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  <meta name="dcterms.date" content="2021-02-10" />
  <title>`std::ranges::contains`</title>
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      code span. { } /* Normal */
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      code span.at { } /* Attribute */
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      code span.bu { color: #9f6807; } /* BuiltIn */
      code span.cf { color: #00607c; } /* ControlFlow */
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      code span.cv { color: #008000; font-style: italic; } /* CommentVar */
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      code span.ex { } /* Extension */
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      code span.im { } /* Import */
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      code.diff {color: #898887}
      code.diff span.va {color: #006e28}
      code.diff span.st {color: #bf0303}
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a {
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color: #4183C4;
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a.hidden_link {
text-decoration: none;
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li {
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margin-bottom: 0.6em;
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h1, h2, h3, h4 {
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line-height: 1;
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ul > li:before {
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:target .codeblock { background-color: #C9FBC9; }
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a.index {
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a.index:before { content: "⟵"; }

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span.term { font-style: italic; }
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span.techterm { font-style: italic; }
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div.numberedTable {
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margin: 2em;
}
div.figure {
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margin: 2em;
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table {
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border-collapse: collapse;
margin-left: auto;
margin-right: auto;
margin-top: 0.8em;
text-align: left;
hyphens: none; 
}
td, th {
padding-left: 1em;
padding-right: 1em;
vertical-align: top;
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td.empty {
padding: 0px;
padding-left: 1px;
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td.left {
text-align: left;
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td.right {
text-align: right;
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td.center {
text-align: center;
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td.justify {
text-align: justify;
}
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tr.rowsep, td.cline {
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tr.even, tr.odd {
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tr.capsep {
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border-top-style: double;
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tr.header {
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border-bottom-style: double;
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th {
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span.centry {
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}
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margin-left: auto;
margin-right: auto;
text-align: center;
width: 90%;
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margin-bottom: 1em;
margin-top: 1em;
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ol.enumeratea { list-style-type: none; background: inherit; }
ol.enumerate { list-style-type: none; background: inherit; }

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<h1 class="title" style="text-align:center"><code class="sourceCode default">std::ranges::contains</code></h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P2302R3</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2021-02-10</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>
      Library Evolution Working Group<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>
    </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></li>
<li><a href="#design"><span class="toc-section-number">4</span> Design<span></span></a>
<ul>
<li><a href="#api"><span class="toc-section-number">4.1</span> API<span></span></a></li>
<li><a href="#naming"><span class="toc-section-number">4.2</span> Naming<span></span></a></li>
<li><a href="#haystack-needle-or-needle-haystack"><span class="toc-section-number">4.3</span> (haystack, needle) or (needle, haystack)?<span></span></a></li>
</ul></li>
<li><a href="#proposed-wording"><span class="toc-section-number">5</span> Proposed wording<span></span></a>
<ul>
<li><a href="#feature-test-macro"><span class="toc-section-number">5.1</span> Feature-test macro<span></span></a></li>
</ul></li>
<li><a href="#implementation-experience"><span class="toc-section-number">6</span> Implementation experience<span></span></a></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" id="abstract"><span class="header-section-number">1</span> Abstract<a href="#abstract" class="self-link"></a></h1>
<p>P2302 proposes two algorithms: one that checks whether or not a range contains an element, and one that checks whether or not a range contains a subrange.</p>
<h1 data-number="2" 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>Renames the subrange overload to <code class="sourceCode default">contains_subrange</code>.</li>
<li>Adds bit about implementation experience.</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>Removes request to have <code class="sourceCode default">basic_string[_view]::contains</code> struck from the C++23 working paper.
<ul>
<li>Adjusts motivation to match.</li>
</ul></li>
<li>Adds section on naming.</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>
<ul>
<li>Initial revision</li>
</ul>
<h1 data-number="3" id="motivation"><span class="header-section-number">3</span> Motivation<a href="#motivation" class="self-link"></a></h1>
<p>At present, it isn’t immediately possible to query whether or not any range —other than a C++23 standard string type— contains an element. <span class="citation" data-cites="P1679R3">[<a href="#ref-P1679R3" role="doc-biblioref">P1679R3</a>]</span> links <code class="sourceCode default">basic_string[_view]::contains</code> with <code class="sourceCode default">basic_string[_view]::starts_with</code> and <code class="sourceCode default">basic_string[_view]::ends_with</code>. Although C++ has done without a dedicated <code class="sourceCode default">contains</code> algorithm for over twenty years, it’d be unfortunate if we only strings had a <code class="sourceCode default">contains</code> algorithm.</p>
<p>This functionality is useful for any input range, when the user wants to know whether or not their range simply contains a value. Up until C++20, we’ve had to write <code class="sourceCode default">stdr::find(r, value) != stdr::end(r)</code> to determine if a single value is inside a range, and to check if a range contains a subrange of interest, we use <code class="sourceCode default">not stdr::search(haystack, needle).empty()</code>. While this is accurate, it isn’t necessarily convenient, and it hardly expresses intent (especially in the latter case). Being able to say <code class="sourceCode default">stdr::contains(r, value)</code> addresses both of these points. Further evidence for the existence of a <code class="sourceCode default">contains</code> algorithm is that the STL gave C++98 an obscurely-named <code class="sourceCode default">contains</code> algorithm called <code class="sourceCode default">binary_search</code>, and <code class="sourceCode default">any_of</code> is “<code class="sourceCode default">contains</code> with a predicate”, showing that there’s prior art for this <code class="sourceCode default">contains</code> to be an algorithm.</p>
<h1 data-number="4" id="design"><span class="header-section-number">4</span> Design<a href="#design" class="self-link"></a></h1>
<p>The proposed usage looks like this:</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="cf">if</span> <span class="op">(</span>std<span class="op">::</span>ranges<span class="op">::</span>contains<span class="op">(</span>haystack, <span class="ch">&#39;o&#39;</span><span class="op">))</span> <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a>  <span class="co">// ...</span></span>
<span id="cb1-3"><a href="#cb1-3"></a><span class="op">}</span></span>
<span id="cb1-4"><a href="#cb1-4"></a></span>
<span id="cb1-5"><a href="#cb1-5"></a><span class="cf">if</span> <span class="op">(</span>std<span class="op">::</span>ranges<span class="op">::</span>contains<span class="op">(</span>haystack<span class="op">.</span>begin<span class="op">()</span>, haystack<span class="op">.</span>end<span class="op">()</span>, <span class="ch">&#39;c&#39;</span><span class="op">))</span> <span class="op">{</span></span>
<span id="cb1-6"><a href="#cb1-6"></a>  <span class="co">// ...</span></span>
<span id="cb1-7"><a href="#cb1-7"></a><span class="op">}</span></span>
<span id="cb1-8"><a href="#cb1-8"></a></span>
<span id="cb1-9"><a href="#cb1-9"></a><span class="cf">if</span> <span class="op">(</span>std<span class="op">::</span>ranges<span class="op">::</span>contains_subrange<span class="op">(</span>haystack<span class="op">.</span>begin<span class="op">()</span>, haystack<span class="op">.</span>end<span class="op">()</span>, long_needle<span class="op">.</span>begin<span class="op">()</span>, long_needle<span class="op">.</span>end<span class="op">()))</span> <span class="op">{</span></span>
<span id="cb1-10"><a href="#cb1-10"></a>  <span class="co">// ...</span></span>
<span id="cb1-11"><a href="#cb1-11"></a><span class="op">}</span></span>
<span id="cb1-12"><a href="#cb1-12"></a></span>
<span id="cb1-13"><a href="#cb1-13"></a><span class="cf">if</span> <span class="op">(</span>std<span class="op">::</span>ranges<span class="op">::</span>contains_subrange<span class="op">(</span>haystack, long_needle<span class="op">))</span> <span class="op">{</span></span>
<span id="cb1-14"><a href="#cb1-14"></a>  <span class="co">// ...</span></span>
<span id="cb1-15"><a href="#cb1-15"></a><span class="op">}</span></span>
<span id="cb1-16"><a href="#cb1-16"></a></span>
<span id="cb1-17"><a href="#cb1-17"></a><span class="cf">if</span> <span class="op">(</span>std<span class="op">::</span>ranges<span class="op">::</span>contains_subrange<span class="op">(</span>haystack<span class="op">.</span>begin<span class="op">()</span>, haystack<span class="op">.</span>end<span class="op">()</span>, long_needle<span class="op">.</span>begin<span class="op">()</span>, long_needle<span class="op">.</span>end<span class="op">()</span>, bind_back<span class="op">(</span>std<span class="op">::</span>modulo<span class="op">()</span>, <span class="dv">4</span><span class="op">)))</span> <span class="op">{</span></span>
<span id="cb1-18"><a href="#cb1-18"></a>  <span class="co">// ...</span></span>
<span id="cb1-19"><a href="#cb1-19"></a><span class="op">}</span></span>
<span id="cb1-20"><a href="#cb1-20"></a></span>
<span id="cb1-21"><a href="#cb1-21"></a><span class="cf">if</span> <span class="op">(</span>std<span class="op">::</span>ranges<span class="op">::</span>contains_subrange<span class="op">(</span>haystack, long_needle, bind_back<span class="op">(</span>std<span class="op">::</span>modulo<span class="op">()</span>, <span class="dv">4</span><span class="op">)))</span> <span class="op">{</span></span>
<span id="cb1-22"><a href="#cb1-22"></a>  <span class="co">// ...</span></span>
<span id="cb1-23"><a href="#cb1-23"></a><span class="op">}</span></span></code></pre></div>
<p>Due to length, the comparison tables below only consider the range-based overloads.</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Before</strong>
</div></th>
<th><div style="text-align:center">
<strong>After</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><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">namespace</span> stdr <span class="op">=</span> std<span class="op">::</span>ranges;</span>
<span id="cb2-2"><a href="#cb2-2"></a>stdr<span class="op">::</span>find<span class="op">(</span>haystack<span class="op">.</span>begin<span class="op">()</span>, haystack<span class="op">.</span>end<span class="op">()</span>, <span class="ch">&#39;o&#39;</span><span class="op">)</span> <span class="op">!=</span> haystack<span class="op">.</span>end<span class="op">()</span></span>
<span id="cb2-3"><a href="#cb2-3"></a>stdr<span class="op">::</span>find<span class="op">(</span>haystack, <span class="ch">&#39;o&#39;</span><span class="op">)</span> <span class="op">!=</span> stdr<span class="op">::</span>end<span class="op">(</span>haystack<span class="op">)</span></span>
<span id="cb2-4"><a href="#cb2-4"></a><span class="kw">not</span> stdr<span class="op">::</span>search<span class="op">(</span>haystack, long_needle<span class="op">).</span>empty<span class="op">()</span></span>
<span id="cb2-5"><a href="#cb2-5"></a><span class="kw">not</span> stdr<span class="op">::</span>search<span class="op">(</span>haystack, long_needle, bind_back<span class="op">(</span>std<span class="op">::</span>modulo<span class="op">()</span>, <span class="dv">4</span><span class="op">)).</span>empty<span class="op">()</span></span></code></pre></div></td>
<td><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">namespace</span> stdr <span class="op">=</span> std<span class="op">::</span>ranges;</span>
<span id="cb3-2"><a href="#cb3-2"></a>stdr<span class="op">::</span>contains<span class="op">(</span>haystack<span class="op">.</span>begin<span class="op">()</span>, haystack<span class="op">.</span>end<span class="op">())</span></span>
<span id="cb3-3"><a href="#cb3-3"></a>stdr<span class="op">::</span>contains<span class="op">(</span>haystack, <span class="ch">&#39;o&#39;</span><span class="op">)</span></span>
<span id="cb3-4"><a href="#cb3-4"></a>stdr<span class="op">::</span>contains_subrange<span class="op">(</span>haystack, long_needle<span class="op">)</span></span>
<span id="cb3-5"><a href="#cb3-5"></a>stdr<span class="op">::</span>contains_subrange<span class="op">(</span>haystack, long_needle, bind_back<span class="op">(</span>std<span class="op">::</span>modulo<span class="op">()</span>, <span class="dv">4</span><span class="op">))</span></span></code></pre></div></td>
</tr>
</tbody>
</table>
<h2 data-number="4.1" id="api"><span class="header-section-number">4.1</span> API<a href="#api" class="self-link"></a></h2>
<p>As this is an algorithm, the author suggests this be placed in namespace <code class="sourceCode default">std::ranges</code>, where new algorithms are currently destined to go. As usual with new algorithms, these ones will also have projections.</p>
<p>The author notes that for an unsorted sequence, a <code class="sourceCode default">contains</code> algorithm is simply one of the following:</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> <span class="kw">const</span> haystack <span class="op">=</span> <span class="st">&quot;the quick brown fox jumps over the lazy dog&quot;</span><span class="bu">sv</span>;</span>
<span id="cb4-2"><a href="#cb4-2"></a></span>
<span id="cb4-3"><a href="#cb4-3"></a>stdr<span class="op">::</span>find<span class="op">(</span>haystack, <span class="ch">&#39;o&#39;</span><span class="op">)</span> <span class="op">!=</span> haystack<span class="op">.</span>end<span class="op">()</span>; <span class="co">// haystack.contains(&#39;o&#39;)</span></span>
<span id="cb4-4"><a href="#cb4-4"></a><span class="kw">not</span> stdr<span class="op">::</span>search<span class="op">(</span>haystack, <span class="st">&quot;red&quot;</span><span class="bu">sv</span><span class="op">).</span>empty<span class="op">()</span>; <span class="co">// haystack.contains(&quot;red&quot;sv)</span></span>
<span id="cb4-5"><a href="#cb4-5"></a></span>
<span id="cb4-6"><a href="#cb4-6"></a><span class="co">// See https://godbolt.org/z/vTendW</span></span></code></pre></div>
<p>As such, <code class="sourceCode default">contains</code> is a wrapper around these two algorithms:</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="kw">namespace</span> std <span class="op">{</span></span>
<span id="cb5-2"><a href="#cb5-2"></a>  <span class="kw">template</span><span class="op">&lt;</span>input_­iterator I, sentinel_­for<span class="op">&lt;</span>I<span class="op">&gt;</span> S, <span class="kw">class</span> T, <span class="kw">class</span> Proj <span class="op">=</span> identity<span class="op">&gt;</span></span>
<span id="cb5-3"><a href="#cb5-3"></a>  <span class="kw">requires</span> indirect_­binary_­predicate<span class="op">&lt;</span>ranges<span class="op">::</span>equal_to, projected<span class="op">&lt;</span>I, Proj<span class="op">&gt;</span>, <span class="kw">const</span> T<span class="op">*&gt;</span></span>
<span id="cb5-4"><a href="#cb5-4"></a>  <span class="kw">constexpr</span> <span class="dt">bool</span> ranges<span class="op">::</span>contains<span class="op">(</span>I first, S last, <span class="kw">const</span> T<span class="op">&amp;</span> value, Proj proj <span class="op">=</span> <span class="op">{})</span>;</span>
<span id="cb5-5"><a href="#cb5-5"></a></span>
<span id="cb5-6"><a href="#cb5-6"></a>  <span class="kw">template</span><span class="op">&lt;</span>input_­range R, <span class="kw">class</span> T, <span class="kw">class</span> Proj <span class="op">=</span> identity<span class="op">&gt;</span></span>
<span id="cb5-7"><a href="#cb5-7"></a>  <span class="kw">requires</span> indirect_­binary_­predicate<span class="op">&lt;</span>ranges<span class="op">::</span>equal_to, projected<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span>R<span class="op">&gt;</span>, Proj<span class="op">&gt;</span>, <span class="kw">const</span> T<span class="op">*&gt;</span></span>
<span id="cb5-8"><a href="#cb5-8"></a>  <span class="kw">constexpr</span> <span class="dt">bool</span> ranges<span class="op">::</span>contains<span class="op">(</span>R<span class="op">&amp;&amp;</span> r, <span class="kw">const</span> T<span class="op">&amp;</span> value, Proj proj <span class="op">=</span> <span class="op">{})</span>;</span>
<span id="cb5-9"><a href="#cb5-9"></a></span>
<span id="cb5-10"><a href="#cb5-10"></a>  <span class="kw">template</span><span class="op">&lt;</span>forward_­iterator I1, sentinel_­for<span class="op">&lt;</span>I1<span class="op">&gt;</span> S1, forward_­iterator I2,</span>
<span id="cb5-11"><a href="#cb5-11"></a>           sentinel_­for<span class="op">&lt;</span>I2<span class="op">&gt;</span> S2, <span class="kw">class</span> Pred <span class="op">=</span> ranges<span class="op">::</span>equal_to,</span>
<span id="cb5-12"><a href="#cb5-12"></a>           <span class="kw">class</span> Proj1 <span class="op">=</span> identity, <span class="kw">class</span> Proj2 <span class="op">=</span> identity<span class="op">&gt;</span></span>
<span id="cb5-13"><a href="#cb5-13"></a>    <span class="kw">requires</span> indirectly_­comparable<span class="op">&lt;</span>I1, I2, Pred, Proj1, Proj2<span class="op">&gt;</span></span>
<span id="cb5-14"><a href="#cb5-14"></a>    <span class="kw">constexpr</span> <span class="dt">bool</span></span>
<span id="cb5-15"><a href="#cb5-15"></a>      ranges<span class="op">::</span>contains_subrange<span class="op">(</span>I1 first1, S1 last1, I2 first2, S2 last2, Pred pred <span class="op">=</span> <span class="op">{}</span>,</span>
<span id="cb5-16"><a href="#cb5-16"></a>                                Proj1 proj1 <span class="op">=</span> <span class="op">{}</span>, Proj2 proj2 <span class="op">=</span> <span class="op">{})</span>;</span>
<span id="cb5-17"><a href="#cb5-17"></a></span>
<span id="cb5-18"><a href="#cb5-18"></a>  <span class="kw">template</span><span class="op">&lt;</span>forward_­range R1, forward_­range R2, <span class="kw">class</span> Pred <span class="op">=</span> ranges<span class="op">::</span>equal_to,</span>
<span id="cb5-19"><a href="#cb5-19"></a>           <span class="kw">class</span> Proj1 <span class="op">=</span> identity, <span class="kw">class</span> Proj2 <span class="op">=</span> identity<span class="op">&gt;</span></span>
<span id="cb5-20"><a href="#cb5-20"></a>    <span class="kw">requires</span> indirectly_­comparable<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span>R1<span class="op">&gt;</span>, iterator_t<span class="op">&lt;</span>R2<span class="op">&gt;</span>, Pred, Proj1, Proj2<span class="op">&gt;</span></span>
<span id="cb5-21"><a href="#cb5-21"></a>    <span class="kw">constexpr</span> <span class="dt">bool</span></span>
<span id="cb5-22"><a href="#cb5-22"></a>      ranges<span class="op">::</span>contains_subrange<span class="op">(</span>R1<span class="op">&amp;&amp;</span> r1, R2<span class="op">&amp;&amp;</span> r2, Pred pred <span class="op">=</span> <span class="op">{}</span>,</span>
<span id="cb5-23"><a href="#cb5-23"></a>                                Proj1 proj1 <span class="op">=</span> <span class="op">{}</span>, Proj2 proj2 <span class="op">=</span> <span class="op">{})</span>;</span>
<span id="cb5-24"><a href="#cb5-24"></a><span class="op">}</span></span></code></pre></div>
<h2 data-number="4.2" id="naming"><span class="header-section-number">4.2</span> Naming<a href="#naming" class="self-link"></a></h2>
<p>The minutes from P2302R0’s discusion note that it may be more approriate to name these functions <code class="sourceCode default">contains_element</code> and <code class="sourceCode default">contains_range</code>, since it could be ambiguous as to whether or not the range-based algorithms are performing a find or a search. Worse, a programmer might intend to use the element <code class="sourceCode default">contains</code> while searching a range of ranges, which without care, will accidentally call the wrong overload. This is a good point, and the names should probably be different.</p>
<p>This paper proposes <code class="sourceCode default">contains</code> instead of <code class="sourceCode default">contains_element</code>, since that is a very common, and much complained-about missing feature in C++. It also elects to rename the suggested <code class="sourceCode default">contains_range</code> as <code class="sourceCode default">contains_subrange</code>, because the author feels it’s a more appropriate name: if the range is found, then it’s a subrange of the haystack.</p>
<h2 data-number="4.3" id="haystack-needle-or-needle-haystack"><span class="header-section-number">4.3</span> (haystack, needle) or (needle, haystack)?<a href="#haystack-needle-or-needle-haystack" class="self-link"></a></h2>
<p>This issue was addressed in <span class="citation" data-cites="P1659R3">[<a href="#ref-P1659R3" role="doc-biblioref">P1659R3</a>]</span>, but it bears repeating.</p>
<blockquote>
<p>Concerns were outlined in all of N3609, P0457, P1679 about the ambiguity of whether we are performing <code class="sourceCode default">starts_with(haystack, needle)</code> or <code class="sourceCode default">starts_with(needle, haystack)</code>. There is prior art in the algorithms library that makes the first range the subject of the operation: <code class="sourceCode default">mismatch</code>, <code class="sourceCode default">equal</code>, <code class="sourceCode default">search</code>, <code class="sourceCode default">find_first_of</code>, and <code class="sourceCode default">lexicographical_compare</code> all take the form <code class="sourceCode default">algorithm(haystack, needle)</code>, so the author remains unconvinced about the ambiguity.</p>
</blockquote>
<p>LEWG approved P1659 in the Cologne 2019 meeting: meaning that this working group agrees that there isn’t any ambiguity, or that LEWG is inconsistent in what it agrees upon.</p>
<h1 data-number="5" id="proposed-wording"><span class="header-section-number">5</span> Proposed wording<a href="#proposed-wording" class="self-link"></a></h1>
<p>Add the following text to <span>25.4
 <a href="https://wg21.link/algorithm.syn">[algorithm.syn]</a></span>:</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="co">// [alg.none.of], none of</span></span>
<span id="cb6-2"><a href="#cb6-2"></a>  <span class="co">// ...</span></span>
<span id="cb6-3"><a href="#cb6-3"></a></span>
<span id="cb6-4"><a href="#cb6-4"></a>  <span class="co">// [alg.contains], contains</span></span>
<span id="cb6-5"><a href="#cb6-5"></a>  <span class="kw">template</span><span class="op">&lt;</span>input_­iterator I, sentinel_­for<span class="op">&lt;</span>I<span class="op">&gt;</span> S, <span class="kw">class</span> T, <span class="kw">class</span> Proj <span class="op">=</span> identity<span class="op">&gt;</span></span>
<span id="cb6-6"><a href="#cb6-6"></a>    <span class="kw">requires</span> indirect_­binary_­predicate<span class="op">&lt;</span>ranges<span class="op">::</span>equal_to, projected<span class="op">&lt;</span>I, Proj<span class="op">&gt;</span>, <span class="kw">const</span> T<span class="op">*&gt;</span></span>
<span id="cb6-7"><a href="#cb6-7"></a>    <span class="kw">constexpr</span> <span class="dt">bool</span> ranges<span class="op">::</span>contains<span class="op">(</span>I first, S last, <span class="kw">const</span> T<span class="op">&amp;</span> value, Proj proj <span class="op">=</span> <span class="op">{})</span>;</span>
<span id="cb6-8"><a href="#cb6-8"></a>  <span class="kw">template</span><span class="op">&lt;</span>input_­range R, <span class="kw">class</span> T, <span class="kw">class</span> Proj <span class="op">=</span> identity<span class="op">&gt;</span></span>
<span id="cb6-9"><a href="#cb6-9"></a>    <span class="kw">requires</span> indirect_­binary_­predicate<span class="op">&lt;</span>ranges<span class="op">::</span>equal_to, projected<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span>R<span class="op">&gt;</span>, Proj<span class="op">&gt;</span>, <span class="kw">const</span> T<span class="op">*&gt;</span></span>
<span id="cb6-10"><a href="#cb6-10"></a>    <span class="kw">constexpr</span> <span class="dt">bool</span> ranges<span class="op">::</span>contains<span class="op">(</span>R<span class="op">&amp;&amp;</span> r, <span class="kw">const</span> T<span class="op">&amp;</span> value, Proj proj <span class="op">=</span> <span class="op">{})</span>;</span>
<span id="cb6-11"><a href="#cb6-11"></a></span>
<span id="cb6-12"><a href="#cb6-12"></a>  <span class="kw">template</span><span class="op">&lt;</span>forward_­iterator I1, sentinel_­for<span class="op">&lt;</span>I1<span class="op">&gt;</span> S1, forward_­iterator I2,</span>
<span id="cb6-13"><a href="#cb6-13"></a>           sentinel_­for<span class="op">&lt;</span>I2<span class="op">&gt;</span> S2, <span class="kw">class</span> Pred <span class="op">=</span> ranges<span class="op">::</span>equal_to,</span>
<span id="cb6-14"><a href="#cb6-14"></a>           <span class="kw">class</span> Proj1 <span class="op">=</span> identity, <span class="kw">class</span> Proj2 <span class="op">=</span> identity<span class="op">&gt;</span></span>
<span id="cb6-15"><a href="#cb6-15"></a>    <span class="kw">requires</span> indirectly_­comparable<span class="op">&lt;</span>I1, I2, Pred, Proj1, Proj2<span class="op">&gt;</span></span>
<span id="cb6-16"><a href="#cb6-16"></a>    <span class="kw">constexpr</span> <span class="dt">bool</span> ranges<span class="op">::</span>contains_subrange<span class="op">(</span>I1 first1, S1 last1, I2 first2, S2 last2,</span>
<span id="cb6-17"><a href="#cb6-17"></a>                                             Pred pred <span class="op">=</span> <span class="op">{}</span>, Proj1 proj1 <span class="op">=</span> <span class="op">{}</span>, Proj2 proj2 <span class="op">=</span> <span class="op">{})</span>;</span>
<span id="cb6-18"><a href="#cb6-18"></a>  <span class="kw">template</span><span class="op">&lt;</span>forward_­range R1, forward_­range R2, <span class="kw">class</span> Pred <span class="op">=</span> ranges<span class="op">::</span>equal_to,</span>
<span id="cb6-19"><a href="#cb6-19"></a>           <span class="kw">class</span> Proj1 <span class="op">=</span> identity, <span class="kw">class</span> Proj2 <span class="op">=</span> identity<span class="op">&gt;</span></span>
<span id="cb6-20"><a href="#cb6-20"></a>    <span class="kw">requires</span> indirectly_­comparable<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span>R1<span class="op">&gt;</span>, iterator_t<span class="op">&lt;</span>R2<span class="op">&gt;</span>, Pred, Proj1, Proj2<span class="op">&gt;</span></span>
<span id="cb6-21"><a href="#cb6-21"></a>    <span class="kw">constexpr</span> <span class="dt">bool</span> ranges<span class="op">::</span>contains_subrange<span class="op">(</span>R1<span class="op">&amp;&amp;</span> r1, R2<span class="op">&amp;&amp;</span> r2, Pred pred <span class="op">=</span> <span class="op">{}</span>,</span>
<span id="cb6-22"><a href="#cb6-22"></a>                                             Proj1 proj1 <span class="op">=</span> <span class="op">{}</span>, Proj2 proj2 <span class="op">=</span> <span class="op">{})</span>;</span>
<span id="cb6-23"><a href="#cb6-23"></a></span>
<span id="cb6-24"><a href="#cb6-24"></a>  <span class="co">// [alg.foreach], for each</span></span></code></pre></div>
<p>Add the following to <span>25.6
 <a href="https://wg21.link/alg.nonmodifying">[alg.nonmodifying]</a></span>:</p>
<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">template</span><span class="op">&lt;</span>input_­iterator I, sentinel_­for<span class="op">&lt;</span>I<span class="op">&gt;</span> S, <span class="kw">class</span> T, <span class="kw">class</span> Proj <span class="op">=</span> identity<span class="op">&gt;</span></span>
<span id="cb7-2"><a href="#cb7-2"></a>  <span class="kw">requires</span> indirect_­binary_­predicate<span class="op">&lt;</span>ranges<span class="op">::</span>equal_to, projected<span class="op">&lt;</span>I, Proj<span class="op">&gt;</span>, <span class="kw">const</span> T<span class="op">*&gt;</span></span>
<span id="cb7-3"><a href="#cb7-3"></a>  <span class="kw">constexpr</span> <span class="dt">bool</span> ranges<span class="op">::</span>contains<span class="op">(</span>I first, S last, <span class="kw">const</span> T<span class="op">&amp;</span> value, Proj proj <span class="op">=</span> <span class="op">{})</span>;</span>
<span id="cb7-4"><a href="#cb7-4"></a></span>
<span id="cb7-5"><a href="#cb7-5"></a><span class="kw">template</span><span class="op">&lt;</span>input_­range R, <span class="kw">class</span> T, <span class="kw">class</span> Proj <span class="op">=</span> identity<span class="op">&gt;</span></span>
<span id="cb7-6"><a href="#cb7-6"></a>  <span class="kw">requires</span> indirect_­binary_­predicate<span class="op">&lt;</span>ranges<span class="op">::</span>equal_to, projected<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span>R<span class="op">&gt;</span>, Proj<span class="op">&gt;</span>, <span class="kw">const</span> T<span class="op">*&gt;</span></span>
<span id="cb7-7"><a href="#cb7-7"></a>  <span class="kw">constexpr</span> <span class="dt">bool</span> ranges<span class="op">::</span>contains<span class="op">(</span>R<span class="op">&amp;&amp;</span> r, <span class="kw">const</span> T<span class="op">&amp;</span> value, Proj proj <span class="op">=</span> <span class="op">{})</span>;</span></code></pre></div>
<p><em>Returns</em>: <code class="sourceCode default">ranges::find(std::move(first), last, value, proj) != last;</code></p>
<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">template</span><span class="op">&lt;</span>forward_­iterator I1, sentinel_­for<span class="op">&lt;</span>I1<span class="op">&gt;</span> S1, forward_­iterator I2,</span>
<span id="cb8-2"><a href="#cb8-2"></a>         sentinel_­for<span class="op">&lt;</span>I2<span class="op">&gt;</span> S2, <span class="kw">class</span> Pred <span class="op">=</span> ranges<span class="op">::</span>equal_to,</span>
<span id="cb8-3"><a href="#cb8-3"></a>         <span class="kw">class</span> Proj1 <span class="op">=</span> identity, <span class="kw">class</span> Proj2 <span class="op">=</span> identity<span class="op">&gt;</span></span>
<span id="cb8-4"><a href="#cb8-4"></a>  <span class="kw">requires</span> indirectly_­comparable<span class="op">&lt;</span>I1, I2, Pred, Proj1, Proj2<span class="op">&gt;</span></span>
<span id="cb8-5"><a href="#cb8-5"></a>  <span class="kw">constexpr</span> <span class="dt">bool</span> ranges<span class="op">::</span>contains_subrange<span class="op">(</span>I1 first1, S1 last1, I2 first2, S2 last2,</span>
<span id="cb8-6"><a href="#cb8-6"></a>                                           Pred pred <span class="op">=</span> <span class="op">{}</span>, Proj1 proj1 <span class="op">=</span> <span class="op">{}</span>, Proj2 proj2 <span class="op">=</span> <span class="op">{})</span>;</span>
<span id="cb8-7"><a href="#cb8-7"></a><span class="kw">template</span><span class="op">&lt;</span>forward_­range R1, forward_­range R2, <span class="kw">class</span> Pred <span class="op">=</span> ranges<span class="op">::</span>equal_to,</span>
<span id="cb8-8"><a href="#cb8-8"></a>         <span class="kw">class</span> Proj1 <span class="op">=</span> identity, <span class="kw">class</span> Proj2 <span class="op">=</span> identity<span class="op">&gt;</span></span>
<span id="cb8-9"><a href="#cb8-9"></a>  <span class="kw">requires</span> indirectly_­comparable<span class="op">&lt;</span>iterator_t<span class="op">&lt;</span>R1<span class="op">&gt;</span>, iterator_t<span class="op">&lt;</span>R2<span class="op">&gt;</span>, Pred, Proj1, Proj2<span class="op">&gt;</span></span>
<span id="cb8-10"><a href="#cb8-10"></a>  <span class="kw">constexpr</span> <span class="dt">bool</span> ranges<span class="op">::</span>contains_subrange<span class="op">(</span>R1<span class="op">&amp;&amp;</span> r1, R2<span class="op">&amp;&amp;</span> r2, Pred pred <span class="op">=</span> <span class="op">{}</span>,</span>
<span id="cb8-11"><a href="#cb8-11"></a>                                           Proj1 proj1 <span class="op">=</span> <span class="op">{}</span>, Proj2 proj2 <span class="op">=</span> <span class="op">{})</span>;</span></code></pre></div>
<p><em>Returns</em>: <code class="sourceCode default">first2 == last2 || !ranges::search(first1, last1, first2, last2, pred, proj1, proj2).empty();</code></p>
<h2 data-number="5.1" id="feature-test-macro"><span class="header-section-number">5.1</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 default">&lt;version&gt;</code> synopsis, with the value selected by the editor to reflect the date of adoption of this paper:</p>
<div class="sourceCode" id="cb9"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb9-1"><a href="#cb9-1"></a><span class="pp">#define __cpp_lib_ranges_contains </span><span class="dv">20</span><span class="er">XXXXL</span><span class="pp"> </span><span class="co">// also in &lt;algorithm&gt;</span></span></code></pre></div>
<h1 data-number="6" id="implementation-experience"><span class="header-section-number">6</span> Implementation experience<a href="#implementation-experience" class="self-link"></a></h1>
<p>This has been implemented in range-v3 and is available as a <a href="https://github.com/ericniebler/range-v3/pull/1675">Pull Request</a>.</p>
<h1 data-number="7" 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 Corentin Jabot for reviewing this proposal.</p>
<h1 data-number="8" 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-P1659R3">
<p>[P1659R3] Christopher Di Bella. 2021-02-19. starts_with and ends_with. <br />
<a href="https://wg21.link/p1659r3">https://wg21.link/p1659r3</a></p>
</div>
<div id="ref-P1679R3">
<p>[P1679R3] Wim Leflere, Paul Fee. 2020-07-22. String Contains function. <br />
<a href="https://wg21.link/p1679r3">https://wg21.link/p1679r3</a></p>
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