<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3702: Should zip_transform_view::iterator remove operator&lt;?</title>
<meta property="og:title" content="Issue 3702: Should zip_transform_view::iterator remove operator&lt;?">
<meta property="og:description" content="C++ library issue. Status: C++23">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3702.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="3702"><a href="lwg-defects.html#3702">3702</a>. Should <code>zip_transform_view::<i>iterator</i></code> remove <code>operator&lt;</code>?</h3>
<p><b>Section:</b> 25.7.26.3 <a href="https://wg21.link/range.zip.transform.iterator">[range.zip.transform.iterator]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-05-21 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</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>
After LWG <a href="lwg-defects.html#3692" title="zip_view::iterator's operator&lt;=&gt; is overconstrained (Status: C++23)">3692</a><sup><a href="https://cplusplus.github.io/LWG/issue3692" title="Latest snapshot">(i)</a></sup>, <code>zip_view::<i>iterator</i></code> only provides <code>operator&lt;=&gt;</code>. 
Since the comparison of <code>zip_transform_view::<i>iterator</i></code> uses <code>zip_view::<i>iterator</i></code>'s 
<code>operator&lt;=&gt;</code>, it is possible to remove <code>zip_transform_view::<i>iterator</i></code>'s 
<code>operator&lt;</code>, <code>&gt;</code>, <code>&lt;=</code>, <code>&gt;=</code> and just detect if <code><i>ziperator</i></code>'s 
<code>operator&lt;=&gt;</code> is available.
<p/>
Since the <code><i>ziperator</i></code>'s <code>operator&lt;=&gt;</code> is valid only when <code>zip_view</code> is a 
<code>random_access_range</code>, we don't need to additionally constrain the <code><i>ziperator</i></code> to be 
<code>three_way_comparable</code>.
</p>

<p><i>[2022-06-21; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after five 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-3702"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 25.7.26.3 <a href="https://wg21.link/range.zip.transform.iterator">[range.zip.transform.iterator]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
[&hellip;]
template&lt;copy_constructible F, input_range... Views&gt;
requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0) &amp;&amp; is_object_v&lt;F&gt; &amp;&amp;
         regular_invocable&lt;F&amp;, range_reference_t&lt;Views&gt;...&gt; &amp;&amp;
         <i>can-reference</i>&lt;invoke_result_t&lt;F&amp;, range_reference_t&lt;Views&gt;...&gt;&gt;
template&lt;bool Const&gt;
class zip_transform_view&lt;F, Views...&gt;::<i>iterator</i> {
  using <i>Parent</i> = <i>maybe-const&lt;</i>Const, zip_transform_view&gt;;        // exposition only
  using <i>Base</i> = <i>maybe-const</i>&lt;Const, <i>InnerView</i>&gt;;                   // exposition only
  <i>Parent</i>* <i>parent_</i> = nullptr;                                    // exposition only
  <i>ziperator</i>&lt;Const&gt; <i>inner_</i>;                                      // exposition only

  constexpr <i>iterator</i>(<i>Parent</i>&amp; parent, <i>ziperator</i>&lt;Const&gt; inner);   // exposition only
public:
  [&hellip;]
  friend constexpr bool operator==(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
    requires equality_comparable&lt;<i>ziperator</i>&lt;Const&gt;&gt;;
  
  <del>friend constexpr bool operator&lt;(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
    requires random_access_range&lt;<i>Base</i>&gt;;
  friend constexpr bool operator&gt;(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
    requires random_access_range&lt;<i>Base</i>&gt;;
  friend constexpr bool operator&lt;=(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
    requires random_access_range&lt;<i>Base</i>&gt;;
  friend constexpr bool operator&gt;=(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
    requires random_access_range&lt;<i>Base</i>&gt;;</del>
  friend constexpr auto operator&lt;=&gt;(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
    requires random_access_range&lt;<i>Base</i>&gt;<del>&amp;&amp; three_way_comparable&lt;<i>ziperator</i>&lt;Const&gt;&gt;</del>;
  [&hellip;]
};
[&hellip;]
}
</pre>
</blockquote>
[&hellip;]
<pre>
friend constexpr bool operator==(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  requires equality_comparable&lt;<i>ziperator</i>&lt;Const&gt;&gt;;
<del>friend constexpr bool operator&lt;(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  requires random_access_range&lt;<i>Base</i>&gt;;
friend constexpr bool operator&gt;(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  requires random_access_range&lt;<i>Base</i>&gt;;
friend constexpr bool operator&lt;=(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  requires random_access_range&lt;<i>Base</i>&gt;;
friend constexpr bool operator&gt;=(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  requires random_access_range&lt;<i>Base</i>&gt;;</del>
friend constexpr auto operator&lt;=&gt;(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  requires random_access_range&lt;<i>Base</i>&gt;<del>&amp;&amp; three_way_comparable&lt;<i>ziperator</i>&lt;Const&gt;&gt;</del>;
</pre>
<blockquote>
<p>
-14- Let <code><i>op</i></code> be the operator.
</p>
<p>
-15- <i>Effects</i>: Equivalent to: <code>return x.<i>inner_</i> <i>op</i> y.<i>inner_</i>;</code>
</p>
</blockquote>
</blockquote>
</li>

</ol>





</body>
</html>
