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<title>A Plan for C++23 Ranges</title>
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<h1 class="title" style="text-align:center">A Plan for C++23 Ranges</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2214R2</td>
</tr>
<tr>
<td>Date:</td>
<td>2022-02-21</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>
LEWG<br>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to:</td>
<td>
Barry Revzin<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
Conor Hoekstra<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
Tim Song<br><<a href="mailto:[email protected]" class="email">[email protected]</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"><span class="toc-section-number">2</span> Introduction<span></span></a></li>
<li><a href="#view-adjuncts"><span class="toc-section-number">3</span> View adjuncts<span></span></a></li>
<li><a href="#views"><span class="toc-section-number">4</span> Views<span></span></a>
<ul>
<li><a href="#full-view-or-not-full-view"><span class="toc-section-number">4.1</span> Full view or not full view<span></span></a></li>
<li><a href="#the-zip-family"><span class="toc-section-number">4.2</span> The <code class="sourceCode cpp">zip</code> family<span></span></a>
<ul>
<li><a href="#a-tuple-that-is-writable"><span class="toc-section-number">4.2.1</span> A <code class="sourceCode cpp">tuple</code> that is <code class="sourceCode cpp">writable</code><span></span></a></li>
<li><a href="#a-tuple-that-is-readable"><span class="toc-section-number">4.2.2</span> A <code class="sourceCode cpp">tuple</code> that is <code class="sourceCode cpp">readable</code><span></span></a></li>
<li><a href="#zip-and-zip_transforms-value_type"><span class="toc-section-number">4.2.3</span> <code class="sourceCode cpp">zip</code> and <code class="sourceCode cpp">zip_transform</code>’s <code class="sourceCode cpp">value_type</code><span></span></a></li>
<li><a href="#enumerates-first-range"><span class="toc-section-number">4.2.4</span> <code class="sourceCode cpp">enumerate</code>’s first range<span></span></a></li>
<li><a href="#does-adjacent-mean-2-or-n"><span class="toc-section-number">4.2.5</span> Does <code class="sourceCode cpp">adjacent</code> mean <code class="sourceCode cpp"><span class="dv">2</span></code> or <code class="sourceCode cpp">n</code>?<span></span></a></li>
</ul></li>
<li><a href="#the-group_by-family"><span class="toc-section-number">4.3</span> The <code class="sourceCode cpp">group_by</code> family<span></span></a></li>
<li><a href="#monadic-binds"><span class="toc-section-number">4.4</span> Monadic binds<span></span></a>
<ul>
<li><a href="#flat_map"><span class="toc-section-number">4.4.1</span> <code class="sourceCode cpp">flat_map</code><span></span></a></li>
<li><a href="#transform_maybe"><span class="toc-section-number">4.4.2</span> <code class="sourceCode cpp">transform_maybe</code><span></span></a></li>
</ul></li>
<li><a href="#the-sliding-family"><span class="toc-section-number">4.5</span> The sliding family<span></span></a></li>
<li><a href="#the-takedrop-family"><span class="toc-section-number">4.6</span> The take/drop family<span></span></a></li>
<li><a href="#generative-factories"><span class="toc-section-number">4.7</span> Generative factories<span></span></a></li>
<li><a href="#other-view-adapters"><span class="toc-section-number">4.8</span> Other view adapters<span></span></a></li>
<li><a href="#derivatives-of-transform"><span class="toc-section-number">4.9</span> Derivatives of <code class="sourceCode cpp">transform</code><span></span></a>
<ul>
<li><a href="#viewsas_const"><span class="toc-section-number">4.9.1</span> <code class="sourceCode cpp">views<span class="op">::</span>as_const</code><span></span></a></li>
<li><a href="#viewsas_rvalue"><span class="toc-section-number">4.9.2</span> <code class="sourceCode cpp">views<span class="op">::</span>as_rvalue</code><span></span></a></li>
</ul></li>
</ul></li>
<li><a href="#algorithms"><span class="toc-section-number">5</span> Algorithms<span></span></a>
<ul>
<li><a href="#algorithms-that-output-a-value-catamorphisms"><span class="toc-section-number">5.1</span> Algorithms that Output a Value (Catamorphisms)<span></span></a>
<ul>
<li><a href="#stdaccumulate-rangesfold"><span class="toc-section-number">5.1.1</span> <code class="sourceCode cpp">std<span class="op">::</span>accumulate</code> → <code class="sourceCode cpp">ranges<span class="op">::</span>fold</code><span></span></a></li>
<li><a href="#rangesreduce"><span class="toc-section-number">5.1.2</span> <code class="sourceCode cpp">ranges<span class="op">::</span>reduce</code><span></span></a></li>
<li><a href="#rangestransform_reduce-and-rangesinner_product"><span class="toc-section-number">5.1.3</span> <code class="sourceCode cpp">ranges<span class="op">::</span>transform_reduce</code> and <code class="sourceCode cpp">ranges<span class="op">::</span>inner_product</code><span></span></a></li>
</ul></li>
<li><a href="#algorithms-that-output-a-range-anamorphisms"><span class="toc-section-number">5.2</span> Algorithms that Output a Range (Anamorphisms)<span></span></a>
<ul>
<li><a href="#shift_left-and-shift_right"><span class="toc-section-number">5.2.1</span> <code class="sourceCode cpp">shift_left</code> and <code class="sourceCode cpp">shift_right</code><span></span></a></li>
<li><a href="#stdadjacent_difference-rangesadjacent_transform"><span class="toc-section-number">5.2.2</span> <code class="sourceCode cpp">std<span class="op">::</span>adjacent_difference</code> → <code class="sourceCode cpp">ranges<span class="op">::</span>adjacent_transform</code><span></span></a></li>
<li><a href="#stdpartial_sum-rangespartial_fold-and-stdinexclusive_scan"><span class="toc-section-number">5.2.3</span> <code class="sourceCode cpp">std<span class="op">::</span>partial_sum</code> → <code class="sourceCode cpp">ranges<span class="op">::</span>partial_fold</code> and <code class="sourceCode cpp">std<span class="op">::{</span>in,ex<span class="op">}</span>clusive_scan</code><span></span></a></li>
<li><a href="#transform_inexclusive_scan"><span class="toc-section-number">5.2.4</span> <code class="sourceCode cpp">transform_<span class="op">{</span>in,ex<span class="op">}</span>clusive_scan</code><span></span></a></li>
</ul></li>
<li><a href="#parallel-algorithms"><span class="toc-section-number">5.3</span> Parallel Algorithms<span></span></a></li>
</ul></li>
<li><a href="#actions"><span class="toc-section-number">6</span> Actions<span></span></a></li>
<li><a href="#plan-summary"><span class="toc-section-number">7</span> Plan Summary<span></span></a>
<ul>
<li><a href="#tier-nan"><span class="toc-section-number">7.1</span> Tier <code class="sourceCode cpp">NaN</code><span></span></a></li>
<li><a href="#tier-1"><span class="toc-section-number">7.2</span> <span class="addu">Tier 1</span><span></span></a></li>
<li><a href="#tier-2"><span class="toc-section-number">7.3</span> <span class="yellow">Tier 2</span><span></span></a></li>
<li><a href="#tier-3"><span class="toc-section-number">7.4</span> <span class="diffdel">Tier 3</span><span></span></a></li>
</ul></li>
<li><a href="#bibliography"><span class="toc-section-number">8</span> References<span></span></a></li>
</ul>
</div>
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<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p>Since <span class="citation" data-cites="P2214R1">[<a href="#ref-P2214R1" role="doc-biblioref">P2214R1</a>]</span>, updating with progress and links to other papers.</p>
<p>Since <span class="citation" data-cites="P2214R0">[<a href="#ref-P2214R0" role="doc-biblioref">P2214R0</a>]</span>, updating with progress and links to other papers. Several changes have been made here:</p>
<ul>
<li>Pipe support for range adaptors has been added as a Tier 1 priority.</li>
<li><code class="sourceCode cpp">cache1</code> has been lowered from Tier 1 to Tier 2 due to inherent issues with its design.</li>
<li><code class="sourceCode cpp">as_const</code> has been upgraded from Tier 3 to Tier 1 due to demand (and renamed from <code class="sourceCode cpp">const_</code>).</li>
<li><code class="sourceCode cpp">flat_map</code> has been lowered to Tier 3 (since now <code class="sourceCode cpp">transform<span class="op">(</span>f<span class="op">)</span> <span class="op">|</span> join</code> is correct for all cases).</li>
<li><code class="sourceCode cpp">transform_maybe</code> has been lowered to Tier 2 (since it depends on <code class="sourceCode cpp">cache1</code>)</li>
<li>the various set range adaptors were omitted previously, and are added as Tier 3.</li>
<li>two more algorithms that weren’t rangified for C++20 added as Tier 1: <code class="sourceCode cpp">shift_left</code> and <code class="sourceCode cpp">shift_right</code>.</li>
<li>added more color to the discussion of <code class="sourceCode cpp">chunk_by</code></li>
</ul>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">2</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>When Ranges was merged into C++20 <span class="citation" data-cites="P0896R4">[<a href="#ref-P0896R4" role="doc-biblioref">P0896R4</a>]</span>, it was knowingly incomplete. While it was based on the implementation experience in range-v3 <span class="citation" data-cites="range-v3">[<a href="#ref-range-v3" role="doc-biblioref">range-v3</a>]</span>, only a small part of that library was adopted into C++20. The Ranges proposal was big enough already, a lot of the pieces were separable and so could be included later.</p>
<p>But now that the core of Ranges has been included, later has come and we have to figure out what to do for C++23. This is a particularly trying period in the committee’s history with the global pandemic and lack of face-to-face meetings. But we do already have a plan for C++23 <span class="citation" data-cites="P0592R4">[<a href="#ref-P0592R4" role="doc-biblioref">P0592R4</a>]</span> which laid out the following priorities:</p>
<div class="quote">
<p>The priority order of handling material is thus:</p>
<ol type="1">
<li>Material that is mentioned in this plan.</li>
<li>Bug fixes, performance improvements, integration fixes for/between existing features, and issue processing.</li>
<li>Material that is not mentioned in this plan.</li>
</ol>
</div>
<p>and</p>
<div class="quote">
<p>Where are nex-gen Ranges in this plan?</p>
<p>We could certainly entertain more Actions and Views in the realm of Ranges. Whether such material appears for standardization is a bit unknown at this point.</p>
</div>
<p>We believe that adding more functionality to Ranges is important, even if it would technically be a 3rd priority item (unless you consider the spirit of the 2nd priority to include integration with itself).</p>
<p>But there is a <em>lot</em> of outstanding functionality that could be added. And while we think all of it should eventually be added (having more algorithms and more views is always a good thing), we realize that this is may be too much even in the C++23 time frame. Rather than having one-off papers that propose one bit of functionality (as in <span class="citation" data-cites="P1255R6">[<a href="#ref-P1255R6" role="doc-biblioref">P1255R6</a>]</span>, <span class="citation" data-cites="P1894R0">[<a href="#ref-P1894R0" role="doc-biblioref">P1894R0</a>]</span>, or <span class="citation" data-cites="P2164R5">[<a href="#ref-P2164R5" role="doc-biblioref">P2164R5</a>]</span>), we think it’s important to take a big picture view of what is out there and triage the various parts into three separate buckets:</p>
<ol type="1">
<li>Functionality that is really important for C++23. That which is frequently used and broadly applicable, and thus whose absence is frequently complained about.</li>
<li>Functionality that would be nice to have for C++23. Not as frequently needed as the first bucket, but still clearly want it in the standard - but more tolerable if this slips. Ideally C++23 anyway, but simply less critical.</li>
<li>Functionality that is less important or needs more design work.</li>
</ol>
<p>This paper provides our opinion for how to categorize Ranges functionality into those three buckets. We go through, in turn: <a href="#view-adjuncts">views-adjacent functionality</a>, <a href="#views">views</a>, <a href="#algorithms">algorithms</a>, and <a href="#actions">actions</a> (which do not exist in C++20).</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="view-adjuncts"><span class="header-section-number">3</span> View adjuncts<a href="#view-adjuncts" class="self-link"></a></h1>
<p>C++20 Ranges, and the range-v3 that birthed it, isn’t just a collection of loosely related views and algorithms. There’s some important other functionality there.</p>
<p>One critical piece of missing functionality is <span class="addu"><code class="sourceCode cpp">ranges<span class="op">::</span>to</code> <span class="citation" data-cites="P1206R7">[<a href="#ref-P1206R7" role="doc-biblioref">P1206R7</a>]</span></span>. It’s not a view, but it is often used as the terminal component of a view pipeline to create a new trailing range - to finally collect the results of the computation being constructed. This is a top priority and is sorely missing.</p>
<p>Another criticial piece of functionality was pointed out by Walter Brown in the telecon discussing the initial draft of this paper: it is simply not possible for users to write their own range adaptors such that they smoothly interact with standard library (or other user) range adaptors. That is, it is possible (but a little tedious) for users to write an adaptor such that they can write <code class="sourceCode cpp">r <span class="op">|</span> my<span class="op">::</span>thing <span class="op">|</span> my<span class="op">::</span>func<span class="op">(</span><span class="dv">2</span><span class="op">)</span></code>, but it is <em>not</em> possible for them to write an adaptor such that they can write:</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> adaptor <span class="op">=</span> my<span class="op">::</span>thing <span class="op">|</span> std<span class="op">::</span>views<span class="op">::</span>transform<span class="op">(</span>f<span class="op">)</span> <span class="op">|</span> my<span class="op">::</span>func<span class="op">(</span><span class="dv">2</span><span class="op">)</span>;</span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="kw">auto</span> result <span class="op">=</span> r <span class="op">|</span> adaptor;</span></code></pre></div>
<p>In order to do that, standard library adaptors need to recognize user-defined adaptors as being adaptors. That needs library help and is also a top priority - since if users can define more adaptors, it becomes less critical that the standard library provide all of them. The standard library needs to provide <span class="addu">pipe support for user-defined adaptors <span class="citation" data-cites="P2387R3">[<a href="#ref-P2387R3" role="doc-biblioref">P2387R3</a>]</span></span></p>
<p>Another important piece of functionality is simply the ability to print views. In range-v3, views were printable, which made it easy to debug programs or to provide meaningful output. For instance, the following program using range-v3 happily compiles:</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="pp">#include </span><span class="im"><range/v3/view/iota.hpp></span></span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="pp">#include </span><span class="im"><range/v3/view/transform.hpp></span></span>
<span id="cb2-3"><a href="#cb2-3"></a><span class="pp">#include </span><span class="im"><range/v3/view/filter.hpp></span></span>
<span id="cb2-4"><a href="#cb2-4"></a><span class="pp">#include </span><span class="im"><iostream></span></span>
<span id="cb2-5"><a href="#cb2-5"></a></span>
<span id="cb2-6"><a href="#cb2-6"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb2-7"><a href="#cb2-7"></a> <span class="kw">namespace</span> rv <span class="op">=</span> ranges<span class="op">::</span>views;</span>
<span id="cb2-8"><a href="#cb2-8"></a> <span class="kw">auto</span> r <span class="op">=</span> rv<span class="op">::</span>iota<span class="op">(</span><span class="dv">0</span>, <span class="dv">20</span><span class="op">)</span></span>
<span id="cb2-9"><a href="#cb2-9"></a> <span class="op">|</span> rv<span class="op">::</span>filter<span class="op">([](</span><span class="dt">int</span> i<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> i <span class="op">%</span> <span class="dv">2</span> <span class="op">==</span> <span class="dv">0</span>; <span class="op">})</span></span>
<span id="cb2-10"><a href="#cb2-10"></a> <span class="op">|</span> rv<span class="op">::</span>transform<span class="op">([](</span><span class="dt">int</span> i<span class="op">){</span> <span class="cf">return</span> i <span class="op">*</span> i; <span class="op">})</span>;</span>
<span id="cb2-11"><a href="#cb2-11"></a> std<span class="op">::</span>cout <span class="op"><<</span> r;</span>
<span id="cb2-12"><a href="#cb2-12"></a><span class="op">}</span></span></code></pre></div>
<p>and prints <code class="sourceCode cpp"><span class="op">[</span><span class="dv">0</span>,<span class="dv">4</span>,<span class="dv">16</span>,<span class="dv">36</span>,<span class="dv">64</span>,<span class="dv">100</span>,<span class="dv">144</span>,<span class="dv">196</span>,<span class="dv">256</span>,<span class="dv">324</span><span class="op">]</span></code>.</p>
<p>Similarly, fmtlib supports printing ranges with <code class="sourceCode cpp">fmt<span class="op">::</span>join</code>, which is slightly more tedious but is at least still a single line:</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="pp">#include </span><span class="im"><range/v3/view/iota.hpp></span></span>
<span id="cb3-2"><a href="#cb3-2"></a><span class="pp">#include </span><span class="im"><range/v3/view/transform.hpp></span></span>
<span id="cb3-3"><a href="#cb3-3"></a><span class="pp">#include </span><span class="im"><range/v3/view/filter.hpp></span></span>
<span id="cb3-4"><a href="#cb3-4"></a><span class="pp">#include </span><span class="im"><fmt/format.h></span></span>
<span id="cb3-5"><a href="#cb3-5"></a></span>
<span id="cb3-6"><a href="#cb3-6"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb3-7"><a href="#cb3-7"></a> <span class="kw">namespace</span> rv <span class="op">=</span> ranges<span class="op">::</span>views;</span>
<span id="cb3-8"><a href="#cb3-8"></a> <span class="kw">auto</span> r <span class="op">=</span> rv<span class="op">::</span>iota<span class="op">(</span><span class="dv">0</span>, <span class="dv">20</span><span class="op">)</span></span>
<span id="cb3-9"><a href="#cb3-9"></a> <span class="op">|</span> rv<span class="op">::</span>filter<span class="op">([](</span><span class="dt">int</span> i<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> i <span class="op">%</span> <span class="dv">2</span> <span class="op">==</span> <span class="dv">0</span>; <span class="op">})</span></span>
<span id="cb3-10"><a href="#cb3-10"></a> <span class="op">|</span> rv<span class="op">::</span>transform<span class="op">([](</span><span class="dt">int</span> i<span class="op">){</span> <span class="cf">return</span> i <span class="op">*</span> i; <span class="op">})</span>;</span>
<span id="cb3-11"><a href="#cb3-11"></a> fmt<span class="op">::</span>print<span class="op">(</span><span class="st">"[{}]"</span>, fmt<span class="op">::</span>join<span class="op">(</span>r, <span class="st">","</span><span class="op">))</span>;</span>
<span id="cb3-12"><a href="#cb3-12"></a><span class="op">}</span></span></code></pre></div>
<p>But neither the ability to stream views directly nor <code class="sourceCode cpp">fmt<span class="op">::</span>join</code> are in C++20, so there is no direct way to print a range at all.</p>
<p>We think it’s important that C++23 provides the ability to <span class="addu">format all the ranges <span class="citation" data-cites="P2286R6">[<a href="#ref-P2286R6" role="doc-biblioref">P2286R6</a>]</span></span>. Since these are all standard library types, it is difficult for the user to be able to actually do this themselves and it’s frustrating to even have to.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="views"><span class="header-section-number">4</span> Views<a href="#views" class="self-link"></a></h1>
<p>C++20 included a bunch of views, but range-v3 has a whole lot more. Views, much like algorithms, are the kind of thing where it’s just generally good to have more of them. The C++20 standard library has over 100 algorithms, but only 17 range adapters and view factories (excluding <code class="sourceCode cpp">all</code> and <code class="sourceCode cpp">ref</code>). We want more.</p>
<p>We’ll start this section by enumerating all the adapters in range-v3 (and a few that aren’t), noting their current status, and ranking them according to our view of their priority for C++23, before describing how we came up with such a ranking.</p>
<table>
<colgroup>
<col style="width: 36%"></col>
<col style="width: 39%"></col>
<col style="width: 24%"></col>
</colgroup>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>View</strong>
</div></th>
<th><div style="text-align:center">
<strong>Current Status</strong>
</div></th>
<th><div style="text-align:center">
<strong>Proposed Priority</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><code class="sourceCode cpp">addressof</code></td>
<td>range-v3</td>
<td>Not proposed</td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">adjacent</code></td>
<td>(not in range-v3)</td>
<td><span class="addu">Tier 1 <span class="citation" data-cites="P2321R2">[<a href="#ref-P2321R2" role="doc-biblioref">P2321R2</a>]</span></span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">adjacent_transform</code></td>
<td>(not in range-v3)</td>
<td><span class="addu">Tier 1 <span class="citation" data-cites="P2321R2">[<a href="#ref-P2321R2" role="doc-biblioref">P2321R2</a>]</span></span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">adjacent_filter</code></td>
<td>range-v3</td>
<td><span class="diffdel">Tier 3</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">adjacent_remove_if</code></td>
<td>range-v3</td>
<td><span class="diffdel">Tier 3</span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">all</code></td>
<td>C++20</td>
<td>C++20</td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">any_view<span class="op"><</span>T<span class="op">></span></code></td>
<td>range-v3</td>
<td>Not proposed</td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">c_str</code></td>
<td>range-v3</td>
<td><span class="diffdel">Tier 3</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">cache1</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2. Possibly renamed as <code class="sourceCode cpp">cache_last</code> or <code class="sourceCode cpp">cache_latest</code></span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">cartesian_product</code></td>
<td>range-v3</td>
<td><span class="addu">Tier 1 <span class="citation" data-cites="P2374R1">[<a href="#ref-P2374R1" role="doc-biblioref">P2374R1</a>]</span></span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">chunk</code></td>
<td>range-v3</td>
<td><span class="addu">Tier 1 <span class="citation" data-cites="P2442R1">[<a href="#ref-P2442R1" role="doc-biblioref">P2442R1</a>]</span></span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">chunk_by</code></td>
<td>range-v3</td>
<td><span class="addu">Tier 1 <span class="citation" data-cites="P2443R1">[<a href="#ref-P2443R1" role="doc-biblioref">P2443R1</a>]</span></span>. This is an improved <a href="#the-group_by-family"><code class="sourceCode cpp">group_by</code></a> recently added to range-v3. <span class="yellow">Also consider a variant <code class="sourceCode cpp">chunk_on</code> as Tier 2</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">common</code></td>
<td>C++20</td>
<td>C++20</td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">concat</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2 <span class="citation" data-cites="P2542R0">[<a href="#ref-P2542R0" role="doc-biblioref">P2542R0</a>]</span></span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">const_</code></td>
<td>range-v3</td>
<td><span class="addu">Tier 1 <span class="citation" data-cites="P2278R2">[<a href="#ref-P2278R2" role="doc-biblioref">P2278R2</a>]</span>, renamed to <code class="sourceCode cpp">as_const</code></span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">counted</code></td>
<td>C++20</td>
<td>C++20</td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">cycle</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2</span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">delimit</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">drop</code></td>
<td>C++20</td>
<td>C++20</td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">drop_last</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">drop_last_while</code></td>
<td>(not in range-v3)</td>
<td><span class="yellow">Tier 2</span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">drop_exactly</code></td>
<td>range-v3</td>
<td><span class="diffdel">Tier 3</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">drop_while</code></td>
<td>C++20</td>
<td>C++20</td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">empty</code></td>
<td>C++20</td>
<td>C++20</td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">enumerate</code></td>
<td>range-v3</td>
<td><span class="addu">Tier 1 <span class="citation" data-cites="P2164R5">[<a href="#ref-P2164R5" role="doc-biblioref">P2164R5</a>]</span></span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">filter</code></td>
<td>C++20</td>
<td>C++20</td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">for_each</code></td>
<td>range-v3</td>
<td><span class="diffdel">Tier 3. Most languages call this <code class="sourceCode cpp">flat_map</code>, but we probably need to call it <code class="sourceCode cpp">transform_join</code>. <br />This is now completely supported as <code class="sourceCode cpp">transform<span class="op">(</span>f<span class="op">)</span> <span class="op">|</span> join</code></span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">generate</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">generate_n</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2</span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">group_by</code></td>
<td>range-v3</td>
<td>Not proposed. See <a href="#the-group_by-family"><code class="sourceCode cpp">group_by</code> discussion</a></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">head</code></td>
<td>(not in range-v3)</td>
<td><span class="diffdel">Tier 3</span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">indirect</code></td>
<td>range-v3</td>
<td>Not proposed</td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">intersperse</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2</span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">ints</code></td>
<td>range-v3</td>
<td>Unnecessary unless people really hate <code class="sourceCode cpp">iota</code>.</td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">iota</code></td>
<td>C++20</td>
<td>C++20</td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">join</code></td>
<td>partially C++20, lacks delimiter ability</td>
<td><span class="addu">Tier 1 (adding delimiter ability via <code class="sourceCode cpp">join_with</code> <span class="citation" data-cites="P2441R2">[<a href="#ref-P2441R2" role="doc-biblioref">P2441R2</a>]</span>)</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">keys</code></td>
<td>C++20</td>
<td>C++20</td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">linear_distribute</code></td>
<td>range-v3</td>
<td><span class="diffdel">Tier 3</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">maybe</code></td>
<td>proposed in <span class="citation" data-cites="P1255R6">[<a href="#ref-P1255R6" role="doc-biblioref">P1255R6</a>]</span></td>
<td>???</td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">move</code></td>
<td>range-v3</td>
<td><span class="addu">Tier 1, renamed to <code class="sourceCode cpp">as_rvalue</code> <span class="citation" data-cites="P2446R2">[<a href="#ref-P2446R2" role="doc-biblioref">P2446R2</a>]</span></span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">partial_sum</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2, but not taking a callable (solely as a specialized form of <code class="sourceCode cpp">scan</code>)</span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">remove</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">remove_if</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2</span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">repeat</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2 <span class="citation" data-cites="P2474R1">[<a href="#ref-P2474R1" role="doc-biblioref">P2474R1</a>]</span></span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">repeat_n</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2, but under the name <code class="sourceCode cpp">repeat</code>, <span class="citation" data-cites="P2474R1">[<a href="#ref-P2474R1" role="doc-biblioref">P2474R1</a>]</span></span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">replace</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">replace_if</code></td>
<td>range-v3</td>
<td><span class="yellow">Tier 2</span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">reverse</code></td>
<td>C++20</td>
<td>C++20</td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">sample</code></td>
<td>range-v3</td>
<td><span class="diffdel">Tier 3</span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">scan</code></td>
<td>(not in range-v3)</td>
<td><span class="yellow">Tier 2, as a rename of what is <code class="sourceCode cpp">partial_sum</code> in range-v3</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">set_difference</code></td>
<td>range-v3</td>
<td><span class="diffdel">Tier 3</span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">set_intersection</code></td>
<td>range-v3</td>
<td><span class="diffdel">Tier 3</span></td>
</tr>
<tr class="odd">
<td><code class="sourceCode cpp">set_union</code></td>
<td>range-v3</td>
<td><span class="diffdel">Tier 3</span></td>
</tr>
<tr class="even">
<td><code class="sourceCode cpp">set_symmetric_difference</code></td>
<td>range-v3</td>
<td><span class="diffdel">Tier 3</span></td>
</tr>
<tr class="odd">