<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3677: Is a cv-qualified pair specially handled in uses-allocator construction?</title>
<meta property="og:title" content="Issue 3677: Is a cv-qualified pair specially handled in uses-allocator construction?">
<meta property="og:description" content="C++ library issue. Status: C++23">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3677.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++23">C++23</a> status.</em></p>
<h3 id="3677"><a href="lwg-defects.html#3677">3677</a>. Is a <i>cv</i>-qualified <code>pair</code> specially handled in uses-allocator construction?</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++23">C++23</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2022-02-16 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>2
</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++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
It seems unclear whether <i>cv</i>-qualified <code>pair</code> specializations are considered as 
specializations of <code>pair</code> in 20.2.8.2 <a href="https://wg21.link/allocator.uses.construction">[allocator.uses.construction]</a>.
<p/>
Currently MSVC STL only considered <i>cv</i>-unqualified pair types as such specializations, 
while libstdc++ accept both <i>cv</i>-unqualified and <code>const</code>-qualified pair types as 
such specializations. The resolution of LWG <a href="lwg-defects.html#3525" title="uses_allocator_construction_args fails to handle types convertible to pair (Status: C++23)">3525</a><sup><a href="https://cplusplus.github.io/LWG/issue3525" title="Latest snapshot">(i)</a></sup> uses <code>remove_cv_t</code>,
which possibly imply that the specialization of pair may be <i>cv</i>-qualified.
<p/>
The difference can be observed via the following program:
</p>
<blockquote><pre>
#include &lt;utility&gt;
#include &lt;memory&gt;
#include &lt;vector&gt;
#include &lt;cassert&gt; 

template&lt;class T&gt;
class payload_ator {

  int payload{};
    
public:
  payload_ator() = default;

  constexpr explicit payload_ator(int n) noexcept : payload{n} {}

  template&lt;class U&gt;
  constexpr explicit payload_ator(payload_ator&lt;U&gt; a) noexcept : payload{a.payload} {}   

  friend bool operator==(payload_ator, payload_ator) = default;

  template&lt;class U&gt;
  friend constexpr bool operator==(payload_ator x, payload_ator&lt;U&gt; y) noexcept
  {
    return x.payload == y.payload;
  }   

  using value_type = T;

  constexpr T* allocate(std::size_t n) { return std::allocator&lt;T&gt;{}.allocate(n); }

  constexpr void deallocate(T* p, std::size_t n) { return std::allocator&lt;T&gt;{}.deallocate(p, n); }   

  constexpr int get_payload() const noexcept { return payload; }
};

bool test()
{
  constexpr int in_v = 42;
  using my_pair_t = std::pair&lt;int, std::vector&lt;int, payload_ator&lt;int&gt;&gt;&gt;;
  auto out_v = std::make_obj_using_allocator&lt;const my_pair_t&gt;(payload_ator&lt;int&gt;{in_v}).second.get_allocator().get_payload();
  return in_v == out_v;
}

int main()
{
  assert(test()); // <span style="color:red;font-weight:bolder">passes only if a const-qualified pair specialization is considered as a pair specialization</span>
}
</pre></blockquote>

<p><i>[2022-03-04; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
</p>

<p><i>[2022-08-24; LWG telecon]</i></p>

<p>Change every <code>T</code> to <code>remove_cv_t&lt;T&gt;</code>.</p>

<p><i>[2022-08-25; Jonathan Wakely provides wording]</i></p>


<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ul>
<li><p>Modify 20.2.8.2 <a href="https://wg21.link/allocator.uses.construction">[allocator.uses.construction]</a> as indicated,
using <code>remove_cv_t</code> in every <i>Constraints</i>: element:</p>

<blockquote>
<p><i>Constraints</i>:
<code><ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins></code> [is|is not]
a specialization of <code>pair</code>
</p>
</blockquote>
</li>
</ul>

</blockquote>

<p><i>[2022-09-23; Jonathan provides improved wording]</i></p>


<p><i>[2022-09-30; moved to Tentatively Ready after seven votes in reflector poll]</i></p>


<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3677"><b>Proposed resolution:</b></p>

<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify 20.2.8.2 <a href="https://wg21.link/allocator.uses.construction">[allocator.uses.construction]</a> as indicated,
using <code>remove_cv_t</code> in every <i>Constraints</i>: element (paragraphs 4, 6, 8, 10, 12, 14, 17):</p>

<blockquote>
<p><i>Constraints</i>:
<code><ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins></code> [is|is not]
a specialization of <code>pair</code>
</p>
</blockquote>
</li>

<li>
<p>Add <code>remove_cv_t</code> in paragraph 5:</p>

<blockquote>
<p>-5- <i>Returns</i>: A <code>tuple</code> value determined as follows: </p>
<p>(5.1) &mdash; If
<code>uses_allocator_v&lt;<ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins>, Alloc&gt;</code>
is <code>false</code> and  <code>is_constructible_v&lt;T, Args...&gt;</code>
is <code>true</code>,
return <code>forward_as_tuple(std::forward&lt;Args&gt;(args)...)</code>.
</p>
<p>(5.2) &mdash;
Otherwise, if <code>uses_allocator_v&lt;<ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins>, Alloc&gt;</code> is <code>true</code>
and <code>is_constructible_v&lt;T, allocator_arg_t, const Alloc&amp;, Args...&gt;</code> is <code>true</code>, return
</p>
<pre><code>tuple&lt;allocator_arg_t, const Alloc&amp;, Args&amp;&amp;...&gt;(
  allocator_arg, alloc, std::forward&lt;Args&gt;(args)...)</code></pre>

<p>(5.3) &mdash;
Otherwise, if <code>uses_allocator_v&lt;<ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins>, Alloc&gt;</code> is <code>true</code>
and <code>is_constructible_v&lt;T, Args..., const Alloc&amp;&gt;</code> is <code>true</code>, return <code>forward_as_tuple(std::forward&lt;Args&gt;(args)..., alloc)</code>.
</p>
<p>(5.4) &mdash; Otherwise, the program is ill-formed. </p>
</blockquote>
</li>

<li>
<p>Rephrase paragraph 7 in terms of the pair member types:</p>

<blockquote>
<p>
<ins> -?-
Let <code>T1</code> be <code>T::first_type</code>.
Let <code>T2</code> be <code>T::second_type</code>.
</ins>
</p>

<p>-6- <i>Constraints</i>:
<code><ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins></code> is
a specialization of <code>pair</code>
</p>
<p>-7- <i>Effects:</i>:
<del>
For <code>T</code> specified as <code>pair&lt;T1, T2&gt;</code>, equivalent
</del>
<ins>Equivalent</ins>
to:
</p>
</blockquote>
</li>

</ol>






</body>
</html>
