<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 2120: What should async do if neither 'async' nor 'deferred' is set in policy?</title>
<meta property="og:title" content="Issue 2120: What should async do if neither 'async' nor 'deferred' is set in policy?">
<meta property="og:description" content="C++ library issue. Status: C++14">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue2120.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++14">C++14</a> status.</em></p>
<h3 id="2120"><a href="lwg-defects.html#2120">2120</a>. What should <code>async</code> do if neither '<code>async</code>' nor '<code>deferred</code>' is set in policy?</h3>
<p><b>Section:</b> 32.10.9 <a href="https://wg21.link/futures.async">[futures.async]</a> <b>Status:</b> <a href="lwg-active.html#C++14">C++14</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2012-01-01 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#futures.async">active issues</a> in [futures.async].</p>
<p><b>View all other</b> <a href="lwg-index.html#futures.async">issues</a> in [futures.async].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++14">C++14</a> status.</p>
<p><b>Discussion:</b></p>

<p>
Implementations already disagree, one returns an invalid future with
no shared state, one chooses <code>policy == async</code> and one chooses <code>policy ==
deferred</code>, see c++std-lib-30839, c++std-lib-30840 and c++std-lib-30844.
It's not clear if returning an invalid future is allowed by the current wording.
</p>
<p>
If the intention is to allow an empty future to be returned, then
32.10.9 <a href="https://wg21.link/futures.async">[futures.async]</a> p3 and p4 should be adjusted to clarify that a
shared state might not be created and an invalid future might be returned.
</p>
<p>
If the intention is that a valid future is always returned, p3 should
say something about the case where none of the conditions applies.
</p>

<p><i>[2012, Portland: move to Review]</i></p>

<p>
We could make it undefined if no launch policy is defined.
</p>
<p>
Hans: If no launch policy is specified the behaviour is undefined
</p>
<p>
Artur: or implementation defined?
</p>
<p>
Hans: no: we don't want people to do this
</p>
<p><i>[Proposed wording]</i></p>

<p>
This wording is relative to N3376
</p>
<p>
Add a third bullet to the end of the list in 30.6.8p3
</p>
<blockquote>
"if no valid launch policy is provided the behaviour is undefined"
</blockquote>
<p>
Moved to review
</p>

<p><i>[2013-04-19, Bristol]</i></p>
 

<p>Detlef provides new wording</p>

<p>Previous wording:</p>

<blockquote class="note">  
<p><i>[This wording is relative to N3376]</i></p>

<p>
Add a third bullet to the end of the list in 32.10.9 <a href="https://wg21.link/futures.async">[futures.async]</a>p3
</p>
<blockquote>
<ins>&ndash; if no valid launch policy is provided the behaviour is undefined</ins>
</blockquote>
</blockquote>

<p><i>[2013-09 Chicago]</i></p>

<p>
If no policy is given, it should be undefined, so moved to Immediate.
</p>
<p>
Accept for Working Paper
</p>



<p id="res-2120"><b>Proposed resolution:</b></p>
<p><i>[This wording is relative to N3485]</i></p>

<p>
Add a third bullet to the end of the list in 32.10.9 <a href="https://wg21.link/futures.async">[futures.async]</a>p3
</p>
<blockquote>
<ins>&ndash; If no value is set in the launch policy, or a value is set that is
neither specified in this International Standard or by the implementation, the behaviour 
is undefined.</ins>
</blockquote>






</body>
</html>
