<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3321: uninitialized_construct_using_allocator should use construct_at</title>
<meta property="og:title" content="Issue 3321: uninitialized_construct_using_allocator should use construct_at">
<meta property="og:description" content="C++ library issue. Status: C++20">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3321.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++20">C++20</a> status.</em></p>
<h3 id="3321"><a href="lwg-defects.html#3321">3321</a>. <code>uninitialized_construct_using_allocator</code> should use <code>construct_at</code></h3>
<p><b>Section:</b> 20.2.8.2 <a href="https://wg21.link/allocator.uses.construction">[allocator.uses.construction]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> United States <b>Opened:</b> 2019-11-06 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View other</b> <a href="lwg-index-open.html#allocator.uses.construction">active issues</a> in [allocator.uses.construction].</p>
<p><b>View all other</b> <a href="lwg-index.html#allocator.uses.construction">issues</a> in [allocator.uses.construction].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses <a href="https://github.com/cplusplus/nbballot/issues/210">US 213</a></b></p>

<p>
<code>uninitialized_construct_using_allocator</code> should use <code>construct_at</code> instead 
of <code>operator new</code>
<p/>
Proposed change:
</p>
<blockquote>
<p>
<i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
return <del>::new(static_cast&lt;void*&gt;(p))</del>
<ins>construct_at(p,</ins>
<del>T(</del>make_obj_using_allocator&lt;T&gt;(alloc,
std::forward&lt;Args&gt;(args)...))<ins>)</ins>;
</pre></blockquote>
</blockquote>
<p>
<b>Tim Song:</b>
<p/>
The proposed wording in the NB comment is incorrect, because it prevents guaranteed elision.
</p>

<p><i>[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]</i></p>



<p id="res-3321"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4835">N4835</a>.</p>

<ol>
<li><p>Modify 20.2.8.2 <a href="https://wg21.link/allocator.uses.construction">[allocator.uses.construction]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class T, class Alloc, class... Args&gt;
  constexpr T* uninitialized_construct_using_allocator(T* p, const Alloc&amp; alloc, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-17- <i>Effects:</i> Equivalent to:
<blockquote><pre>
return <del>::new(static_cast&lt;void*&gt;(p))
  T(make_obj_using_allocator&lt;T&gt;(</del><ins>apply([&amp;](auto&amp;&amp;...xs) {
         return construct_at(p, std::forward&lt;decltype(xs)&gt;(xs)...);
     }, uses_allocator_construction_args&lt;T&gt;(</ins>alloc, std::forward&lt;Args&gt;(args)...));
</pre></blockquote>
</p>
</blockquote>
</blockquote>
</li>

</ol>





</body>
</html>
