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<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang xml:lang>
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<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
<meta name="dcterms.date" content="2019-11-07" />
<title>Concept traits should be named after concepts</title>
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<h1 class="title" style="text-align:center">Concept traits should be named after concepts</h1>
<table style="border:none;float:right">
<tr>
<td>Document #: </td>
<td>P1871R1</td>
</tr>
<tr>
<td>Date: </td>
<td>2019-11-07</td>
</tr>
<tr>
<td style="vertical-align:top">Project: </td>
<td>Programming Language C++<br>
LEWG<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">
<h1 id="revision-history" 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 <span class="citation" data-cites="P1871R0">[<a href="#ref-P1871R0" role="doc-biblioref">P1871R0</a>]</span> was presented in Belfast and originally proposed making all of the variable template opt-ins for concepts named <code class="sourceCode cpp">enable_NAME</code>. LEWG rejected this direction. However, one of the other changes in the paper was to ensure at least that the traits were named the same as the concepts. This used to be the case, except when we renamed the concept to <code class="sourceCode cpp">sized_sentinel_for</code> we kept the trait named <code class="sourceCode cpp">disable_sized_sentinel</code>, so this paper proposes simply updating that trait to have the same name - so that all such traits are either <code class="sourceCode cpp">enable_NAME</code> or <code class="sourceCode cpp">disable_NAME</code>.</p>
<h1 id="motivation" style="border-bottom:1px solid #cccccc"><span class="header-section-number">2</span> Motivation<a href="#motivation" class="self-link"></a></h1>
<p>The concept <code class="sourceCode cpp">sized_sentinel_for</code>, from <a href="http://eel.is/c++draft/iterator.concept.sizedsentinel">[iterator.concept.sizedsentinel]</a>, is defined as:</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">template</span><span class="op"><</span><span class="kw">class</span> S, <span class="kw">class</span> I<span class="op">></span></span>
<span id="cb1-2"><a href="#cb1-2"></a> <span class="kw">concept</span> sized_sentinel_for <span class="op">=</span></span>
<span id="cb1-3"><a href="#cb1-3"></a> sentinel_for<span class="op"><</span>S, I<span class="op">></span> <span class="op">&&</span></span>
<span id="cb1-4"><a href="#cb1-4"></a> <span class="op">!</span>disable_sized_sentinel<span class="op"><</span>remove_cv_t<span class="op"><</span>S<span class="op">></span>, remove_cv_t<span class="op"><</span>I<span class="op">>></span> <span class="op">&&</span></span>
<span id="cb1-5"><a href="#cb1-5"></a> <span class="kw">requires</span><span class="op">(</span><span class="kw">const</span> I<span class="op">&</span> i, <span class="kw">const</span> S<span class="op">&</span> s<span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-6"><a href="#cb1-6"></a> <span class="op">{</span> s <span class="op">-</span> i <span class="op">}</span> <span class="op">-></span> same_as<span class="op"><</span>iter_difference_t<span class="op"><</span>I<span class="op">>></span>;</span>
<span id="cb1-7"><a href="#cb1-7"></a> <span class="op">{</span> i <span class="op">-</span> s <span class="op">}</span> <span class="op">-></span> same_as<span class="op"><</span>iter_difference_t<span class="op"><</span>I<span class="op">>></span>;</span>
<span id="cb1-8"><a href="#cb1-8"></a> <span class="op">}</span>;</span></code></pre></div>
<p>Note that the concept is named <code class="sourceCode cpp">sized_sentinel_for</code> but the trait is named <code class="sourceCode cpp">disable_sized_sentinel</code>. These used to be closely related because the concept used to be named <code class="sourceCode cpp">SizedSentinel</code> and got the suffix <code class="sourceCode cpp">_for</code> added as part of <span class="citation" data-cites="P1754R1">[<a href="#ref-P1754R1" role="doc-biblioref">P1754R1</a>]</span>.</p>
<p>This paper proposes that the trait and the concept should have the same name.</p>
<h1 id="wording" style="border-bottom:1px solid #cccccc"><span class="header-section-number">3</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Change 23.2 [iterator.synopsis]:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb2"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb2-1"><a href="#cb2-1"></a> // [iterator.concept.sizedsentinel], concept sized_sentinel_for</span>
<span id="cb2-2"><a href="#cb2-2"></a> template<class S, class I></span>
<span id="cb2-3"><a href="#cb2-3"></a><span class="st">- inline constexpr bool disable_sized_sentinel = false;</span></span>
<span id="cb2-4"><a href="#cb2-4"></a><span class="va">+ inline constexpr bool disable_sized_sentinel<span class="diffins">_for</span> = false;</span></span></code></pre></div>
</div>
</blockquote>
<p>and later:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb3"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb3-1"><a href="#cb3-1"></a> template<class Iterator1, class Iterator2></span>
<span id="cb3-2"><a href="#cb3-2"></a> requires (!sized_sentinel_for<Iterator1, Iterator2>)</span>
<span id="cb3-3"><a href="#cb3-3"></a><span class="st">- inline constexpr bool disable_sized_sentinel<reverse_iterator<Iterator1>,</span></span>
<span id="cb3-4"><a href="#cb3-4"></a><span class="st">- reverse_iterator<Iterator2>> = true;</span></span>
<span id="cb3-5"><a href="#cb3-5"></a><span class="va">+ inline constexpr bool disable_sized_sentinel<span class="diffins">_for</span><reverse_iterator<Iterator1>,</span></span>
<span id="cb3-6"><a href="#cb3-6"></a><span class="va">+ reverse_iterator<Iterator2>> = true;</span></span></code></pre></div>
</div>
</blockquote>
<p>Change 23.3.4.8 [iterator.concept.sizedsentinel]:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> The <code class="sourceCode cpp">sized_sentinel_for</code> concept specifies requirements on an <code class="sourceCode cpp">input_or_output_iterator</code> and a corresponding <code class="sourceCode cpp">sentinel_for</code> that allow the use of the <code class="sourceCode cpp"><span class="op">-</span></code> operator to compute the distance between them in constant time.</p>
<div>
<div class="sourceCode" id="cb4"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb4-1"><a href="#cb4-1"></a>template<class S, class I></span>
<span id="cb4-2"><a href="#cb4-2"></a> concept sized_sentinel_for =</span>
<span id="cb4-3"><a href="#cb4-3"></a> sentinel_for<S, I> &&</span>
<span id="cb4-4"><a href="#cb4-4"></a><span class="st">- !disable_sized_sentinel<remove_cv_t<S>, remove_cv_t<I>> &&</span></span>
<span id="cb4-5"><a href="#cb4-5"></a><span class="va">+ !disable_sized_sentinel<span class="diffins">_for</span><remove_cv_t<S>, remove_cv_t<I>> &&</span></span>
<span id="cb4-6"><a href="#cb4-6"></a> requires(const I& i, const S& s) {</span>
<span id="cb4-7"><a href="#cb4-7"></a> { s - i } -> same_as<iter_difference_t<I>>;</span>
<span id="cb4-8"><a href="#cb4-8"></a> { i - s } -> same_as<iter_difference_t<I>>;</span>
<span id="cb4-9"><a href="#cb4-9"></a> };</span></code></pre></div>
</div>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> Let <code class="sourceCode cpp">i</code> be an iterator of type <code class="sourceCode cpp">I</code>, and <code class="sourceCode cpp">s</code> a sentinel of type <code class="sourceCode cpp">S</code> such that <code class="sourceCode cpp"><span class="op">[</span>i, s<span class="op">)</span></code> denotes a range. Let <code class="sourceCode cpp">N</code> be the smallest number of applications of <code class="sourceCode cpp"><span class="op">++</span>i</code> necessary to make <code class="sourceCode cpp"><span class="dt">bool</span><span class="op">(</span>i <span class="op">==</span> s<span class="op">)</span></code> be <code class="sourceCode cpp"><span class="kw">true</span></code>. <code class="sourceCode cpp">S</code> and <code class="sourceCode cpp">I</code> model <code class="sourceCode cpp">sized_sentinel_for<span class="op"><</span>S, I<span class="op">></span></code> only if</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb5-1"><a href="#cb5-1"></a>- [2.1]{.pnum} If `N` is representable by `iter_difference_t<I>`, then `s - i` is well-defined and equals `N`.</span>
<span id="cb5-2"><a href="#cb5-2"></a>- [2.2]{.pnum} If `−N` is representable by `iter_difference_t<I>`, then `i - s` is well-defined and equals `−N`.</span></code></pre></div>
<div>
<div class="sourceCode" id="cb6"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb6-1"><a href="#cb6-1"></a> template<class S, class I></span>
<span id="cb6-2"><a href="#cb6-2"></a><span class="st">- inline constexpr bool disable_sized_sentinel = false;</span></span>
<span id="cb6-3"><a href="#cb6-3"></a><span class="va">+ inline constexpr bool disable_sized_sentinel<span class="diffins">_for</span> = false;</span></span></code></pre></div>
</div>
<p><span class="marginalizedparent"><a class="marginalized">3</a></span> <em>Remarks</em>: Pursuant to [namespace.std], users may specialize <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode cpp">disable_sized_sentinel</code></span></del></span> <span class="addu"><code class="sourceCode cpp">disable_sized_sentinel_for</code></span> for <em>cv</em>-unqualified non-array object types <code class="sourceCode cpp">S</code> and <code class="sourceCode cpp">I</code> if <code class="sourceCode cpp">S</code> and/or <code class="sourceCode cpp">I</code> is a program-defined type. Such specializations shall be usable in constant expressions ([expr.const]) and have type <code class="sourceCode cpp"><span class="kw">const</span> <span class="dt">bool</span></code>.</p>
<p><span class="marginalizedparent"><a class="marginalized">4</a></span> [ <em>Note</em>: <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode cpp">disable_sized_sentinel</code></span></del></span> <span class="addu"><code class="sourceCode cpp">disable_sized_sentinel_for</code></span> allows use of sentinels and iterators with the library that satisfy but do not in fact model <code class="sourceCode cpp">sized_sentinel_for</code>. — <em>end note</em> ]</p>
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<h1 id="references" style="border-bottom:1px solid #cccccc"><span class="header-section-number">4</span> References<a href="#references" class="self-link"></a></h1>
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<div id="ref-P1754R1">
<p>[P1754R1] Herb Sutter, Casey Carter, Gabriel Dos Reis, Eric Niebler, Bjarne Stroustrup, Andrew Sutton, Ville Voutilainen. 2019. Rename concepts to standard_case for C++20, while we still can. <br />
<a href="https://wg21.link/p1754r1">https://wg21.link/p1754r1</a></p>
</div>
<div id="ref-P1871R0">
<p>[P1871R0] Barry Revzin. 2019. Should concepts be enabled or disabled? <br />
<a href="https://wg21.link/p1871r0">https://wg21.link/p1871r0</a></p>
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