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<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
<meta name="dcterms.date" content="2020-02-19" />
<title>Conditionally borrowed ranges</title>
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<h1 class="title" style="text-align:center">Conditionally borrowed ranges</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2017R1</td>
</tr>
<tr>
<td>Date:</td>
<td>2020-02-19</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>
LWG<br>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to:</td>
<td>
Barry Revzin<br><<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.com</a>><br>
</td>
</tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#revision-history"><span class="toc-section-number">1</span> Revision History<span></span></a></li>
<li><a href="#introduction-and-motivation"><span class="toc-section-number">2</span> Introduction and Motivation<span></span></a></li>
<li><a href="#history-and-status-quo"><span class="toc-section-number">3</span> History and Status Quo<span></span></a>
<ul>
<li><a href="#implementation-strategy"><span class="toc-section-number">3.1</span> Implementation Strategy<span></span></a></li>
</ul></li>
<li><a href="#proposal"><span class="toc-section-number">4</span> Proposal<span></span></a></li>
<li><a href="#wording"><span class="toc-section-number">5</span> Wording<span></span></a></li>
<li><a href="#implementation"><span class="toc-section-number">6</span> Implementation<span></span></a></li>
<li><a href="#bibliography"><span class="toc-section-number">7</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history" data-number="1" style="border-bottom:1px solid #cccccc"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p>R0 to R1: <code class="sourceCode cpp">safe_range</code> was renamed to <code class="sourceCode cpp">borrowed_range</code> in Prague by resolving <span class="citation" data-cites="LWG3379">[<a href="#ref-LWG3379" role="doc-biblioref">LWG3379</a>]</span>. This paper has been updated to reflect that.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction-and-motivation" data-number="2" style="border-bottom:1px solid #cccccc"><span class="header-section-number">2</span> Introduction and Motivation<a href="#introduction-and-motivation" class="self-link"></a></h1>
<p>Consider the following approach to trimming a <code class="sourceCode cpp">std<span class="op">::</span>string</code>:</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">auto</span> trim<span class="op">(</span>std<span class="op">::</span>string <span class="kw">const</span><span class="op">&</span> s<span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a> <span class="kw">auto</span> isalpha <span class="op">=</span> <span class="op">[](</span><span class="dt">unsigned</span> <span class="dt">char</span> c<span class="op">){</span> <span class="cf">return</span> std<span class="op">::</span>isalpha<span class="op">(</span>c<span class="op">)</span>; <span class="op">}</span>;</span>
<span id="cb1-3"><a href="#cb1-3"></a> <span class="kw">auto</span> b <span class="op">=</span> ranges<span class="op">::</span>find_if<span class="op">(</span>s, isalpha<span class="op">)</span>;</span>
<span id="cb1-4"><a href="#cb1-4"></a> <span class="kw">auto</span> e <span class="op">=</span> ranges<span class="op">::</span>find_if<span class="op">(</span>s <span class="op">|</span> views<span class="op">::</span>reverse, isalpha<span class="op">).</span>base<span class="op">()</span>;</span>
<span id="cb1-5"><a href="#cb1-5"></a> <span class="cf">return</span> subrange<span class="op">(</span>b, e<span class="op">)</span>;</span>
<span id="cb1-6"><a href="#cb1-6"></a><span class="op">}</span></span></code></pre></div>
<p>This is a fairly nice and, importantly, safe way to implement <code class="sourceCode cpp">trim</code>. The iterators <code class="sourceCode cpp">b</code> and <code class="sourceCode cpp">e</code> returned from <code class="sourceCode cpp">find_if</code> will not dangle, since they point into the string <code class="sourceCode cpp">s</code> whose lifetime outlives the function.</p>
<p>Except this code will not compile at the moment, either in C++20 or in range-v3, failing on the declaration of <code class="sourceCode cpp">e</code>. The algorithm <code class="sourceCode cpp">find_if</code> is in <span>25.5.5 <a href="https://wg21.link/alg.find">[alg.find]</a></span> is declared as:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="kw">template</span><span class="op"><</span>input_range R, <span class="kw">class</span> Proj <span class="op">=</span> identity,</span>
<span id="cb2-2"><a href="#cb2-2"></a> indirect_unary_predicate<span class="op"><</span>projected<span class="op"><</span>iterator_t<span class="op"><</span>R<span class="op">></span>, Proj<span class="op">>></span> Pred<span class="op">></span></span>
<span id="cb2-3"><a href="#cb2-3"></a> <span class="kw">constexpr</span> borrowed_iterator_t<span class="op"><</span>R<span class="op">></span></span>
<span id="cb2-4"><a href="#cb2-4"></a> ranges<span class="op">::</span>find_if<span class="op">(</span>R<span class="op">&&</span> r, Pred pred, Proj proj <span class="op">=</span> <span class="op">{})</span>;</span></code></pre></div>
<p><code class="sourceCode cpp">R</code> will deduce as <code class="sourceCode cpp">reverse_view<span class="op"><</span>ref_view<span class="op"><</span>std<span class="op">::</span>string <span class="kw">const</span><span class="op">>></span></code>, which does <em>not</em> satisfy <code class="sourceCode cpp">borrowed_range</code> (it is neither an lvalue reference, nor does <code class="sourceCode cpp">reverse_view</code> currently opt-in to being a <code class="sourceCode cpp">borrowed_range</code>) hence the return type <code class="sourceCode cpp">borrowed_iterator_t<span class="op"><</span>R<span class="op">></span></code> is the type <code class="sourceCode cpp">dangling</code> rather than being the type <code class="sourceCode cpp">iterator_t<span class="op"><</span>R<span class="op">></span></code>. Instead of getting the reverse iterator we might have expected, that we need to call <code class="sourceCode cpp"><span class="op">.</span>base<span class="op">()</span></code> on, we get effectively nothing. We are forced to rewrite the above as:</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> trim<span class="op">(</span>std<span class="op">::</span>string <span class="kw">const</span><span class="op">&</span> s<span class="op">)</span> <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2"></a> <span class="kw">auto</span> isalpha <span class="op">=</span> <span class="op">[](</span><span class="dt">unsigned</span> <span class="dt">char</span> c<span class="op">){</span> <span class="cf">return</span> std<span class="op">::</span>isalpha<span class="op">(</span>c<span class="op">)</span>; <span class="op">}</span>;</span>
<span id="cb3-3"><a href="#cb3-3"></a> <span class="kw">auto</span> b <span class="op">=</span> ranges<span class="op">::</span>find_if<span class="op">(</span>s, isalpha<span class="op">)</span>;</span>
<span id="cb3-4"><a href="#cb3-4"></a> <span class="kw">auto</span> reversed <span class="op">=</span> s <span class="op">|</span> views<span class="op">::</span>reverse;</span>
<span id="cb3-5"><a href="#cb3-5"></a> <span class="kw">auto</span> e <span class="op">=</span> ranges<span class="op">::</span>find_if<span class="op">(</span>reversed, isalpha<span class="op">).</span>base<span class="op">()</span>;</span>
<span id="cb3-6"><a href="#cb3-6"></a> <span class="cf">return</span> subrange<span class="op">(</span>b, e<span class="op">)</span>;</span>
<span id="cb3-7"><a href="#cb3-7"></a><span class="op">}</span></span></code></pre></div>
<p>Which is an unnecessary code indirection. The goal of this paper is to make the initial example just work. We clearly have a borrowed range that is not marked as such, so I consider this to be a library defect.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="history-and-status-quo" data-number="3" style="border-bottom:1px solid #cccccc"><span class="header-section-number">3</span> History and Status Quo<a href="#history-and-status-quo" class="self-link"></a></h1>
<p>Ranges introduced with it the concept <em><code class="sourceCode cpp">forwarding<span class="op">-</span>range</code></em>. This was then renamed to <code class="sourceCode cpp">safe_range</code> by <span class="citation" data-cites="P1870R1">[<a href="#ref-P1870R1" role="doc-biblioref">P1870R1</a>]</span> in Belfast, and again to <code class="sourceCode cpp">borrowed_range</code> by resolving <span class="citation" data-cites="LWG3379">[<a href="#ref-LWG3379" role="doc-biblioref">LWG3379</a>]</span> in Prague, but the core concept remains the same. A range is a <code class="sourceCode cpp">borrowed_range</code> when you can hold onto its iterators after the range goes out of scope. There are two kinds of borrowed ranges:</p>
<ul>
<li>lvalue references to ranges are always borrowed ranges. It’s not the lvalue reference itself which owns the data, so if the reference dies, we’re fine.</li>
<li>ranges which must necessarily opt in to being considered borrowed, by way of the new <code class="sourceCode cpp">enable_borrowed_range</code> variable template, which defaults to <code class="sourceCode cpp"><span class="kw">false</span></code> for all types.</li>
</ul>
<p>There are several borrowed ranges which do this opt in today:</p>
<ul>
<li><code class="sourceCode cpp">ref_view</code> is a borrowed range. It’s basically a reference - the iterators it gives out are iterators to the range it refers to, which it has no ownership of. If the <code class="sourceCode cpp">ref_view</code> dies, the referred to range can still be around.</li>
<li><code class="sourceCode cpp">string_view</code> is a borrowed range. Like <code class="sourceCode cpp">ref_view</code>, it just refers to data - the iterators it gives out are iterators into some other containers. <code class="sourceCode cpp">span</code> and <code class="sourceCode cpp">subrange</code> are similar.</li>
<li><code class="sourceCode cpp">empty_view</code> doesn’t even have any data, so it’s trivially borrowed.</li>
<li><code class="sourceCode cpp">iota_view</code> works by having the iterators themselves own the “counter”, so having the iterators stick around is sufficient.</li>
</ul>
<p>And that’s it. We have six, <em>unconditionally</em> borrowed ranges. All other ranges and views in the standard library are <em>unconditionally</em> not borrowed. But this is far too strict. As the opening example demonstrates, there are many more kinds of borrowed ranges you can construct than <em>just</em> the chosen six.</p>
<p>This issue was first pointed out by Johel Ernesto Guerrero Peña in <span class="citation" data-cites="stl2.640">[<a href="#ref-stl2.640" role="doc-biblioref">stl2.640</a>]</span>.</p>
<h2 data-number="3.1" id="implementation-strategy" data-number="3.1"><span class="header-section-number">3.1</span> Implementation Strategy<a href="#implementation-strategy" class="self-link"></a></h2>
<p>A range is going to be borrowed if its iterators do not in any way refer to it. For the ranges in the working draft which are unconditionally borrowed, this follows directly from how they actually work. But for some other ranges, it might depend on implementation strategy.</p>
<p>One can imagine different specifications for views like <code class="sourceCode cpp">transform_view</code> and even <code class="sourceCode cpp">filter_view</code> that might allow them to be borrowed, but that’s beyond the scope of this paper which is limited to those views that are already borrowed as specified.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="proposal" data-number="4" style="border-bottom:1px solid #cccccc"><span class="header-section-number">4</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>Several range adapters semantically behave as if they have a single member of some templated view type. If that underlying view type is a <code class="sourceCode cpp">borrowed_range</code>, the range adapter itself can be transitively borrowed. For example, <code class="sourceCode cpp">s <span class="op">|</span> views<span class="op">::</span>reverse</code> has the type <code class="sourceCode cpp">reverse_view<span class="op"><</span>ref_view<span class="op"><</span>string <span class="kw">const</span><span class="op">>></span></code>. This can be a <code class="sourceCode cpp">borrowed_range</code> because <code class="sourceCode cpp">ref_view<span class="op"><</span>string <span class="kw">const</span><span class="op">></span></code> is a <code class="sourceCode cpp">borrowed_range</code>. Likewise, <code class="sourceCode cpp">s <span class="op">|</span> views<span class="op">::</span>reverse <span class="op">|</span> views<span class="op">::</span>take<span class="op">(</span><span class="dv">3</span><span class="op">)</span></code> can also be a <code class="sourceCode cpp">borrowed_range</code> by extending this logic further.</p>
<p>Here is a table of all the range adapters and factories in the current working draft, what their current <code class="sourceCode cpp">borrowed_range</code> status is, and what this paper proposes.</p>
<table>
<tr>
<th>
Name
</th>
<th>
Current Status
</th>
<th>
Proposed
</th>
</tr>
<tr>
<td>
<code class="sourceCode cpp">empty</code>
</td>
<td>
Borrowed
</td>
<td>
No change
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">single</code>
</td>
<td>
Not Borrowed
</td>
<td>
No change. It’s the view which holds the element, not the iterators.
</th>
</tr>
<tr>
<td>
<code class="sourceCode cpp">iota</code>
</td>
<td>
Borrowed
</td>
<td>
No change.
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">istream</code>
</td>
<td>
Not Borrowed
</th>
<td>
No change. The iterators need to refer to parent view, which holds onto the element.
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">ref</code>
</td>
<td>
Borrowed
</td>
<td>
No change.
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">filter</code>
</td>
<td>
Not Borrowed
</th>
<td>
No change. The view needs to own the predicate.
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">transform</code>
</td>
<td>
Not Borrowed
</th>
<td>
No change, as above.
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">take</code>
</td>
<td>
Not Borrowed
</th>
<td>
<span class="addu">Conditionally borrowed, based on the underlying view. The iterators are just iterators into the underlying view (or thin wrappers thereof).</span>.
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">take_while</code>
</td>
<td>
Not Borrowed
</th>
<td>
No change, same as <code class="sourceCode cpp">filter</code>.
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">drop</code>
</td>
<td>
Not Borrowed
</th>
<td>
<span class="addu">Conditionally borrowed, same as <code class="sourceCode cpp">take</code></span>
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">drop_while</code>
</td>
<td>
Not Borrowed
</th>
<td>
<span class="addu">Conditionally borrowed. Unlike <code class="sourceCode cpp">take_while</code> or <code class="sourceCode cpp">filter</code>, we only need the predicate to find the new begin. Once we found it, it’s just transparent.</span>
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">join</code>
</td>
<td>
Not Borrowed
</th>
<td>
No change. This one is quite complex and iterators need to refer the <code class="sourceCode cpp">join_view</code>.
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">split</code>
</td>
<td>
Not Borrowed
</th>
<td>
No change, as with <code class="sourceCode cpp">join</code>.
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">counted</code>
</td>
<td colspan="2">
Not actually its own view, <code class="sourceCode cpp">counted<span class="op">(</span>r, n<span class="op">)</span></code> is actually either some <code class="sourceCode cpp">subrange</code> or ill-formed, so it’s already borrowed.
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">common</code>
</td>
<td>
Not Borrowed
</th>
<td>
<span class="addu">Conditionally borrowed based on the underlying view. All it does is propagate iterators.</span>
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">reverse</code>
</td>
<td>
Not Borrowed
</th>
<td>
<span class="addu">Conditionally borrowed based on the underlying view. All it does is propagate reverse iterators.</span>
</td>
</tr>
<tr>
<td>
<code class="sourceCode cpp">elements</code>/<code class="sourceCode cpp">keys</code>/<code class="sourceCode cpp">values</code>
</td>
<td>
Not Borrowed
</th>
<td>
<span class="addu">Conditionally borrowed based on the underlying view. This is a special case of <code class="sourceCode cpp">transform_view</code> where the transform is actually encoded into the type, so it doesn’t need to be held onto by the view itself.</span>
</td>
</tr>
</table>
<h1 data-number="5" style="border-bottom:1px solid #cccccc" id="wording" data-number="5" style="border-bottom:1px solid #cccccc"><span class="header-section-number">5</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Add six variable template specializations to <span>24.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a></span>:</p>
<div>
<div class="sourceCode" id="cb4"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb4-1"><a href="#cb4-1"></a>#include <initializer_list></span>
<span id="cb4-2"><a href="#cb4-2"></a>#include <iterator></span>
<span id="cb4-3"><a href="#cb4-3"></a></span>
<span id="cb4-4"><a href="#cb4-4"></a>namespace std::ranges {</span>
<span id="cb4-5"><a href="#cb4-5"></a> // [...]</span>
<span id="cb4-6"><a href="#cb4-6"></a> </span>
<span id="cb4-7"><a href="#cb4-7"></a></span>
<span id="cb4-8"><a href="#cb4-8"></a> // [range.take], take view</span>
<span id="cb4-9"><a href="#cb4-9"></a> template<view> class take_view;</span>
<span id="cb4-10"><a href="#cb4-10"></a> </span>
<span id="cb4-11"><a href="#cb4-11"></a><span class="va">+ template<class T></span></span>
<span id="cb4-12"><a href="#cb4-12"></a><span class="va">+ inline constexpr bool enable_borrowed_range<take_view<T>> = enable_borrowed_range<T>; </span></span>
<span id="cb4-13"><a href="#cb4-13"></a></span>
<span id="cb4-14"><a href="#cb4-14"></a> namespace views { inline constexpr <em>unspecified</em> take = <em>unspecified</em>; } </span>
<span id="cb4-15"><a href="#cb4-15"></a> </span>
<span id="cb4-16"><a href="#cb4-16"></a> // [...]</span>
<span id="cb4-17"><a href="#cb4-17"></a> </span>
<span id="cb4-18"><a href="#cb4-18"></a> // [range.drop], drop view</span>
<span id="cb4-19"><a href="#cb4-19"></a> template<view V></span>
<span id="cb4-20"><a href="#cb4-20"></a> class drop_view;</span>
<span id="cb4-21"><a href="#cb4-21"></a> </span>
<span id="cb4-22"><a href="#cb4-22"></a><span class="va">+ template<class T></span></span>
<span id="cb4-23"><a href="#cb4-23"></a><span class="va">+ inline constexpr bool enable_borrowed_range<drop_view<T>> = enable_borrowed_range<T>; </span></span>
<span id="cb4-24"><a href="#cb4-24"></a></span>
<span id="cb4-25"><a href="#cb4-25"></a> namespace views { inline constexpr <em>unspecified</em> drop = <em>unspecified</em>; } </span>
<span id="cb4-26"><a href="#cb4-26"></a> </span>
<span id="cb4-27"><a href="#cb4-27"></a> // [range.drop.while], drop while view</span>
<span id="cb4-28"><a href="#cb4-28"></a> template<view V, class Pred></span>
<span id="cb4-29"><a href="#cb4-29"></a> requires input_range<V> && is_object_v<Pred> &&</span>
<span id="cb4-30"><a href="#cb4-30"></a> indirect_unary_predicate<const Pred, iterator_t<V>></span>
<span id="cb4-31"><a href="#cb4-31"></a> class drop_while_view;</span>
<span id="cb4-32"><a href="#cb4-32"></a></span>
<span id="cb4-33"><a href="#cb4-33"></a><span class="va">+ template<class T, class Pred></span></span>
<span id="cb4-34"><a href="#cb4-34"></a><span class="va">+ inline constexpr bool enable_borrowed_range<drop_while_view<T, Pred>> = enable_borrowed_range<T>; </span></span>
<span id="cb4-35"><a href="#cb4-35"></a></span>
<span id="cb4-36"><a href="#cb4-36"></a> namespace views { inline constexpr <em>unspecified</em> drop_while = <em>unspecified</em>; }</span>
<span id="cb4-37"><a href="#cb4-37"></a></span>
<span id="cb4-38"><a href="#cb4-38"></a> // [...] </span>
<span id="cb4-39"><a href="#cb4-39"></a> </span>
<span id="cb4-40"><a href="#cb4-40"></a> // [range.common], common view</span>
<span id="cb4-41"><a href="#cb4-41"></a> template<view V></span>
<span id="cb4-42"><a href="#cb4-42"></a> requires (!common_range<V> && copyable<iterator_t<V>>)</span>
<span id="cb4-43"><a href="#cb4-43"></a> class common_view;</span>
<span id="cb4-44"><a href="#cb4-44"></a> </span>
<span id="cb4-45"><a href="#cb4-45"></a><span class="va">+ template<class T></span></span>
<span id="cb4-46"><a href="#cb4-46"></a><span class="va">+ inline constexpr bool enable_borrowed_range<common_view<T>> = enable_borrowed_range<T>; </span></span>
<span id="cb4-47"><a href="#cb4-47"></a></span>
<span id="cb4-48"><a href="#cb4-48"></a> namespace views { inline constexpr <em>unspecified</em> common = <em>unspecified</em>; }</span>
<span id="cb4-49"><a href="#cb4-49"></a></span>
<span id="cb4-50"><a href="#cb4-50"></a> // [range.reverse], reverse view</span>
<span id="cb4-51"><a href="#cb4-51"></a> template<view V></span>
<span id="cb4-52"><a href="#cb4-52"></a> requires bidirectional_range<V></span>
<span id="cb4-53"><a href="#cb4-53"></a> class reverse_view;</span>
<span id="cb4-54"><a href="#cb4-54"></a> </span>
<span id="cb4-55"><a href="#cb4-55"></a><span class="va">+ template<class T></span></span>
<span id="cb4-56"><a href="#cb4-56"></a><span class="va">+ inline constexpr bool enable_borrowed_range<reverse_view<T>> = enable_borrowed_range<T>; </span></span>
<span id="cb4-57"><a href="#cb4-57"></a></span>
<span id="cb4-58"><a href="#cb4-58"></a> namespace views { inline constexpr <em>unspecified</em> reverse = <em>unspecified</em>; }</span>
<span id="cb4-59"><a href="#cb4-59"></a></span>
<span id="cb4-60"><a href="#cb4-60"></a> // [range.elements], elements view</span>
<span id="cb4-61"><a href="#cb4-61"></a> template<input_range V, size_t N></span>
<span id="cb4-62"><a href="#cb4-62"></a> requires <em>see below</em>;</span>
<span id="cb4-63"><a href="#cb4-63"></a> class elements_view;</span>
<span id="cb4-64"><a href="#cb4-64"></a> </span>
<span id="cb4-65"><a href="#cb4-65"></a><span class="va">+ template<class T, size_t N></span></span>
<span id="cb4-66"><a href="#cb4-66"></a><span class="va">+ inline constexpr bool enable_borrowed_range<elements_view<T, N>> = enable_borrowed_range<T>; </span></span>
<span id="cb4-67"><a href="#cb4-67"></a></span>
<span id="cb4-68"><a href="#cb4-68"></a> template<class R></span>
<span id="cb4-69"><a href="#cb4-69"></a> using keys_view = elements_view<all_view<R>, 0>;</span>
<span id="cb4-70"><a href="#cb4-70"></a> template<class R></span>
<span id="cb4-71"><a href="#cb4-71"></a> using values_view = elements_view<all_view<R>, 1>;</span>
<span id="cb4-72"><a href="#cb4-72"></a></span>
<span id="cb4-73"><a href="#cb4-73"></a> namespace views {</span>
<span id="cb4-74"><a href="#cb4-74"></a> template<size_t N></span>
<span id="cb4-75"><a href="#cb4-75"></a> inline constexpr <em>unspecified</em> elements = <em>unspecified</em> ;</span>
<span id="cb4-76"><a href="#cb4-76"></a> inline constexpr <em>unspecified</em> keys = <em>unspecified</em> ;</span>
<span id="cb4-77"><a href="#cb4-77"></a> inline constexpr <em>unspecified</em> values = <em>unspecified</em> ;</span>
<span id="cb4-78"><a href="#cb4-78"></a> }</span>
<span id="cb4-79"><a href="#cb4-79"></a>}</span></code></pre></div>
</div>
<h1 data-number="6" style="border-bottom:1px solid #cccccc" id="implementation" data-number="6" style="border-bottom:1px solid #cccccc"><span class="header-section-number">6</span> Implementation<a href="#implementation" class="self-link"></a></h1>
<p>This has been implemented in range-v3 <span class="citation" data-cites="range-v3.1405">[<a href="#ref-range-v3.1405" role="doc-biblioref">range-v3.1405</a>]</span>. The PR includes the six range adapters in this paper, along with a smattering of other range adapters from range-v3 that can also be made conditionally borrowed in this manner (<code class="sourceCode cpp"><span class="kw">const</span></code>, <code class="sourceCode cpp">chunk</code>, <code class="sourceCode cpp">delimit</code>, <code class="sourceCode cpp">drop_exactly</code>, <code class="sourceCode cpp">indirect</code>, <code class="sourceCode cpp">intersperse</code>, <code class="sourceCode cpp">move</code>, <code class="sourceCode cpp">slice</code>, <code class="sourceCode cpp">sliding</code>, <code class="sourceCode cpp">tail</code>, <code class="sourceCode cpp">trim</code>, <code class="sourceCode cpp">unbounded</code>, and <code class="sourceCode cpp">zip</code>) as well as a bunch of other range adapters that can be additionally made conditionally borrowed based on both the underlying range and the shape of invocables that they rely on (<code class="sourceCode cpp">group_by</code>, all the <code class="sourceCode cpp">set_algorithm_view</code> adapters, <code class="sourceCode cpp">split_when</code>, <code class="sourceCode cpp">take_while</code>/<code class="sourceCode cpp">iter_take_while</code>, <code class="sourceCode cpp">transform</code>, and <code class="sourceCode cpp">zip_view</code>/<code class="sourceCode cpp">iter_zip_view</code>).</p>
<h1 data-number="7" style="border-bottom:1px solid #cccccc" id="bibliography" data-number="7" style="border-bottom:1px solid #cccccc"><span class="header-section-number">7</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-LWG3379">
<p>[LWG3379] Casey Carter. “safe” in several library names is misleading. <br />
<a href="https://wg21.link/lwg3379">https://wg21.link/lwg3379</a></p>
</div>
<div id="ref-P1870R1">
<p>[P1870R1] Barry Revzin. 2019. forwarding-range is too subtle. <br />
<a href="https://wg21.link/p1870r1">https://wg21.link/p1870r1</a></p>
</div>
<div id="ref-range-v3.1405">
<p>[range-v3.1405] Barry Revzin. 2020. Making more range adapters safe. <br />
<a href="https://github.com/ericniebler/range-v3/pull/1405">https://github.com/ericniebler/range-v3/pull/1405</a></p>
</div>
<div id="ref-stl2.640">
<p>[stl2.640] Johel Ernesto Guerrero Peña. 2019. Unsafe views that are actually safe. <br />
<a href="https://github.com/ericniebler/stl2/issues/640">https://github.com/ericniebler/stl2/issues/640</a></p>
</div>
</div>
</div>
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