<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 2347: reverse_iterator::operator[] calls const version of current[]</title>
<meta property="og:title" content="Issue 2347: reverse_iterator::operator[] calls const version of current[]">
<meta property="og:description" content="C++ library issue. Status: NAD">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue2347.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#NAD">NAD</a> status.</em></p>
<h3 id="2347"><a href="lwg-closed.html#2347">2347</a>. <code>reverse_iterator::operator[]</code> calls const version of <code>current[]</code></h3>
<p><b>Section:</b> 24.5.1.6 <a href="https://wg21.link/reverse.iter.elem">[reverse.iter.elem]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Timo Bingmann <b>Opened:</b> 2013-11-11 <b>Last modified:</b> 2021-06-06</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#reverse.iter.elem">issues</a> in [reverse.iter.elem].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently <code>reverse_iterator::operator[]()</code> returns "<code>current[-n-1]</code>" and has
an "<em>unspecified</em>" return type.
<p/>
When <code>Iterator</code> is a mutable random access iterator, the expression
"<code>current[-n-1]</code>" calls "<code>Iterator::operator[] const</code>", which returns a
const reference. This const reference cannot be converted back to a
mutable reference.
<p/>
This issue is related to the "<em>unspecified</em>" return value of
<code>reverse_iterator::operator[]</code>, see defect <a href="lwg-defects.html#386" title="Reverse iterator's operator[] has impossible return type (Status: CD1)">386</a><sup><a href="https://cplusplus.github.io/LWG/issue386" title="Latest snapshot">(i)</a></sup>.
<p/>
The -1 is due to "current" pointing one item beyond the
<code>reverse_iterator</code>'s real current value.
<p/>
The current libstdc++ implementation reads "<code>*(current + n)</code>" for
<code>reverse_iterator::operator[]</code>.
<p/>
This copied <code>current</code>, advances (backwards) via <code>operator+</code> and
dereferences. It bypasses the issues due to <code>reverse_iterator::operator[]</code>
being const by copying the iterator.
</p>

<p><i>[2014-02-13 Issaquah : close as NAD]</i></p>



<p id="res-2347"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3797.</p>

<ol>
<li><p>Edit 24.5.1.2 <a href="https://wg21.link/reverse.iterator">[reverse.iterator]</a>, class template <code>reverse_iterator</code> synopsis, as indicated:</p>

<blockquote><pre>
namespace std {
  template &lt;class Iterator&gt;
  class reverse_iterator : public
  iterator&lt;typename iterator_traits&lt;Iterator&gt;::iterator_category,
    typename iterator_traits&lt;Iterator&gt;::value_type,
    typename iterator_traits&lt;Iterator&gt;::difference_type,
    typename iterator_traits&lt;Iterator&gt;::pointer,
    typename iterator_traits&lt;Iterator&gt;::reference> {
  public:
    [&hellip;]
    <del><em>unspecified</em></del><ins>reference</ins> operator[](difference_type n) const;
    [&hellip;]
  };
  [&hellip;]
}
</pre></blockquote>
</li>

<li><p>Edit  [reverse.iter.opindex] as indicated:</p>

<blockquote><pre>
<del><em>unspecified</em></del><ins>reference</ins> operator[](
  typename reverse_iterator&lt;Iterator&gt;::difference_type n) const;
</pre><blockquote>
<p>
-1- <i>Returns:</i> <code><del>current[-n-1]</del><ins>*(current + n)</ins></code>.
</p>
</blockquote></blockquote>
</li>

</ol>






</body>
</html>
