<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 4002: The definition of iota_view::iterator::iterator_concept should be improved</title>
<meta property="og:title" content="Issue 4002: The definition of iota_view::iterator::iterator_concept should be improved">
<meta property="og:description" content="C++ library issue. Status: New">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue4002.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#New">New</a> status.</em></p>
<h3 id="4002"><a href="lwg-active.html#4002">4002</a>. The definition of <code>iota_view::<i>iterator</i>::iterator_concept</code> should be improved</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#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-10-27 <b>Last modified:</b> 2023-11-07</p>
<p><b>Priority: </b>3
</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#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
When <code>W</code> models <code>incrementable</code>, the <code>iota_view</code> iterator will be a <code>forward_iterator</code>, 
which accidentally makes <code>iota_view&lt;istream_iterator&lt;int&gt;&gt;</code> satisfies <code>forward_range</code>, 
which is obviously incorrect.
<p/>
Since iterator types are well within the domain of <code>W</code>, it seems more appropriate and accurate to define 
the <code>iterator_concept</code> through the iterator concept in this case.
</p>

<p><i>[2023-11-07; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
"This would change the iterator concept for types which model <code>advanceable</code> but do not model <code>random_access_iterator</code>. A type with +=
and -= and operator* will currently get <code>random_access_iterator_tag</code>
but this proposed resolution would change it to <code>bidirectional_iterator_tag</code>."
"If we want to handle this case (and not leave it as IFNDR - the type
satisfies-but-not-models <code>incrementable</code>) then we need to account
for it everywhere we are currently using <code>incrementable</code>
(and possibly <code>equality_comparable</code>?), and not just in the concept."
</p>



<p id="res-4002"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4964" title=" Working Draft, Programming Languages — C++">N4964</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 class="note">
<p>
[<i>Drafting note</i>: When <code>W</code> only models <code>input_or_output_iterator</code>, it implies that its 
pre- and post-increment are not equality-preserving, so <code>iterator_concept</code> should be <code>input_iterator_tag</code>.]
</p>
</blockquote>

<blockquote>
<p>
-1- <code><i>iterator</i>::iterator_concept</code> is defined as follows:
</p>
<ol style="list-style-type: none">
<li><p>(?.?) <ins>&mdash; If <code>W</code> models <code>input_or_output_iterator</code>, then</ins></p>
<ol style="list-style-type: none">
<li><p>(?.?) <ins>&mdash; if <code>W</code> models <code>random_access_iterator</code>, then <code>iterator_concept</code> is 
<code>random_access_iterator_tag</code>;</ins></p></li>
<li><p>(?.?) <ins>&mdash; otherwise, if <code>W</code> models <code>bidirectional_iterator</code>, then <code>iterator_concept</code> 
is <code>bidirectional_iterator_tag</code>;</ins></p></li>
<li><p>(?.?) <ins>&mdash; otherwise, if <code>W</code> models <code>forward_iterator</code>, then <code>iterator_concept</code> 
is <code>forward_iterator_tag</code>;</ins></p></li>
<li><p>(?.?) <ins>&mdash; otherwise, <code>iterator_concept</code> is <code>input_iterator_tag</code>.</ins></p></li>
</ol>
</li>
<li><p>(1.1) &mdash; <ins>Otherwise, i</ins><del>I</del>f <code>W</code> models <code><i>advanceable</i></code>, then 
<code>iterator_concept</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_concept</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_concept</code> is 
<code>forward_iterator_tag</code>.</p></li>
<li><p>(1.4) &mdash; Otherwise, <code>iterator_concept</code> is <code>input_iterator_tag</code>.</p></li>
</ol>
</blockquote>

</li>

</ol>





</body>
</html>
