<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3596: ranges::starts_with and ranges::ends_with are underspecified</title>
<meta property="og:title" content="Issue 3596: ranges::starts_with and ranges::ends_with are underspecified">
<meta property="og:description" content="C++ library issue. Status: NAD">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3596.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#NAD">NAD</a> status.</em></p>
<h3 id="3596"><a href="lwg-closed.html#3596">3596</a>. <code>ranges::starts_with</code> and <code>ranges::ends_with</code> are underspecified</h3>
<p><b>Section:</b> 26.6.16 <a href="https://wg21.link/alg.starts.with">[alg.starts.with]</a>, 26.6.17 <a href="https://wg21.link/alg.ends.with">[alg.ends.with]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Michael Schellenberger Costa <b>Opened:</b> 2021-09-17 <b>Last modified:</b> 2021-09-24</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
When implementing C++23 new <code>ranges::starts_with</code> and <code>ranges::ends_with</code> 
<a href="https://github.com/microsoft/STL/pull/1997">for MSVC</a> we came around some issues regarding infinite ranges.
<p/>
In short, there is no possible answers to <code>ends_with</code>, if the second range is infinite. Similarly, there 
is no meaningful answer to <code>starts_with</code> if both ranges are infinite.
<p/>
We should constraint both algorithms to forbid those corner cases (preferred resolution), or explicitly specify a 
given return value for those (The reasonable choice would be "<code>return false;</code>")
</p>
<p><i>[2021-09-24 infinite ranges are invalid by [iterator.requirements.general] p10, so undefined by p12. Might be nice to relax this in future, but needs a paper. Status changed: New &rarr; NAD.]</i></p>



<p id="res-3596"><b>Proposed resolution:</b></p>





</body>
</html>
