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<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang xml:lang>
<head>
<meta charset="utf-8" />
<meta name="generator" content="mpark/wg21" />
<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
<meta name="dcterms.date" content="2020-03-01" />
<title>Generalized pack declaration and usage</title>
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<h1 class="title" style="text-align:center">Generalized pack declaration and usage</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P1858R2</td>
</tr>
<tr>
<td>Date:</td>
<td>2020-03-01</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>
EWG<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>
</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="#packs-packs-packs"><span class="toc-section-number">3</span> Packs, packs, packs<span></span></a>
<ul>
<li><a href="#member-packs"><span class="toc-section-number">3.1</span> Member packs<span></span></a></li>
<li><a href="#empty-variable-pack"><span class="toc-section-number">3.2</span> Empty variable pack<span></span></a></li>
<li><a href="#constructor-and-initializer-packs"><span class="toc-section-number">3.3</span> Constructor and initializer packs<span></span></a></li>
<li><a href="#pack-indexing"><span class="toc-section-number">3.4</span> Pack Indexing<span></span></a></li>
<li><a href="#extending-structured-bindings"><span class="toc-section-number">3.5</span> Extending Structured Bindings<span></span></a></li>
<li><a href="#packs-at-block-scope"><span class="toc-section-number">3.6</span> Packs at block scope<span></span></a></li>
<li><a href="#implementing-variant"><span class="toc-section-number">3.7</span> Implementing variant<span></span></a></li>
<li><a href="#other-examples"><span class="toc-section-number">3.8</span> Other Examples<span></span></a></li>
</ul></li>
<li><a href="#expanding-a-type-into-a-pack"><span class="toc-section-number">4</span> Expanding a type into a pack<span></span></a>
<ul>
<li><a href="#generalized-unpacking-with"><span class="toc-section-number">4.1</span> Generalized unpacking with <code class="sourceCode cpp"><span class="op">[:]</span></code><span></span></a></li>
<li><a href="#syntax-free-unpacking"><span class="toc-section-number">4.2</span> Syntax-free unpacking?<span></span></a></li>
<li><a href="#other-examples-of-unpacking"><span class="toc-section-number">4.3</span> Other examples of unpacking<span></span></a></li>
<li><a href="#generalizing-slicing-further"><span class="toc-section-number">4.4</span> Generalizing slicing further<span></span></a></li>
<li><a href="#examples-with-boost.mp11"><span class="toc-section-number">4.5</span> Examples with Boost.Mp11<span></span></a></li>
</ul></li>
<li><a href="#can-functions-return-a-pack"><span class="toc-section-number">5</span> Can functions return a pack?<span></span></a></li>
<li><a href="#disambiguation"><span class="toc-section-number">6</span> Disambiguation<span></span></a>
<ul>
<li><a href="#disambiguating-dependent"><span class="toc-section-number">6.1</span> Disambiguating Dependent Packs<span></span></a></li>
<li><a href="#disambiguating-packs"><span class="toc-section-number">6.2</span> Disambiguating packs of tuples<span></span></a></li>
<li><a href="#nested-pack-expansions"><span class="toc-section-number">6.3</span> Nested pack expansions<span></span></a></li>
</ul></li>
<li><a href="#what-about-reflection"><span class="toc-section-number">7</span> What about Reflection?<span></span></a></li>
<li><a href="#what-about-stdpair"><span class="toc-section-number">8</span> What about <code class="sourceCode cpp">std<span class="op">:</span>pair</code>?<span></span></a></li>
<li><a href="#one-paper-or-many"><span class="toc-section-number">9</span> One paper or many<span></span></a></li>
<li><a href="#proposal"><span class="toc-section-number">10</span> Proposal<span></span></a>
<ul>
<li><a href="#pack-declarations"><span class="toc-section-number">10.1</span> Pack declarations<span></span></a></li>
<li><a href="#dependent-packs"><span class="toc-section-number">10.2</span> Dependent packs<span></span></a></li>
<li><a href="#pack-indexing-1"><span class="toc-section-number">10.3</span> Pack Indexing<span></span></a></li>
<li><a href="#adding-a-layer-of-packness"><span class="toc-section-number">10.4</span> Adding a layer of packness<span></span></a></li>
<li><a href="#pack-slicing"><span class="toc-section-number">10.5</span> Pack slicing<span></span></a></li>
</ul></li>
<li><a href="#acknowledgments"><span class="toc-section-number">11</span> Acknowledgments<span></span></a></li>
<li><a href="#bibliography"><span class="toc-section-number">12</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>R1 <span class="citation" data-cites="P1858R1">[<a href="#ref-P1858R1" role="doc-biblioref">P1858R1</a>]</span> was presented in EWGI in Prague <span class="citation" data-cites="EWGI.Prague">[<a href="#ref-EWGI.Prague" role="doc-biblioref">EWGI.Prague</a>]</span>. An ambiguity in pack indexing was pointed out, which is resolved here (see <a href="#pack-indexing-ambiguity">pack indexing</a>). The structured bindings simplification was split out into its own paper, <span class="citation" data-cites="P2120R0">[<a href="#ref-P2120R0" role="doc-biblioref">P2120R0</a>]</span>, and this paper includes a new <a href="#one-paper-or-many">discussion</a> on the merits of splitting it into multiple papers.</p>
<p>R0 <span class="citation" data-cites="P1858R0">[<a href="#ref-P1858R0" role="doc-biblioref">P1858R0</a>]</span> was presented in EWGI in Belfast <span class="citation" data-cites="EWGI.Belfast">[<a href="#ref-EWGI.Belfast" role="doc-biblioref">EWGI.Belfast</a>]</span>, where further work was encouraged (9-4-1-0-0). Since then, several substantial changes have been made to this paper.</p>
<ul>
<li>The overloadable <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">...()</span></code> and <code class="sourceCode cpp"><span class="kw">using</span> <span class="op">...</span></code> were removed.</li>
<li>Pack expansion is now driven through structured bindings, which are extended on the library side as well.</li>
<li><code class="sourceCode cpp"><span class="kw">constexpr</span></code> ranges removed from this paper, should be handled by expansion statements.</li>
<li>Discussion of functions returning packs - no longer being proposed due to ambiguity of what it actually could mean.</li>
<li>Discussion of pack literals.</li>
</ul>
<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>C++11 introduced variadic templates, one of the truly transformational language features introduced that standard. Despite pretty tight restrictions on where packs could be declared and how they could be used, this feature has proven incredibly successful. Three standards later, there hasn’t even been much change. C++17 added a couple new ways to use packs (fold expressions <span class="citation" data-cites="N4191">[<a href="#ref-N4191" role="doc-biblioref">N4191</a>]</span> and using-declarations <span class="citation" data-cites="P0195R2">[<a href="#ref-P0195R2" role="doc-biblioref">P0195R2</a>]</span>), and C++20 will add a new way to introduce them (in lambda capture <span class="citation" data-cites="P0780R2">[<a href="#ref-P0780R2" role="doc-biblioref">P0780R2</a>]</span>). A proposal to iterate over them (expansion statements <span class="citation" data-cites="P1306R1">[<a href="#ref-P1306R1" role="doc-biblioref">P1306R1</a>]</span>) didn’t quite make it. That’s it.</p>
<p>There have been many papers in the interlude about trying to enhance pack functionality: a language typelist <span class="citation" data-cites="N3728">[<a href="#ref-N3728" role="doc-biblioref">N3728</a>]</span>, fixed size and homogeneous packs <span class="citation" data-cites="N4072">[<a href="#ref-N4072" role="doc-biblioref">N4072</a>]</span> (and later <span class="citation" data-cites="P1219R1">[<a href="#ref-P1219R1" role="doc-biblioref">P1219R1</a>]</span>), indexing and slicing into packs <span class="citation" data-cites="N4235">[<a href="#ref-N4235" role="doc-biblioref">N4235</a>]</span> and <span class="citation" data-cites="P0535R0">[<a href="#ref-P0535R0" role="doc-biblioref">P0535R0</a>]</span>, being able to declare packs in more places <span class="citation" data-cites="P0341R0">[<a href="#ref-P0341R0" role="doc-biblioref">P0341R0</a>]</span> and other places <span class="citation" data-cites="P1061R0">[<a href="#ref-P1061R0" role="doc-biblioref">P1061R0</a>]</span>.</p>
<p>In short, there’s been work in this space, although not all of these papers have been discussed by Evolution. Although, many of these have been received favorably and then never followed up on.</p>
<p>Yet the features that keep getting hinted at and requested again and again are still missing from our feature set:</p>
<ol type="1">
<li>the ability to declare a variable pack at class, namespace, or local scope</li>
<li>the ability to index into a pack</li>
<li>the ability to unpack a tuple, or tuple-like type, inline</li>
</ol>
<p>All efficiently, from a compile time perspective. Instead, for (1) we have to use <code class="sourceCode cpp">std<span class="op">::</span>tuple</code>, for (2) we have to use <code class="sourceCode cpp">std<span class="op">::</span>get</code>, <code class="sourceCode cpp">std<span class="op">::</span>tuple_element</code>, or, if we’re only dealing with types, something like <code class="sourceCode cpp">mp_at_c</code> <span class="citation" data-cites="Boost.Mp11">[<a href="#ref-Boost.Mp11" role="doc-biblioref">Boost.Mp11</a>]</span>, for (3) we have to use <code class="sourceCode cpp">std<span class="op">::</span>apply<span class="op">()</span></code>, which necessarily introduces a new scope. <code class="sourceCode cpp">std<span class="op">::</span>apply<span class="op">()</span></code> is actually worse than that, since it doesn’t play well with callables that aren’t objects and even worse if you want to use additional arguments on top of that:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a>std<span class="op">::</span>tuple args<span class="op">(</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">)</span>;</span>
<span id="cb1-2"><a href="#cb1-2"></a></span>
<span id="cb1-3"><a href="#cb1-3"></a><span class="co">// I want to call f with args... and then 4</span></span>
<span id="cb1-4"><a href="#cb1-4"></a>std<span class="op">::</span>apply<span class="op">([&](</span><span class="kw">auto</span><span class="op">...</span> vs<span class="op">){</span> <span class="cf">return</span> f<span class="op">(</span>vs<span class="op">...</span>, <span class="dv">4</span><span class="op">)</span>; <span class="op">}</span>, args<span class="op">)</span>;</span></code></pre></div>
<p>Matt Calabrese is working on a library facility to help address the shortcomings here <span class="citation" data-cites="Calabrese.Argot">[<a href="#ref-Calabrese.Argot" role="doc-biblioref">Calabrese.Argot</a>]</span>.</p>
<p>This paper attempts to provide a solution to these problems, building on the work of prior paper authors. The goal of this paper is to provide a better implementations for a library <code class="sourceCode cpp">tuple</code> and <code class="sourceCode cpp">variant</code>, ones that ends up being much easier to implement, more compiler friendly, and more ergonomic. The paper will piecewise introduce the necessary language features, increasing in complexity as it goes, and is divided into two broad sections:</p>
<ul>
<li>introducing the ability to declare packs in more places and to index into them</li>
<li>introducing the ability to convert types to packs and to index into them</li>
</ul>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="packs-packs-packs" data-number="3" style="border-bottom:1px solid #cccccc"><span class="header-section-number">3</span> Packs, packs, packs<a href="#packs-packs-packs" class="self-link"></a></h1>
<p>This section will propose several new language features, with the motivating example being a far simpler implementation of <code class="sourceCode cpp">tuple</code>, but which are generally applicable to all uses of variadic templates in C++ today.</p>
<h2 data-number="3.1" id="member-packs" data-number="3.1"><span class="header-section-number">3.1</span> Member packs<a href="#member-packs" class="self-link"></a></h2>
<p>This paper proposes the ability to declare a variable pack wherever we can declare a variable today:</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">namespace</span> xstd <span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb2-3"><a href="#cb2-3"></a> <span class="kw">struct</span> tuple <span class="op">{</span></span>
<span id="cb2-4"><a href="#cb2-4"></a> Ts<span class="op">...</span> elems;</span>
<span id="cb2-5"><a href="#cb2-5"></a> <span class="op">}</span>;</span>
<span id="cb2-6"><a href="#cb2-6"></a><span class="op">}</span></span></code></pre></div>
<p>That gives us all the members that we need, using a syntax that arguably has obvious meaning to any reader familiar with C++ packs. All the usual rules follow directly from there. That class template is an aggregate, so we can use aggregate initialization:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a>xstd<span class="op">::</span>tuple<span class="op"><</span><span class="dt">int</span>, <span class="dt">int</span>, <span class="dt">int</span><span class="op">></span> x<span class="op">{</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">}</span>;</span></code></pre></div>
<p>Or, in C++20:</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a>xstd<span class="op">::</span>tuple y<span class="op">{</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">}</span>;</span></code></pre></div>
<h2 data-number="3.2" id="empty-variable-pack" data-number="3.2"><span class="header-section-number">3.2</span> Empty variable pack<a href="#empty-variable-pack" class="self-link"></a></h2>
<p>What does <code class="sourceCode cpp">xstd<span class="op">::</span>tuple<span class="op"><></span> t;</code> mean here? The same way that an empty function parameter pack means a function taking no arguments, an empty member variable pack means no member variables. <code class="sourceCode cpp">xstd<span class="op">::</span>tuple<span class="op"><></span></code> is an empty type.</p>
<h2 data-number="3.3" id="constructor-and-initializer-packs" data-number="3.3"><span class="header-section-number">3.3</span> Constructor and initializer packs<a href="#constructor-and-initializer-packs" class="self-link"></a></h2>
<p>But <code class="sourceCode cpp">tuple</code> has constructors. <code class="sourceCode cpp">tuple</code> has <em>lots</em> of constructors. We’re not going to go through all of them in this paper, just the interesting ones. But let’s at least start with the easy ones:</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> xstd <span class="op">{</span></span>
<span id="cb5-2"><a href="#cb5-2"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb5-3"><a href="#cb5-3"></a> <span class="kw">class</span> tuple <span class="op">{</span></span>
<span id="cb5-4"><a href="#cb5-4"></a> <span class="kw">public</span><span class="op">:</span></span>
<span id="cb5-5"><a href="#cb5-5"></a> <span class="kw">constexpr</span> tuple<span class="op">()</span> <span class="kw">requires</span> <span class="op">(</span>std<span class="op">::</span>default_constructible<span class="op"><</span>Ts<span class="op">></span> <span class="op">&&</span> <span class="op">...)</span></span>
<span id="cb5-6"><a href="#cb5-6"></a> <span class="op">:</span> elems<span class="op">()...</span></span>
<span id="cb5-7"><a href="#cb5-7"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb5-8"><a href="#cb5-8"></a> </span>
<span id="cb5-9"><a href="#cb5-9"></a> <span class="kw">constexpr</span> tuple<span class="op">(</span>Ts <span class="kw">const</span><span class="op">&...</span> args<span class="op">)</span></span>
<span id="cb5-10"><a href="#cb5-10"></a> <span class="kw">requires</span> <span class="op">(</span>std<span class="op">::</span>copy_constructible<span class="op"><</span>Ts<span class="op">></span> <span class="op">&&</span> <span class="op">...)</span></span>
<span id="cb5-11"><a href="#cb5-11"></a> <span class="op">:</span> elems<span class="op">(</span>args<span class="op">)...</span></span>
<span id="cb5-12"><a href="#cb5-12"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb5-13"><a href="#cb5-13"></a></span>
<span id="cb5-14"><a href="#cb5-14"></a> <span class="kw">private</span><span class="op">:</span></span>
<span id="cb5-15"><a href="#cb5-15"></a> Ts<span class="op">...</span> elems;</span>
<span id="cb5-16"><a href="#cb5-16"></a> <span class="op">}</span>;</span>
<span id="cb5-17"><a href="#cb5-17"></a><span class="op">}</span></span></code></pre></div>
<p>Let’s pick a more complex constructor. A <code class="sourceCode cpp">std<span class="op">::</span>tuple<span class="op"><</span>Ts<span class="op">...></span></code> can be constructed from a <code class="sourceCode cpp">std<span class="op">::</span>pair<span class="op"><</span>T, U<span class="op">></span></code> if <code class="sourceCode cpp"><span class="kw">sizeof</span><span class="op">...(</span>Ts<span class="op">)</span> <span class="op">==</span> <span class="dv">2</span></code> and the two corresponding types are convertible. How would we implement that? To do that check, we need to get the corresponding types. How do we get the first and second types from <code class="sourceCode cpp">Ts</code>?</p>
<h2 data-number="3.4" id="pack-indexing" data-number="3.4"><span class="header-section-number">3.4</span> Pack Indexing<a href="#pack-indexing" class="self-link"></a></h2>
<p>This paper proposes a “simple selection” facility similar to the one initially introduced in <span class="citation" data-cites="N4235">[<a href="#ref-N4235" role="doc-biblioref">N4235</a>]</span> (and favorably received in Urbana 2014): <code class="sourceCode cpp">T<span class="op">...[</span>I<span class="op">]</span></code> is the <code class="sourceCode cpp">I</code>th element of the pack <code class="sourceCode cpp">T</code>, which is a type or value or template based on what kind of pack <code class="sourceCode cpp">T</code> is. <a href="#disambiguating-packs">Later sections</a> of this paper will discuss why this paper diverges from that original proposal in choice of syntax.</p>
<p>Such indexing allows for implementing the pair converting constructor:</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">namespace</span> xstd <span class="op">{</span></span>
<span id="cb6-2"><a href="#cb6-2"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb6-3"><a href="#cb6-3"></a> <span class="kw">class</span> tuple <span class="op">{</span></span>
<span id="cb6-4"><a href="#cb6-4"></a> <span class="kw">public</span><span class="op">:</span></span>
<span id="cb6-5"><a href="#cb6-5"></a> <span class="kw">template</span> <span class="op"><</span>std<span class="op">::</span>convertible_to<span class="op"><</span>Ts<span class="op">...[</span><span class="dv">0</span><span class="op">]></span> T,</span>
<span id="cb6-6"><a href="#cb6-6"></a> std<span class="op">::</span>convertible_to<span class="op"><</span>Ts<span class="op">...[</span><span class="dv">1</span><span class="op">]></span> U<span class="op">></span></span>
<span id="cb6-7"><a href="#cb6-7"></a> <span class="kw">requires</span> <span class="kw">sizeof</span><span class="op">...(</span>Ts<span class="op">)</span> <span class="op">==</span> <span class="dv">2</span></span>
<span id="cb6-8"><a href="#cb6-8"></a> <span class="kw">constexpr</span> tuple<span class="op">(</span>std<span class="op">::</span>pair<span class="op"><</span>T, U<span class="op">></span> <span class="kw">const</span><span class="op">&</span> p<span class="op">)</span></span>
<span id="cb6-9"><a href="#cb6-9"></a> <span class="op">:</span> elems<span class="op">...[</span><span class="dv">0</span><span class="op">](</span>p<span class="op">.</span>first<span class="op">)</span></span>
<span id="cb6-10"><a href="#cb6-10"></a> , elems<span class="op">...[</span><span class="dv">1</span><span class="op">](</span>p<span class="op">.</span>second<span class="op">)</span></span>
<span id="cb6-11"><a href="#cb6-11"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb6-12"><a href="#cb6-12"></a> <span class="kw">private</span><span class="op">:</span></span>
<span id="cb6-13"><a href="#cb6-13"></a> Ts<span class="op">...</span> elems;</span>
<span id="cb6-14"><a href="#cb6-14"></a> <span class="op">}</span>;</span>
<span id="cb6-15"><a href="#cb6-15"></a><span class="op">}</span></span></code></pre></div>
<p>Notably, in an earlier example we constructed the pack as a single entity and here we are constructing each element of the pack separately. Both are fine. We’ll see a simpler way to do this <a href="#tuple-pair">later</a>.</p>
<p>A properly constrained converting constructor from a pack:</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">namespace</span> xstd <span class="op">{</span></span>
<span id="cb7-2"><a href="#cb7-2"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb7-3"><a href="#cb7-3"></a> <span class="kw">class</span> tuple <span class="op">{</span></span>
<span id="cb7-4"><a href="#cb7-4"></a> <span class="kw">public</span><span class="op">:</span></span>
<span id="cb7-5"><a href="#cb7-5"></a> <span class="kw">template</span> <span class="op"><</span>std<span class="op">::</span>constructible<span class="op"><</span>Ts<span class="op">>...</span> Us<span class="op">></span> <span class="co">// everything is convertible</span></span>
<span id="cb7-6"><a href="#cb7-6"></a> <span class="kw">requires</span> <span class="op">(</span><span class="kw">sizeof</span><span class="op">...(</span>Us<span class="op">)</span> <span class="op">></span> <span class="dv">1</span> <span class="op">||</span> <span class="co">// exclude the copy ctor match</span></span>
<span id="cb7-7"><a href="#cb7-7"></a> <span class="op">!</span>std<span class="op">::</span>derived_from<span class="op"><</span>std<span class="op">::</span>remove_cvref_t<span class="op"><</span>Us<span class="op">...[</span><span class="dv">0</span><span class="op">]></span>, tuple<span class="op">>)</span></span>
<span id="cb7-8"><a href="#cb7-8"></a> <span class="kw">constexpr</span> tuple<span class="op">(</span>Us<span class="op">&&...</span> us<span class="op">)</span></span>
<span id="cb7-9"><a href="#cb7-9"></a> <span class="op">:</span> elems<span class="op">(</span>std<span class="op">::</span>forward<span class="op"><</span>Us<span class="op">>(</span>us<span class="op">))...</span></span>
<span id="cb7-10"><a href="#cb7-10"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb7-11"><a href="#cb7-11"></a> <span class="kw">private</span><span class="op">:</span></span>
<span id="cb7-12"><a href="#cb7-12"></a> Ts<span class="op">...</span> elems;</span>
<span id="cb7-13"><a href="#cb7-13"></a> <span class="op">}</span>;</span>
<span id="cb7-14"><a href="#cb7-14"></a><span class="op">}</span></span></code></pre></div>
<p>As well as implementing <code class="sourceCode cpp">tuple_element</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">namespace</span> std <span class="op">{</span></span>
<span id="cb8-2"><a href="#cb8-2"></a> <span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I, <span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb8-3"><a href="#cb8-3"></a> <span class="kw">struct</span> tuple_element<span class="op"><</span>I, xstd<span class="op">::</span>tuple<span class="op"><</span>Ts<span class="op">...>></span></span>
<span id="cb8-4"><a href="#cb8-4"></a> <span class="op">{</span></span>
<span id="cb8-5"><a href="#cb8-5"></a> <span class="kw">using</span> type <span class="op">=</span> Ts<span class="op">...[</span>I<span class="op">]</span>;</span>
<span id="cb8-6"><a href="#cb8-6"></a> <span class="op">}</span>;</span>
<span id="cb8-7"><a href="#cb8-7"></a><span class="op">}</span></span></code></pre></div>
<p>And a member <code class="sourceCode cpp">get</code>:</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="kw">namespace</span> xstd <span class="op">{</span></span>
<span id="cb9-2"><a href="#cb9-2"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb9-3"><a href="#cb9-3"></a> <span class="kw">class</span> tuple <span class="op">{</span></span>
<span id="cb9-4"><a href="#cb9-4"></a> <span class="kw">public</span><span class="op">:</span></span>
<span id="cb9-5"><a href="#cb9-5"></a> <span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I<span class="op">></span></span>
<span id="cb9-6"><a href="#cb9-6"></a> <span class="kw">auto</span> get<span class="op">()</span> <span class="kw">const</span><span class="op">&</span> <span class="op">-></span> Ts<span class="op">...[</span>I<span class="op">]</span> <span class="kw">const</span><span class="op">&</span> <span class="op">{</span></span>
<span id="cb9-7"><a href="#cb9-7"></a> <span class="cf">return</span> elems<span class="op">...[</span>I<span class="op">]</span>;</span>
<span id="cb9-8"><a href="#cb9-8"></a> <span class="op">}</span></span>
<span id="cb9-9"><a href="#cb9-9"></a> </span>
<span id="cb9-10"><a href="#cb9-10"></a> <span class="kw">private</span><span class="op">:</span></span>
<span id="cb9-11"><a href="#cb9-11"></a> Ts<span class="op">...</span> elems;</span>
<span id="cb9-12"><a href="#cb9-12"></a> <span class="op">}</span>;</span>
<span id="cb9-13"><a href="#cb9-13"></a><span class="op">}</span></span></code></pre></div>
<p>This is a lot nicer than status quo. Consider just the implementation complexity difference between these two versions (where the status quo is taken from the libc++ implementation). I’m including just the pieces I’ve shown up ’til now: the default constructor, n-ary converting constructor, <code class="sourceCode cpp">pair</code> converting constructor, and the structured bindings opt-in as a member.</p>
<p>When considering the differences below, don’t just look at the difference in number of lines. Consider also the difference in complexity. You almost don’t even really have to think about how to implement the “proposed” version, whereas the status quo, you really have to plan quite carefully and think a lot at every step of the way.</p>
<p>Also I’m cheating a little and using Boost.Mp11 for pack indexing.</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Status Quo</strong>
</div></th>
<th><div style="text-align:center">
<strong>Proposed</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div class="sourceCode" id="cb10"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb10-1"><a href="#cb10-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...></span> <span class="kw">class</span> tuple;</span>
<span id="cb10-2"><a href="#cb10-2"></a></span>
<span id="cb10-3"><a href="#cb10-3"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb10-4"><a href="#cb10-4"></a><span class="kw">struct</span> std<span class="op">::</span>tuple_size<span class="op"><</span>tuple<span class="op"><</span>Ts<span class="op">...>></span></span>
<span id="cb10-5"><a href="#cb10-5"></a> <span class="op">:</span> integral_constant<span class="op"><</span><span class="dt">size_t</span>, <span class="kw">sizeof</span><span class="op">...(</span>Ts<span class="op">)></span></span>
<span id="cb10-6"><a href="#cb10-6"></a><span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb10-7"><a href="#cb10-7"></a></span>
<span id="cb10-8"><a href="#cb10-8"></a><span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span>, <span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb10-9"><a href="#cb10-9"></a><span class="kw">struct</span> std<span class="op">::</span>tuple_element<span class="op"><</span>I, tuple<span class="op"><</span>Ts<span class="op">...>></span></span>
<span id="cb10-10"><a href="#cb10-10"></a><span class="op">{</span></span>
<span id="cb10-11"><a href="#cb10-11"></a> <span class="kw">using</span> type <span class="op">=</span> mp_at_c<span class="op"><</span>I, mp_list<span class="op"><</span>Ts<span class="op">...>></span>;</span>
<span id="cb10-12"><a href="#cb10-12"></a><span class="op">}</span>;</span>
<span id="cb10-13"><a href="#cb10-13"></a></span>
<span id="cb10-14"><a href="#cb10-14"></a><span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> Idx, <span class="kw">typename</span> T<span class="op">></span></span>
<span id="cb10-15"><a href="#cb10-15"></a><span class="kw">class</span> tuple_leaf <span class="op">{</span></span>
<span id="cb10-16"><a href="#cb10-16"></a> <span class="op">[[</span><span class="at">no_unique_address</span><span class="op">]]</span> T value;</span>
<span id="cb10-17"><a href="#cb10-17"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...></span> <span class="kw">friend</span> <span class="kw">class</span> tuple;</span>
<span id="cb10-18"><a href="#cb10-18"></a> </span>
<span id="cb10-19"><a href="#cb10-19"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb10-20"><a href="#cb10-20"></a> <span class="kw">constexpr</span> tuple_leaf<span class="op">()</span></span>
<span id="cb10-21"><a href="#cb10-21"></a> <span class="kw">requires</span> std<span class="op">::</span>is_default_constructible<span class="op"><</span>T<span class="op">></span></span>
<span id="cb10-22"><a href="#cb10-22"></a> <span class="op">:</span> value<span class="op">()</span></span>
<span id="cb10-23"><a href="#cb10-23"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb10-24"><a href="#cb10-24"></a> </span>
<span id="cb10-25"><a href="#cb10-25"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> U<span class="op">></span></span>
<span id="cb10-26"><a href="#cb10-26"></a> <span class="kw">constexpr</span> tuple_leaf<span class="op">(</span>std<span class="op">::</span>in_place_t, U<span class="op">&&</span> u<span class="op">)</span></span>
<span id="cb10-27"><a href="#cb10-27"></a> <span class="op">:</span> value<span class="op">(</span>std<span class="op">::</span>forward<span class="op"><</span>U<span class="op">>(</span>u<span class="op">))</span></span>
<span id="cb10-28"><a href="#cb10-28"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb10-29"><a href="#cb10-29"></a><span class="op">}</span>;</span>
<span id="cb10-30"><a href="#cb10-30"></a></span>
<span id="cb10-31"><a href="#cb10-31"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> Indices, <span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb10-32"><a href="#cb10-32"></a><span class="kw">class</span> tuple_impl;</span>
<span id="cb10-33"><a href="#cb10-33"></a></span>
<span id="cb10-34"><a href="#cb10-34"></a><span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span><span class="op">...</span> Is, <span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb10-35"><a href="#cb10-35"></a><span class="kw">class</span> tuple_impl<span class="op"><</span>index_sequence<span class="op"><</span>Is<span class="op">...></span>, Ts<span class="op">...></span></span>
<span id="cb10-36"><a href="#cb10-36"></a> <span class="op">:</span> <span class="kw">public</span> tuple_leaf<span class="op"><</span>Is, Ts<span class="op">>...</span></span>
<span id="cb10-37"><a href="#cb10-37"></a><span class="op">{</span></span>
<span id="cb10-38"><a href="#cb10-38"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb10-39"><a href="#cb10-39"></a> tuple_impl<span class="op">()</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb10-40"><a href="#cb10-40"></a> </span>
<span id="cb10-41"><a href="#cb10-41"></a> <span class="kw">template</span> <span class="op"><</span>std<span class="op">::</span>constructible<span class="op"><</span>Ts<span class="op">>...</span> Us<span class="op">></span></span>
<span id="cb10-42"><a href="#cb10-42"></a> <span class="kw">requires</span> <span class="op">(</span><span class="kw">sizeof</span><span class="op">...(</span>Us<span class="op">)</span> <span class="op">></span> <span class="dv">1</span> <span class="op">||</span></span>
<span id="cb10-43"><a href="#cb10-43"></a> <span class="op">!</span>std<span class="op">::</span>derived_from<span class="op"><</span></span>
<span id="cb10-44"><a href="#cb10-44"></a> std<span class="op">::</span>remove_cvref_t<span class="op"><</span>Us<span class="op">...[</span><span class="dv">0</span><span class="op">]></span>, </span>
<span id="cb10-45"><a href="#cb10-45"></a> tuple_impl<span class="op">>)</span></span>
<span id="cb10-46"><a href="#cb10-46"></a> <span class="kw">constexpr</span> tuple_impl<span class="op">(</span>Us<span class="op">&&...</span> us<span class="op">)</span></span>
<span id="cb10-47"><a href="#cb10-47"></a> <span class="op">:</span> tuple_leaf<span class="op"><</span>Is, Ts<span class="op">>(</span></span>
<span id="cb10-48"><a href="#cb10-48"></a> std<span class="op">::</span>in_place, std<span class="op">::</span>forward<span class="op"><</span>Us<span class="op">>(</span>us<span class="op">))...</span></span>
<span id="cb10-49"><a href="#cb10-49"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb10-50"><a href="#cb10-50"></a> </span>
<span id="cb10-51"><a href="#cb10-51"></a> <span class="kw">template</span> <span class="op"><</span>std<span class="op">::</span>convertible_to<span class="op"><</span></span>
<span id="cb10-52"><a href="#cb10-52"></a> mp_at_c<span class="op"><</span><span class="dv">0</span>, mp_list<span class="op"><</span>Ts<span class="op">...>></span> T,</span>
<span id="cb10-53"><a href="#cb10-53"></a> std<span class="op">::</span>convertible_to<span class="op"><</span></span>
<span id="cb10-54"><a href="#cb10-54"></a> mp_at_c<span class="op"><</span><span class="dv">1</span>, mp_list<span class="op"><</span>Ts<span class="op">...>></span> U<span class="op">></span></span>
<span id="cb10-55"><a href="#cb10-55"></a> <span class="kw">requires</span> <span class="op">(</span><span class="kw">sizeof</span><span class="op">...(</span>Ts<span class="op">)</span> <span class="op">==</span> <span class="dv">2</span><span class="op">)</span></span>
<span id="cb10-56"><a href="#cb10-56"></a> <span class="kw">constexpr</span> tuple_impl<span class="op">(</span>std<span class="op">::</span>pair<span class="op"><</span>T, U<span class="op">></span> <span class="kw">const</span><span class="op">&</span> p<span class="op">)</span></span>
<span id="cb10-57"><a href="#cb10-57"></a> <span class="op">:</span> tuple_impl<span class="op">(</span>p<span class="op">.</span>first, p<span class="op">.</span>second<span class="op">)</span></span>
<span id="cb10-58"><a href="#cb10-58"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb10-59"><a href="#cb10-59"></a><span class="op">}</span>;</span>
<span id="cb10-60"><a href="#cb10-60"></a></span>
<span id="cb10-61"><a href="#cb10-61"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb10-62"><a href="#cb10-62"></a><span class="kw">class</span> tuple <span class="op">:</span> <span class="kw">public</span> tuple_impl<span class="op"><</span></span>
<span id="cb10-63"><a href="#cb10-63"></a> make_index_sequence<span class="op"><</span><span class="kw">sizeof</span><span class="op">...(</span>Ts<span class="op">)></span>, Ts<span class="op">...></span></span>
<span id="cb10-64"><a href="#cb10-64"></a><span class="op">{</span> </span>
<span id="cb10-65"><a href="#cb10-65"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb10-66"><a href="#cb10-66"></a> <span class="kw">using</span> tuple<span class="op">::</span>tuple_impl<span class="op">::</span>tuple_impl;</span>
<span id="cb10-67"><a href="#cb10-67"></a></span>
<span id="cb10-68"><a href="#cb10-68"></a> <span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I<span class="op">></span></span>
<span id="cb10-69"><a href="#cb10-69"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> get<span class="op">()</span> <span class="op">-></span> tuple_element_t<span class="op"><</span>I, tuple<span class="op">></span> <span class="kw">const</span><span class="op">&</span> <span class="op">{</span></span>
<span id="cb10-70"><a href="#cb10-70"></a> <span class="kw">using</span> leaf <span class="op">=</span> tuple_leaf<span class="op"><</span>I, tuple_element_t<span class="op"><</span>I, tuple<span class="op">>></span>;</span>
<span id="cb10-71"><a href="#cb10-71"></a> <span class="cf">return</span> <span class="kw">static_cast</span><span class="op"><</span>leaf <span class="kw">const</span><span class="op">&>(*</span><span class="kw">this</span><span class="op">).</span>value;</span>
<span id="cb10-72"><a href="#cb10-72"></a> <span class="op">}</span></span>
<span id="cb10-73"><a href="#cb10-73"></a><span class="op">}</span>;</span></code></pre></div></td>
<td><div class="sourceCode" id="cb11"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb11-1"><a href="#cb11-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...></span> <span class="kw">class</span> tuple;</span>
<span id="cb11-2"><a href="#cb11-2"></a></span>
<span id="cb11-3"><a href="#cb11-3"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb11-4"><a href="#cb11-4"></a><span class="kw">struct</span> tuple_size<span class="op"><</span>tuple<span class="op"><</span>Ts<span class="op">...>></span></span>
<span id="cb11-5"><a href="#cb11-5"></a> <span class="op">:</span> integral_constant<span class="op"><</span><span class="dt">size_t</span>, <span class="kw">sizeof</span><span class="op">...(</span>Ts<span class="op">)></span></span>
<span id="cb11-6"><a href="#cb11-6"></a><span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb11-7"><a href="#cb11-7"></a></span>
<span id="cb11-8"><a href="#cb11-8"></a><span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I, <span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb11-9"><a href="#cb11-9"></a><span class="kw">struct</span> tuple_element<span class="op"><</span>I, tuple<span class="op"><</span>Ts<span class="op">...>></span></span>
<span id="cb11-10"><a href="#cb11-10"></a><span class="op">{</span></span>
<span id="cb11-11"><a href="#cb11-11"></a> <span class="kw">using</span> type <span class="op">=</span> Ts<span class="op">...[</span>I<span class="op">]</span>;</span>
<span id="cb11-12"><a href="#cb11-12"></a><span class="op">}</span>;</span>
<span id="cb11-13"><a href="#cb11-13"></a></span>
<span id="cb11-14"><a href="#cb11-14"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb11-15"><a href="#cb11-15"></a><span class="kw">class</span> tuple <span class="op">{</span></span>
<span id="cb11-16"><a href="#cb11-16"></a> <span class="op">[[</span><span class="at">no_unique_address</span><span class="op">]]</span> Ts<span class="op">...</span> elems;</span>
<span id="cb11-17"><a href="#cb11-17"></a></span>
<span id="cb11-18"><a href="#cb11-18"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb11-19"><a href="#cb11-19"></a> <span class="kw">constexpr</span> tuple<span class="op">()</span></span>
<span id="cb11-20"><a href="#cb11-20"></a> <span class="kw">requires</span> <span class="op">(</span>std<span class="op">::</span>default_constructible<span class="op"><</span>Ts<span class="op">></span> <span class="op">&&</span> <span class="op">...)</span></span>
<span id="cb11-21"><a href="#cb11-21"></a> <span class="op">:</span> elems<span class="op">()...</span></span>
<span id="cb11-22"><a href="#cb11-22"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb11-23"><a href="#cb11-23"></a></span>
<span id="cb11-24"><a href="#cb11-24"></a> <span class="kw">template</span> <span class="op"><</span>std<span class="op">::</span>constructible<span class="op"><</span>Ts<span class="op">>...</span> Us<span class="op">></span></span>
<span id="cb11-25"><a href="#cb11-25"></a> <span class="kw">requires</span> <span class="op">(</span><span class="kw">sizeof</span><span class="op">...(</span>Us<span class="op">)</span> <span class="op">></span> <span class="dv">1</span> <span class="op">||</span></span>
<span id="cb11-26"><a href="#cb11-26"></a> <span class="op">!</span>std<span class="op">::</span>derived_from<span class="op"><</span></span>
<span id="cb11-27"><a href="#cb11-27"></a> std<span class="op">::</span>remove_cvref_t<span class="op"><</span>Us<span class="op">...[</span><span class="dv">0</span><span class="op">]></span>, </span>
<span id="cb11-28"><a href="#cb11-28"></a> tuple<span class="op">>)</span></span>
<span id="cb11-29"><a href="#cb11-29"></a> <span class="kw">constexpr</span> tuple<span class="op">(</span>Us<span class="op">&&...</span> us<span class="op">)</span></span>
<span id="cb11-30"><a href="#cb11-30"></a> <span class="op">:</span> elems<span class="op">(</span>std<span class="op">::</span>forward<span class="op"><</span>Us<span class="op">>(</span>us<span class="op">))...</span></span>
<span id="cb11-31"><a href="#cb11-31"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb11-32"><a href="#cb11-32"></a></span>
<span id="cb11-33"><a href="#cb11-33"></a> <span class="kw">template</span> <span class="op"><</span>std<span class="op">::</span>convertible_to<span class="op"><</span>Ts<span class="op">...[</span><span class="dv">0</span><span class="op">]></span> T,</span>
<span id="cb11-34"><a href="#cb11-34"></a> std<span class="op">::</span>convertible_to<span class="op"><</span>Ts<span class="op">...[</span><span class="dv">1</span><span class="op">]></span> U<span class="op">></span></span>
<span id="cb11-35"><a href="#cb11-35"></a> <span class="kw">requires</span> <span class="op">(</span><span class="kw">sizeof</span><span class="op">...(</span>Ts<span class="op">)</span> <span class="op">==</span> <span class="dv">2</span><span class="op">)</span></span>
<span id="cb11-36"><a href="#cb11-36"></a> <span class="kw">constexpr</span> tuple<span class="op">(</span>std<span class="op">::</span>pair<span class="op"><</span>T, U<span class="op">></span> <span class="kw">const</span><span class="op">&</span> p<span class="op">)</span></span>
<span id="cb11-37"><a href="#cb11-37"></a> <span class="op">:</span> elems<span class="op">...[</span><span class="dv">0</span><span class="op">](</span>p<span class="op">.</span>first<span class="op">)</span></span>
<span id="cb11-38"><a href="#cb11-38"></a> , elems<span class="op">...[</span><span class="dv">1</span><span class="op">](</span>p<span class="op">.</span>second<span class="op">)</span></span>
<span id="cb11-39"><a href="#cb11-39"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb11-40"><a href="#cb11-40"></a> </span>
<span id="cb11-41"><a href="#cb11-41"></a> <span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I<span class="op">></span></span>
<span id="cb11-42"><a href="#cb11-42"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> get<span class="op">()</span> <span class="kw">const</span><span class="op">&</span> <span class="op">-></span> Ts<span class="op">...[</span>I<span class="op">]</span> <span class="kw">const</span><span class="op">&</span> <span class="op">{</span></span>
<span id="cb11-43"><a href="#cb11-43"></a> <span class="cf">return</span> elems<span class="op">...[</span>I<span class="op">]</span>;</span>
<span id="cb11-44"><a href="#cb11-44"></a> <span class="op">}</span> </span>
<span id="cb11-45"><a href="#cb11-45"></a><span class="op">}</span>;</span></code></pre></div></td>
</tr>
</tbody>
</table>
<h3 data-number="3.4.1" id="pack-indexing-ambiguity" data-number="3.4.1"><span class="header-section-number">3.4.1</span> Pack Indexing Ambiguity<a href="#pack-indexing-ambiguity" class="self-link"></a></h3>
<p>In EWGI session in Prague, James Touton pointed out the following example:</p>
<div class="sourceCode" id="cb12"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb12-1"><a href="#cb12-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb12-2"><a href="#cb12-2"></a><span class="kw">struct</span> X <span class="op">{</span></span>
<span id="cb12-3"><a href="#cb12-3"></a> <span class="dt">void</span> f<span class="op">(</span>Ts<span class="op">...[</span><span class="dv">1</span><span class="op">])</span>;</span>
<span id="cb12-4"><a href="#cb12-4"></a><span class="op">}</span>;</span></code></pre></div>
<p>Since C++11, this is valid syntax. <code class="sourceCode cpp">X<span class="op"><</span><span class="dt">int</span>, <span class="dt">char</span><span class="op">>::</span>f</code> is a non-template member function that takes two arguments of types <code class="sourceCode cpp"><span class="dt">int</span><span class="op">*</span></code> and <code class="sourceCode cpp"><span class="dt">char</span><span class="op">*</span></code>. This would be more clear if we provided a name to the function parameter and instead spelled it <code class="sourceCode cpp">Ts<span class="op">...</span> p<span class="op">[</span><span class="dv">1</span><span class="op">]</span></code>.</p>
<p>With this paper, <code class="sourceCode cpp">Ts<span class="op">...[</span><span class="dv">1</span><span class="op">]</span></code> would have different meaning: it would be the single type referring to the 2nd type in the pack <code class="sourceCode cpp">Ts</code>. In other words, <code class="sourceCode cpp">X<span class="op"><</span><span class="dt">int</span>, <span class="dt">char</span><span class="op">>::</span>f</code> would instead become a non-template member function that takes a single argument of type <code class="sourceCode cpp"><span class="dt">char</span></code>.</p>
<p>This is a problem. Thankfully, it’s a very small one. Neither gcc or msvc even allow this syntax today (clang and icc do), so that severely limits how much code exists in the wild that uses it. It’s additionally unlikely to be used much since it’s really just a very strange way to write <code class="sourceCode cpp">Ts<span class="op">*...</span></code> without any real benefit over this more direct formulation. Searching for <code class="sourceCode cpp"><span class="op">...[</span></code> on codesearch.isocpp finds 11 uses, all of which are in compiler test cases.</p>
<p>The proposed resolution is that <code class="sourceCode cpp">Ts<span class="op">...[</span><span class="dv">1</span><span class="op">]</span></code> <em>always</em> means pack indexing. If a user really wants the C++11 meaning, they can either name the parameter (as in <code class="sourceCode cpp">Ts<span class="op">...</span> p<span class="op">[</span><span class="dv">1</span><span class="op">]</span></code>) or spell it as a pointer instead (as in <code class="sourceCode cpp">Ts<span class="op">*...</span></code>).</p>
<h2 data-number="3.5" id="extending-structured-bindings" data-number="3.5"><span class="header-section-number">3.5</span> Extending Structured Bindings<a href="#extending-structured-bindings" class="self-link"></a></h2>
<p>See <span class="citation" data-cites="P2120R0">[<a href="#ref-P2120R0" role="doc-biblioref">P2120R0</a>]</span>.</p>
<h2 data-number="3.6" id="packs-at-block-scope" data-number="3.6"><span class="header-section-number">3.6</span> Packs at block scope<a href="#packs-at-block-scope" class="self-link"></a></h2>
<p>This section has thus far proposed the ability to declare member variable packs and member alias packs, which can greatly help implementing a type like <code class="sourceCode cpp">tuple</code>. For thoroughness, this paper also proposes the ability to declare variable packs and alias packs at block scope. That is:</p>
<div class="sourceCode" id="cb13"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb13-1"><a href="#cb13-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb13-2"><a href="#cb13-2"></a><span class="dt">void</span> foo<span class="op">(</span>Ts<span class="op">&&...</span> ts<span class="op">)</span> <span class="op">{</span></span>
<span id="cb13-3"><a href="#cb13-3"></a> <span class="kw">using</span> <span class="op">...</span>decayed <span class="op">=</span> std<span class="op">::</span>decay_t<span class="op"><</span>Ts<span class="op">></span>;</span>
<span id="cb13-4"><a href="#cb13-4"></a> <span class="kw">auto</span><span class="op">...</span> copies <span class="op">=</span> std<span class="op">::</span>forward<span class="op"><</span>Ts<span class="op">>(</span>ts<span class="op">)</span>;</span>
<span id="cb13-5"><a href="#cb13-5"></a><span class="op">}</span></span></code></pre></div>
<h2 data-number="3.7" id="implementing-variant" data-number="3.7"><span class="header-section-number">3.7</span> Implementing variant<a href="#implementing-variant" class="self-link"></a></h2>
<p>The ability to declare a member pack and index into packs would also make it substantially easier to implement <code class="sourceCode cpp">variant</code>. In the same way that this paper is proposing allowing a member pack of a class type:</p>
<div class="sourceCode" id="cb14"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb14-1"><a href="#cb14-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb14-2"><a href="#cb14-2"></a><span class="kw">struct</span> tuple <span class="op">{</span></span>
<span id="cb14-3"><a href="#cb14-3"></a> Ts<span class="op">...</span> vals;</span>
<span id="cb14-4"><a href="#cb14-4"></a><span class="op">}</span>;</span></code></pre></div>
<p>This paper also proposes allowing a member pack of a union type:</p>
<div class="sourceCode" id="cb15"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb15-1"><a href="#cb15-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb15-2"><a href="#cb15-2"></a><span class="kw">union</span> variant <span class="op">{</span></span>
<span id="cb15-3"><a href="#cb15-3"></a> Ts<span class="op">...</span> alts;</span>
<span id="cb15-4"><a href="#cb15-4"></a><span class="op">}</span>;</span></code></pre></div>
<p>This doesn’t just fall out naturally in the same way the class type case does. Packs behave as a single entity in the language today, and yet here we would need to have a part of an entity - we wouldn’t initialize the pack <code class="sourceCode cpp">alts<span class="op">...</span></code>, we would necessarily only have to initialize exactly one of element of that pack. But I don’t think that’s a huge stretch. And it makes for a substantial improvement in how user-friendly the implementation is.</p>
<p>As with the earlier tuple example, with the proposed language changes, this is something you can just sit down and implement. It practically rolls off the page, because the language would let you express your intent better. With the status quo, this is just a lot more design work in order to eventually produce a lot more code that is harder to understand and takes longer to compile.</p>
<p>The following is an implementation solely of the default constructor, the destructor, and <code class="sourceCode cpp">get_if</code>:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Status Quo</strong>
</div></th>
<th><div style="text-align:center">
<strong>Proposed</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div class="sourceCode" id="cb16"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb16-1"><a href="#cb16-1"></a><span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I, <span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb16-2"><a href="#cb16-2"></a><span class="kw">union</span> impl <span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb16-3"><a href="#cb16-3"></a></span>
<span id="cb16-4"><a href="#cb16-4"></a><span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I, <span class="kw">typename</span> T, <span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb16-5"><a href="#cb16-5"></a><span class="kw">union</span> impl<span class="op"><</span>I, T, Ts<span class="op">...></span></span>
<span id="cb16-6"><a href="#cb16-6"></a><span class="op">{</span></span>
<span id="cb16-7"><a href="#cb16-7"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb16-8"><a href="#cb16-8"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Args<span class="op">></span></span>
<span id="cb16-9"><a href="#cb16-9"></a> <span class="kw">constexpr</span> impl<span class="op">(</span>in_place_index_t<span class="op"><</span><span class="dv">0</span><span class="op">></span>, Args<span class="op">&&...</span> args<span class="op">)</span></span>
<span id="cb16-10"><a href="#cb16-10"></a> <span class="op">:</span> head<span class="op">(</span>std<span class="op">::</span>forward<span class="op"><</span>Args<span class="op">>(</span>args<span class="op">)...)</span></span>
<span id="cb16-11"><a href="#cb16-11"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb16-12"><a href="#cb16-12"></a> </span>
<span id="cb16-13"><a href="#cb16-13"></a> <span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> J, <span class="kw">typename</span><span class="op">...</span> Args<span class="op">></span></span>
<span id="cb16-14"><a href="#cb16-14"></a> <span class="kw">constexpr</span> impl<span class="op">(</span>in_place_index_t<span class="op"><</span>J<span class="op">></span>,</span>
<span id="cb16-15"><a href="#cb16-15"></a> Args<span class="op">&&...</span> args<span class="op">)</span></span>
<span id="cb16-16"><a href="#cb16-16"></a> <span class="op">:</span> tail<span class="op">(</span>in_place_index<span class="op"><</span>J<span class="op">-</span><span class="dv">1</span><span class="op">></span>,</span>
<span id="cb16-17"><a href="#cb16-17"></a> std<span class="op">::</span>forward<span class="op"><</span>Args<span class="op">>(</span>args<span class="op">)...)</span></span>
<span id="cb16-18"><a href="#cb16-18"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb16-19"><a href="#cb16-19"></a> </span>
<span id="cb16-20"><a href="#cb16-20"></a> <span class="op">~</span>impl<span class="op">()</span></span>
<span id="cb16-21"><a href="#cb16-21"></a> <span class="kw">requires</span> std<span class="op">::</span>is_trivially_destructible_v<span class="op"><</span>T<span class="op">></span></span>
<span id="cb16-22"><a href="#cb16-22"></a> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb16-23"><a href="#cb16-23"></a> <span class="op">~</span>impl<span class="op">()</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb16-24"><a href="#cb16-24"></a> </span>
<span id="cb16-25"><a href="#cb16-25"></a> <span class="kw">auto</span> get<span class="op">(</span>in_place_index_t<span class="op"><</span><span class="dv">0</span><span class="op">>)</span> <span class="op">-></span> T<span class="op">&</span> <span class="op">{</span></span>
<span id="cb16-26"><a href="#cb16-26"></a> <span class="cf">return</span> head;</span>
<span id="cb16-27"><a href="#cb16-27"></a> <span class="op">}</span></span>
<span id="cb16-28"><a href="#cb16-28"></a> <span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> J<span class="op">></span></span>
<span id="cb16-29"><a href="#cb16-29"></a> <span class="kw">auto</span> get<span class="op">(</span>in_place_index_t<span class="op"><</span>J<span class="op">>)</span> <span class="op">-></span> <span class="kw">auto</span><span class="op">&</span> <span class="op">{</span></span>
<span id="cb16-30"><a href="#cb16-30"></a> <span class="cf">return</span> tail<span class="op">.</span>get<span class="op">(</span>in_place_index<span class="op"><</span>J<span class="op">-</span><span class="dv">1</span><span class="op">>)</span>;</span>
<span id="cb16-31"><a href="#cb16-31"></a> <span class="op">}</span></span>
<span id="cb16-32"><a href="#cb16-32"></a><span class="kw">private</span><span class="op">:</span></span>
<span id="cb16-33"><a href="#cb16-33"></a> <span class="dt">char</span> _;</span>
<span id="cb16-34"><a href="#cb16-34"></a> T head;</span>
<span id="cb16-35"><a href="#cb16-35"></a> impl<span class="op"><</span>I<span class="op">+</span><span class="dv">1</span>, Ts<span class="op">...></span> tail;</span>
<span id="cb16-36"><a href="#cb16-36"></a><span class="op">}</span>;</span>
<span id="cb16-37"><a href="#cb16-37"></a></span>
<span id="cb16-38"><a href="#cb16-38"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb16-39"><a href="#cb16-39"></a><span class="kw">class</span> variant <span class="op">{</span></span>
<span id="cb16-40"><a href="#cb16-40"></a> <span class="dt">int</span> index_;</span>
<span id="cb16-41"><a href="#cb16-41"></a> impl impl_;</span>
<span id="cb16-42"><a href="#cb16-42"></a></span>
<span id="cb16-43"><a href="#cb16-43"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb16-44"><a href="#cb16-44"></a> <span class="kw">constexpr</span> variant<span class="op">()</span></span>
<span id="cb16-45"><a href="#cb16-45"></a> <span class="kw">requires</span> std<span class="op">::</span>default_constructible<span class="op"><</span></span>
<span id="cb16-46"><a href="#cb16-46"></a> mp_at_c<span class="op"><</span><span class="dv">0</span>, mp_list<span class="op"><</span>Ts<span class="op">...>></span></span>
<span id="cb16-47"><a href="#cb16-47"></a> <span class="op">:</span> index_<span class="op">(</span><span class="dv">0</span><span class="op">)</span></span>
<span id="cb16-48"><a href="#cb16-48"></a> , impl_<span class="op">(</span>in_place_index<span class="op"><</span><span class="dv">0</span><span class="op">>)</span></span>
<span id="cb16-49"><a href="#cb16-49"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb16-50"><a href="#cb16-50"></a> </span>
<span id="cb16-51"><a href="#cb16-51"></a> <span class="op">~</span>variant<span class="op">()</span></span>
<span id="cb16-52"><a href="#cb16-52"></a> <span class="kw">requires</span> <span class="op">(</span>std<span class="op">::</span>is_trivially_destructible_v<span class="op"><</span>Ts<span class="op">></span> <span class="op">&&</span> <span class="op">...)</span></span>
<span id="cb16-53"><a href="#cb16-53"></a> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb16-54"><a href="#cb16-54"></a> <span class="op">~</span>variant<span class="op">()</span> <span class="op">{</span></span>
<span id="cb16-55"><a href="#cb16-55"></a> mp_with_index<span class="op"><</span><span class="kw">sizeof</span><span class="op">...(</span>Ts<span class="op">)>(</span>index_,</span>
<span id="cb16-56"><a href="#cb16-56"></a> <span class="op">[](</span><span class="kw">auto</span> I<span class="op">)</span> <span class="op">{</span></span>
<span id="cb16-57"><a href="#cb16-57"></a> <span class="kw">auto</span><span class="op">&</span> alt <span class="op">=</span> impl<span class="op">.</span>get<span class="op">(</span>in_place_index<span class="op"><</span>I<span class="op">>)</span>;</span>
<span id="cb16-58"><a href="#cb16-58"></a> std<span class="op">::</span>destroy_at<span class="op">(&</span>alt<span class="op">)</span>;</span>
<span id="cb16-59"><a href="#cb16-59"></a> <span class="op">})</span>;</span>
<span id="cb16-60"><a href="#cb16-60"></a> <span class="op">}</span></span>
<span id="cb16-61"><a href="#cb16-61"></a><span class="op">}</span>;</span>
<span id="cb16-62"><a href="#cb16-62"></a></span>
<span id="cb16-63"><a href="#cb16-63"></a><span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I, <span class="kw">typename</span><span class="op">...</span> Types<span class="op">></span></span>
<span id="cb16-64"><a href="#cb16-64"></a><span class="kw">constexpr</span> variant_alternative_t<span class="op"><</span>I, variant<span class="op"><</span>Types<span class="op">...>>*</span></span>
<span id="cb16-65"><a href="#cb16-65"></a>get_if<span class="op">(</span>variant<span class="op"><</span>Types<span class="op">...>*</span> v<span class="op">)</span> <span class="kw">noexcept</span> <span class="op">{</span></span>
<span id="cb16-66"><a href="#cb16-66"></a> <span class="cf">if</span> <span class="op">(</span>v <span class="op">&&</span> v<span class="op">-></span>index_ <span class="op">==</span> I<span class="op">)</span> <span class="op">{</span></span>
<span id="cb16-67"><a href="#cb16-67"></a> <span class="cf">return</span> <span class="op">&</span>v<span class="op">-></span>impl<span class="op">.</span>get<span class="op">(</span>in_place_index<span class="op"><</span>I<span class="op">>)</span>;</span>
<span id="cb16-68"><a href="#cb16-68"></a> <span class="op">}</span> <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb16-69"><a href="#cb16-69"></a> <span class="cf">return</span> <span class="kw">nullptr</span>;</span>
<span id="cb16-70"><a href="#cb16-70"></a> <span class="op">}</span></span>
<span id="cb16-71"><a href="#cb16-71"></a><span class="op">}</span></span></code></pre></div></td>
<td><div class="sourceCode" id="cb17"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb17-1"><a href="#cb17-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb17-2"><a href="#cb17-2"></a><span class="kw">class</span> variant <span class="op">{</span></span>
<span id="cb17-3"><a href="#cb17-3"></a> <span class="dt">int</span> index_;</span>
<span id="cb17-4"><a href="#cb17-4"></a> <span class="kw">union</span> <span class="op">{</span></span>
<span id="cb17-5"><a href="#cb17-5"></a> Ts<span class="op">...</span> alts_;</span>
<span id="cb17-6"><a href="#cb17-6"></a> <span class="op">}</span>;</span>
<span id="cb17-7"><a href="#cb17-7"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb17-8"><a href="#cb17-8"></a> <span class="kw">constexpr</span> variant<span class="op">()</span></span>
<span id="cb17-9"><a href="#cb17-9"></a> <span class="kw">requires</span> std<span class="op">::</span>default_constructible<span class="op"><</span>Ts<span class="op">...[</span><span class="dv">0</span><span class="op">]></span></span>
<span id="cb17-10"><a href="#cb17-10"></a> <span class="op">:</span> index_<span class="op">(</span><span class="dv">0</span><span class="op">)</span></span>
<span id="cb17-11"><a href="#cb17-11"></a> , alts_<span class="op">...[</span><span class="dv">0</span><span class="op">]()</span></span>
<span id="cb17-12"><a href="#cb17-12"></a> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb17-13"><a href="#cb17-13"></a></span>
<span id="cb17-14"><a href="#cb17-14"></a> <span class="op">~</span>variant<span class="op">()</span></span>
<span id="cb17-15"><a href="#cb17-15"></a> <span class="kw">requires</span> <span class="op">(</span>std<span class="op">::</span>is_trivially_destructible_v<span class="op"><</span>Ts<span class="op">></span> <span class="op">&&</span> <span class="op">...)</span></span>