<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 2513: Missing requirements for basic_string::value_type</title>
<meta property="og:title" content="Issue 2513: Missing requirements for basic_string::value_type">
<meta property="og:description" content="C++ library issue. Status: New">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue2513.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#New">New</a> status.</em></p>
<h3 id="2513"><a href="lwg-active.html#2513">2513</a>. Missing requirements for <code>basic_string::value_type</code></h3>
<p><b>Section:</b> 27.1 <a href="https://wg21.link/strings.general">[strings.general]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2015-06-26 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>4
</p>
<p><b>View all other</b> <a href="lwg-index.html#strings.general">issues</a> in [strings.general].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The allocator-aware container requirements in Table 98 impose no
<code>MoveAssignable</code> requirements on the <code>value_type</code> when
<code>propagate_on_container_move_assignment</code> is true, because typically the
container's storage would be moved by just exchanging some pointers.
<p/>
However for a <code>basic_string</code> using the small string optimization move
assignment may need to assign individual characters into the small
string buffer, even when the allocator propagates.
<p/>
The only requirement on the char-like objects stored in a <code>basic_string</code>
are that they are non-array POD types and <code>Destructible</code>, which means
that a POD type with a deleted move assignment operator should be
usable in a <code>basic_string</code>, despite it being impossible to move assign:
</p>
<blockquote><pre>
#include &lt;string&gt;

struct odd_pod 
{
  odd_pod() = default;
  odd_pod&amp; operator=(odd_pod&amp;&amp;) = delete;
};

static_assert(std::is_pod&lt;odd_pod&gt;::value, "POD");

int main()
{
  using S = std::basic_string&lt;odd_pod&gt;;
  S s;
  s = S{};       // fails
}
</pre></blockquote>
<p>
Using libstdc++ <code>basic_string&lt;odd_pod&gt;</code> cannot even be
default-constructed because the constructor attempts to assign the
null terminator to the first element of the small string buffer.
<p/>
Similar problems exist with POD types with a deleted default constructor.
<p/>
I believe that <code>basic_string</code> should require its <code>value_type</code> to be at
least <code>DefaultConstructible</code> and <code>MoveAssignable</code>.
</p>

<p><i>[2016-06, Oulu]</i></p>

<p>This should be resolved by <a href="https://wg21.link/P0178">P0178</a></p>
<p>Note: P0178 was sent back to LEWG in Oulu.</p>


<p id="res-2513"><b>Proposed resolution:</b></p>





</body>
</html>
