7-49"><a href="#cb7-49"></a>    <span class="op">}</span></span>
<span id="cb7-50"><a href="#cb7-50"></a></span>
<span id="cb7-51"><a href="#cb7-51"></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="cb7-52"><a href="#cb7-52"></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="cb7-53"><a href="#cb7-53"></a>  <span class="op">}</span>;</span>
<span id="cb7-54"><a href="#cb7-54"></a></span>
<span id="cb7-55"><a href="#cb7-55"></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="cb7-56"><a href="#cb7-56"></a>    stride_view<span class="op">(</span>R<span class="op">&amp;&amp;</span>, range_difference_t<span class="op">&lt;</span>R<span class="op">&gt;)</span> <span class="op">-&gt;</span> stride_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>
<span id="cb7-57"><a href="#cb7-57"></a><span class="op">}</span></span></code></pre></div>
<p><span class="draftnote" style="color: #01796F">[ Drafting note: <code class="sourceCode cpp">end<span class="op">()</span></code> cannot compute <code class="sourceCode cpp">missing</code> for input-only ranges because <code class="sourceCode cpp">ranges<span class="op">::</span>size</code> (and <code class="sourceCode cpp">ranges<span class="op">::</span>distance</code> by extension) is not required to be valid after <code class="sourceCode cpp">ranges<span class="op">::</span>begin</code> is called, but <code class="sourceCode cpp">end<span class="op">()</span></code> must remain callable. ]</span></p>
<div>
<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> stride_view<span class="op">(</span>V base, range_difference_t<span class="op">&lt;</span>V<span class="op">&gt;</span> stride<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>Preconditions</em>: <code class="sourceCode cpp">stride <span class="op">&gt;</span> <span class="dv">0</span></code> is <code class="sourceCode cpp"><span class="kw">true</span></code>.</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">2</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> and <code class="sourceCode cpp"><em>stride_</em></code> with <code class="sourceCode cpp">stride</code>.</p>
</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> range_difference_t<span class="op">&lt;</span>V<span class="op">&gt;</span> stride<span class="op">()</span> <span class="kw">const</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_6" id="pnum_6">3</a></span> <em>Returns</em>: <code class="sourceCode cpp"><em>stride_</em></code>.</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="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="cb10-2"><a href="#cb10-2"></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="cb11"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb11-1"><a href="#cb11-1"></a><span class="cf">return</span> <em>to-unsigned-like</em><span class="op">(</span><em>div-ceil</em><span class="op">(</span>ranges<span class="op">::</span>distance<span class="op">(</span><em>base_</em><span class="op">)</span>, <em>stride_</em><span class="op">))</span>;</span></code></pre></div>
</blockquote>
</blockquote>
</div>
<h4 class="unnumbered" data-number id="class-template-stride_viewiterator-range.stride.iterator">24.7.?.3 Class template <code class="sourceCode cpp">stride_view<span class="op">::</span><em>iterator</em></code> [range.stride.iterator]<a href="#class-template-stride_viewiterator-range.stride.iterator" class="self-link"></a></h4>
<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">namespace</span> std<span class="op">::</span>ranges <span class="op">{</span></span>
<span id="cb12-2"><a href="#cb12-2"></a>  <span class="kw">template</span><span class="op">&lt;</span>input_range V<span class="op">&gt;</span></span>
<span id="cb12-3"><a href="#cb12-3"></a>    <span class="kw">requires</span> view<span class="op">&lt;</span>V<span class="op">&gt;</span></span>
<span id="cb12-4"><a href="#cb12-4"></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="cb12-5"><a href="#cb12-5"></a>  <span class="kw">class</span> stride_view<span class="op">&lt;</span>V<span class="op">&gt;::</span><em>iterator</em> <span class="op">{</span></span>
<span id="cb12-6"><a href="#cb12-6"></a>    <span class="kw">using</span> <em>Parent</em> <span class="op">=</span> <em>maybe-const</em><span class="op">&lt;</span>Const, chunk_view<span class="op">&gt;</span>;                <span class="co">// exposition only</span></span>
<span id="cb12-7"><a href="#cb12-7"></a>    <span class="kw">using</span> <em>Base</em> <span class="op">=</span> <em>maybe-const</em><span class="op">&lt;</span>Const, V<span class="op">&gt;</span>;                           <span class="co">// exposition only</span></span>
<span id="cb12-8"><a href="#cb12-8"></a></span>
<span id="cb12-9"><a href="#cb12-9"></a>    iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> <em>current_</em> <span class="op">=</span> iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;()</span>;               <span class="co">// exposition only</span></span>
<span id="cb12-10"><a href="#cb12-10"></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">// exposition only</span></span>
<span id="cb12-11"><a href="#cb12-11"></a>    range_difference_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> <em>stride_</em> <span class="op">=</span> <span class="dv">0</span>;                         <span class="co">// exposition only</span></span>
<span id="cb12-12"><a href="#cb12-12"></a>    range_difference_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> <em>missing_</em> <span class="op">=</span> <span class="dv">0</span>;                        <span class="co">// exposition only</span></span>
<span id="cb12-13"><a href="#cb12-13"></a></span>
<span id="cb12-14"><a href="#cb12-14"></a>    <span class="kw">constexpr</span> <em>iterator</em><span class="op">(</span><em>Parent</em><span class="op">*</span> parent, iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> current,  <span class="co">// exposition only</span></span>
<span id="cb12-15"><a href="#cb12-15"></a>                       range_difference_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> missing <span class="op">=</span> <span class="dv">0</span><span class="op">)</span>;</span>
<span id="cb12-16"><a href="#cb12-16"></a>  <span class="kw">public</span><span class="op">:</span></span>
<span id="cb12-17"><a href="#cb12-17"></a>    <span class="kw">using</span> difference_type <span class="op">=</span> range_difference_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-18"><a href="#cb12-18"></a>    <span class="kw">using</span> value_type <span class="op">=</span> range_value_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-19"><a href="#cb12-19"></a>    <span class="kw">using</span> iterator_concept <span class="op">=</span> <em>see below</em>;</span>
<span id="cb12-20"><a href="#cb12-20"></a>    <span class="kw">using</span> iterator_category <span class="op">=</span> <em>see below</em>; <span class="co">// not always present</span></span>
<span id="cb12-21"><a href="#cb12-21"></a></span>
<span id="cb12-22"><a href="#cb12-22"></a>    <em>iterator</em><span class="op">()</span> <span class="kw">requires</span> default_initializable<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;=</span> <span class="cf">default</span>;</span>
<span id="cb12-23"><a href="#cb12-23"></a></span>
<span id="cb12-24"><a href="#cb12-24"></a>    <span class="kw">constexpr</span> <em>iterator</em><span class="op">(</span><em>iterator</em><span class="op">&lt;!</span>Const<span class="op">&gt;</span> other<span class="op">)</span></span>
<span id="cb12-25"><a href="#cb12-25"></a>      <span class="kw">requires</span> Const <span class="op">&amp;&amp;</span> convertible_to<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span>V<span class="op">&gt;</span>, iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span></span>
<span id="cb12-26"><a href="#cb12-26"></a>                     <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="cb12-27"><a href="#cb12-27"></a></span>
<span id="cb12-28"><a href="#cb12-28"></a>    <span class="kw">constexpr</span> iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> base<span class="op">()</span> <span class="op">&amp;&amp;</span>;</span>
<span id="cb12-29"><a href="#cb12-29"></a>    <span class="kw">constexpr</span> <span class="kw">const</span> iterator_t<span class="op">&lt;</span><em>Base</em><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="cb12-30"><a href="#cb12-30"></a></span>
<span id="cb12-31"><a href="#cb12-31"></a>    <span class="kw">constexpr</span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> <span class="kw">operator</span><span class="op">*()</span> <span class="kw">const</span> <span class="op">{</span> <span class="cf">return</span> <span class="op">*</span><em>current_</em>; <span class="op">}</span></span>
<span id="cb12-32"><a href="#cb12-32"></a></span>
<span id="cb12-33"><a href="#cb12-33"></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="cb12-34"><a href="#cb12-34"></a></span>
<span id="cb12-35"><a href="#cb12-35"></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="cb12-36"><a href="#cb12-36"></a>    <span class="kw">constexpr</span> <em>iterator</em> <span class="kw">operator</span><span class="op">++(</span><span class="dt">int</span><span class="op">)</span> <span class="kw">requires</span> forward_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-37"><a href="#cb12-37"></a></span>
<span id="cb12-38"><a href="#cb12-38"></a>    <span class="kw">constexpr</span> <em>iterator</em><span class="op">&amp;</span> <span class="kw">operator</span><span class="op">--()</span> <span class="kw">requires</span> bidirectional_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-39"><a href="#cb12-39"></a>    <span class="kw">constexpr</span> <em>iterator</em> <span class="kw">operator</span><span class="op">--(</span><span class="dt">int</span><span class="op">)</span> <span class="kw">requires</span> bidirectional_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-40"><a href="#cb12-40"></a></span>
<span id="cb12-41"><a href="#cb12-41"></a>    <span class="kw">constexpr</span> <em>iterator</em><span class="op">&amp;</span> <span class="kw">operator</span><span class="op">+=(</span>difference_type n<span class="op">)</span> <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-42"><a href="#cb12-42"></a>    <span class="kw">constexpr</span> <em>iterator</em><span class="op">&amp;</span> <span class="kw">operator</span><span class="op">-=(</span>difference_type n<span class="op">)</span> <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-43"><a href="#cb12-43"></a></span>
<span id="cb12-44"><a href="#cb12-44"></a>    <span class="kw">constexpr</span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> <span class="kw">operator</span><span class="op">[](</span>difference_type n<span class="op">)</span> <span class="kw">const</span></span>
<span id="cb12-45"><a href="#cb12-45"></a>      <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span></span>
<span id="cb12-46"><a href="#cb12-46"></a>    <span class="op">{</span> <span class="cf">return</span> <span class="op">*(*</span><span class="kw">this</span> <span class="op">+</span> n<span class="op">)</span>; <span class="op">}</span></span>
<span id="cb12-47"><a href="#cb12-47"></a></span>
<span id="cb12-48"><a href="#cb12-48"></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, default_sentinel<span class="op">)</span>;</span>
<span id="cb12-49"><a href="#cb12-49"></a></span>
<span id="cb12-50"><a href="#cb12-50"></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>iterator</em><span class="op">&amp;</span> y<span class="op">)</span></span>
<span id="cb12-51"><a href="#cb12-51"></a>      <span class="kw">requires</span> equality_comparable<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span>;</span>
<span id="cb12-52"><a href="#cb12-52"></a></span>
<span id="cb12-53"><a href="#cb12-53"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">&lt;(</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="cb12-54"><a href="#cb12-54"></a>      <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-55"><a href="#cb12-55"></a></span>
<span id="cb12-56"><a href="#cb12-56"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">&gt;(</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="cb12-57"><a href="#cb12-57"></a>      <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-58"><a href="#cb12-58"></a></span>
<span id="cb12-59"><a href="#cb12-59"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">&lt;=(</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="cb12-60"><a href="#cb12-60"></a>      <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-61"><a href="#cb12-61"></a></span>
<span id="cb12-62"><a href="#cb12-62"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">&gt;=(</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="cb12-63"><a href="#cb12-63"></a>      <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-64"><a href="#cb12-64"></a></span>
<span id="cb12-65"><a href="#cb12-65"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">&lt;=&gt;(</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="cb12-66"><a href="#cb12-66"></a>        <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> <span class="op">&amp;&amp;</span> three_way_comparable<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span>;</span>
<span id="cb12-67"><a href="#cb12-67"></a></span>
<span id="cb12-68"><a href="#cb12-68"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> <em>iterator</em><span class="op">&amp;</span> <span class="kw">operator</span><span class="op">+(</span><span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> x, difference_type n<span class="op">)</span></span>
<span id="cb12-69"><a href="#cb12-69"></a>      <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-70"><a href="#cb12-70"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> <em>iterator</em><span class="op">&amp;</span> <span class="kw">operator</span><span class="op">+(</span>difference_type n, <span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> x<span class="op">)</span></span>
<span id="cb12-71"><a href="#cb12-71"></a>      <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-72"><a href="#cb12-72"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> <em>iterator</em><span class="op">&amp;</span> <span class="kw">operator</span><span class="op">-(</span><span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> x, difference_type n<span class="op">)</span></span>
<span id="cb12-73"><a href="#cb12-73"></a>      <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb12-74"><a href="#cb12-74"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> difference_type <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>iterator</em><span class="op">&amp;</span> y<span class="op">)</span></span>
<span id="cb12-75"><a href="#cb12-75"></a>      <span class="kw">requires</span> sized_sentinel_for<span class="op">&lt;</span>iterator_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="cb12-76"><a href="#cb12-76"></a></span>
<span id="cb12-77"><a href="#cb12-77"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> difference_type <span class="kw">operator</span><span class="op">-(</span>default_sentinel_t y, <span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> x<span class="op">)</span></span>
<span id="cb12-78"><a href="#cb12-78"></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="cb12-79"><a href="#cb12-79"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> difference_type <span class="kw">operator</span><span class="op">-(</span><span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> x, default_sentinel_t y<span class="op">)</span></span>
<span id="cb12-80"><a href="#cb12-80"></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="cb12-81"><a href="#cb12-81"></a></span>
<span id="cb12-82"><a href="#cb12-82"></a>    <span class="kw">friend</span> <span class="kw">constexpr</span> range_rvalue_reference_t<span class="op">&lt;</span><em>Base</em><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="cb12-83"><a href="#cb12-83"></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="cb12-84"><a href="#cb12-84"></a></span>
<span id="cb12-85"><a href="#cb12-85"></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="cb12-86"><a href="#cb12-86"></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="cb12-87"><a href="#cb12-87"></a>      <span class="kw">requires</span> indirectly_swappable<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span>;</span>
<span id="cb12-88"><a href="#cb12-88"></a>  <span class="op">}</span>;</span>
<span id="cb12-89"><a href="#cb12-89"></a><span class="op">}</span></span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_8" id="pnum_8">1</a></span> <code class="sourceCode cpp"><em>iterator</em><span class="op">::</span>iterator_concept</code> is defined as follows:</p>
<ul>
<li><p><span class="marginalizedparent"><a class="marginalized" href="#pnum_9" id="pnum_9">(1.1)</a></span> If <code class="sourceCode cpp"><em>Base</em></code> models <code class="sourceCode cpp">random_access_range</code>, then <code class="sourceCode cpp">iterator_concept</code> denotes <code class="sourceCode cpp">random_access_iterator_tag</code>.</p></li>
<li><p><span class="marginalizedparent"><a class="marginalized" href="#pnum_10" id="pnum_10">(1.2)</a></span> Otherwise, if <code class="sourceCode cpp"><em>Base</em></code> models <code class="sourceCode cpp">bidirectional_range</code>, then <code class="sourceCode cpp">iterator_concept</code> denotes <code class="sourceCode cpp">bidirectional_iterator_tag</code>.</p></li>
<li><p><span class="marginalizedparent"><a class="marginalized" href="#pnum_11" id="pnum_11">(1.3)</a></span> Otherwise, if <code class="sourceCode cpp"><em>Base</em></code> models <code class="sourceCode cpp">forward_range</code>, then <code class="sourceCode cpp">iterator_concept</code> denotes <code class="sourceCode cpp">forward_iterator_tag</code>.</p></li>
<li><p><span class="marginalizedparent"><a class="marginalized" href="#pnum_12" id="pnum_12">(1.4)</a></span> Otherwise, <code class="sourceCode cpp">iterator_concept</code> denotes <code class="sourceCode cpp">input_iterator_tag</code>.</p></li>
</ul>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_13" id="pnum_13">2</a></span> The member <em>typedef-name</em> <code class="sourceCode cpp">iterator_category</code> is defined if and only if <code class="sourceCode cpp"><em>Base</em></code> models <code class="sourceCode cpp">forward_range</code>. In that case, <code class="sourceCode cpp"><em>iterator</em><span class="op">::</span>iterator_category</code> is defined as follows:</p>
<ul>
<li><p><span class="marginalizedparent"><a class="marginalized" href="#pnum_14" id="pnum_14">(2.1)</a></span> Let <code class="sourceCode cpp">C</code> denote the type <code class="sourceCode cpp">iterator_traits<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;::</span>iterator_category</code>.</p></li>
<li><p><span class="marginalizedparent"><a class="marginalized" href="#pnum_15" id="pnum_15">(2.2)</a></span> If <code class="sourceCode cpp">C</code> models <code class="sourceCode cpp">derived_from<span class="op">&lt;</span>random_access_iterator_tag<span class="op">&gt;</span></code>, then <code class="sourceCode cpp">iterator_category</code> denotes <code class="sourceCode cpp">random_access_iterator_tag</code>.</p></li>
<li><p><span class="marginalizedparent"><a class="marginalized" href="#pnum_16" id="pnum_16">(2.3)</a></span> Otherwise, <code class="sourceCode cpp">iterator_category</code> denotes <code class="sourceCode cpp">C</code>.</p></li>
</ul>
<div>
<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">constexpr</span> <em>iterator</em><span class="op">(</span><em>Parent</em><span class="op">*</span> parent, iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> current,</span>
<span id="cb13-2"><a href="#cb13-2"></a>                   range_difference_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> missing <span class="op">=</span> <span class="dv">0</span><span class="op">)</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_17" id="pnum_17">3</a></span> <em>Effects</em>: Initializes <code class="sourceCode cpp"><em>current_</em></code> with <code class="sourceCode cpp">std<span class="op">::</span>move<span class="op">(</span>current<span class="op">)</span></code>, <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">-&gt;</span><em>base_</em><span class="op">)</span></code>, <code class="sourceCode cpp"><em>stride_</em></code> with <code class="sourceCode cpp">parent<span class="op">-&gt;</span><em>stride_</em></code>, and <code class="sourceCode cpp"><em>missing_</em></code> with <code class="sourceCode cpp">missing</code>.</p>
</blockquote>
<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> <em>iterator</em><span class="op">(</span><em>iterator</em><span class="op">&lt;!</span>Const<span class="op">&gt;</span> i<span class="op">)</span></span>
<span id="cb14-2"><a href="#cb14-2"></a>  <span class="kw">requires</span> Const <span class="op">&amp;&amp;</span> convertible_to<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span>V<span class="op">&gt;</span>, iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span></span>
<span id="cb14-3"><a href="#cb14-3"></a>                 <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>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_18" id="pnum_18">4</a></span> <em>Effects</em>: Initializes <code class="sourceCode cpp"><em>current_</em></code> with <code class="sourceCode cpp">std<span class="op">::</span>move<span class="op">(</span>i<span class="op">.</span><em>current_</em><span class="op">)</span></code>, <code class="sourceCode cpp"><em>end_</em></code> with <code class="sourceCode cpp">std<span class="op">::</span>move<span class="op">(</span>i<span class="op">.</span><em>end_</em><span class="op">)</span></code>, <code class="sourceCode cpp"><em>stride_</em></code> with <code class="sourceCode cpp">i<span class="op">.</span><em>stride_</em></code>, and <code class="sourceCode cpp"><em>missing_</em></code> with <code class="sourceCode cpp">i<span class="op">.</span><em>missing_</em></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><em>Base</em><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_19" id="pnum_19">5</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><em>Base</em><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_20" id="pnum_20">6</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_21" id="pnum_21">7</a></span> <em>Preconditions:</em> <code class="sourceCode cpp"><em>current_</em> <span class="op">!=</span> <em>end_</em></code> is <code class="sourceCode cpp"><span class="kw">true</span></code>.</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_22" id="pnum_22">8</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>   <em>missing_</em> <span class="op">=</span> ranges<span class="op">::</span>advance<span class="op">(</span><em>current_</em>, <em>stride_</em>, <em>end_</em><span class="op">)</span>;</span>
<span id="cb18-2"><a href="#cb18-2"></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_23" id="pnum_23">9</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> <em>iterator</em> <span class="kw">operator</span><span class="op">++(</span><span class="dt">int</span><span class="op">)</span> <span class="kw">requires</span> forward_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_24" id="pnum_24">10</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="kw">auto</span> tmp <span class="op">=</span> <span class="op">*</span><span class="kw">this</span>;</span>
<span id="cb21-2"><a href="#cb21-2"></a>  <span class="op">++*</span><span class="kw">this</span>;</span>
<span id="cb21-3"><a href="#cb21-3"></a>  <span class="cf">return</span> tmp;</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">constexpr</span> <em>iterator</em><span class="op">&amp;</span> <span class="kw">operator</span><span class="op">--()</span> <span class="kw">requires</span> bidirectional_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_25" id="pnum_25">11</a></span> <em>Effects:</em> Equivalent to:</p>
<blockquote>
<div class="sourceCode" id="cb23"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb23-1"><a href="#cb23-1"></a>   ranges<span class="op">::</span>advance<span class="op">(</span><em>current_</em>, <em>missing_</em> <span class="op">-</span> <em>stride_</em><span class="op">)</span>;</span>
<span id="cb23-2"><a href="#cb23-2"></a>   <em>missing_</em> <span class="op">=</span> <span class="dv">0</span>;</span>
<span id="cb23-3"><a href="#cb23-3"></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="cb24"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb24-1"><a href="#cb24-1"></a><span class="kw">constexpr</span> <em>iterator</em> <span class="kw">operator</span><span class="op">--(</span><span class="dt">int</span><span class="op">)</span> <span class="kw">requires</span> bidirectional_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span></code></pre></div>
<blockquote>
<blockquote>
<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">auto</span> tmp <span class="op">=</span> <span class="op">*</span><span class="kw">this</span>;</span>
<span id="cb25-2"><a href="#cb25-2"></a>  <span class="op">--*</span><span class="kw">this</span>;</span>
<span id="cb25-3"><a href="#cb25-3"></a>  <span class="cf">return</span> tmp;</span></code></pre></div>
</blockquote>
</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> <em>iterator</em><span class="op">&amp;</span> <span class="kw">operator</span><span class="op">+=(</span>difference_type n<span class="op">)</span> <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_26" id="pnum_26">12</a></span> <em>Preconditions</em>: If <code class="sourceCode cpp">n</code> is positive, <code class="sourceCode cpp">ranges<span class="op">::</span>distance<span class="op">(</span><em>current_</em>, <em>end_</em><span class="op">)</span> <span class="op">&gt;</span> <em>stride_</em> <span class="op">*</span> <span class="op">(</span>n <span class="op">-</span> <span class="dv">1</span><span class="op">)</span></code> is <code class="sourceCode cpp"><span class="kw">true</span></code>. <span class="note"><span>[ <em>Note 1:</em> </span>If <code class="sourceCode cpp">n</code> is negative, the <em>Effects:</em> paragraph implies a precondition.<span> — <em>end note</em> ]</span></span></p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_27" id="pnum_27">13</a></span> <em>Effects:</em> Equivalent to:</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="cf">if</span> <span class="op">(</span>n <span class="op">&gt;</span> <span class="dv">0</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb27-2"><a href="#cb27-2"></a>  <em>missing_</em> <span class="op">=</span> ranges<span class="op">::</span>advance<span class="op">(</span><em>current_</em>, <em>stride_</em> <span class="op">*</span> n, <em>end_</em><span class="op">)</span>;</span>
<span id="cb27-3"><a href="#cb27-3"></a><span class="op">}</span></span>
<span id="cb27-4"><a href="#cb27-4"></a><span class="cf">else</span> <span class="cf">if</span> <span class="op">(</span>n <span class="op">&lt;</span> <span class="dv">0</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb27-5"><a href="#cb27-5"></a>  ranges<span class="op">::</span>advance<span class="op">(</span><em>current_</em>, <em>stride_</em> <span class="op">*</span> n <span class="op">+</span> <em>missing_</em><span class="op">)</span>;</span>
<span id="cb27-6"><a href="#cb27-6"></a>  <em>missing_</em> <span class="op">=</span> <span class="dv">0</span>;</span>
<span id="cb27-7"><a href="#cb27-7"></a><span class="op">}</span></span>
<span id="cb27-8"><a href="#cb27-8"></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="cb28"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb28-1"><a href="#cb28-1"></a><span class="kw">constexpr</span> <em>iterator</em><span class="op">&amp;</span> <span class="kw">operator</span><span class="op">-=(</span>difference_type x<span class="op">)</span></span>
<span id="cb28-2"><a href="#cb28-2"></a>  <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_28" id="pnum_28">14</a></span> <em>Effects:</em> Equivalent to: <code class="sourceCode cpp"><span class="cf">return</span> <span class="op">*</span><span class="kw">this</span> <span class="op">+=</span> <span class="op">-</span>x;</code></p>
</blockquote>
<div class="sourceCode" id="cb29"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb29-1"><a href="#cb29-1"></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, default_sentinel<span class="op">)</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_29" id="pnum_29">15</a></span> <em>Returns:</em> <code class="sourceCode cpp">x<span class="op">.</span><em>current_</em> <span class="op">==</span> x<span class="op">.</span><em>end_</em>;</code></p>
</blockquote>
<div class="sourceCode" id="cb30"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb30-1"><a href="#cb30-1"></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>iterator</em><span class="op">&amp;</span> y<span class="op">)</span></span>
<span id="cb30-2"><a href="#cb30-2"></a>      <span class="kw">requires</span> equality_comparable<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_30" id="pnum_30">16</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>current_</em></code>.</p>
</blockquote>
<div class="sourceCode" id="cb31"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb31-1"><a href="#cb31-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">&lt;(</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="cb31-2"><a href="#cb31-2"></a>  <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_31" id="pnum_31">17</a></span> <em>Returns</em>: <code class="sourceCode cpp">x<span class="op">.</span><em>current_</em> <span class="op">&lt;</span> y<span class="op">.</span><em>current_</em></code>.</p>
</blockquote>
<div class="sourceCode" id="cb32"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb32-1"><a href="#cb32-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">&gt;(</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="cb32-2"><a href="#cb32-2"></a>  <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_32" id="pnum_32">18</a></span> <em>Effects</em>: Equivalent to: <code class="sourceCode cpp"><span class="cf">return</span> y <span class="op">&lt;</span> x;</code></p>
</blockquote>
<div class="sourceCode" id="cb33"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb33-1"><a href="#cb33-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">&lt;=(</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="cb33-2"><a href="#cb33-2"></a>  <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_33" id="pnum_33">19</a></span> <em>Effects</em>: Equivalent to: <code class="sourceCode cpp"><span class="cf">return</span> <span class="op">!(</span>y <span class="op">&lt;</span> x<span class="op">)</span>;</code></p>
</blockquote>
<div class="sourceCode" id="cb34"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb34-1"><a href="#cb34-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">&gt;=(</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="cb34-2"><a href="#cb34-2"></a>  <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_34" id="pnum_34">20</a></span> <em>Effects</em>: Equivalent to: <code class="sourceCode cpp"><span class="cf">return</span> <span class="op">!(</span>x <span class="op">&lt;</span> y<span class="op">)</span>;</code></p>
</blockquote>
<div class="sourceCode" id="cb35"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb35-1"><a href="#cb35-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">&lt;=&gt;(</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="cb35-2"><a href="#cb35-2"></a>  <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span> <span class="op">&amp;&amp;</span></span>
<span id="cb35-3"><a href="#cb35-3"></a>           three_way_comparable<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_35" id="pnum_35">21</a></span> <em>Returns</em>: <code class="sourceCode cpp">x<span class="op">.</span><em>current_</em> <span class="op">&lt;=&gt;</span> y<span class="op">.</span><em>current_</em></code>.</p>
</blockquote>
<div class="sourceCode" id="cb36"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb36-1"><a href="#cb36-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> <em>iterator</em> <span class="kw">operator</span><span class="op">+(</span><span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> i, difference_type n<span class="op">)</span></span>
<span id="cb36-2"><a href="#cb36-2"></a>  <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span>
<span id="cb36-3"><a href="#cb36-3"></a><span class="kw">friend</span> <span class="kw">constexpr</span> <em>iterator</em> <span class="kw">operator</span><span class="op">+(</span>difference_type n, <span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> i<span class="op">)</span></span>
<span id="cb36-4"><a href="#cb36-4"></a>  <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_36" id="pnum_36">22</a></span> <em>Effects</em>: Equivalent to:</p>
<blockquote>
<div class="sourceCode" id="cb37"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb37-1"><a href="#cb37-1"></a>  <span class="kw">auto</span> r <span class="op">=</span> i;</span>
<span id="cb37-2"><a href="#cb37-2"></a>  r <span class="op">+=</span> n;</span>
<span id="cb37-3"><a href="#cb37-3"></a>  <span class="cf">return</span> r;</span></code></pre></div>
</blockquote>
</blockquote>
<div class="sourceCode" id="cb38"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb38-1"><a href="#cb38-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> <em>iterator</em> <span class="kw">operator</span><span class="op">-(</span><span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> i, difference_type n<span class="op">)</span></span>
<span id="cb38-2"><a href="#cb38-2"></a>  <span class="kw">requires</span> random_access_range<span class="op">&lt;</span><em>Base</em><span class="op">&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_37" id="pnum_37">23</a></span> <em>Effects</em>: Equivalent to:</p>
<blockquote>
<div class="sourceCode" id="cb39"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb39-1"><a href="#cb39-1"></a>  <span class="kw">auto</span> r <span class="op">=</span> i;</span>
<span id="cb39-2"><a href="#cb39-2"></a>  r <span class="op">-=</span> n;</span>
<span id="cb39-3"><a href="#cb39-3"></a>  <span class="cf">return</span> r;</span></code></pre></div>
</blockquote>
</blockquote>
<div class="sourceCode" id="cb40"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb40-1"><a href="#cb40-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> difference_type <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>iterator</em><span class="op">&amp;</span> y<span class="op">)</span></span>
<span id="cb40-2"><a href="#cb40-2"></a>  <span class="kw">requires</span> sized_sentinel_for<span class="op">&lt;</span>iterator_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>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_38" id="pnum_38">24</a></span> <em>Returns:</em> Let <code class="sourceCode cpp">N</code> be <code class="sourceCode cpp">x<span class="op">.</span><em>current_</em> <span class="op">-</span> y<span class="op">.</span><em>current_</em></code>.</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_39" id="pnum_39">(24.1)</a></span> If <code class="sourceCode cpp"><em>Base</em></code> models <code class="sourceCode cpp">forward_range</code>, <code class="sourceCode cpp"><span class="op">(</span>N <span class="op">+</span> x<span class="op">.</span><em>missing_</em> <span class="op">-</span> y<span class="op">.</span><em>missing_</em><span class="op">)</span> <span class="op">/</span> x<span class="op">.</span><em>stride_</em></code>.</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_40" id="pnum_40">(24.2)</a></span> Otherwise, if <code class="sourceCode cpp">N</code> is negative, <code class="sourceCode cpp"><span class="op">-</span><em>div-ceil</em><span class="op">(-</span>N, x<span class="op">.</span><em>stride_</em><span class="op">)</span></code>.</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_41" id="pnum_41">(24.3)</a></span> Otherwise, <code class="sourceCode cpp"><em>div-ceil</em><span class="op">(</span>N, x<span class="op">.</span><em>stride_</em><span class="op">)</span></code>.</li>
</ul>
<p><span class="draftnote" style="color: #01796F">[ Drafting note: When <code class="sourceCode cpp"><em>Base</em></code> is input-only, the value of <code class="sourceCode cpp"><em>missing_</em></code> is unreliable. ]</span></p>
</blockquote>
<div class="sourceCode" id="cb41"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb41-1"><a href="#cb41-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> difference_type <span class="kw">operator</span><span class="op">-(</span>default_sentinel_t y, <span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> x<span class="op">)</span></span>
<span id="cb41-2"><a href="#cb41-2"></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>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_42" id="pnum_42">25</a></span> <em>Returns</em>: <code class="sourceCode cpp"><em>div-ceil</em><span class="op">(</span>x<span class="op">.</span><em>end_</em> <span class="op">-</span> x<span class="op">.</span><em>current_</em>, x<span class="op">.</span><em>stride_</em><span class="op">)</span></code>.</p>
</blockquote>
<div class="sourceCode" id="cb42"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb42-1"><a href="#cb42-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> difference_type <span class="kw">operator</span><span class="op">-(</span><span class="kw">const</span> <em>iterator</em><span class="op">&amp;</span> x, default_sentinel_t y<span class="op">)</span></span>
<span id="cb42-2"><a href="#cb42-2"></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>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_43" id="pnum_43">26</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 class="sourceCode" id="cb43"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb43-1"><a href="#cb43-1"></a><span class="kw">friend</span> <span class="kw">constexpr</span> range_rvalue_reference_t<span class="op">&lt;</span><em>Base</em><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="cb43-2"><a href="#cb43-2"></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_44" id="pnum_44">27</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="cb44"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb44-1"><a href="#cb44-1"></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="cb44-2"><a href="#cb44-2"></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="cb44-3"><a href="#cb44-3"></a>  <span class="kw">requires</span> indirectly_swappable<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span><em>Base</em><span class="op">&gt;&gt;</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_45" id="pnum_45">28</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>
<h2 data-number="6.3" id="feature-test-macro"><span class="header-section-number">6.3</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>, header <code class="sourceCode cpp"><span class="op">&lt;</span>version<span class="op">&gt;</span></code> synopsis, with the value selected by the editor to reflect the date of adoption of this paper:</p>
<div class="sourceCode" id="cb45"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb45-1"><a href="#cb45-1"></a><span class="pp">#define __cpp_lib_ranges_stride </span><span class="dv">20</span><span class="er">XXXXL</span><span class="pp"> </span><span class="co">// also in &lt;ranges&gt;</span></span></code></pre></div>
<h1 data-number="7" style="border-bottom:1px solid #cccccc" id="acknowledgements"><span class="header-section-number">7</span> Acknowledgements<a href="#acknowledgements" class="self-link"></a></h1>
<p>The author would like to thank Tristan Brindle for providing editorial commentary on P1899, and also those who reviewed material for, or attended the aforementioned CppCon session or post-conference class, for their input on the design of the proposed <code class="sourceCode cpp">stride_view</code>.</p>
<h1 data-number="8" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">8</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-P2442R1">
<p>[P2442R1] Tim Song. 2021. Windowing range adaptors: <code class="sourceCode cpp">views<span class="op">::</span>chunk</code> and <code class="sourceCode cpp">views<span class="op">::</span>slide</code>. <br />
<a href="https://wg21.link/P2442R1">https://wg21.link/P2442R1</a></p>
</div>
</div>
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