<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3762: generator::iterator::operator== should pass by reference</title>
<meta property="og:title" content="Issue 3762: generator::iterator::operator== should pass by reference">
<meta property="og:description" content="C++ library issue. Status: C++23">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3762.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="3762"><a href="lwg-defects.html#3762">3762</a>. <code>generator::<i>iterator</i>::operator==</code> should pass by reference</h3>
<p><b>Section:</b> 25.8.6 <a href="https://wg21.link/coro.generator.iterator">[coro.generator.iterator]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-08-27 <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#coro.generator.iterator">active issues</a> in [coro.generator.iterator].</p>
<p><b>View all other</b> <a href="lwg-index.html#coro.generator.iterator">issues</a> in [coro.generator.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>
Currently, <code>generator::<i>iterator</i>::operator==</code> passes <code><i>iterator</i></code> by value.
Given that <code><i>iterator</i></code> is a move-only type, this makes it impossible for 
<code>generator</code> to model <code>range</code>, since <code>default_sentinel</code> cannot be compared 
to the <code><i>iterator</i></code> of <code>const</code> lvalue and is not eligible to be its sentinel.
</p>

<p><i>[2022-09-23; Reflector poll]</i></p>

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

<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3762"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>

<li><p>Modify 25.8.6 <a href="https://wg21.link/coro.generator.iterator">[coro.generator.iterator]</a> as indicated:</p>
<blockquote>
<blockquote>
<pre>
namespace std {
  template&lt;class Ref, class V, class Allocator&gt;
  class generator&lt;Ref, V, Allocator&gt;::<i>iterator</i> {
  public:
    using value_type = <i>value</i>;
    using difference_type = ptrdiff_t;

    <i>iterator</i>(<i>iterator</i>&amp;&amp; other) noexcept;
    <i>iterator</i>&amp; operator=(<i>iterator</i>&amp;&amp; other) noexcept;
    
    <i>reference</i> operator*() const noexcept(is_nothrow_copy_constructible_v&lt;<i>reference</i>&gt;);
    <i>iterator</i>&amp; operator++();
    void operator++(int);

    friend bool operator==(<ins>const</ins> <i>iterator</i><ins>&amp;</ins> i, default_sentinel_t);

  private:
    coroutine_handle&lt;promise_type&gt; <i>coroutine_</i>; <i>// exposition only</i>
  };
}
</pre>
</blockquote>
<p>
[&hellip;]
</p>
<pre>
friend bool operator==(<ins>const</ins> <i>iterator</i><ins>&amp;</ins> i, default_sentinel_t);
</pre>
<blockquote>
<p>
-10- <i>Effects</i>: Equivalent to: <code>return i.<i>coroutine_</i>.done();</code>
</p>
</blockquote>
</blockquote>

</li>

</ol>





</body>
</html>
