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<title>Simplified structured bindings protocol with pack aliases</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">Simplified structured bindings protocol with pack aliases</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2120R0</td>
</tr>
<tr>
<td>Date:</td>
<td>2020-02-17</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="#introduction-and-motivation"><span class="toc-section-number">1</span> Introduction and Motivation<span></span></a></li>
<li><a href="#proposal"><span class="toc-section-number">2</span> Proposal<span></span></a></li>
<li><a href="#wording"><span class="toc-section-number">3</span> Wording<span></span></a></li>
<li><a href="#bibliography"><span class="toc-section-number">4</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="introduction-and-motivation" data-number="1" style="border-bottom:1px solid #cccccc"><span class="header-section-number">1</span> Introduction and Motivation<a href="#introduction-and-motivation" class="self-link"></a></h1>
<p>When <span class="citation" data-cites="P1858R1">[<a href="#ref-P1858R1" role="doc-biblioref">P1858R1</a>]</span> was presented to EWGI in Prague <span class="citation" data-cites="EWGI.Prague">[<a href="#ref-EWGI.Prague" role="doc-biblioref">EWGI.Prague</a>]</span>, that group requested that the structured bindings extension in that proposal was split off into its own paper. This is that paper, and the original paper continues on as an R2 <span class="citation" data-cites="P1858R2">[<a href="#ref-P1858R2" role="doc-biblioref">P1858R2</a>]</span>.</p>
<p>Assuming the original paper gets adopted, and we end up with facilities allowing both declaring packs and indexing into them, it becomes a lot easier to implement something like <code class="sourceCode cpp">tuple</code> and opt it into structured bindings support:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="kw">class</span> tuple <span class="op">{</span></span>
<span id="cb1-3"><a href="#cb1-3"></a> Ts<span class="op">...</span> elems;</span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb1-5"><a href="#cb1-5"></a> <span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I<span class="op">></span></span>
<span id="cb1-6"><a href="#cb1-6"></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="cb1-7"><a href="#cb1-7"></a> <span class="cf">return</span> elems<span class="op">...[</span>I<span class="op">]</span>;</span>
<span id="cb1-8"><a href="#cb1-8"></a> <span class="op">}</span></span>
<span id="cb1-9"><a href="#cb1-9"></a><span class="op">}</span>;</span>
<span id="cb1-10"><a href="#cb1-10"></a></span>
<span id="cb1-11"><a href="#cb1-11"></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="cb1-12"><a href="#cb1-12"></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="cb1-13"><a href="#cb1-13"></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="cb1-14"><a href="#cb1-14"></a><span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb1-15"><a href="#cb1-15"></a></span>
<span id="cb1-16"><a href="#cb1-16"></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="cb1-17"><a href="#cb1-17"></a><span class="kw">struct</span> tuple_element<span class="op"><</span>I, tuple<span class="op"><</span>Ts<span class="op">...>></span> <span class="op">{</span></span>
<span id="cb1-18"><a href="#cb1-18"></a> <span class="kw">using</span> type <span class="op">=</span> Ts<span class="op">...[</span>I<span class="op">]</span>;</span>
<span id="cb1-19"><a href="#cb1-19"></a><span class="op">}</span>;</span></code></pre></div>
<p>That’s short, easy to read, easy to write, and easy to follow - dramatically more so than the status quo without P1858.</p>
<p>But there’s quite a bit of redundancy there. And a problem with the tuple-like protocol here is that we need to instantiate a lot of templates. A declaration like:</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">auto</span> <span class="op">[</span>v<sub>1</sub>, v<sub>2</sub>, <span class="op">...</span>, v<sub>N</sub><span class="op">]</span> <span class="op">=</span> tuple;</span></code></pre></div>
<p>requires <code class="sourceCode cpp"><span class="dv">2</span><span class="er">N</span><span class="op">+</span><span class="dv">1</span></code> template instantiations: one for <code class="sourceCode cpp">std<span class="op">::</span>tuple_size</code>, <code class="sourceCode cpp">N</code> for <code class="sourceCode cpp">std<span class="op">::</span>tuple_element</code>, and another <code class="sourceCode cpp">N</code> for all the <code class="sourceCode cpp">get</code>s). That’s pretty wasteful. Additionally, the tuple-like protocol is tedious for users to implement. There was a proposal to reduce the customization mechanism by dropping <code class="sourceCode cpp">std<span class="op">::</span>tuple_element</code> <span class="citation" data-cites="P1096R0">[<a href="#ref-P1096R0" role="doc-biblioref">P1096R0</a>]</span>, which was… close. 13-7 in San Diego.</p>
<p>What do <code class="sourceCode cpp">tuple_size</code> and <code class="sourceCode cpp">tuple_element</code> do? They give you a number of types and then each of those types in turn. But we already have a mechanism in the language that provides this information more directly: we can provide a pack of types.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="proposal" data-number="2" style="border-bottom:1px solid #cccccc"><span class="header-section-number">2</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>Currently, there are three kinds of types that can be used with structured bindings <span class="citation" data-cites="P0144R2">[<a href="#ref-P0144R2" role="doc-biblioref">P0144R2</a>]</span>:</p>
<ol type="1">
<li><p>Arrays (specifically <code class="sourceCode cpp">T<span class="op">[</span>N<span class="op">]</span></code> and not <code class="sourceCode cpp">std<span class="op">::</span>array<span class="op"><</span>T, N<span class="op">></span></code>).</p></li>
<li><p>Tuple-like: those types that specialize <code class="sourceCode cpp">std<span class="op">::</span>tuple_size</code>, <code class="sourceCode cpp">std<span class="op">::</span>tuple_element</code>, and either provide a member or non-member <code class="sourceCode cpp">get<span class="op">()</span></code>.</p></li>
<li><p>Types where all of their members are public members of the same class (approximately).</p></li>
</ol>
<p>This paper suggests extending the Tuple-like category by allowing types to opt-in by <em>either</em> providing a member pack alias named <code class="sourceCode cpp">tuple_elements</code> <em>or</em>, if not that, then the status quo of specialization both <code class="sourceCode cpp">std<span class="op">::</span>tuple_size</code> and <code class="sourceCode cpp">std<span class="op">::</span>tuple_element</code>.</p>
<p>In other words, a complete opt-in to structured bindings for our <code class="sourceCode cpp">tuple</code> would become:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>With P1858</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="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></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="cb3-2"><a href="#cb3-2"></a><span class="kw">class</span> tuple <span class="op">{</span></span>
<span id="cb3-3"><a href="#cb3-3"></a> Ts<span class="op">...</span> elems;</span>
<span id="cb3-4"><a href="#cb3-4"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb3-5"><a href="#cb3-5"></a> <span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I<span class="op">></span></span>
<span id="cb3-6"><a href="#cb3-6"></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="cb3-7"><a href="#cb3-7"></a> <span class="cf">return</span> elems<span class="op">...[</span>I<span class="op">]</span>;</span>
<span id="cb3-8"><a href="#cb3-8"></a> <span class="op">}</span></span>
<span id="cb3-9"><a href="#cb3-9"></a><span class="op">}</span>;</span>
<span id="cb3-10"><a href="#cb3-10"></a></span>
<span id="cb3-11"><a href="#cb3-11"></a><span class="kw">namespace</span> std <span class="op">{</span></span>
<span id="cb3-12"><a href="#cb3-12"></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="cb3-13"><a href="#cb3-13"></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="cb3-14"><a href="#cb3-14"></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="cb3-15"><a href="#cb3-15"></a> <span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb3-16"><a href="#cb3-16"></a></span>
<span id="cb3-17"><a href="#cb3-17"></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="cb3-18"><a href="#cb3-18"></a> <span class="kw">struct</span> tuple_element<span class="op"><</span>I, tuple<span class="op"><</span>Ts<span class="op">...>></span> <span class="op">{</span></span>
<span id="cb3-19"><a href="#cb3-19"></a> <span class="kw">using</span> type <span class="op">=</span> Ts<span class="op">...[</span>I<span class="op">]</span>;</span>
<span id="cb3-20"><a href="#cb3-20"></a> <span class="op">}</span>;</span>
<span id="cb3-21"><a href="#cb3-21"></a><span class="op">}</span></span></code></pre></div></td>
<td><div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb4-2"><a href="#cb4-2"></a><span class="kw">class</span> tuple <span class="op">{</span></span>
<span id="cb4-3"><a href="#cb4-3"></a> Ts<span class="op">...</span> elems;</span>
<span id="cb4-4"><a href="#cb4-4"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb4-5"><a href="#cb4-5"></a> <span class="kw">using</span> <span class="op">...</span>tuple_elements <span class="op">=</span> Ts;</span>
<span id="cb4-6"><a href="#cb4-6"></a></span>
<span id="cb4-7"><a href="#cb4-7"></a> <span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I<span class="op">></span></span>
<span id="cb4-8"><a href="#cb4-8"></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="cb4-9"><a href="#cb4-9"></a> <span class="cf">return</span> elems<span class="op">...[</span>I<span class="op">]</span>;</span>
<span id="cb4-10"><a href="#cb4-10"></a> <span class="op">}</span></span>
<span id="cb4-11"><a href="#cb4-11"></a><span class="op">}</span>;</span></code></pre></div></td>
</tr>
</tbody>
</table>
<p>This would also help those cases where we need to opt-in to the tuple protocol in cases where we do not even have a pack:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>With P1858</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="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span><span class="op">></span> <span class="kw">struct</span> pair_get;</span>
<span id="cb5-2"><a href="#cb5-2"></a></span>
<span id="cb5-3"><a href="#cb5-3"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> T, <span class="kw">typename</span> U<span class="op">></span></span>
<span id="cb5-4"><a href="#cb5-4"></a><span class="kw">struct</span> pair <span class="op">{</span></span>
<span id="cb5-5"><a href="#cb5-5"></a> T first;</span>
<span id="cb5-6"><a href="#cb5-6"></a> U second;</span>
<span id="cb5-7"><a href="#cb5-7"></a> </span>
<span id="cb5-8"><a href="#cb5-8"></a> <span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I<span class="op">></span></span>
<span id="cb5-9"><a href="#cb5-9"></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> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span></span>
<span id="cb5-10"><a href="#cb5-10"></a> <span class="op">{</span></span>
<span id="cb5-11"><a href="#cb5-11"></a> <span class="cf">return</span> pair_get<span class="op"><</span>I<span class="op">>::</span>get<span class="op">(*</span><span class="kw">this</span><span class="op">)</span>;</span>
<span id="cb5-12"><a href="#cb5-12"></a> <span class="op">}</span></span>
<span id="cb5-13"><a href="#cb5-13"></a><span class="op">}</span>;</span>
<span id="cb5-14"><a href="#cb5-14"></a></span>
<span id="cb5-15"><a href="#cb5-15"></a><span class="kw">template</span> <span class="op"><></span></span>
<span id="cb5-16"><a href="#cb5-16"></a><span class="kw">struct</span> pair_get<span class="op"><</span><span class="dv">0</span><span class="op">></span> <span class="op">{</span></span>
<span id="cb5-17"><a href="#cb5-17"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> T, <span class="kw">typename</span> U<span class="op">></span></span>
<span id="cb5-18"><a href="#cb5-18"></a> <span class="kw">static</span> <span class="kw">constexpr</span> <span class="kw">auto</span> get<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="cb5-19"><a href="#cb5-19"></a> <span class="op">-></span> T <span class="kw">const</span><span class="op">&</span></span>
<span id="cb5-20"><a href="#cb5-20"></a> <span class="op">{</span></span>
<span id="cb5-21"><a href="#cb5-21"></a> <span class="cf">return</span> p<span class="op">.</span>first;</span>
<span id="cb5-22"><a href="#cb5-22"></a> <span class="op">}</span></span>
<span id="cb5-23"><a href="#cb5-23"></a><span class="op">}</span>;</span>
<span id="cb5-24"><a href="#cb5-24"></a></span>
<span id="cb5-25"><a href="#cb5-25"></a><span class="kw">template</span> <span class="op"><></span></span>
<span id="cb5-26"><a href="#cb5-26"></a><span class="kw">struct</span> pair_get<span class="op"><</span><span class="dv">1</span><span class="op">></span> <span class="op">{</span></span>
<span id="cb5-27"><a href="#cb5-27"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> T, <span class="kw">typename</span> U<span class="op">></span></span>
<span id="cb5-28"><a href="#cb5-28"></a> <span class="kw">static</span> <span class="kw">constexpr</span> <span class="kw">auto</span> get<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="cb5-29"><a href="#cb5-29"></a> <span class="op">-></span> U <span class="kw">const</span><span class="op">&</span></span>
<span id="cb5-30"><a href="#cb5-30"></a> <span class="op">{</span></span>
<span id="cb5-31"><a href="#cb5-31"></a> <span class="cf">return</span> p<span class="op">.</span>second;</span>
<span id="cb5-32"><a href="#cb5-32"></a> <span class="op">}</span></span>
<span id="cb5-33"><a href="#cb5-33"></a><span class="op">}</span>;</span>
<span id="cb5-34"><a href="#cb5-34"></a></span>
<span id="cb5-35"><a href="#cb5-35"></a><span class="kw">namespace</span> std <span class="op">{</span></span>
<span id="cb5-36"><a href="#cb5-36"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> T, <span class="kw">typename</span> U<span class="op">></span></span>
<span id="cb5-37"><a href="#cb5-37"></a> <span class="kw">struct</span> tuple_size<span class="op"><</span>pair<span class="op"><</span>T, U<span class="op">>></span></span>
<span id="cb5-38"><a href="#cb5-38"></a> <span class="op">:</span> integral_constant<span class="op"><</span><span class="dt">size_t</span>, <span class="dv">2</span><span class="op">></span></span>
<span id="cb5-39"><a href="#cb5-39"></a> <span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb5-40"><a href="#cb5-40"></a> </span>
<span id="cb5-41"><a href="#cb5-41"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> T, <span class="kw">typename</span> U<span class="op">></span></span>
<span id="cb5-42"><a href="#cb5-42"></a> <span class="kw">struct</span> tuple_element<span class="op"><</span><span class="dv">0</span>, pair<span class="op"><</span>T, U<span class="op">>></span> <span class="op">{</span></span>
<span id="cb5-43"><a href="#cb5-43"></a> <span class="kw">using</span> type <span class="op">=</span> T;</span>
<span id="cb5-44"><a href="#cb5-44"></a> <span class="op">}</span>;</span>
<span id="cb5-45"><a href="#cb5-45"></a> </span>
<span id="cb5-46"><a href="#cb5-46"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> T, <span class="kw">typename</span> U<span class="op">></span></span>
<span id="cb5-47"><a href="#cb5-47"></a> <span class="kw">struct</span> tuple_element<span class="op"><</span><span class="dv">0</span>, pair<span class="op"><</span>T, U<span class="op">>></span> <span class="op">{</span></span>
<span id="cb5-48"><a href="#cb5-48"></a> <span class="kw">using</span> type <span class="op">=</span> U;</span>
<span id="cb5-49"><a href="#cb5-49"></a> <span class="op">}</span>; </span>
<span id="cb5-50"><a href="#cb5-50"></a><span class="op">}</span></span></code></pre></div></td>
<td><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">template</span> <span class="op"><</span><span class="kw">typename</span> T, <span class="kw">typename</span> U<span class="op">></span></span>
<span id="cb6-2"><a href="#cb6-2"></a><span class="kw">struct</span> pair <span class="op">{</span></span>
<span id="cb6-3"><a href="#cb6-3"></a> T first;</span>
<span id="cb6-4"><a href="#cb6-4"></a> U second;</span>
<span id="cb6-5"><a href="#cb6-5"></a> </span>
<span id="cb6-6"><a href="#cb6-6"></a> <span class="kw">using</span> <span class="op">...</span>tuple_elements <span class="op">=</span> tuple<span class="op"><</span>T, U<span class="op">>::[:]</span>;</span>
<span id="cb6-7"><a href="#cb6-7"></a></span>
<span id="cb6-8"><a href="#cb6-8"></a> <span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span> I<span class="op">></span></span>
<span id="cb6-9"><a href="#cb6-9"></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> tuple_elements<span class="op">...[</span>I<span class="op">]</span> <span class="kw">const</span><span class="op">&</span></span>
<span id="cb6-10"><a href="#cb6-10"></a> <span class="op">{</span></span>
<span id="cb6-11"><a href="#cb6-11"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>I <span class="op">==</span> <span class="dv">0</span><span class="op">)</span> <span class="cf">return</span> first;</span>
<span id="cb6-12"><a href="#cb6-12"></a> <span class="cf">else</span> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>I <span class="op">==</span> <span class="dv">1</span><span class="op">)</span> <span class="cf">return</span> second;</span>
<span id="cb6-13"><a href="#cb6-13"></a> <span class="op">}</span></span>
<span id="cb6-14"><a href="#cb6-14"></a><span class="op">}</span>;</span></code></pre></div></td>
</tr>
</tbody>
</table>
<p>Note that the whole <code class="sourceCode cpp">pair_get</code> implementation on the left can be replaced by introducing a pack alias as on the right anyway. And if that’s already a useful thing to do to help implement a feature, it’d be nice to go that extra one step and make that already useful solution even more useful.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="wording" data-number="3" style="border-bottom:1px solid #cccccc"><span class="header-section-number">3</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Change <span>9.6 <a href="https://wg21.link/dcl.struct.bind">[dcl.struct.bind]</a></span>/4:</p>
<blockquote>
<p>Otherwise, if <span class="addu">either</span></p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(4.1)</a></span> <span class="addu">the <em>qualified-id</em> <code class="sourceCode cpp">E<span class="op">::</span>tuple_elements</code> names an alias pack, or</span></li>
<li><span class="marginalizedparent"><a class="marginalized">(4.2)</a></span> the <em>qualified-id</em> <code class="sourceCode cpp">std<span class="op">::</span>tuple_size<span class="op"><</span>E<span class="op">></span></code> names a complete class type with a member named <code class="sourceCode cpp">value</code>,</li>
</ul>
<p><span class="addu">then the number and types of the elements are determined as follows. If in the first case, the number of elements in the <em>identifier-list</em> shall be equal to the value of <code class="sourceCode cpp"><span class="kw">sizeof</span><span class="op">...(</span>E<span class="op">::</span>tuple_elements<span class="op">)</span></code> and let <code class="sourceCode cpp">T<sub>i</sub></code> designate the type <code class="sourceCode cpp">E<span class="op">::</span>tuple_elements<span class="op">...[</span>i<span class="op">]</span></code>. Otherwise,</span> the expression <code class="sourceCode cpp">std<span class="op">::</span>tuple_size<span class="op"><</span>E<span class="op">></span><span class="op">::</span>value</code> shall be a well-formed integral constant expression <span class="rm" style="color: #bf0303"><del>and</del></span> <span class="addu">,</span> the number of elements in the <em>identifier-list</em> shall be equal to the value of that expression<span class="addu">, and let <code class="sourceCode cpp">T<sub>i</sub></code> designate the type <code class="sourceCode cpp">std<span class="op">::</span>tuple_element<span class="op"><</span>i, E<span class="op">>::</span>type</code></span>. Let <code class="sourceCode cpp">i</code> be an index prvalue of type <code class="sourceCode cpp">std<span class="op">::</span>size_t</code> corresponding to <code class="sourceCode cpp">v<sub>i</sub></code>. The <em>unqualified-id</em> <code class="sourceCode cpp">get</code> is looked up in the scope of <code class="sourceCode cpp">E</code> by class member access lookup ([basic.lookup.classref]), and if that finds at least one declaration that is a function template whose first template parameter is a non-type parameter, the initializer is <code class="sourceCode cpp">e<span class="op">.</span>get<span class="op"><</span>i<span class="op">>()</span></code>. Otherwise, the initializer is <code class="sourceCode cpp">get<span class="op"><</span>i<span class="op">>(</span>e<span class="op">)</span></code>, where get is looked up in the associated namespaces ([basic.lookup.argdep]). In either case, <code class="sourceCode cpp">get<span class="op"><</span>i<span class="op">></span></code> is interpreted as a <em>template-id</em>. [ <em>Note</em>: Ordinary unqualified lookup ([basic.lookup.unqual]) is not performed. — <em>end note</em> ] In either case, <code class="sourceCode cpp">e</code> is an lvalue if the type of the entity <code class="sourceCode cpp">e</code> is an lvalue reference and an xvalue otherwise. Given <span class="rm" style="color: #bf0303"><del>the type <span><code class="sourceCode default">T<sub>i</sub></code></span> designated by <span><code class="sourceCode default">std::tuple_element<i, E>::type</code></span> and</del></span> the type <code class="sourceCode cpp">U<sub>i</sub></code> designated by either <code class="sourceCode cpp">T<sub>i</sub><span class="op">&</span></code> or <code class="sourceCode cpp">T<sub>i</sub><span class="op">&&</span></code>, where <code class="sourceCode cpp">U<sub>i</sub></code> is an lvalue reference if the initializer is an lvalue and an rvalue reference otherwise, variables are introduced with unique names <code class="sourceCode cpp">r<sub>i</sub></code> as follows:</p>
<div class="sourceCode" id="cb7"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb7-1"><a href="#cb7-1"></a>S U<sub>i</sub> r<sub>i</sub> = <em>initializer</em> ;</span></code></pre></div>
<p>Each <code class="sourceCode cpp">v<sub>i</sub></code> is the name of an lvalue of type <code class="sourceCode cpp">T<sub>i</sub></code> that refers to the object bound to <code class="sourceCode cpp">r<sub>i</sub></code>; the referenced type is <code class="sourceCode cpp">T<sub>i</sub></code>.</p>
</blockquote>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="bibliography" data-number="4" style="border-bottom:1px solid #cccccc"><span class="header-section-number">4</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-EWGI.Prague">
<p>[EWGI.Prague] EWGI. 2020. EWGI Discussion of P1858R1. <br />
<a href="http://wiki.edg.com/bin/view/Wg21prague/P1858R1SG17">http://wiki.edg.com/bin/view/Wg21prague/P1858R1SG17</a></p>
</div>
<div id="ref-P0144R2">
<p>[P0144R2] Herb Sutter. 2016. Structured Bindings. <br />
<a href="https://wg21.link/p0144r2">https://wg21.link/p0144r2</a></p>
</div>
<div id="ref-P1096R0">
<p>[P1096R0] Timur Doumler. 2018. Simplify the customization point for structured bindings. <br />
<a href="https://wg21.link/p1096r0">https://wg21.link/p1096r0</a></p>
</div>
<div id="ref-P1858R1">
<p>[P1858R1] Barry Revzin. 2020. Generalized pack declaration and usage. <br />
<a href="https://wg21.link/p1858r1">https://wg21.link/p1858r1</a></p>
</div>
<div id="ref-P1858R2">
<p>[P1858R2] Barry Revzin. 2020. Generalized pack declaration and usage. <br />
<a href="https://wg21.link/p1858r2">https://wg21.link/p1858r2</a></p>
</div>
</div>
</div>
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