<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3509: Range adaptor objects are underspecified</title>
<meta property="og:title" content="Issue 3509: Range adaptor objects are underspecified">
<meta property="og:description" content="C++ library issue. Status: Resolved">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3509.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#Resolved">Resolved</a> status.</em></p>
<h3 id="3509"><a href="lwg-defects.html#3509">3509</a>. Range adaptor objects are underspecified</h3>
<p><b>Section:</b> 25.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2020-12-15 <b>Last modified:</b> 2021-06-14</p>
<p><b>Priority: </b>2
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.adaptor.object">active issues</a> in [range.adaptor.object].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.adaptor.object">issues</a> in [range.adaptor.object].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
There is implementation divergence in the handling of this example:
</p>
<blockquote><pre>
template&lt;class F&gt;
auto filter(F f) {
  return std::views::filter(f);
}
std::vector&lt;int&gt; v = {1, 2, 3, 4};
auto f = filter([i = std::vector{4}] (auto x) { return x == i[0]; });
auto x = v | f;
</pre></blockquote>
<p>
libstdc++'s <code>views::filter</code> stores a reference to the argument if it is
passed as an lvalue, so <code>f</code> contains a dangling reference in the above
example. MSVC's <code>views::filter</code> always stores a copy, and forwards that
as either an lvalue or an rvalue depending on the range adaptor closure object's 
value category.
<p/>
MSVC's behavior here seems desirable and is consistent with range-v3's
behavior as well: the ability to form range adapter pipelines like
this is an important feature, and there's nothing in the above example
that even hints at a dangling reference problem.
<p/>
However, the wording in 25.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a> is unclear at best
about exactly how the arguments are stored in the <code>adaptor(args...)</code>
case; arguably, the requirement that <code>adaptor(range, args...)</code> be
equivalent to <code>adaptor(args...)(range)</code> may even rule out the ability to
make an extra copy. Certainly nothing in the wording guarantees that
it is safe to reuse lvalue range pipelines (that is, <code>v | f</code> doesn't
move from <code>f</code>).
</p>

<p><i>[2021-01-15; Telecon prioritization]</i></p>

<p>
Set priority to 2 following reflector and telecon discussions.
</p>
<p><i>[2021-06-13 Resolved by the adoption of <a href="https://wg21.link/P2281R1" title=" Clarifying range adaptor objects">P2281R1</a> at the June 2021 plenary. Status changed: New &rarr; Resolved.]</i></p>



<p id="res-3509"><b>Proposed resolution:</b></p>





</body>
</html>
