<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 528: const_iterator iterator issue when they are the same type</title>
<meta property="og:title" content="Issue 528: const_iterator iterator issue when they are the same type">
<meta property="og:description" content="C++ library issue. Status: NAD">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue528.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="528"><a href="lwg-closed.html#528">528</a>. <code>const_iterator</code> <code>iterator</code> issue when they are the same type</h3>
<p><b>Section:</b> 23.5 <a href="https://wg21.link/unord">[unord]</a>, 99 [tr.unord.unord] <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Paolo Carlini <b>Opened:</b> 2005-10-12 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#unord">issues</a> in [unord].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
while implementing the resolution of issue 6.19 I'm noticing the
following: according to 6.3.4.3/2 (and 6.3.4.5/2), for unordered_set and
unordered_multiset:
</p>

<blockquote><p>
    "The iterator and const_iterator types are both const types. It is
unspecified whether they are the same type"
</p></blockquote>

<p>
Now, according to the resolution of 6.19, we have overloads of insert
with hint and erase (single and range) both for iterator and
const_iterator, which, AFAICS, can be meaningful at the same time *only*
if iterator and const_iterator *are* in fact different types.
</p>
<p>
Then, iterator and const_iterator are *required* to be different types?
Or that is an unintended consequence? Maybe the overloads for plain
iterators should be added only to unordered_map and unordered_multimap?
Or, of course, I'm missing something?
</p>



<p id="res-528"><b>Proposed resolution:</b></p>
<p>
Add to 6.3.4.3p2 (and 6.3.4.5p2):
</p>
<p>
2  ... The iterator and const_iterator types are both <del>const</del>
<ins>constant</ins> iterator types.
It is unspecified whether they are the same type. 
</p>

<p>
Add a new subsection to 17.4.4 [lib.conforming]:
</p>

<blockquote>
<p>
An implementation shall not supply an overloaded function
       signature specified in any library clause if such a signature
       would be inherently ambiguous during overload resolution
       due to two library types referring to the same type.
</p>
<p>
       [Note: For example, this occurs when a container's iterator
       and const_iterator types are the same. -- end note]
</p>
</blockquote>

<p><i>[
Post-Berlin: Beman supplied wording.
]</i></p>




<p><b>Rationale:</b></p><p>
Toronto:  The first issue has been fixed by N2350 (the insert and erase members
are collapsed into one signature).  Alisdair to open a separate issue on the
chapter 17 wording.
</p>




</body>
</html>
