<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3670: Cpp17InputIterators don't have integer-class difference types</title>
<meta property="og:title" content="Issue 3670: Cpp17InputIterators don't have integer-class difference types">
<meta property="og:description" content="C++ library issue. Status: C++23">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3670.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++23">C++23</a> status.</em></p>
<h3 id="3670"><a href="lwg-defects.html#3670">3670</a>. <i>Cpp17InputIterator</i>s don't have integer-class difference types</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++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2022-02-04 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</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++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
25.6.4.3 <a href="https://wg21.link/range.iota.iterator">[range.iota.iterator]</a> defines 
</p>
<blockquote><pre>
using iterator_category = input_iterator_tag; <i>// present only if W models incrementable</i>
</pre></blockquote>
<p>
but when <code>difference_type</code> is an integer-class type the iterator does not meet the 
<i>Cpp17InputIterator</i> requirements.
</p>

<p><i>[2022-02-07; Daniel comments]</i></p>

<p>
As requested by LWG <a href="lwg-defects.html#3376" title="&quot;integer-like class type&quot; is too restrictive (Status: Resolved)">3376</a><sup><a href="https://cplusplus.github.io/LWG/issue3376" title="Latest snapshot">(i)</a></sup> and wording implemented by <a href="https://wg21.link/P2393R1" title=" Cleaning up integer-class types">P2393R1</a>,
integer-class types are no longer required to have class type.
</p>

<p>
<strong>Previous resolution [SUPERSEDED]:</strong>
</p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>.
</p>

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

<blockquote>
<pre>
namespace std::ranges {
  template&lt;weakly_incrementable W, semiregular Bound&gt;
    requires <i>weakly-equality-comparable-with</i>&lt;W, Bound&gt; &amp;&amp; copyable&lt;W&gt;
  struct iota_view&lt;W, Bound&gt;::<i>iterator</i> {
    [&hellip;]
    using iterator_category = input_iterator_tag; <i>// present only if W models incrementable <ins>and</ins></i> 
                                                  <i><ins>// IOTA-DIFF-T(W) is not a class type</ins></i>
    using value_type = W;
    using difference_type = <i>IOTA-DIFF-T</i>(W);
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2022-02-07; Casey Carter provides improved wording]</i></p>


<p><i>[2022-03-04; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after seven votes in favour during reflector poll.
</p>

<p><i>[2022-07-15; LWG telecon: move to Ready]</i></p>


<p><i>[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3670"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>.
</p>

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

<blockquote>
<pre>
namespace std::ranges {
  template&lt;weakly_incrementable W, semiregular Bound&gt;
    requires <i>weakly-equality-comparable-with</i>&lt;W, Bound&gt; &amp;&amp; copyable&lt;W&gt;
  struct iota_view&lt;W, Bound&gt;::<i>iterator</i> {
    [&hellip;]
    using iterator_category = input_iterator_tag; <i>// present only if W models incrementable <ins>and</ins></i> 
                                                  <i><ins>// IOTA-DIFF-T(W) is an integral type</ins></i>
    using value_type = W;
    using difference_type = <i>IOTA-DIFF-T</i>(W);
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>
</ol>





</body>
</html>
