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  <title>C-Array Interoperability of MDSpan</title>
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<h1 class="title" style="text-align:center"><code>C-Array Interoperability of MDSpan</code></h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #: </td>
    <td>PXXXX</td>
  </tr>
  <tr>
    <td>Date: </td>
    <td>2022-02-14</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project: </td>
    <td>Programming Language C++<br>
      LEWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to: </td>
    <td>
      Christian Trott<br>&lt;<a href="mailto:crtrott@sandia.gov" class="email">crtrott@sandia.gov</a>&gt;<br>
      Damien Lebrun-Grandie<br>&lt;<a href="mailto:lebrungrandt@ornl.gov" class="email">lebrungrandt@ornl.gov</a>&gt;<br>
      Mark Hoemmen<br>&lt;<a href="mailto:mhoemmen@stellarscience.com" class="email">mhoemmen@stellarscience.com</a>&gt;<br>
      K. R. Walker<br>&lt;<a href="mailto:krwalker@stellarscience.com" class="email">krwalker@stellarscience.com</a>&gt;<br>
      Daniel Sunderland<br>&lt;<a href="mailto:dansunderland@gmail.com" class="email">dansunderland@gmail.com</a>&gt;<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>
<ul>
<li><a href="#initial-version-2022-02-mailing"><span class="toc-section-number">1.1</span> Initial Version 2022-02
Mailing<span></span></a></li>
</ul></li>
<li><a href="#description"><span class="toc-section-number">2</span>
Description<span></span></a>
<ul>
<li><a href="#the-issue"><span class="toc-section-number">2.1</span> The
Issue<span></span></a></li>
<li><a href="#proposal"><span class="toc-section-number">2.2</span>
Proposal<span></span></a></li>
<li><a href="#implementation-prototype"><span class="toc-section-number">2.3</span> Implementation
Prototype<span></span></a></li>
</ul></li>
<li><a href="#wording"><span class="toc-section-number">3</span>
Wording<span></span></a>
<ul>
<li><a href="#in-subsection-22.7.x.1-mdspan.mdspan.overview"><span class="toc-section-number">3.1</span> In SubSection 22.7.X.1
[mdspan.mdspan.overview]<span></span></a></li>
<li><a href="#in-subsection-22.7.x.2-mdspan.mdspan.cons"><span class="toc-section-number">3.2</span> In SubSection 22.7.X.2
[mdspan.mdspan.cons]<span></span></a></li>
</ul></li>
</ul>
</div>
<h1 data-number="1" id="revision-history"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<h2 data-number="1.1" id="initial-version-2022-02-mailing"><span class="header-section-number">1.1</span> Initial Version 2022-02
Mailing<a href="#initial-version-2022-02-mailing" class="self-link"></a></h2>
<h1 data-number="2" id="description"><span class="header-section-number">2</span> Description<a href="#description" class="self-link"></a></h1>
<h2 data-number="2.1" id="the-issue"><span class="header-section-number">2.1</span> The Issue<a href="#the-issue" class="self-link"></a></h2>
<p>Currently one cannot construct an <code>mdspan</code> from a
multidimensional c-array.</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> a<span class="op">[</span><span class="dv">3</span><span class="op">][</span><span class="dv">2</span><span class="op">]</span> <span class="op">=</span> <span class="op">{</span><span class="dv">1</span>,<span class="dv">2</span>,<span class="dv">3</span>,<span class="dv">4</span>,<span class="dv">5</span>,<span class="dv">6</span><span class="op">}</span>;</span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>mdspan<span class="op">&lt;</span><span class="dt">int</span>,dextents<span class="op">&lt;</span><span class="dv">2</span><span class="op">&gt;&gt;</span> b<span class="op">(</span>a,<span class="dv">3</span>,<span class="dv">2</span><span class="op">)</span>; <span class="co">// fail</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>mdspan<span class="op">&lt;</span><span class="dt">int</span>,extents<span class="op">&lt;</span><span class="dv">3</span>,<span class="dv">2</span><span class="op">&gt;&gt;</span> c<span class="op">(</span>a<span class="op">)</span>; <span class="co">// fail</span></span></code></pre></div>
<p>It only works for 1D c-arrays, which decay to pointers. However the
deduction guide when using just the 1D c-array with no length argument,
would deduce a rank-0 <code>mdspan</code>, which may be surprising to
users:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> a<span class="op">[</span><span class="dv">6</span><span class="op">]</span> <span class="op">=</span> <span class="op">{</span><span class="dv">1</span>,<span class="dv">2</span>,<span class="dv">3</span>,<span class="dv">4</span>,<span class="dv">5</span>,<span class="dv">6</span><span class="op">}</span>;</span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a>mdspan b<span class="op">(</span>a<span class="op">)</span>;</span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a><span class="kw">static_assert</span><span class="op">(</span><span class="kw">decltype</span><span class="op">(</span>b<span class="op">)::</span>rank<span class="op">()==</span><span class="dv">0</span><span class="op">)</span>;</span></code></pre></div>
<h2 data-number="2.2" id="proposal"><span class="header-section-number">2.2</span> Proposal<a href="#proposal" class="self-link"></a></h2>
<p>We cannot currently fix the multidimensional c-array construction,
since it is UB to alias a nested C-Array with a element type pointer -
per discussion on the C++ committee reflector in January 2022. However,
in practice it works (on the compilers we tested e.g. clang-based and
gcc) - and it may be something the committee changes in the future -
i.e. make it not-UB. We propotyped this capability, which requires an
additional constructor from c-array and a few deduction guides.</p>
<p>What we can fix today is the deduction from 1D c-array, by adding a
deduction guide from c-array constraint to rank-1 arrays.</p>
<p>This would enable the following:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> a<span class="op">[</span><span class="dv">6</span><span class="op">]</span> <span class="op">=</span> <span class="op">{</span><span class="dv">1</span>,<span class="dv">2</span>,<span class="dv">3</span>,<span class="dv">4</span>,<span class="dv">5</span>,<span class="dv">6</span><span class="op">}</span>;</span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a>  mdspan b<span class="op">(</span>a<span class="op">)</span>;</span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a>  <span class="kw">static_assert</span><span class="op">(</span><span class="kw">decltype</span><span class="op">(</span>b<span class="op">)::</span>rank<span class="op">()==</span><span class="dv">1</span><span class="op">)</span>;</span>
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a>  <span class="kw">static_assert</span><span class="op">(</span><span class="kw">decltype</span><span class="op">(</span>b<span class="op">)::</span>static_extent<span class="op">(</span><span class="dv">0</span><span class="op">)==</span><span class="dv">6</span><span class="op">)</span>;</span>
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>It is currently possible, and will continue to be allowed to do the
following with 1D c-arrays (via the decay to pointer code path):</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="op">{</span></span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> a<span class="op">[</span><span class="dv">6</span><span class="op">]</span> <span class="op">=</span> <span class="op">{</span><span class="dv">1</span>,<span class="dv">2</span>,<span class="dv">3</span>,<span class="dv">4</span>,<span class="dv">5</span>,<span class="dv">6</span><span class="op">}</span>;</span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a>  mdspan b<span class="op">(</span>a,<span class="dv">3</span><span class="op">)</span>;</span>
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a>  <span class="kw">static_assert</span><span class="op">(</span><span class="kw">decltype</span><span class="op">(</span>b<span class="op">)::</span>rank<span class="op">()==</span><span class="dv">1</span><span class="op">)</span>;</span>
<span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a>  <span class="kw">static_assert</span><span class="op">(</span><span class="kw">decltype</span><span class="op">(</span>b<span class="op">)::</span>static_extent<span class="op">(</span><span class="dv">0</span><span class="op">)==</span>dynamic_extent;</span>
<span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a>  <span class="co">// b.extent(0)==3</span></span>
<span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb4-8"><a href="#cb4-8" aria-hidden="true" tabindex="-1"></a><span class="op">{</span></span>
<span id="cb4-9"><a href="#cb4-9" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> a<span class="op">[</span><span class="dv">6</span><span class="op">]</span> <span class="op">=</span> <span class="op">{</span><span class="dv">1</span>,<span class="dv">2</span>,<span class="dv">3</span>,<span class="dv">4</span>,<span class="dv">5</span>,<span class="dv">6</span><span class="op">}</span>;</span>
<span id="cb4-10"><a href="#cb4-10" aria-hidden="true" tabindex="-1"></a>  mdspan<span class="op">&lt;</span><span class="dt">int</span>,extents<span class="op">&lt;</span><span class="dv">3</span><span class="op">&gt;&gt;</span> b<span class="op">(</span>a<span class="op">)</span>;</span>
<span id="cb4-11"><a href="#cb4-11" aria-hidden="true" tabindex="-1"></a>  <span class="kw">static_assert</span><span class="op">(</span><span class="kw">decltype</span><span class="op">(</span>b<span class="op">)::</span>rank<span class="op">()==</span><span class="dv">1</span><span class="op">)</span>;</span>
<span id="cb4-12"><a href="#cb4-12" aria-hidden="true" tabindex="-1"></a>  <span class="kw">static_assert</span><span class="op">(</span><span class="kw">decltype</span><span class="op">(</span>b<span class="op">)::</span>static_extent<span class="op">(</span><span class="dv">0</span><span class="op">)==</span><span class="dv">3</span>;</span>
<span id="cb4-13"><a href="#cb4-13" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>Adding this deduction guide now, means that if we later fix the
general issue of constructing an mdspan from a nested c-array (which
would require a core language change), we do not break prior
behavior.</p>
<h2 data-number="2.3" id="implementation-prototype"><span class="header-section-number">2.3</span> Implementation Prototype<a href="#implementation-prototype" class="self-link"></a></h2>
<p>Based on the reference implementation of P0009 we implemented the
changes in a godbolt example: <a href="https://godbolt.org/z/TWW8vbc5P">godbolt</a>.</p>
<h1 data-number="3" id="wording"><span class="header-section-number">3</span> Wording<a href="#wording" class="self-link"></a></h1>
<h2 data-number="3.1" id="in-subsection-22.7.x.1-mdspan.mdspan.overview"><span class="header-section-number">3.1</span> In SubSection 22.7.X.1
[mdspan.mdspan.overview]<a href="#in-subsection-22.7.x.1-mdspan.mdspan.overview" class="self-link"></a></h2>
<ul>
<li>Add the following deduction guides after the class definition:</li>
</ul>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> CArray<span class="op">&gt;</span></span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a>mdspan<span class="op">(</span>CArray<span class="op">&amp;)</span></span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a>  <span class="op">-&gt;</span> mdspan<span class="op">&lt;</span>see below<span class="op">&gt;</span>;</span></code></pre></div>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> Pointer<span class="op">&gt;</span></span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a>mdspan<span class="op">(</span>Pointer<span class="op">&amp;)</span></span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a>  <span class="op">-&gt;</span> mdspan<span class="op">&lt;</span>see below<span class="op">&gt;</span>;</span></code></pre></div>
<h2 data-number="3.2" id="in-subsection-22.7.x.2-mdspan.mdspan.cons"><span class="header-section-number">3.2</span> In SubSection 22.7.X.2
[mdspan.mdspan.cons]<a href="#in-subsection-22.7.x.2-mdspan.mdspan.cons" class="self-link"></a></h2>
<ul>
<li>Insert after paragraph 19:</li>
</ul>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> CArray<span class="op">&gt;</span></span>
<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a>mdspan<span class="op">(</span>CArray<span class="op">&amp;)</span></span>
<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a>  <span class="op">-&gt;</span> mdspan<span class="op">&lt;</span>see below<span class="op">&gt;</span>;</span></code></pre></div>
<p><em>Constraints:</em></p>
<ul>
<li><code>is_array_v&lt;CArray&gt;</code> is <code>true</code>.</li>
<li><code>rank_v&lt;CArray&gt;==1</code> is <code>true</code>.</li>
</ul>
<p><em>Remarks:</em></p>
<ul>
<li>The deduced type is:
<code>mdspan&lt; std::remove_all_extents_t&lt;CArray&gt;, extents&lt;extent_v&lt;CArray,0&gt;&gt; &gt;</code></li>
</ul>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> Pointer<span class="op">&gt;</span></span>
<span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a>mdspan<span class="op">(</span>Pointer<span class="op">&amp;)</span></span>
<span id="cb8-3"><a href="#cb8-3" aria-hidden="true" tabindex="-1"></a>  <span class="op">-&gt;</span> mdspan<span class="op">&lt;</span>see below<span class="op">&gt;</span>;</span></code></pre></div>
<p><em>Constraints:</em></p>
<ul>
<li><p><code>is_array_v&lt;Pointer&gt;</code> is
<code>false</code>.</p></li>
<li><p><code>is_pointer_v&lt;Pointer&gt;</code> is
<code>true</code>.</p></li>
</ul>
<p><em>Remarks:</em></p>
<ul>
<li><p>The deduced type is:
<code>mdspan&lt; std::remove_pointer_t&lt;Pointer&gt;, extents&lt;&gt; &gt;</code></p></li>
<li><p>Modify paragraph 20 to be:</p></li>
</ul>
<p><em>Constraints:</em>
<code>(is_convertible_v&lt;Integrals, size_type&gt; &amp;&amp; ...)</code>
is <code>true</code> and <code>sizeof...(Integrals) &gt; 0</code> is
<code>true</code>.</p>
</div>
</div>
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