<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 4012: common_view::begin/end are missing the simple-view check</title>
<meta property="og:title" content="Issue 4012: common_view::begin/end are missing the simple-view check">
<meta property="og:description" content="C++ library issue. Status: WP">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue4012.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#WP">WP</a> status.</em></p>
<h3 id="4012"><a href="lwg-defects.html#4012">4012</a>. <code>common_view::begin/end</code> are missing the <code><i>simple-view</i></code> check</h3>
<p><b>Section:</b> 25.7.20.2 <a href="https://wg21.link/range.common.view">[range.common.view]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-11-11 <b>Last modified:</b> 2024-04-02</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.common.view">issues</a> in [range.common.view].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>common_view::begin/end</code> have exactly the same implementation as their corresponding <code>const</code> versions, 
which implies that when the underlying <code>V</code> satisfies <code><i>simple-view</i></code>, it is sufficient to 
just provide <code>const</code>-qualified members.
</p>

<p><i>[2024-03-11; Reflector poll]</i></p>

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

<p><i>[Tokyo 2024-03-23; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4012"><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 25.7.20.2 <a href="https://wg21.link/range.common.view">[range.common.view]</a> as indicated:</p>

<blockquote><pre>
namespace std::ranges {
  template&lt;view V&gt;
    requires (!common_range&lt;V&gt; &amp;&amp; copyable&lt;iterator_t&lt;V&gt;&gt;)
  class common_view : public view_interface&lt;common_view&lt;V&gt;&gt; {
  private:
    V <i>base_</i> = V();  // <i>exposition only</i>

  public:
    [&hellip;]
    constexpr auto begin() <ins>requires (!<i>simple-view</i>&lt;V&gt;)</ins> {
      if constexpr (random_access_range&lt;V&gt; &amp;&amp; sized_range&lt;V&gt;)
        return ranges::begin(<i>base_</i>);
      else
        return common_iterator&lt;iterator_t&lt;V&gt;, sentinel_t&lt;V&gt;&gt;(ranges::begin(<i>base_</i>));
    }

    constexpr auto begin() const requires range&lt;const V&gt; {
      if constexpr (random_access_range&lt;const V&gt; &amp;&amp; sized_range&lt;const V&gt;)
        return ranges::begin(<i>base_</i>);
      else
        return common_iterator&lt;iterator_t&lt;const V&gt;, sentinel_t&lt;const V&gt;&gt;(ranges::begin(<i>base_</i>));
    }
    
    constexpr auto end() <ins>requires (!<i>simple-view</i>&lt;V&gt;)</ins> {
      if constexpr (random_access_range&lt;V&gt; &amp;&amp; sized_range&lt;V&gt;)
        return ranges::begin(<i>base_</i>) + ranges::distance(<i>base_</i>);
      else
        return common_iterator&lt;iterator_t&lt;V&gt;, sentinel_t&lt;V&gt;&gt;(ranges::end(<i>base_</i>));
    }

    constexpr auto end() const requires range&lt;const V&gt; {
      if constexpr (random_access_range&lt;const V&gt; &amp;&amp; sized_range&lt;const V&gt;)
        return ranges::begin(<i>base_</i>) + ranges::distance(<i>base_</i>);
      else
        return common_iterator&lt;iterator_t&lt;const V&gt;, sentinel_t&lt;const V&gt;&gt;(ranges::end(<i>base_</i>));
    }

    [&hellip;]
  };
[&hellip;]
}
</pre></blockquote>

</li>

</ol>





</body>
</html>
