<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 2876: shared_ptr::shared_ptr(const weak_ptr&lt;Y&gt;&amp;) constructor should be constrained</title>
<meta property="og:title" content="Issue 2876: shared_ptr::shared_ptr(const weak_ptr&lt;Y&gt;&amp;) constructor should be constrained">
<meta property="og:description" content="C++ library issue. Status: C++17">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue2876.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="2876"><a href="lwg-defects.html#2876">2876</a>. <code>shared_ptr::shared_ptr(const weak_ptr&lt;Y&gt;&amp;)</code> constructor should be constrained</h3>
<p><b>Section:</b> 20.3.2.2.2 <a href="https://wg21.link/util.smartptr.shared.const">[util.smartptr.shared.const]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> United States <b>Opened:</b> 2017-02-03 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#util.smartptr.shared.const">active issues</a> in [util.smartptr.shared.const].</p>
<p><b>View all other</b> <a href="lwg-index.html#util.smartptr.shared.const">issues</a> in [util.smartptr.shared.const].</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><b>Addresses US 129</b></p>

<p>Paragraph 22: This constructor should not participate in overload 
resolution unless the requirements are satisfied, in 
order to give correct results from the <code>is_constructible</code> trait.</p>

<p>
Proposed change:
</p>
<p>
Add a <i>Remarks:</i> clause to constrain this constructor 
not to participate in overload resolution unless the 
<i>Requires</i> clause is satisfied.
</p>

<p><i>[2017-02-23, Jonathan provides wording]</i></p>


<p><i>[Kona 2017-02-27]</i></p>

<p>Accepted as Immediate to resolve NB comment.</p>


<p id="res-2876"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4640">N4640</a>.
</p>

<ol>
<li><p>Modify 20.3.2.2.2 <a href="https://wg21.link/util.smartptr.shared.const">[util.smartptr.shared.const]</a> as indicated:</p>
<blockquote>
<pre>
template&lt;class Y&gt; explicit shared_ptr(const weak_ptr&lt;Y&gt;&amp; r);
</pre>
<blockquote>
<p>
<del>-22- <i>Requires:</i> <code>Y*</code> shall be compatible with <code>T*</code>.</del>
<p/>
[&hellip;]
<p/>
-25- <i>Throws:</i> [&hellip;]
<p/>
<ins>-?- <i>Remarks:</i> This constructor shall not participate in overload resolution
unless <code>Y*</code> is compatible with <code>T*</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>






</body>
</html>
