<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 1163: shared_future is inconsistent with shared_ptr</title>
<meta property="og:title" content="Issue 1163: shared_future is inconsistent with shared_ptr">
<meta property="og:description" content="C++ library issue. Status: Resolved">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue1163.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#Resolved">Resolved</a> status.</em></p>
<h3 id="1163"><a href="lwg-defects.html#1163">1163</a>. <code>shared_future</code> is inconsistent with <code>shared_ptr</code></h3>
<p><b>Section:</b> 32.10.8 <a href="https://wg21.link/futures.shared.future">[futures.shared.future]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> LWG <b>Opened:</b> 2009-06-28 <b>Last modified:</b> 2021-06-06</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#futures.shared.future">issues</a> in [futures.shared.future].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>

<p><b>Addresses UK 338</b></p>

<p><b>Description</b></p>

        <p><code>shared_future</code> is currently
        <code>CopyConstructible</code>, but not <code>CopyAssignable</code>. This is
        inconsistent with <code>shared_ptr</code>, and will surprise users.
        Users will then write work-arounds to provide this
        behaviour. We should provide it simply and efficiently as
        part of shared_future. Note that since the shared_future
        member functions for accessing the state are all declared
        const, the original usage of an immutable shared_future
        value that can be freely copied by multiple threads can be
        retained by declaring such an instance as "<code>const
        shared_future</code>".</p>
<p><b>Suggestion</b></p>
        <p>Remove "=delete"
        from the copy-assignment operator of shared_future. Add a
        move-constructor <code>shared_future(shared_future&amp;&amp;
        rhs)</code>, and a move-assignment operator <code>shared_future&amp;
        operator=(shared_future&amp;&amp; rhs)</code>. The postcondition
        for the copy-assignment operator is that <code>*this</code> has the same
        associated state as <code>rhs</code>. The postcondition for the
        move-constructor and move assignment is that <code>*this</code> has the
        same associated as <code>rhs</code> had before the
        constructor&#47;assignment call and that <code>rhs</code> has no associated
        state.</p>
<p><b>Notes</b></p>
<p>Create an issue. Detlef will look into it.</p>

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


<blockquote><p>
Pending a paper from Anthony Williams &#47; Detlef Vollmann.
</p></blockquote>

<p><i>[
2009-10-14 Pending paper:
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2967.html">N2967</a>.
]</i></p>


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


<blockquote><p>
<del>NAD Editorial</del><ins>Resolved</ins>.  Adressed by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2997.htm">N2997</a>.
</p></blockquote>



<p id="res-1163"><b>Proposed resolution:</b></p>





</body>
</html>
