<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 1047: Ensure that future's get() blocks when not ready</title>
<meta property="og:title" content="Issue 1047: Ensure that future's get() blocks when not ready">
<meta property="og:description" content="C++ library issue. Status: Resolved">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue1047.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="1047"><a href="lwg-defects.html#1047">1047</a>. Ensure that future's <code>get()</code> blocks when not ready</h3>
<p><b>Section:</b> 32.10.7 <a href="https://wg21.link/futures.unique.future">[futures.unique.future]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2009-03-12 <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.unique.future">issues</a> in [futures.unique.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 334 [CD1]</b></p>

<p>
Behaviour of <code>get()</code> is undefined if calling <code>get()</code> while
not <code>is_ready()</code>. The intent is that <code>get()</code> is a blocking
call, and will wait for the future to become ready.
</p>

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


<blockquote>
<p>
Agree, move to Review.
</p>
</blockquote>

<p><i>[
2009-04-03 Thomas J. Gritzan adds:
]</i></p>


<blockquote>
<p>
This issue also applies to <code>shared_future::get()</code>.
</p>

<p>
Suggested wording:
</p>

<p>
Add a paragraph to  [futures.shared_future]:
</p>

<blockquote><pre>
void shared_future&lt;void&gt;::get() const;
</pre>
<blockquote><p>
<i>Effects:</i> If <code>is_ready()</code> would return <code>false</code>, block on the asynchronous
result associated with <code>*this</code>.
</p></blockquote>
</blockquote>
</blockquote>

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

<blockquote><p>
It is not clear to us that this is an issue,
because the proposed resolution's Effects clause seems to duplicate
information already present in the Synchronization clause.
Keep in Review status.
</p></blockquote>

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


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



<p id="res-1047"><b>Proposed resolution:</b></p>
<p>
Add a paragraph to  [futures.unique_future]:
</p>

<blockquote><pre>
R&amp;&amp; unique_future::get();
R&amp; unique_future&lt;R&amp;&gt;::get();
void unique_future&lt;void&gt;::get();
</pre>
<blockquote>
<p><i>Note:</i>...</p>
<p>
<ins><i>Effects:</i> If <code>is_ready()</code> would return <code>false</code>,
block on the asynchronous result associated with <code>*this</code>.</ins>
</p>
<p>
<i>Synchronization:</i> if <code>*this</code> is associated with a
<code>promise</code> object, the completion of <code>set_value()</code> or
<code>set_exception()</code> to that <code>promise</code> happens before (1.10)
<code>get()</code> returns.
</p>
</blockquote>
</blockquote>





</body>
</html>
