<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 2443: std::array member functions should be constexpr</title>
<meta property="og:title" content="Issue 2443: std::array member functions should be constexpr">
<meta property="og:description" content="C++ library issue. Status: Resolved">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue2443.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<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#Resolved">Resolved</a> status.</em></p>
<h3 id="2443"><a href="lwg-defects.html#2443">2443</a>. <code>std::array</code> member functions should be <code>constexpr</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#Resolved">Resolved</a>
 <b>Submitter:</b> Peter Sommerlad <b>Opened:</b> 2014-10-06 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>Not Prioritized
</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#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
When experimenting with C++14 relaxed <code>constexpr</code> functions I made the observation that I couldn't 
use <code>std::array</code> to create a table of data at compile time directly using loops in a function. 
However, a simple substitute I could use instead:
</p>
<blockquote>
<pre>
template &lt;typename T, size_t n&gt;
struct ar {
  T a[n];
  constexpr ar() : a{{}}{}
  constexpr auto data() const { return &amp;a[0];}
  constexpr T const &amp; operator[](size_t i) const { return a[i]; }
  constexpr T &amp; operator[](size_t i) { return a[i]; }
};

template &lt;size_t n&gt;
using arr = ar&lt;size_t, n&gt;; // std::array&lt;size_t, n&gt;;

template &lt;size_t n&gt;
constexpr auto make_tab(){
  arr&lt;n&gt; result;
  for(size_t i=0; i &lt; n; ++i)
    result[i] = (i+1)*(i+1); // cannot define operator[] for mutable array...
  return result;
}

template &lt;size_t n&gt;
constexpr auto squares=make_tab&lt; n&gt;();

int main() {
  int dummy[squares&lt;5&gt;[3]];
}
</pre>
</blockquote>
<p>
Therefore, I suggest that all member functions of <code>std::array</code> should be made <code>constexpr</code> 
to make the type usable in <code>constexpr</code> functions.
<p/>
Wording should be straight forward, may be with the exception of <code>fill</code>, which would require 
<code>fill_n</code> to be <code>constexpr</code> as well.
</p>

<p><i>[2014-11 Urbana]</i></p>

<p>Move to LEWG</p>
<p>
The extent to which <code>constexpr</code> becomes a part of the Library design is a policy
matter best handled initially by LEWG.
</p>

<p><i>[08-2016, Post-Chicago]</i></p>

<p>Move to Tentatively Resolved</p>


<p id="res-2443"><b>Proposed resolution:</b></p>
<p>This functionality is provided by <a href="https://wg21.link/P0031R0">P0031R0</a></p>





</body>
</html>
