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<meta name="dcterms.date" content="2022-10-13" />
<title>Structured Bindings can introduce a Pack</title>
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<h1 class="title" style="text-align:center">Structured Bindings can introduce a Pack</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P1061R3</td>
</tr>
<tr>
<td>Date:</td>
<td>2022-10-13</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>
Jonathan Wakely<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="#motivation"><span class="toc-section-number">2</span> Motivation<span></span></a></li>
<li><a href="#proposal"><span class="toc-section-number">3</span> Proposal<span></span></a>
<ul>
<li><a href="#other-languages"><span class="toc-section-number">3.1</span> Other Languages<span></span></a></li>
<li><a href="#implementation-burden"><span class="toc-section-number">3.2</span> Implementation Burden<span></span></a></li>
<li><a href="#implementation-experience"><span class="toc-section-number">3.3</span> Implementation Experience<span></span></a></li>
<li><a href="#handling-non-dependent-packs-in-the-wording"><span class="toc-section-number">3.4</span> Handling non-dependent packs in the wording<span></span></a></li>
<li><a href="#namespace-scope-packs"><span class="toc-section-number">3.5</span> Namespace-scope packs<span></span></a></li>
</ul></li>
<li><a href="#wording"><span class="toc-section-number">4</span> Wording<span></span></a></li>
<li><a href="#acknowledgements"><span class="toc-section-number">5</span> Acknowledgements<span></span></a></li>
<li><a href="#bibliography"><span class="toc-section-number">6</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p>R3 removes the exclusion of namespace-scope per <a href="https://github.com/cplusplus/papers/issues/294#issuecomment-1234578812">EWG guidance</a>.</p>
<p>R2 adds a section about implementation complexity, implementation experience, and wording.</p>
<p>R1 of this paper <span class="citation" data-cites="P1061R1">[<a href="#ref-P1061R1" role="doc-biblioref">P1061R1</a>]</span> was presented to EWG in Belfast 2019 <span class="citation" data-cites="P1061R1.Minutes">[<a href="#ref-P1061R1.Minutes" role="doc-biblioref">P1061R1.Minutes</a>]</span> which approved the direction as presented (12-5-2-0-1).</p>
<p>R0 of this paper <span class="citation" data-cites="P1061R0">[<a href="#ref-P1061R0" role="doc-biblioref">P1061R0</a>]</span> was presented to EWGI in Kona 2019 <span class="citation" data-cites="P1061R0.Minutes">[<a href="#ref-P1061R0.Minutes" role="doc-biblioref">P1061R0.Minutes</a>]</span>, who reviewed it favorably and thought this was a good investment of our time (4-3-4-1-0). The consensus in the room was that the restriction that the introduced pack need not be the trailing identifier.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="motivation"><span class="header-section-number">2</span> Motivation<a href="#motivation" class="self-link"></a></h1>
<p>Function parameter packs and tuples are conceptually very similar. Both are heterogeneous sequences of objects. Some problems are easier to solve with a parameter pack, some are easier to solve with a <code class="sourceCode cpp">tuple</code>. Today, it’s trivial to convert a pack to a <code class="sourceCode cpp">tuple</code>, but it’s somewhat more involved to convert a <code class="sourceCode cpp">tuple</code> to a pack. You have to go through <code class="sourceCode cpp">std<span class="op">::</span>apply<span class="op">()</span></code> <span class="citation" data-cites="N3915">[<a href="#ref-N3915" role="doc-biblioref">N3915</a>]</span>:</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<span class="op"><</span>A, B, C<span class="op">></span> tup <span class="op">=</span> <span class="op">...</span>;</span>
<span id="cb1-2"><a href="#cb1-2"></a>std<span class="op">::</span>apply<span class="op">([&](</span><span class="kw">auto</span><span class="op">&&...</span> elems<span class="op">){</span></span>
<span id="cb1-3"><a href="#cb1-3"></a> <span class="co">// now I have a pack</span></span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="op">}</span>, tup<span class="op">)</span>;</span></code></pre></div>
<p>This is great for cases where we just need to call a [non-overloaded] function or function object, but rapidly becomes much more awkward as we dial up the complexity. Not to mention if I want to return from the outer scope based on what these elements have to be.</p>
<p>How do we compute the dot product of two <code class="sourceCode cpp">tuple</code>s? It’s a choose your own adventure of awkward choices:</p>
<table style="width:100%">
<tr>
<th style="width:50%">
Nested <code class="sourceCode cpp">apply<span class="op">()</span></code>
</th>
<th style="width:50%">
Using <code class="sourceCode cpp">index_sequence</code>
</th>
</tr>
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<td>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">></span></span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="kw">auto</span> dot_product<span class="op">(</span>P p, Q q<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-3"><a href="#cb2-3"></a> <span class="cf">return</span> std<span class="op">::</span>apply<span class="op">([&](</span><span class="kw">auto</span><span class="op">...</span> p_elems<span class="op">){</span></span>
<span id="cb2-4"><a href="#cb2-4"></a> <span class="cf">return</span> std<span class="op">::</span>apply<span class="op">([&](</span><span class="kw">auto</span><span class="op">...</span> q_elems<span class="op">){</span></span>
<span id="cb2-5"><a href="#cb2-5"></a> <span class="cf">return</span> <span class="op">(...</span> <span class="op">+</span> <span class="op">(</span>p_elems <span class="op">*</span> q_elems<span class="op">))</span>;</span>
<span id="cb2-6"><a href="#cb2-6"></a> <span class="op">}</span>, q<span class="op">)</span></span>
<span id="cb2-7"><a href="#cb2-7"></a> <span class="op">}</span>, p<span class="op">)</span>;</span>
<span id="cb2-8"><a href="#cb2-8"></a><span class="op">}</span></span></code></pre></div>
</td>
<td>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">template</span> <span class="op"><</span><span class="dt">size_t</span><span class="op">...</span> Is, <span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">></span></span>
<span id="cb3-2"><a href="#cb3-2"></a><span class="kw">auto</span> dot_product<span class="op">(</span>std<span class="op">::</span>index_sequence<span class="op"><</span>Is<span class="op">...></span>, P p, Q, q<span class="op">)</span> <span class="op">{</span></span>
<span id="cb3-3"><a href="#cb3-3"></a> <span class="cf">return</span> <span class="op">(...</span> <span class="op">+</span> <span class="op">(</span>std<span class="op">::</span>get<span class="op"><</span>Is<span class="op">>(</span>p<span class="op">)</span> <span class="op">*</span> std<span class="op">::</span>get<span class="op"><</span>Is<span class="op">>(</span>q<span class="op">)))</span>;</span>
<span id="cb3-4"><a href="#cb3-4"></a><span class="op">}</span></span>
<span id="cb3-5"><a href="#cb3-5"></a></span>
<span id="cb3-6"><a href="#cb3-6"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">></span></span>
<span id="cb3-7"><a href="#cb3-7"></a><span class="kw">auto</span> dot_product<span class="op">(</span>P p, Q q<span class="op">)</span> <span class="op">{</span></span>
<span id="cb3-8"><a href="#cb3-8"></a> <span class="cf">return</span> dot_product<span class="op">(</span></span>
<span id="cb3-9"><a href="#cb3-9"></a> std<span class="op">::</span>make_index_sequence<span class="op"><</span>std<span class="op">::</span>tuple_size<span class="op"><</span>P<span class="op">>::</span>value<span class="op">>{}</span>,</span>
<span id="cb3-10"><a href="#cb3-10"></a> p, q<span class="op">)</span>;</span>
<span id="cb3-11"><a href="#cb3-11"></a><span class="op">}</span></span></code></pre></div>
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</table>
<p>Regardless of which option you dislike the least, both are limited to only <code class="sourceCode cpp">std<span class="op">::</span>tuple</code>s. We don’t have the ability to do this at all for any of the other kinds of types that can be used in a structured binding declaration <span class="citation" data-cites="P0144R2">[<a href="#ref-P0144R2" role="doc-biblioref">P0144R2</a>]</span> - because we need to explicit list the correct number of identifiers, and we might not know how many there are.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="proposal"><span class="header-section-number">3</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>We propose to extend the structured bindings syntax to allow the user to introduce a pack as (at most) one of the identifiers:</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a>std<span class="op">::</span>tuple<span class="op"><</span>X, Y, Z<span class="op">></span> f<span class="op">()</span>;</span>
<span id="cb4-2"><a href="#cb4-2"></a></span>
<span id="cb4-3"><a href="#cb4-3"></a><span class="kw">auto</span> <span class="op">[</span>x,y,z<span class="op">]</span> <span class="op">=</span> f<span class="op">()</span>; <span class="co">// OK today</span></span>
<span id="cb4-4"><a href="#cb4-4"></a><span class="kw">auto</span> <span class="op">[...</span>xs<span class="op">]</span> <span class="op">=</span> f<span class="op">()</span>; <span class="co">// proposed: xs is a pack of length three containing an X, Y, and a Z</span></span>
<span id="cb4-5"><a href="#cb4-5"></a><span class="kw">auto</span> <span class="op">[</span>x, <span class="op">...</span>rest<span class="op">]</span> <span class="op">=</span> f<span class="op">()</span>; <span class="co">// proposed: x is an X, rest is a pack of length two (Y and Z)</span></span>
<span id="cb4-6"><a href="#cb4-6"></a><span class="kw">auto</span> <span class="op">[</span>x,y,z, <span class="op">...</span>rest<span class="op">]</span> <span class="op">=</span> f<span class="op">()</span>; <span class="co">// proposed: rest is an empty pack</span></span>
<span id="cb4-7"><a href="#cb4-7"></a><span class="kw">auto</span> <span class="op">[</span>x, <span class="op">...</span>rest, z<span class="op">]</span> <span class="op">=</span> f<span class="op">()</span>; <span class="co">// proposed: x is an X, rest is a pack of length one</span></span>
<span id="cb4-8"><a href="#cb4-8"></a> <span class="co">// consisting of the Y, z is a Z</span></span>
<span id="cb4-9"><a href="#cb4-9"></a><span class="kw">auto</span> <span class="op">[...</span>a, <span class="op">...</span>b<span class="op">]</span> <span class="op">=</span> f<span class="op">()</span>; <span class="co">// ill-formed: multiple packs</span></span></code></pre></div>
<p>If we additionally add the structured binding customization machinery to <code class="sourceCode cpp">std<span class="op">::</span>integer_sequence</code>, this could greatly simplify generic code:</p>
<table style="width:100%">
<tr>
<th>
Today
</th>
<th>
Proposed
</th>
</tr>
<tr>
<td colspan="2">
<center>
Implementing <code class="sourceCode cpp">std<span class="op">::</span>apply<span class="op">()</span></code>
</center>
</td>
</tr>
<tr>
<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">namespace</span> detail <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">class</span> F, <span class="kw">class</span> Tuple, std<span class="op">::</span><span class="dt">size_t</span><span class="op">...</span> I<span class="op">></span></span>
<span id="cb5-3"><a href="#cb5-3"></a> <span class="kw">constexpr</span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> apply_impl<span class="op">(</span>F <span class="op">&&</span>f, Tuple <span class="op">&&</span>t,</span>
<span id="cb5-4"><a href="#cb5-4"></a> std<span class="op">::</span>index_sequence<span class="op"><</span>I<span class="op">...>)</span></span>
<span id="cb5-5"><a href="#cb5-5"></a> <span class="op">{</span></span>
<span id="cb5-6"><a href="#cb5-6"></a> <span class="cf">return</span> std<span class="op">::</span>invoke<span class="op">(</span>std<span class="op">::</span>forward<span class="op"><</span>F<span class="op">>(</span>f<span class="op">)</span>,</span>
<span id="cb5-7"><a href="#cb5-7"></a> std<span class="op">::</span>get<span class="op"><</span>I<span class="op">>(</span>std<span class="op">::</span>forward<span class="op"><</span>Tuple<span class="op">>(</span>t<span class="op">))...)</span>;</span>
<span id="cb5-8"><a href="#cb5-8"></a> <span class="op">}</span></span>
<span id="cb5-9"><a href="#cb5-9"></a><span class="op">}</span></span>
<span id="cb5-10"><a href="#cb5-10"></a></span>
<span id="cb5-11"><a href="#cb5-11"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">class</span> F, <span class="kw">class</span> Tuple<span class="op">></span></span>
<span id="cb5-12"><a href="#cb5-12"></a><span class="kw">constexpr</span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> apply<span class="op">(</span>F <span class="op">&&</span>f, Tuple <span class="op">&&</span>t<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 class="cf">return</span> detail<span class="op">::</span>apply_impl<span class="op">(</span></span>
<span id="cb5-15"><a href="#cb5-15"></a> std<span class="op">::</span>forward<span class="op"><</span>F<span class="op">>(</span>f<span class="op">)</span>, std<span class="op">::</span>forward<span class="op"><</span>Tuple<span class="op">>(</span>t<span class="op">)</span>,</span>
<span id="cb5-16"><a href="#cb5-16"></a> std<span class="op">::</span>make_index_sequence<span class="op"><</span>std<span class="op">::</span>tuple_size_v<span class="op"><</span></span>
<span id="cb5-17"><a href="#cb5-17"></a> std<span class="op">::</span>decay_t<span class="op"><</span>Tuple<span class="op">>>>{})</span>;</span>
<span id="cb5-18"><a href="#cb5-18"></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">class</span> F, <span class="kw">class</span> Tuple<span class="op">></span></span>
<span id="cb6-2"><a href="#cb6-2"></a><span class="kw">constexpr</span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> apply<span class="op">(</span>F <span class="op">&&</span>f, Tuple <span class="op">&&</span>t<span class="op">)</span></span>
<span id="cb6-3"><a href="#cb6-3"></a><span class="op">{</span></span>
<span id="cb6-4"><a href="#cb6-4"></a> <span class="kw">auto</span><span class="op">&&</span> <span class="op">[...</span>elems<span class="op">]</span> <span class="op">=</span> t;</span>
<span id="cb6-5"><a href="#cb6-5"></a> <span class="cf">return</span> std<span class="op">::</span>invoke<span class="op">(</span>std<span class="op">::</span>forward<span class="op"><</span>F<span class="op">>(</span>f<span class="op">)</span>,</span>
<span id="cb6-6"><a href="#cb6-6"></a> forward_like<span class="op"><</span>Tuple, <span class="kw">decltype</span><span class="op">(</span>elems<span class="op">)>(</span>elems<span class="op">)...)</span>;</span>
<span id="cb6-7"><a href="#cb6-7"></a><span class="op">}</span></span></code></pre></div>
</td>
</tr>
<tr>
<td colspan="2">
<center>
<code class="sourceCode cpp">dot_product<span class="op">()</span></code>, nested
</center>
</td>
</tr>
<tr>
<td>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">></span></span>
<span id="cb7-2"><a href="#cb7-2"></a><span class="kw">auto</span> dot_product<span class="op">(</span>P p, Q q<span class="op">)</span> <span class="op">{</span></span>
<span id="cb7-3"><a href="#cb7-3"></a> <span class="cf">return</span> std<span class="op">::</span>apply<span class="op">([&](</span><span class="kw">auto</span><span class="op">...</span> p_elems<span class="op">){</span></span>
<span id="cb7-4"><a href="#cb7-4"></a> <span class="cf">return</span> std<span class="op">::</span>apply<span class="op">([&](</span><span class="kw">auto</span><span class="op">...</span> q_elems<span class="op">){</span></span>
<span id="cb7-5"><a href="#cb7-5"></a> <span class="cf">return</span> <span class="op">(...</span> <span class="op">+</span> <span class="op">(</span>p_elems <span class="op">*</span> q_elems<span class="op">))</span>;</span>
<span id="cb7-6"><a href="#cb7-6"></a> <span class="op">}</span>, q<span class="op">)</span></span>
<span id="cb7-7"><a href="#cb7-7"></a> <span class="op">}</span>, p<span class="op">)</span>;</span>
<span id="cb7-8"><a href="#cb7-8"></a><span class="op">}</span></span></code></pre></div>
</td>
<td>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">></span></span>
<span id="cb8-2"><a href="#cb8-2"></a><span class="kw">auto</span> dot_product<span class="op">(</span>P p, Q q<span class="op">)</span> <span class="op">{</span></span>
<span id="cb8-3"><a href="#cb8-3"></a> <span class="co">// no indirection!</span></span>
<span id="cb8-4"><a href="#cb8-4"></a> <span class="kw">auto</span><span class="op">&&</span> <span class="op">[...</span>p_elems<span class="op">]</span> <span class="op">=</span> p;</span>
<span id="cb8-5"><a href="#cb8-5"></a> <span class="kw">auto</span><span class="op">&&</span> <span class="op">[...</span>q_elems<span class="op">]</span> <span class="op">=</span> q;</span>
<span id="cb8-6"><a href="#cb8-6"></a> <span class="cf">return</span> <span class="op">(...</span> <span class="op">+</span> <span class="op">(</span>p_elems <span class="op">*</span> q_elems<span class="op">))</span>;</span>
<span id="cb8-7"><a href="#cb8-7"></a><span class="op">}</span></span></code></pre></div>
</td>
</tr>
<tr>
<td colspan="2">
<center>
<code class="sourceCode cpp">dot_product<span class="op">()</span></code>, with <code class="sourceCode cpp">index_sequence</code>
</center>
</td>
</tr>
<tr>
<td>
<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">template</span> <span class="op"><</span><span class="dt">size_t</span><span class="op">...</span> Is, <span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">></span></span>
<span id="cb9-2"><a href="#cb9-2"></a><span class="kw">auto</span> dot_product<span class="op">(</span>std<span class="op">::</span>index_sequence<span class="op"><</span>Is<span class="op">...></span>, P p, Q, q<span class="op">)</span> <span class="op">{</span></span>
<span id="cb9-3"><a href="#cb9-3"></a> <span class="cf">return</span> <span class="op">(...</span> <span class="op">+</span> <span class="op">(</span>std<span class="op">::</span>get<span class="op"><</span>Is<span class="op">>(</span>p<span class="op">)</span> <span class="op">*</span> std<span class="op">::</span>get<span class="op"><</span>Is<span class="op">>(</span>q<span class="op">)))</span>;</span>
<span id="cb9-4"><a href="#cb9-4"></a><span class="op">}</span></span>
<span id="cb9-5"><a href="#cb9-5"></a></span>
<span id="cb9-6"><a href="#cb9-6"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">></span></span>
<span id="cb9-7"><a href="#cb9-7"></a><span class="kw">auto</span> dot_product<span class="op">(</span>P p, Q q<span class="op">)</span> <span class="op">{</span></span>
<span id="cb9-8"><a href="#cb9-8"></a> <span class="cf">return</span> dot_product<span class="op">(</span></span>
<span id="cb9-9"><a href="#cb9-9"></a> std<span class="op">::</span>make_index_sequence<span class="op"><</span>std<span class="op">::</span>tuple_size_v<span class="op"><</span>P<span class="op">>>{}</span>,</span>
<span id="cb9-10"><a href="#cb9-10"></a> p, q<span class="op">)</span>;</span>
<span id="cb9-11"><a href="#cb9-11"></a><span class="op">}</span></span></code></pre></div>
</td>
<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">class</span> P, <span class="kw">class</span> Q<span class="op">></span></span>
<span id="cb10-2"><a href="#cb10-2"></a><span class="kw">auto</span> dot_product<span class="op">(</span>P p, Q q<span class="op">)</span> <span class="op">{</span></span>
<span id="cb10-3"><a href="#cb10-3"></a> <span class="co">// no helper function necessary!</span></span>
<span id="cb10-4"><a href="#cb10-4"></a> <span class="kw">auto</span> <span class="op">[...</span>Is<span class="op">]</span> <span class="op">=</span> std<span class="op">::</span>make_index_sequence<span class="op"><</span></span>
<span id="cb10-5"><a href="#cb10-5"></a> std<span class="op">::</span>tuple_size_v<span class="op"><</span>P<span class="op">>>{}</span>;</span>
<span id="cb10-6"><a href="#cb10-6"></a> <span class="cf">return</span> <span class="op">(...</span> <span class="op">+</span> <span class="op">(</span>std<span class="op">::</span>get<span class="op"><</span>Is<span class="op">>(</span>p<span class="op">)</span> <span class="op">*</span> std<span class="op">::</span>get<span class="op"><</span>Is<span class="op">>(</span>q<span class="op">)))</span>;</span>
<span id="cb10-7"><a href="#cb10-7"></a><span class="op">}</span></span></code></pre></div>
</td>
</tr>
</table>
<p>Not only are these implementations more concise, but they are also more functional. I can just as easily use <code class="sourceCode cpp">apply<span class="op">()</span></code> with user-defined types as I can with <code class="sourceCode cpp">std<span class="op">::</span>tuple</code>:</p>
<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">struct</span> Point <span class="op">{</span></span>
<span id="cb11-2"><a href="#cb11-2"></a> <span class="dt">int</span> x, y, z;</span>
<span id="cb11-3"><a href="#cb11-3"></a><span class="op">}</span>;</span>
<span id="cb11-4"><a href="#cb11-4"></a></span>
<span id="cb11-5"><a href="#cb11-5"></a>Point getPoint<span class="op">()</span>;</span>
<span id="cb11-6"><a href="#cb11-6"></a><span class="dt">double</span> calc<span class="op">(</span><span class="dt">int</span>, <span class="dt">int</span>, <span class="dt">int</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="dt">double</span> result <span class="op">=</span> std<span class="op">::</span>apply<span class="op">(</span>calc, getPoint<span class="op">())</span>; <span class="co">// ill-formed today, ok with proposed implementation</span></span></code></pre></div>
<h2 data-number="3.1" id="other-languages"><span class="header-section-number">3.1</span> Other Languages<a href="#other-languages" class="self-link"></a></h2>
<p>Python 2 had always allowed for a syntax similar to C++17 structured bindings, where you have to provide all the identifiers:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb1-1"><a href="#cb1-1"></a><span class="op">>>></span> a, b, c, d, e <span class="op">=</span> <span class="bu">range</span>(<span class="dv">5</span>) <span class="co"># ok</span></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="op">>>></span> a, <span class="op">*</span>b <span class="op">=</span> <span class="bu">range</span>(<span class="dv">3</span>)</span>
<span id="cb1-3"><a href="#cb1-3"></a> File <span class="st">"<stdin>"</span>, line <span class="dv">1</span></span>
<span id="cb1-4"><a href="#cb1-4"></a> a, <span class="op">*</span>b <span class="op">=</span> <span class="bu">range</span>(<span class="dv">3</span>)</span>
<span id="cb1-5"><a href="#cb1-5"></a> <span class="op">^</span></span>
<span id="cb1-6"><a href="#cb1-6"></a><span class="pp">SyntaxError</span>: invalid syntax</span></code></pre></div>
<p>But you could not do any more than that. Python 3 went one step further by way of PEP-3132 <span class="citation" data-cites="PEP.3132">[<a href="#ref-PEP.3132" role="doc-biblioref">PEP.3132</a>]</span>. That proposal allowed for a single starred identifier to be used, which would bind to all the elements as necessary:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb2-1"><a href="#cb2-1"></a><span class="op">>>></span> a, <span class="op">*</span>b, c <span class="op">=</span> <span class="bu">range</span>(<span class="dv">5</span>)</span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="op">>>></span> a</span>
<span id="cb2-3"><a href="#cb2-3"></a><span class="dv">0</span></span>
<span id="cb2-4"><a href="#cb2-4"></a><span class="op">>>></span> c</span>
<span id="cb2-5"><a href="#cb2-5"></a><span class="dv">4</span></span>
<span id="cb2-6"><a href="#cb2-6"></a><span class="op">>>></span> b</span>
<span id="cb2-7"><a href="#cb2-7"></a>[<span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span>]</span></code></pre></div>
<p>The Python 3 behavior is synonymous with what is being proposed here. Notably, from that PEP:</p>
<blockquote>
<p>Possible changes discussed were:</p>
<ul>
<li>Only allow a starred expression as the last item in the exprlist. This would simplify the unpacking code a bit and allow for the starred expression to be assigned an iterator. This behavior was rejected because it would be too surprising.</li>
</ul>
</blockquote>
<p>R0 of this proposal only allowed a pack to be introduced as the last item, which was changed in R1.</p>
<h2 data-number="3.2" id="implementation-burden"><span class="header-section-number">3.2</span> Implementation Burden<a href="#implementation-burden" class="self-link"></a></h2>
<p>Unfortunately, this proposal has some implementation complexity. The issue is not so much this aspect:</p>
<blockquote>
<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>typeanme Tuple<span class="op">></span></span>
<span id="cb12-2"><a href="#cb12-2"></a><span class="kw">auto</span> sum_template<span class="op">(</span>Tuple tuple<span class="op">)</span> <span class="op">{</span></span>
<span id="cb12-3"><a href="#cb12-3"></a> <span class="kw">auto</span> <span class="op">[...</span>elems<span class="op">]</span> <span class="op">=</span> tuple;</span>
<span id="cb12-4"><a href="#cb12-4"></a> <span class="cf">return</span> <span class="op">(...</span> <span class="op">+</span> elems<span class="op">)</span>;</span>
<span id="cb12-5"><a href="#cb12-5"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>This part is more or less straightforward - we have a dependent type and we introduce a pack from it, but we’re already in a template context where dealing with packs is just a normal thing.</p>
<p>The problem is this aspect:</p>
<blockquote>
<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">auto</span> sum_non_template<span class="op">(</span>SomeConreteType tuple<span class="op">)</span> <span class="op">{</span></span>
<span id="cb13-2"><a href="#cb13-2"></a> <span class="kw">auto</span> <span class="op">[...</span>elems<span class="op">]</span> <span class="op">=</span> tuple;</span>
<span id="cb13-3"><a href="#cb13-3"></a> <span class="cf">return</span> <span class="op">(...</span> <span class="op">+</span> elems<span class="op">)</span>;</span>
<span id="cb13-4"><a href="#cb13-4"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>We have not yet in the history of C++ had this notion of packs outside of dependent contexts. This is completely novel, and imposes a burden on implementations to have to track packs outside of templates where they previously had not.</p>
<p>However, in our estimation, this functionality is going to come to C++ in one form or other fairly soon. Reflection, in the latest form of <span class="citation" data-cites="P1240R2">[<a href="#ref-P1240R2" role="doc-biblioref">P1240R2</a>]</span>, has many examples of introducing packs in non-template contexts as well - through the notion of a <em>reflection range</em>. That paper introduces several reifiers that can manipilate a newly-introduced pack, such as:</p>
<blockquote>
<div class="sourceCode" id="cb14"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb14-1"><a href="#cb14-1"></a>std<span class="op">::</span>meta<span class="op">::</span>info t_args<span class="op">[]</span> <span class="op">=</span> <span class="op">{</span> <span class="op">^</span><span class="dt">int</span>, <span class="op">^</span><span class="dv">42</span> <span class="op">}</span>;</span>
<span id="cb14-2"><a href="#cb14-2"></a><span class="kw">template</span><span class="op"><</span><span class="kw">typename</span> T, T<span class="op">></span> <span class="kw">struct</span> X <span class="op">{}</span>;</span>
<span id="cb14-3"><a href="#cb14-3"></a>X<span class="op"><...[:</span>t_args<span class="op">:]...></span> x; <span class="co">// Same as "X<int, 42> x;".</span></span>
<span id="cb14-4"><a href="#cb14-4"></a><span class="kw">template</span><span class="op"><</span><span class="kw">typename</span>, <span class="kw">typename</span><span class="op">></span> <span class="kw">struct</span> Y <span class="op">{}</span>;</span>
<span id="cb14-5"><a href="#cb14-5"></a>Y<span class="op"><...[:</span>t_args<span class="op">:]...></span> y; <span class="co">// Error: same as "Y<int, 42> y;".</span></span></code></pre></div>
</blockquote>
<p>As with the structured bindings example in this paper - we have a non-dependent object outside of a template that we’re using to introduce a pack.</p>
<p>Furthermore, unlike some of the reflection examples, and some of the more generic pack facilities proposed in <span class="citation" data-cites="P1858R2">[<a href="#ref-P1858R2" role="doc-biblioref">P1858R2</a>]</span>, this paper offers a nice benefit: all packs must still be declared before use. Even in the <code class="sourceCode cpp">sum_non_template</code> example which, as the name suggests, is not a template in any way, the pack <code class="sourceCode cpp">elems</code> needs an initial declaration. So any machinery that implementations need to track packs doesn’t need to be enabled everywhere - only when a pack declaration has been seen.</p>
<h2 data-number="3.3" id="implementation-experience"><span class="header-section-number">3.3</span> Implementation Experience<a href="#implementation-experience" class="self-link"></a></h2>
<p>Jason Rice has implemented this in a <a href="https://github.com/ricejasonf/llvm-project/commits/ricejasonf/p1061">clang</a>. As far as we’ve been able to ascertain, it works great.</p>
<p>It is also <a href="https://godbolt.org/z/Tnz4e1dY9">available on Compiler Explorer</a>.</p>
<h2 data-number="3.4" id="handling-non-dependent-packs-in-the-wording"><span class="header-section-number">3.4</span> Handling non-dependent packs in the wording<a href="#handling-non-dependent-packs-in-the-wording" class="self-link"></a></h2>
<p>The strategy the wording takes to handle <code class="sourceCode cpp">sum_non_template</code> above is to designate <code class="sourceCode cpp">elems</code> as a <em>non-dependent pack</em> and to state that non-dependent packs are instantiated immediately. In that example, <code class="sourceCode cpp">elems</code> is obviously non-dependent (nothing anywhere is dependent), so we just instantiate the <em>fold-expression</em> immediately.</p>
<p>But defining “non-dependent pack” is non-trivial. Examples from Richard Smith:</p>
<blockquote>
<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="dt">int</span><span class="op">></span> <span class="kw">struct</span> X <span class="op">{</span> <span class="kw">using</span> type <span class="op">=</span> <span class="dt">int</span>; <span class="op">}</span>;</span>
<span id="cb15-2"><a href="#cb15-2"></a><span class="kw">template</span><span class="op"><</span><span class="kw">typename</span> T<span class="op">></span> <span class="dt">void</span> f<span class="op">()</span> <span class="op">{</span></span>
<span id="cb15-3"><a href="#cb15-3"></a> <span class="kw">struct</span> Y <span class="op">{</span> <span class="dt">int</span> a, b; <span class="op">}</span>;</span>
<span id="cb15-4"><a href="#cb15-4"></a> <span class="kw">auto</span> <span class="op">[...</span>v<span class="op">]</span> <span class="op">=</span> Y<span class="op">()</span>;</span>
<span id="cb15-5"><a href="#cb15-5"></a> X<span class="op"><</span><span class="kw">sizeof</span><span class="op">...(</span>v<span class="op">)>::</span>type x; <span class="co">// need typename or no?</span></span>
<span id="cb15-6"><a href="#cb15-6"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Is <code class="sourceCode cpp">X<span class="op"><</span><span class="kw">sizeof</span><span class="op">...(</span>v<span class="op">)></span></code> a dependent type or is the pack <code class="sourceCode cpp">v</code> expanded eagerly because we already know its size?</p>
<p>One approach could be to say that packs are instantiated immediately only if they appear outside of <em>any</em> template. But then:</p>
<blockquote>
<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">struct</span> Z <span class="op">{</span> <span class="dt">int</span> a, b; <span class="op">}</span>;</span>
<span id="cb16-2"><a href="#cb16-2"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> T<span class="op">></span> <span class="dt">void</span> g<span class="op">()</span> <span class="op">{</span></span>
<span id="cb16-3"><a href="#cb16-3"></a> <span class="kw">auto</span> <span class="op">[...</span>v<span class="op">]</span> <span class="op">=</span> Z<span class="op">()</span>;</span>
<span id="cb16-4"><a href="#cb16-4"></a> X<span class="op"><</span><span class="kw">sizeof</span><span class="op">...(</span>v<span class="op">)>::</span>type x; <span class="co">// need typename or no?</span></span>
<span id="cb16-5"><a href="#cb16-5"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Here, despite being in a template, nothing is actually dependent.</p>
<p>I think the right line to draw here is to follow <span class="citation" data-cites="CWG2074">[<a href="#ref-CWG2074" role="doc-biblioref">CWG2074</a>]</span> - which asks about this example:</p>
<div class="quote">
<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> T<span class="op">></span></span>
<span id="cb17-2"><a href="#cb17-2"></a><span class="dt">void</span> f<span class="op">()</span> <span class="op">{</span></span>
<span id="cb17-3"><a href="#cb17-3"></a> <span class="kw">struct</span> X <span class="op">{</span></span>
<span id="cb17-4"><a href="#cb17-4"></a> <span class="kw">typedef</span> <span class="dt">int</span> type;</span>
<span id="cb17-5"><a href="#cb17-5"></a> <span class="pp">#ifdef DEPENDENT</span></span>
<span id="cb17-6"><a href="#cb17-6"></a> T x;</span>
<span id="cb17-7"><a href="#cb17-7"></a> <span class="pp">#endif</span></span>
<span id="cb17-8"><a href="#cb17-8"></a> <span class="op">}</span>;</span>
<span id="cb17-9"><a href="#cb17-9"></a> X<span class="op">::</span>type y; <span class="co">// #1</span></span>
<span id="cb17-10"><a href="#cb17-10"></a><span class="op">}</span></span>
<span id="cb17-11"><a href="#cb17-11"></a></span>
<span id="cb17-12"><a href="#cb17-12"></a><span class="dt">void</span> g<span class="op">()</span> <span class="op">{</span> f<span class="op"><</span><span class="dt">int</span><span class="op">>()</span>; <span class="op">}</span></span></code></pre></div>
<p>there is implementation variance in the treatment of <code class="sourceCode cpp"><span class="pp">#1</span></code>, but whether or not <code class="sourceCode cpp">DEPENDENT</code> is defined appears to make no difference.</p>
<p>[…]</p>
<p>Perhaps the right answer is that the types should be dependent but a member of the current instantiation, permitting name lookup without typename.</p>
</div>
<p>I think the best rule would be to follow the suggested answer in this core issue: a structured bindings pack is dependent if: the type of its initializer (the <code class="sourceCode cpp">E</code> in <span>9.6 <a href="https://wg21.link/dcl.struct.bind">[dcl.struct.bind]</a></span>) is dependent and it is not a member of the current instantiaton. This would make neither of the <code class="sourceCode cpp"><span class="op">...</span>v</code> packs dependent, which seems conceptually correct.</p>
<h2 data-number="3.5" id="namespace-scope-packs"><span class="header-section-number">3.5</span> Namespace-scope packs<a href="#namespace-scope-packs" class="self-link"></a></h2>
<p>In addition to non-dependent packs, this paper also seems like it would offer the ability to declare a pack at <em>namespace</em> scope:</p>
<blockquote>
<div class="sourceCode" id="cb18"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb18-1"><a href="#cb18-1"></a><span class="kw">struct</span> Point <span class="op">{</span> <span class="dt">int</span> x, y; <span class="op">}</span>;</span>
<span id="cb18-2"><a href="#cb18-2"></a><span class="kw">auto</span> <span class="op">[...</span> parts<span class="op">]</span> <span class="op">=</span> Point<span class="op">{</span><span class="dv">1</span>, <span class="dv">2</span><span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p>Structured bindings in namespace scope are a little odd to begin with, since they currently cannot be declared <code class="sourceCode cpp"><span class="kw">inline</span></code>. A structured binding pack at namespace scope adds that much more complexity. For now, this paper simply rejects such a declaration since we are now very far removed from the primary motivation of the paper, and this only serves to add additional complexity.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="wording"><span class="header-section-number">4</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Add a new grammar option for <em>simple-declaration</em> to <span>9.1 <a href="https://wg21.link/dcl.pre">[dcl.pre]</a></span>:</p>
<blockquote>
<blockquote>
<div class="line-block"><span class="addu"><em>sb-pack-identifier</em>:</span><br />
<span class="addu"><code class="sourceCode cpp"><span class="op">...</span></code> <em>identifier</em></span></div>
</blockquote>
<blockquote>
<div class="line-block"><span class="addu"><em>sb-identifier-list</em>:</span><br />
<span class="addu"><em>identifier</em></span><br />
<span class="addu"><em>sb-pack-identifier</em></span><br />
<span class="addu"><em>sb-identifier-list</em> <code class="sourceCode cpp">,</code> <em>identifier</em></span><br />
<span class="addu"><em>sb-identifier-list</em> <code class="sourceCode cpp">,</code> <em>sb-pack-identifier</em></span><br />
<br />
<em>simple-declaration</em>:<br />
<em>decl-specifier-seq</em> <em>init-declarator-list</em><sub>opt</sub> <code class="sourceCode cpp">;</code><br />
<em>attribute-specifier-seq</em> <em>decl-specifier-seq</em> <em>init-declarator-list</em> <code class="sourceCode cpp">;</code><br />
<em>attribute-specifier-seq</em><sub>opt</sub> <em>decl-specifier-seq</em> <em>ref-qualifier</em><sub>opt</sub> <code class="sourceCode cpp"><span class="op">[</span></code> <span class="rm" style="color: #bf0303"><del><em>identifier-list</em></del></span> <span class="addu"><em>sb-identifier-list</em></span> <code class="sourceCode cpp"><span class="op">]</span></code> <em>initializer</em> <code class="sourceCode cpp">;</code></div>
</blockquote>
</blockquote>
<p>Change <span>9.1 <a href="https://wg21.link/dcl.pre">[dcl.pre]</a></span> paragraph 8:</p>
<blockquote>
<p>A <em>simple-declaration</em> with an <span class="rm" style="color: #bf0303"><del><em>identifier-list</em></del></span> <span class="addu"><em>sb-identifier-list</em></span> is called a structured binding declaration ( [dcl.struct.bind]). The <em>decl-specifier-seq</em> shall contain only the <em>type-specifier</em> <code class="sourceCode cpp"><span class="kw">auto</span></code> and <em>cv-qualifiers</em>. The <em>initializer</em> shall be of the form “<code class="sourceCode cpp"><span class="op">=</span> <em>assignment-expression</em></code>”, of the form “<code class="sourceCode cpp"><span class="op">{</span> <em>assignment-expression</em> <span class="op">}</span></code>”, or of the form “<code class="sourceCode cpp"><span class="op">(</span> <em>assignment-expression</em> <span class="op">)</span></code>”, where the <em>assignment-expression</em> is of array or non-union class type.</p>
</blockquote>
<p>Change <span>9.6 <a href="https://wg21.link/dcl.struct.bind">[dcl.struct.bind]</a></span> paragraph 1:</p>
<blockquote>
<p>A structured binding declaration introduces the <i>identifier</i>s v<sub>0</sub>, v<sub>1</sub>, v<sub>2</sub>, … of the <span class="rm" style="color: #bf0303"><del><i>identifier-list</i></del></span> <span class="addu"><i>sb-identifier-list</i></span> as names ([basic.scope.declarative]) of <i>structured bindings</i>. <span class="addu">The declaration shall contain at most one <em>sb-pack-identifier</em>.</span> Let <i>cv</i> denote the <i>cv-qualifiers</i> in the <i>decl-specifier-seq</i>.</p>
</blockquote>
<p>Introduce new paragraphs after <span>9.6 <a href="https://wg21.link/dcl.struct.bind">[dcl.struct.bind]</a></span> paragraph 1, introducing the terms “structured binding size” and SB<sub><em>i</em></sub>:</p>
<blockquote>
<div class="addu">
<p>The <em>structured binding size</em> of a type <code class="sourceCode cpp">E</code> is the required number of names that need to be introduced by the structured binding declaration, as defined below. If there is no structured binding pack, then the number of elements in the <em>sb-identifier-list</em> shall be equal to the structured binding size. Otherwise, the number of elements of the structured binding pack is the structured binding size less the number of elements in the <em>sb-identifier-list</em>.</p>
<p>Let SB<sub><em>i</em></sub> denote <em>i</em><sup>th</sup> <em>identifier</em> in the structured binding declaration after expanding the structured binding pack, if any [ <em>Note</em>: if there is no structured binding pack, then the SB<sub><em>i</em></sub> denotes v<sub><em>i</em></sub>. - <em>end note</em> ] [ <em>Example</em>:</p>
<div class="sourceCode" id="cb19"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb19-1"><a href="#cb19-1"></a>struct C { int x, y, z; };</span>
<span id="cb19-2"><a href="#cb19-2"></a></span>
<span id="cb19-3"><a href="#cb19-3"></a>auto [a, b, c] = C(); // SB<sub>0</sub> is a, SB<sub>1</sub> is b, and SB<sub>2</sub> is c</span>
<span id="cb19-4"><a href="#cb19-4"></a>auto [d, ...e] = C(); // SB<sub>0</sub> is d, the pack e (v<sub>1</sub>) contains two identifiers: SB<sub>1</sub> and SB<sub>2</sub></span>
<span id="cb19-5"><a href="#cb19-5"></a>auto [...f, g] = C(); // the pack f (v<sub>0</sub>) contains two identifiers: SB<sub>0</sub> and SB<sub>1</sub>, and SB<sub>2</sub> is g</span></code></pre></div>
<ul>
<li><em>end example</em> ]</li>
</ul>
</div>
</blockquote>
<p>Change <span>9.6 <a href="https://wg21.link/dcl.struct.bind">[dcl.struct.bind]</a></span> paragraph 3 to define a structured binding size and extend the example:</p>
<blockquote>
<p>If <code>E</code> is an array type with element type <code>T</code>, <span class="rm" style="color: #bf0303"><del>the number of elements in the <i>identifier-list</i></del></span> <span class="addu">the structured binding size of <code>E</code></span> shall be equal to the number of elements of <code>E</code>. Each <span class="rm" style="color: #bf0303"><del><i>v<sub>i</sub></i></del></span> <span class="addu">SB<sub><em>i</em></sub></span> is the name of an lvalue that refers to the element <i>i</i> of the array and whose type is <code>T</code>; the referenced type is <code>T</code>. [<em>Note</em>: The top-level <em>cv</em>-qualifiers of <code class="sourceCode cpp">T</code> are <em>cv</em>. — <em>end note</em>] [<em>Example</em>:</p>
<div>
<div class="sourceCode" id="cb20"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb20-1"><a href="#cb20-1"></a>auto f() -> int(&)[2];</span>
<span id="cb20-2"><a href="#cb20-2"></a>auto [ x, y ] = f(); // x and y refer to elements in a copy of the array return value</span>
<span id="cb20-3"><a href="#cb20-3"></a>auto& [ xr, yr ] = f(); // xr and yr refer to elements in the array referred to by f's return value</span>
<span id="cb20-4"><a href="#cb20-4"></a></span>
<span id="cb20-5"><a href="#cb20-5"></a><span class="va">+ auto g() -> int(&)[4];</span></span>
<span id="cb20-6"><a href="#cb20-6"></a><span class="va">+ auto [a, ...b, c] = g(); // a is the first element of the array, b is a pack containing the second and</span></span>
<span id="cb20-7"><a href="#cb20-7"></a><span class="va">+ // third elements, and c is the fourth element</span></span>
<span id="cb20-8"><a href="#cb20-8"></a><span class="va">+ auto& [...e] = g(); // e is a pack referring to the four elements of the array</span></span></code></pre></div>
</div>
<p>— <em>end example</em>]</p>
</blockquote>
<p>Change <span>9.6 <a href="https://wg21.link/dcl.struct.bind">[dcl.struct.bind]</a></span> paragraph 4 to define a structured binding size:</p>
<blockquote>
<p>Otherwise, if the <i>qualified-id</i> <code>std::tuple_size<E></code> names a complete type, the expression <code class="language-cpp">std::tuple_size<E>::value</code> shall be a well-formed integral constant expression and the <span class="rm" style="color: #bf0303"><del>number of elements in the <i>identifier-list</i></del></span> <span class="addu">structured binding size of <code>E</code></span> shall be equal to the value of that expression. […] Each <span class="rm" style="color: #bf0303"><del><i>v<sub>i</sub></i></del></span> <span class="addu">SB<sub><em>i</em></sub></span> is the name of an lvalue of type <code class>T<sub>i</sub></code> that refers to the object bound to <code class>r<sub>i</sub></code>; the referenced type is <code class>T<sub>i</sub></code>.</p>
</blockquote>
<p>Change <span>9.6 <a href="https://wg21.link/dcl.struct.bind">[dcl.struct.bind]</a></span> paragraph 5 to define a structured binding size:</p>
<blockquote>
<p>Otherwise, all of <code>E</code>’s non-static data members shall be direct members of <code>E</code> or of the same base class of <code>E</code>, well-formed when named as <code>e.name</code> in the context of the structured binding, <code>E</code> shall not have an anonymous union member, and the <span class="rm" style="color: #bf0303"><del>number of elements in the <i>identifier-list</i></del></span> <span class="addu">structured binding size of <code>E</code></span> shall be equal to the number of non-static data members of <code>E</code>. Designating the non-static data members of <code>E</code> as <code class>m<sub>0</sub>, m<sub>1</sub>, m<sub>2</sub>, . . .</code> (in declaration order), each <span class="rm" style="color: #bf0303"><del><code class>v<sub>i</i></code></del></span> <span class="addu">SB<sub><em>i</em></sub></span> is the name of an lvalue that refers to the member <code class>m<sub>i</sub></code> of <code>e</code> and whose type is <i>cv</i> <code class>T<sub>i</sub></code>, where <code class>T<sub>i</sub></code> is the declared type of that member; the referenced type is <i>cv</i> <code class>T<sub>i</sub></code>. The lvalue is a bit-field if that member is a bit-field.</p>
</blockquote>
<p>Add a new clause to <span>13.7.4 <a href="https://wg21.link/temp.variadic">[temp.variadic]</a></span>, after paragraph 3:</p>
<blockquote>
<div class="addu">
<p>A <i>structured binding pack</i> is an <i>identifier</i> that introduces zero or more <i>structured binding</i>s ([dcl.struct.bind]). <i>[ Example</i></p>
<div class="sourceCode" id="cb21"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb21-1"><a href="#cb21-1"></a>auto foo() -> int(&)[2];</span>
<span id="cb21-2"><a href="#cb21-2"></a>auto [...a] = foo(); // a is a structured binding pack containing 2 elements</span>
<span id="cb21-3"><a href="#cb21-3"></a>auto [b, c, ...d] = foo(); // d is a structured binding pack containing 0 elements</span>
<span id="cb21-4"><a href="#cb21-4"></a>auto [e, f, g, ...h] = foo(); // error: too many identifiers</span></code></pre></div>
<ul>
<li><em>end example]</em></li>
</ul>
</div>
</blockquote>
<p>In <span>13.7.4 <a href="https://wg21.link/temp.variadic">[temp.variadic]</a></span>, change paragraph 4:</p>
<blockquote>
<p>A <em>pack</em> is a template parameter pack, a function parameter pack, <span class="rm" style="color: #bf0303"><del>or</del></span> an <em>init-capture</em> pack<span class="addu">, or a structured binding pack</span>. The number of elements of a template parameter pack or a function parameter pack is the number of arguments provided for the parameter pack. The number of elements of an <em>init-capture</em> pack is the number of elements in the pack expansion of its <em>initializer</em>.</p>
</blockquote>
<p>In <span>13.7.4 <a href="https://wg21.link/temp.variadic">[temp.variadic]</a></span>, paragraph 5 (describing pack expansions) remains unchanged.</p>
<p>In <span>13.7.4 <a href="https://wg21.link/temp.variadic">[temp.variadic]</a></span>, add a bullet to paragraph 8:</p>
<blockquote>
<p>Such an element, in the context of the instantiation, is interpreted as follows:</p>
<ul>
<li>if the pack is a template parameter pack, the element is a template parameter ([temp.param]) of the corresponding kind (type or non-type) designating the <i>i</i><sup>th</sup> corresponding type or value template argument;</li>
<li>if the pack is a function parameter pack, the element is an <i>id-expression</i> designating the <i>i</i><sup>th</sup> function parameter that resulted from instantiation of the function parameter pack declaration; <span class="rm" style="color: #bf0303"><del>otherwise</del></span></li>
<li>if the pack is an <i>init-capture</i> pack, the element is an <i>id-expression</i> designating the variable introduced by the <i>i</i><sup>th</sup>th <i>init-capture</i> that resulted from instantiation of the <i>init-capture</i> pack<span class="rm" style="color: #bf0303"><del>.</del></span> <span class="addu">; otherwise</span></li>
<li><span class="addu">if the pack is a structured binding pack, the element is an <i>id-expression</i> designating the <i>i</i><sup>th</sup> structured binding that resulted from the structured binding declaration.</span></li>
</ul>
</blockquote>
<p>Add a new paragraph after <span>13.7.4 <a href="https://wg21.link/temp.variadic">[temp.variadic]</a></span></p>
<blockquote>
<div class="addu">
<p>A structured bindings pack is dependent if, letting <code class="sourceCode cpp">E</code> denote the type of its initializer:</p>
<ul>
<li><code class="sourceCode cpp">E</code> is dependent ([temp.dep.type]), and</li>
<li><code class="sourceCode cpp">E</code> is not a member of the current instantiation, and</li>
<li>either <code class="sourceCode cpp">E</code> is not a local class or <code class="sourceCode cpp">E</code> has a dependent base class.</li>
</ul>
<p>[ <em>Example:</em></p>
<div class="sourceCode" id="cb22"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb22-1"><a href="#cb22-1"></a>struct B { int i; };</span>
<span id="cb22-2"><a href="#cb22-2"></a></span>
<span id="cb22-3"><a href="#cb22-3"></a>template <typeanme T></span>
<span id="cb22-4"><a href="#cb22-4"></a>struct C { T t; };</span>
<span id="cb22-5"><a href="#cb22-5"></a></span>
<span id="cb22-6"><a href="#cb22-6"></a>template <typename T></span>
<span id="cb22-7"><a href="#cb22-7"></a>void f(T t) {</span>
<span id="cb22-8"><a href="#cb22-8"></a> auto [...a] = t; // a is dependent</span>
<span id="cb22-9"><a href="#cb22-9"></a> auto [...b] = B{1}; // b is not dependent</span>
<span id="cb22-10"><a href="#cb22-10"></a> auto [...c] = C<T>{t}; // c is dependent</span>
<span id="cb22-11"><a href="#cb22-11"></a></span>
<span id="cb22-12"><a href="#cb22-12"></a> struct D { T t; };</span>
<span id="cb22-13"><a href="#cb22-13"></a> auto [...d] = D{t}; // d is not dependent</span>
<span id="cb22-14"><a href="#cb22-14"></a></span>
<span id="cb22-15"><a href="#cb22-15"></a> struct E : T {};</span>
<span id="cb22-16"><a href="#cb22-16"></a> auto [...e] = E{t}; // e is dependent</span>
<span id="cb22-17"><a href="#cb22-17"></a></span>
<span id="cb22-18"><a href="#cb22-18"></a>}</span></code></pre></div>
<ul>
<li><em>end example</em> ]</li>
</ul>
<p>If all of the packs expanded by a pack expansion are structured bindings packs that are not dependent, the pack expansion is instantiated immediately.</p>
<p>[ <em>Example:</em></p>
<div class="sourceCode" id="cb23"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb23-1"><a href="#cb23-1"></a>struct E { int i, j; };</span>
<span id="cb23-2"><a href="#cb23-2"></a></span>
<span id="cb23-3"><a href="#cb23-3"></a>constexpr int sum(E e) {</span>
<span id="cb23-4"><a href="#cb23-4"></a> auto [...v] = e;</span>
<span id="cb23-5"><a href="#cb23-5"></a> return (... + v);</span>
<span id="cb23-6"><a href="#cb23-6"></a>}</span>
<span id="cb23-7"><a href="#cb23-7"></a></span>
<span id="cb23-8"><a href="#cb23-8"></a>static_assert(sum({.i=1, .j=2}) == 3); // ok</span></code></pre></div>
<ul>
<li><em>end example</em> ]</li>
</ul>
</div>
</blockquote>