<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 2475: Allow overwriting of std::basic_string terminator with charT() to allow 
cleaner interoperation with legacy APIs</title>
<meta property="og:title" content="Issue 2475: Allow overwriting of std::basic_string terminator with charT() to allow 
cleaner interoperation with legacy APIs">
<meta property="og:description" content="C++ library issue. Status: C++17">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue2475.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#C++17">C++17</a> status.</em></p>
<h3 id="2475"><a href="lwg-defects.html#2475">2475</a>. Allow overwriting of <code>std::basic_string</code> terminator with <code>charT()</code> to allow 
cleaner interoperation with legacy APIs</h3>
<p><b>Section:</b> 27.4.3.6 <a href="https://wg21.link/string.access">[string.access]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Matt Weber <b>Opened:</b> 2015-02-21 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#string.access">issues</a> in [string.access].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
It is often desirable to use a <code>std::basic_string</code> object as a buffer when interoperating with libraries 
that mutate null-terminated arrays of characters. In many cases, these legacy APIs write a null terminator at 
the specified end of the provided buffer. Providing such a function with an appropriately-sized 
<code>std::basic_string</code> results in undefined behavior when the <code>charT</code> object at the <code>size()</code> 
position is overwritten, even if the value remains unchanged.
<p/>
Absent the ability to allow for this, applications are forced into pessimizations such as: providing 
appropriately-sized <code>std::vectors</code> of <code>charT</code> for interoperating with the legacy API, and then 
copying the <code>std::vector</code> to a <code>std::basic_string</code>; providing an oversized <code>std::basic_string</code> 
object and then calling <code>resize()</code> later.
<p/>
A trivial example:
</p>
<blockquote>
<pre>
#include &lt;string&gt;
#include &lt;vector&gt;

void legacy_function(char *out, size_t count) {
  for (size_t i = 0; i &lt; count; ++i) {
    *out++ = '0' + (i % 10);
  }
  *out = '\0'; // <span style="color:#C80000;font-weight:bold">if size() == count, this results in undefined behavior</span>
}

int main() {
  std::string s(10, '\0');
  legacy_function(&amp;s[0], s.size()); // <span style="color:#C80000;font-weight:bold">undefined behavior</span>

  std::vector&lt;char&gt; buffer(11);
  legacy_function(&amp;buffer[0], buffer.size() - 1);
  std::string t(&amp;buffer[0], buffer.size() - 1); // potentially expensive copy

  std::string u(11, '\0');
  legacy_function(&amp;u[0], u.size() - 1);
  u.resize(u.size() - 1); // needlessly complicates the program's logic
}
</pre>
</blockquote>
<p>
A slight relaxation of the requirement on the returned object from the element access operator would 
allow for this interaction with no semantic change to existing programs.
</p>

<p><i>[2016-08 Chicago]</i></p>

<p>Tues PM: This should also apply to non-const <code>data()</code>. Billy to update wording.</p>
<p>Fri PM: Move to Tentatively Ready</p>


<p id="res-2475"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4296.</p>

<ol>
<li>
<p>Edit 27.4.3.6 <a href="https://wg21.link/string.access">[string.access]</a> as indicated:</p>
<blockquote>
<pre>
const_reference operator[](size_type pos) const;
reference operator[](size_type pos);
</pre>
<blockquote>
<p>
-1- <i>Requires</i>: [&hellip;]
<p/>
-2- <i>Returns</i>: <code>*(begin() + pos)</code> if <code>pos &lt; size()</code>. Otherwise, returns a reference to an object of type
<code>charT</code> with value <code>charT()</code>, where modifying the object <ins>to any value other than <code>charT()</code></ins> 
leads to undefined behavior.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>





</body>
</html>
