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  <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>P1061R1</td>
  </tr>
  <tr>
    <td>Date: </td>
    <td>2019-10-06</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project: </td>
    <td>Programming Language C++<br>
      EWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to: </td>
    <td>
      Barry Revzin<br>&lt;<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.com</a>&gt;<br>
      Jonathan Wakely<br>&lt;<a href="mailto:jonathan.wakely@gmail.com" class="email">jonathan.wakely@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<h1 id="revision-history" 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>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 id="motivation" style="border-bottom:1px solid #cccccc"><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">&lt;</span>A, B, C<span class="op">&gt;</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">([&amp;](</span><span class="kw">auto</span><span class="op">&amp;&amp;...</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">&lt;</span><span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">&gt;</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">([&amp;](</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">([&amp;](</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">&lt;</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">&gt;</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">&lt;</span>Is<span class="op">...&gt;</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">&lt;</span>Is<span class="op">&gt;(</span>p<span class="op">)</span> <span class="op">*</span> std<span class="op">::</span>get<span class="op">&lt;</span>Is<span class="op">&gt;(</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">&lt;</span><span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">&gt;</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">&lt;</span>std<span class="op">::</span>tuple_size<span class="op">&lt;</span>P<span class="op">&gt;::</span>value<span class="op">&gt;{}</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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<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 id="proposal" style="border-bottom:1px solid #cccccc"><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">&lt;</span>X, Y, Z<span class="op">&gt;</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">&lt;</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">&gt;</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">&amp;&amp;</span>f, Tuple <span class="op">&amp;&amp;</span>t,</span>
<span id="cb5-4"><a href="#cb5-4"></a>        std<span class="op">::</span>index_sequence<span class="op">&lt;</span>I<span class="op">...&gt;)</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">&lt;</span>F<span class="op">&gt;(</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">&lt;</span>I<span class="op">&gt;(</span>std<span class="op">::</span>forward<span class="op">&lt;</span>Tuple<span class="op">&gt;(</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">&lt;</span><span class="kw">class</span> F, <span class="kw">class</span> Tuple<span class="op">&gt;</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">&amp;&amp;</span>f, Tuple <span class="op">&amp;&amp;</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">&lt;</span>F<span class="op">&gt;(</span>f<span class="op">)</span>, std<span class="op">::</span>forward<span class="op">&lt;</span>Tuple<span class="op">&gt;(</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">&lt;</span>std<span class="op">::</span>tuple_size_v<span class="op">&lt;</span></span>
<span id="cb5-17"><a href="#cb5-17"></a>            std<span class="op">::</span>decay_t<span class="op">&lt;</span>Tuple<span class="op">&gt;&gt;&gt;{})</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">&lt;</span><span class="kw">class</span> F, <span class="kw">class</span> Tuple<span class="op">&gt;</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">&amp;&amp;</span>f, Tuple <span class="op">&amp;&amp;</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">&amp;&amp;</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">&lt;</span>F<span class="op">&gt;(</span>f<span class="op">)</span>,</span>
<span id="cb6-6"><a href="#cb6-6"></a>        forward_like<span class="op">&lt;</span>Tuple, <span class="kw">decltype</span><span class="op">(</span>elems<span class="op">)&gt;(</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">&lt;</span><span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">&gt;</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">([&amp;](</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">([&amp;](</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">&lt;</span><span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">&gt;</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">&amp;&amp;</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">&amp;&amp;</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">&lt;</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">&gt;</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">&lt;</span>Is<span class="op">...&gt;</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">&lt;</span>Is<span class="op">&gt;(</span>p<span class="op">)</span> <span class="op">*</span> std<span class="op">::</span>get<span class="op">&lt;</span>Is<span class="op">&gt;(</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">&lt;</span><span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">&gt;</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">&lt;</span>std<span class="op">::</span>tuple_size_v<span class="op">&lt;</span>P<span class="op">&gt;&gt;{}</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">&lt;</span><span class="kw">class</span> P, <span class="kw">class</span> Q<span class="op">&gt;</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">&lt;</span></span>
<span id="cb10-5"><a href="#cb10-5"></a>        std<span class="op">::</span>tuple_size_v<span class="op">&lt;</span>P<span class="op">&gt;&gt;{}</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">&lt;</span>Is<span class="op">&gt;(</span>p<span class="op">)</span> <span class="op">*</span> std<span class="op">::</span>get<span class="op">&lt;</span>Is<span class="op">&gt;(</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 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">&gt;&gt;&gt;</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">&gt;&gt;&gt;</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">&quot;&lt;stdin&gt;&quot;</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">&gt;&gt;&gt;</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">&gt;&gt;&gt;</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">&gt;&gt;&gt;</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">&gt;&gt;&gt;</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>
<h1 id="wording" style="border-bottom:1px solid #cccccc"><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 9 [dcl.dcl]:</p>
<blockquote>
<blockquote>
<div class="line-block"><span class="addu"><em>sb-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-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-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 9 [dcl.dcl] 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 “= <em>assignment-expression</em>”, of the form “{ <em>assignment-expression</em> }”, or of the form “( <em>assignment-expression</em> )”, where the <em>assignment-expression</em> is of array or non-union class type.</p>
</blockquote>
<p>Change 9.5 [dcl.struct.bind] paragraph 1:</p>
<blockquote>
A structured binding declaration introduces the identifiers 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-identifier</em>. If the declaration contains an <em>sb-identifier</em>, the declaration introduces a <em>structured binding pack</em> ([temp.variadic]).</span> Let <i>cv</i> denote the <i>cv-qualifiers</i> in the <i>decl-specifier-seq</i>.
</blockquote>
<p>Introduce a new paragraph after 9.5 [dcl.struct.bind] paragraph 1, introducing the term “structured binding size”:</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>
</div>
</blockquote>
<p>Change 9.5 [dcl.struct.bind] paragraph 3 to define a structured binding size:</p>
<blockquote>
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>. <span class="rm" style="color: #bf0303"><del>Each <i>v<sub>i</sub></i></del></span> <span class="addu">The <i>i</i><sup>th</sup> <i>identifier</i></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>.
</blockquote>
<p>Change 9.5 [dcl.struct.bind] paragraph 3 to define a structured binding size:</p>
<blockquote>
Otherwise, if the <i>qualified-id</i> <code>std::tuple_size&lt;E&gt;</code> names a complete type, the expression <code class="language-cpp">std::tuple_size&lt;E&gt;::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. […] <span class="rm" style="color: #bf0303"><del>Each <i>v<sub>i</sub></i></del></span> <span class="addu">The <i>i</i><sup>th</sup> <i>identifier</i></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>.
</blockquote>
<p>Change 9.5 [dcl.struct.bind] paragraph 5 to define a structured binding size:</p>
<blockquote>
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), <span class="rm" style="color: #bf0303"><del>each <code class>v<sub>i</i></code></del></span> <span class="addu">the <i>i</i><sup>th</sup> <i>identifier</i></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.
</blockquote>
<p>Add a new clause to 13.6.3 [temp.variadic], 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="cb14"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb14-1"><a href="#cb14-1"></a>auto foo() -&gt; int(&amp;)[2];</span>
<span id="cb14-2"><a href="#cb14-2"></a>auto [...a] = foo();          // a is a structured binding pack containing 2 elements</span>
<span id="cb14-3"><a href="#cb14-3"></a>auto [b, c, ...d] = foo();    // d is a structured binding pack containing 0 elements</span>
<span id="cb14-4"><a href="#cb14-4"></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 13.6.3 [temp.variadic], 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 13.6.3 [temp.variadic], paragraph 5 (describing pack expansions) remains unchanged.</p>
<p>In 13.6.3 [temp.variadic], add a bullet to paragraph 8:</p>
<blockquote>
Such an element, in the context of the instantiation, is interpreted as follows:
<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>
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>
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>
<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>
</ul>
</blockquote>
<h1 id="acknowledgements" style="border-bottom:1px solid #cccccc"><span class="header-section-number">5</span> Acknowledgements<a href="#acknowledgements" class="self-link"></a></h1>
<p>Thanks to Michael Park and Tomasz Kamiński for their helpful feedback.</p>
<h1 id="references" style="border-bottom:1px solid #cccccc"><span class="header-section-number">6</span> References<a href="#references" class="self-link"></a></h1>

<div id="refs" role="doc-bibliography">
<div id="ref-N3915">
<p>[N3915] Peter Sommerlad. 2014. apply() call a function with arguments from a tuple (V3). <br />
<a href="https://wg21.link/n3915">https://wg21.link/n3915</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-P1061R0">
<p>[P1061R0] Barry Revzin, Jonathan Wakely. 2018. Structured Bindings can introduce a Pack. <br />
<a href="https://wg21.link/p1061r0">https://wg21.link/p1061r0</a></p>
</div>
<div id="ref-P1061R0.Minutes">
<p>[P1061R0.Minutes] EWGI. 2019. Kona 2019 EWGI: P1061R0. <br />
<a href="http://wiki.edg.com/bin/view/Wg21kona2019/P1061">http://wiki.edg.com/bin/view/Wg21kona2019/P1061</a></p>
</div>
<div id="ref-PEP.3132">
<p>[PEP.3132] Georg Brandl. 2007. PEP 3132 – Extended Iterable Unpacking. <br />
<a href="https://www.python.org/dev/peps/pep-3132/">https://www.python.org/dev/peps/pep-3132/</a></p>
</div>
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