<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3291: iota_view::iterator has the wrong iterator_category</title>
<meta property="og:title" content="Issue 3291: iota_view::iterator has the wrong iterator_category">
<meta property="og:description" content="C++ library issue. Status: C++20">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3291.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#C++20">C++20</a> status.</em></p>
<h3 id="3291"><a href="lwg-defects.html#3291">3291</a>. <code>iota_view::iterator</code> has the wrong <code>iterator_category</code></h3>
<p><b>Section:</b> 25.6.4.3 <a href="https://wg21.link/range.iota.iterator">[range.iota.iterator]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Eric Niebler <b>Opened:</b> 2019-09-13 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.iota.iterator">active issues</a> in [range.iota.iterator].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.iota.iterator">issues</a> in [range.iota.iterator].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In the old way of looking at the world, forward iterators need to return real references. Since 
dereferencing <code>iota_view</code>'s iterators returns by value, it cannot be a C++17 forward 
iterator. (It can, however, be a C++20 <code>forward_iterator</code>.) However, <code>iota_view</code>'s 
iterator has an <code>iterator_category</code> that (sometimes) falsely claims that it is forward or 
better (depending on the properties of the <code>weakly_incrementable</code> type it wraps).
</p>

<p><i>[2019-10-19 Issue Prioritization]</i></p>

<p>Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.</p>


<p id="res-3291"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 25.6.4.3 <a href="https://wg21.link/range.iota.iterator">[range.iota.iterator]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;class W, class Bound&gt;
  struct iota_view&lt;W, Bound&gt;::iterator {
  private:
    [&hellip;]
  public:
    using iterator_c<ins>oncept</ins><del>ategory</del> = <i>see below</i>;
    <ins>using iterator_category = input_iterator_tag;</ins>
    using value_type = W;
    using difference_type = <i>IOTA-DIFF-T</i>(W);
    [&hellip;]
  };
  [&hellip;]
}
</pre>
</blockquote>
<p>
-1- <code>iterator::iterator_c<ins>oncept</ins><del>ategory</del></code> is defined as follows:
</p>
<ol style="list-style-type: none">
<li><p>(1.1) &mdash; If <code>W</code> models <code><i>advanceable</i></code>, then 
<code>iterator_c<ins>oncept</ins><del>ategory</del></code> is <code>random_access_iterator_tag</code>.</p></li>
<li><p>(1.2) &mdash; Otherwise, if <code>W</code> models <code><i>decrementable</i></code>, then 
<code>iterator_c<ins>oncept</ins><del>ategory</del></code> is <code>bidirectional_iterator_tag</code>.</p></li>
<li><p>(1.3) &mdash; Otherwise, if <code>W</code> models <code>incrementable</code>, then 
<code>iterator_c<ins>oncept</ins><del>ategory</del></code> is <code>forward_iterator_tag</code>.</p></li>
<li><p>(1.4) &mdash; Otherwise, <code>iterator_c<ins>oncept</ins><del>ategory</del></code> is 
<code>input_iterator_tag</code>.</p></li>
</ol>

</blockquote>
</li>

</ol>





</body>
</html>
