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<title>Issue 2457: std::begin() and std::end() do not support multi-dimensional arrays correctly</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="2457"><a href="lwg-closed.html#2457">2457</a>. <code>std::begin()</code> and <code>std::end()</code> do not support multi-dimensional arrays correctly</h3>
<p><b>Section:</b> 24.7 <a href="https://wg21.link/iterator.range">[iterator.range]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Janez &#x17d;emva <b>Opened:</b> 2014-11-16 <b>Last modified:</b> 2024-06-24</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#iterator.range">active issues</a> in [iterator.range].</p>
<p><b>View all other</b> <a href="lwg-index.html#iterator.range">issues</a> in [iterator.range].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The following code:
</p>
<blockquote><pre>
#include &lt;algorithm&gt;
#include &lt;iterator&gt;
#include &lt;iostream&gt;
#include &lt;cassert&gt;

int main() 
{
  int a[2][3][4] = { { { 1,  2,  3,  4}, { 5,  6,  7,  8}, { 9, 10, 11, 12} },
                     { {13, 14, 15, 16}, {17, 18, 19, 20}, {21, 22, 23, 24} } };
  int b[2][3][4];

  assert(std::distance(std::begin(a), std::end(a)) == 2 * 3 * 4);
  std::copy(std::begin(a), std::end(a), std::begin(b));
  std::copy(std::begin(b), std::end(b), std::ostream_iterator&lt;int&gt;(std::cout, ","));
}
</pre></blockquote>
<p>
does not compile. 
<p/>
A possible way to remedy this would be to add the following overloads of
<code>begin</code>, <code>end</code>, <code>rbegin</code>, and <code>rend</code> to 24.7 <a href="https://wg21.link/iterator.range">[iterator.range]</a>, 
relying on recursive evaluation:
</p>
<blockquote><pre>
namespace std {

  template &lt;typename T, size_t M, size_t N&gt;
  constexpr remove_all_extents_t&lt;T&gt;*
  begin(T (&amp;array)[M][N])
  {
    return begin(*array);
  }
  
  template &lt;typename T, size_t M, size_t N&gt;
  constexpr remove_all_extents_t&lt;T&gt;*
  end(T (&amp;array)[M][N])
  {
    return end(array[M - 1]);
  }

  template &lt;typename T, size_t M, size_t N&gt;
  reverse_iterator&lt;remove_all_extents_t&lt;T&gt;*&gt;
  rbegin(T (&amp;array)[M][N])
  {
    return decltype(rbegin(array))(end(array[M - 1]));
  }
  
  template &lt;typename T, size_t M, size_t N&gt;
  reverse_iterator&lt;remove_all_extents_t&lt;T&gt;*&gt;
  rend(T (&amp;array)[M][N])
  {
    return decltype(rend(array))(begin(*array));
  }

}
</pre></blockquote>

<p><i>[2023-04-06; LWG reflector poll in November 2021]</i></p>

<p>
Changed to Tentatively NAD after 12 votes in favour.
Use <code>views::join</code> or <code>mdspan</code> instead.
</p>

<p><i>[St. Louis 2024-06-24 Status changed: Tentatively NAD &rarr; NAD.]</i></p>



<p id="res-2457"><b>Proposed resolution:</b></p>





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