<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 215: Can a map's key_type be const?</title>
<meta property="og:title" content="Issue 215: Can a map's key_type be const?">
<meta property="og:description" content="C++ library issue. Status: NAD">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue215.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="215"><a href="lwg-closed.html#215">215</a>. Can a map's key_type be const?</h3>
<p><b>Section:</b> 23.2.7 <a href="https://wg21.link/associative.reqmts">[associative.reqmts]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Judy Ward <b>Opened:</b> 2000-02-29 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#associative.reqmts">active issues</a> in [associative.reqmts].</p>
<p><b>View all other</b> <a href="lwg-index.html#associative.reqmts">issues</a> in [associative.reqmts].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>A user noticed that this doesn't compile with the Rogue Wave library because
the rb_tree class declares a key_allocator, and allocator&lt;const int&gt; is
not legal, I think:</p>
<blockquote>
  <pre>map &lt; const int, ... &gt; // legal?</pre>
</blockquote>
<p>which made me wonder whether it is legal for a map's key_type to be const. In
email from Matt Austern he said:</p>
<blockquote>
<p>I'm not sure whether it's legal to declare a map with a const key type. I
hadn't thought about that question until a couple weeks ago. My intuitive
feeling is that it ought not to be allowed, and that the standard ought to say
so. It does turn out to work in SGI's library, though, and someone in the
compiler group even used it. Perhaps this deserves to be written up as an issue
too.</p>
</blockquote>


<p><b>Rationale:</b></p>
<p>The &quot;key is assignable&quot; requirement from table 69 in
23.2.7 <a href="https://wg21.link/associative.reqmts">[associative.reqmts]</a> already implies the key cannot be const.</p>




</body>
</html>
