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<title>Issue 2335: array&lt;array&lt;int, 3&gt;, 4&gt; should be layout-compatible with int[4][3]</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#NAD">NAD</a> status.</em></p>
<h3 id="2335"><a href="lwg-closed.html#2335">2335</a>. <code>array&lt;array&lt;int, 3&gt;, 4&gt;</code> should be layout-compatible with <code>int[4][3]</code></h3>
<p><b>Section:</b> 23.3.3 <a href="https://wg21.link/array">[array]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Jeffrey Yasskin <b>Opened:</b> 2013-10-04 <b>Last modified:</b> 2020-11-09</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#array">issues</a> in [array].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In order to replace some uses of C arrays with <code>std::array</code>, we need it
to be possible to cast from a <code>std::array&lt;&gt;</code> to an equivalent C array.
Core wording doesn't appear to be in quite the right state to allow
casting, but if we specify that appropriate types are
layout-compatible, we can at least write:
</p>
<blockquote><pre>
union {
  array&lt;array&lt;array&lt;int, 2&gt;, 3&gt;, 4&gt; arr;
  int carr[4][3][2];
};
</pre></blockquote>
<p>
to view memory as the other type: C++14 CD [class.mem]p18.
</p>
I believe it's sufficient to add "<code>array&lt;T, N&gt;</code> shall be
layout-compatible (6.9 <a href="https://wg21.link/basic.types">[basic.types]</a>) with <code>T[N]</code>." to 
23.3.3.1 <a href="https://wg21.link/array.overview">[array.overview]</a>, but we might also need some extension to 
11.4 <a href="https://wg21.link/class.mem">[class.mem]</a> to address the possibility of layout-compatibility 
between struct and array types.
<p>
I checked that libc++ on MacOS already implements this, although it
would be good for someone else to double-check; I haven't checked any
other standard libraries.
</p>

<p><i>[2020-02-14, Prague]</i></p>

<p>
LWG discussions and decision for NAD. 
</p>
<p><i>[2020-11-09 Status changed: Tentatively NAD &rarr; NAD.]</i></p>



<p><b>Rationale:</b></p>
The desire to use <code>std::array</code> like this seems like an "XY problem". The goal 
should be "replace C arrays" not "replace C arrays with <code>std::array</code>", because 
<code>std::array</code> is not suitable here. There are superior solutions being proposed, 
and will be available in a future version of C++ (e.g. using <code>mdspan</code> as a 
multi-dimensional view on an array).


<p id="res-2335"><b>Proposed resolution:</b></p>





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