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<title>Issue 1505: Synchronization between promise::set_value and future::get</title>
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<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="1505"><a href="lwg-defects.html#1505">1505</a>. Synchronization between <code>promise::set_value</code> and <code>future::get</code></h3>
<p><b>Section:</b> 32.10.6 <a href="https://wg21.link/futures.promise">[futures.promise]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> INCITS <b>Opened:</b> 2010-08-25 <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#futures.promise">issues</a> in [futures.promise].</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 US-197</b></p>
<p>
There is no defined synchronization between <code>promise::set_value</code> and <code>future::get</code> 
(32.10.6 <a href="https://wg21.link/futures.promise">[futures.promise]</a> p. 21, 25).
</p>
<p><i>[
Resolution proposed by ballot comment:
]</i></p>

<blockquote><p>
Replace "[<i>Note</i>: and they synchronize and
serialize with other functions through the referred
associated asynchronous state. &mdash; <i>end note</i>]" with
the normative "They synchronize with (1.10) any
operation on a future object with the same
associated asynchronous state marked ready."
</p></blockquote>

<p><i>[
2010-11-02 Daniel translates proposal into proper wording changes
]</i></p>


<p><i>[2011-03-19: Detlef comments]</i></p>


<p>The proposed resolution for <a href="lwg-defects.html#1507" title="promise::XXX_at_thread_exit functions have no
synchronization requirements (Status: Resolved)">1507</a><sup><a href="https://cplusplus.github.io/LWG/issue1507" title="Latest snapshot">(i)</a></sup> would cover this issue as well.
Effectively it will reject the request but a clarification is added that the 
normative wording is already in 32.10.5 <a href="https://wg21.link/futures.state">[futures.state]</a>.</p>

<blockquote class="note">
<ol>
<li>Change 32.10.6 <a href="https://wg21.link/futures.promise">[futures.promise]</a> p. 21 as indicated:
<blockquote><p>
21 <em>Synchronization</em>: calls to <code>set_value</code> and <code>set_exception</code> on a single 
<code>promise</code> object are serialized.
<del>[ <em>Note</em>: and they synchronize and serialize with other functions through the referred associated asynchronous
state. &mdash; <em>end note</em> ]</del><ins>They <em>synchronize with</em> ([intro.multithread]) any
operation on a future object with the same associated asynchronous state marked ready.</ins>
</p></blockquote>
</li>
<li>Change 32.10.6 <a href="https://wg21.link/futures.promise">[futures.promise]</a> p. 25 as indicated:
<blockquote><p>
25 <em>Synchronization</em>: calls to <code>set_value</code> and <code>set_exception</code> on a single 
<code>promise</code> object are serialized.
<del>[ <em>Note</em>: and they synchronize and serialize with other functions through the referred associated asynchronous
state. &mdash; <em>end note</em> ]</del><ins>They <em>synchronize with</em> ([intro.multithread]) any
operation on a future object with the same associated asynchronous state marked ready.</ins>
</p></blockquote>
</li>
</ol>
</blockquote>


<p id="res-1505"><b>Proposed resolution:</b></p><p>
Resolved 2001-03 Madrid by issue 1507.
</p>



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