<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3185: Uses-allocator construction functions missing constexpr and noexcept</title>
<meta property="og:title" content="Issue 3185: Uses-allocator construction functions missing constexpr and noexcept">
<meta property="og:description" content="C++ library issue. Status: C++20">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3185.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="3185"><a href="lwg-defects.html#3185">3185</a>. Uses-allocator construction functions missing <code>constexpr</code> and <code>noexcept</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> Pablo Halpern <b>Opened:</b> 2019-01-29 <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>
The uses-allocator construction functions introduced into WP when <a href="https://wg21.link/p0591r4">P0591r4</a> 
was accepted (Nov 2018, San Diego) should all be <code>constexpr</code>. All but two should also be <code>noexcept</code>. 
Getting this right is an important part of correctly adding <code>constexpr</code> memory allocation into the WP.
<p/>
The minimal change is to add the <code>constexpr</code> to all of the new functions except 
<code>uninitialized_construct_using_allocator</code> and <code>noexcept</code> to all 
of the overloads of <code>uses_allocator_construction_args</code>. Optionally, we could consider adding conditional 
<code>noexcept</code> to the remaining two functions. If <a href="https://wg21.link/p0784">p0784</a> is accepted, 
then also add <code>constexpr</code> to <code>uninitialized_construct_using_allocator</code>.
</p>

<p><i>[2019-02-12 Priority to 0 and Status to Tentatively Ready after six positive votes on the reflector.]</i></p>




<p id="res-3185"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4800">N4800</a>.</p>

<ol>
<li><p>Change header <code>&lt;memory&gt;</code> synopsis, 20.2.2 <a href="https://wg21.link/memory.syn">[memory.syn]</a>, as indicated:</p>
<blockquote>
<pre>
[&hellip;]
<i>// 20.2.8.2 <a href="https://wg21.link/allocator.uses.construction">[allocator.uses.construction]</a>, uses-allocator construction</i>
template &lt;class T, class Alloc, class... Args&gt;
<ins>constexpr</ins> auto uses_allocator_construction_args(const Alloc&amp; alloc, Args&amp;&amp;... args) <ins>noexcept</ins> -&gt; <i>see below</i>;
template &lt;class T, class Alloc, class Tuple1, class Tuple2&gt;
<ins>constexpr</ins> auto uses_allocator_construction_args(const Alloc&amp; alloc, piecewise_construct_t,
                                      Tuple1&amp;&amp; x, Tuple2&amp;&amp; y) <ins>noexcept</ins> -&gt; <i>see below</i>;
template &lt;class T, class Alloc&gt;
<ins>constexpr</ins> auto uses_allocator_construction_args(const Alloc&amp; alloc) <ins>noexcept</ins> -&gt; <i>see below</i>;
template &lt;class T, class Alloc, class U, class V&gt;
<ins>constexpr</ins> auto uses_allocator_construction_args(const Alloc&amp; alloc, U&amp;&amp; u, V&amp;&amp; v) <ins>noexcept</ins> -&gt; <i>see below</i>;
template &lt;class T, class Alloc, class U, class V&gt;
<ins>constexpr</ins> auto uses_allocator_construction_args(const Alloc&amp; alloc, const pair&lt;U,V&gt;&amp; pr) <ins>noexcept</ins> -&gt; <i>see below</i>;
template &lt;class T, class Alloc, class U, class V&gt;
<ins>constexpr</ins> auto uses_allocator_construction_args(const Alloc&amp; alloc, pair&lt;U,V&gt;&amp;&amp; pr) <ins>noexcept</ins> -&gt; <i>see below</i>;
template &lt;class T, class Alloc, class... Args&gt;
<ins>constexpr</ins> T make_obj_using_allocator(const Alloc&amp; alloc, Args&amp;&amp;... args);
template &lt;class T, class Alloc, class... Args&gt;
T* uninitialized_construct_using_allocator(T* p, const Alloc&amp; alloc, Args&amp;&amp;... args);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Change 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;
  <ins>constexpr</ins> auto uses_allocator_construction_args(const Alloc&amp; alloc, Args&amp;&amp;... args) <ins>noexcept</ins> -&gt; <i>see below</i>;
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class T, class Alloc, class Tuple1, class Tuple2&gt;
  <ins>constexpr</ins> auto uses_allocator_construction_args(const Alloc&amp; alloc, piecewise_construct_t,
                                        Tuple1&amp;&amp; x, Tuple2&amp;&amp; y) <ins>noexcept</ins> -&gt; <i>see below</i>;
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class T, class Alloc&gt;
  <ins>constexpr</ins> auto uses_allocator_construction_args(const Alloc&amp; alloc) <ins>noexcept</ins> -&gt; <i>see below</i>;
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class T, class Alloc, class U, class V&gt;
  <ins>constexpr</ins> auto uses_allocator_construction_args(const Alloc&amp; alloc, U&amp;&amp; u, V&amp;&amp; v) <ins>noexcept</ins> -&gt; <i>see below</i>;
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class T, class Alloc, class U, class V&gt;
  <ins>constexpr</ins> auto uses_allocator_construction_args(const Alloc&amp; alloc, const pair&lt;U,V&gt;&amp; pr) <ins>noexcept</ins> -&gt; <i>see below</i>;
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class T, class Alloc, class U, class V&gt;
  <ins>constexpr</ins> auto uses_allocator_construction_args(const Alloc&amp; alloc, pair&lt;U,V&gt;&amp;&amp; pr) <ins>noexcept</ins> -&gt; <i>see below</i>;
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class T, class Alloc, class... Args&gt;
  <ins>constexpr</ins> T make_obj_using_allocator(const Alloc&amp; alloc, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class T, class Alloc, class... Args&gt;
  T* uninitialized_construct_using_allocator(T* p, const Alloc&amp; alloc, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>




</body>
</html>
