<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 854: default_delete converting constructor underspecified</title>
<meta property="og:title" content="Issue 854: default_delete converting constructor underspecified">
<meta property="og:description" content="C++ library issue. Status: C++11">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue854.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++11">C++11</a> status.</em></p>
<h3 id="854"><a href="lwg-defects.html#854">854</a>. <code>default_delete</code> converting constructor underspecified</h3>
<p><b>Section:</b> 20.3.1.2.2 <a href="https://wg21.link/unique.ptr.dltr.dflt">[unique.ptr.dltr.dflt]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2008-06-18 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#unique.ptr.dltr.dflt">issues</a> in [unique.ptr.dltr.dflt].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p>
No relationship between <code>U</code> and <code>T</code> in the converting constructor for <code>default_delete</code> template.
</p>
<p>
Requirements: <code>U*</code> is convertible to <code>T*</code> and <code>has_virtual_destructor&lt;T&gt;</code>;
the latter should also become a concept.
</p>
<p>
Rules out cross-casting.
</p>
<p>
The requirements for <code>unique_ptr</code> conversions should be the same as those on the deleter.
</p>

<p><i>[
Howard adds 2008-11-26:
]</i></p>


<blockquote>
<p>
I believe we need to be careful to not outlaw the following use case, and
I believe the current proposed wording
(<code>requires Convertible&lt;U*, T*&gt; &amp;&amp; HasVirtualDestructor&lt;T&gt;</code>) does so:
</p>

<blockquote><pre>
#include &lt;memory&gt;

int main()
{
    std::unique_ptr&lt;int&gt; p1(new int(1));
    std::unique_ptr&lt;const int&gt; p2(move(p1));
    int i = *p2;
<span style="color:#C80000">//    *p2 = i;  // should not compile</span>
}
</pre></blockquote>

<p>
I've removed "<code>&amp;&amp; HasVirtualDestructor&lt;T&gt;</code>" from the
<code>requires</code> clause in the proposed wording.
</p>

</blockquote>

<p><i>[
Post Summit:
]</i></p>


<blockquote>
<p>
Alisdair: This issue has to stay in review pending a paper constraining
<code>unique_ptr</code>.
</p>
<p>
Consensus: We agree with the resolution, but <code>unique_ptr</code> needs
to be constrained, too.
</p>
<p>
Recommend Keep in Review.
</p>
</blockquote>

<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote><p>
Keep in Review status for the reasons cited.
</p></blockquote>

<p><i>[
2009-07 Frankfurt:
]</i></p>


<blockquote>
<p>
The proposed resolution uses concepts. Howard needs to rewrite the
proposed resolution.
</p>
<p>
Move back to Open.
</p>
</blockquote>

<p><i>[
2009-07-26 Howard provided rewritten proposed wording and moved to Review.
]</i></p>


<p><i>[
2009-10 Santa Cruz:
]</i></p>


<blockquote><p>
Move to Ready.
</p></blockquote>



<p id="res-854"><b>Proposed resolution:</b></p>
<p>
Add after 20.3.1.2.2 <a href="https://wg21.link/unique.ptr.dltr.dflt">[unique.ptr.dltr.dflt]</a>, p1:
</p>

<blockquote><pre>
template &lt;class U&gt; default_delete(const default_delete&lt;U&gt;&amp; other);
</pre>
<blockquote>
<p>
-1- <i>Effects:</i> ...
</p>
<p><ins>
<i>Remarks:</i> This constructor shall participate in overload resolution
if and only if <code>U*</code> is implicitly convertible to <code>T*</code>.
</ins></p>
</blockquote>
</blockquote>






</body>
</html>
