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<title>Issue 1502: Specification of [futures.state] unclear</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="1502"><a href="lwg-defects.html#1502">1502</a>. Specification of [futures.state] unclear</h3>
<p><b>Section:</b> 32.10.5 <a href="https://wg21.link/futures.state">[futures.state]</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.state">issues</a> in [futures.state].</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-195</b></p>

<p>The intent and meaning of 32.10.5 <a href="https://wg21.link/futures.state">[futures.state]</a> p10 is not apparent.</p>

<blockquote><p>
10 Accesses to the same shared state conflict (1.10).
</p></blockquote>

<p><i>[
2011-03-07 Jonathan Wakely adds:
]</i></p>

<blockquote><p>
It's not clear which paragraph this refers to, I had to go to the
ballot comments where US-195 reveals it's para 8, which in the FCD
(N3092) says:
</p><blockquote><p>
 Accesses to the same associated asynchronous state conflict (1.10).
</p></blockquote>
<p>This is now para 10 in N3242:</p>
<blockquote><p>
 Accesses to the same shared state conflict (1.10).
</p></blockquote>
</blockquote>

<p><i>[2011-03-07: Lawrence comments and drafts wording]</i></p>


<p>The intent of this paragraph is to deal with operations,
such as <code>shared_future::get()</code>, that return a reference
to a value held in the shared state.  User code could potentially
conflict when accessing that value.</p>

<p>Lawrence proposed resolution:</p>

<p>Modify 32.10.5 <a href="https://wg21.link/futures.state">[futures.state]</a> p10 as follows:</p>

<blockquote><p>
10 <del>Accesses to the same shared state conflict (6.10.2 <a href="https://wg21.link/intro.multithread">[intro.multithread]</a>).</del><ins>Some 
  operations, e.g. <code>shared_future::get()</code>
 ( [futures.shared_future]), may return a reference to a
 value held in their shared state. Accesses and modifications through
 those references by concurrent threads to the same 
 shared state may potentially conflict (6.10.2 <a href="https://wg21.link/intro.multithread">[intro.multithread]</a>).
 [<i>Note:</i> As a consequence, accesses must either use
 read-only operations or provide additional synchronization.
 &mdash; <i>end note</i>]</ins>
</p></blockquote>

<p><i>[2011-03-19: Detlef suggests an alternative resolution, shown below.]</i></p>


<blockquote class="note">
<p>Proposed Resolution</p>
<p>Modify 32.10.5 <a href="https://wg21.link/futures.state">[futures.state]</a> p10 as follows:</p>

<blockquote><p>
10 Accesses to the same shared state conflict (6.10.2 <a href="https://wg21.link/intro.multithread">[intro.multithread]</a>). <ins>[<i>Note</i>: 
This explicitely specifies that the shared state is visible in the objects that reference this state 
in the sense of data race avoidance 16.4.6.10 <a href="https://wg21.link/res.on.data.races">[res.on.data.races]</a>. &mdash; <i>end note</i>]</ins>
</p></blockquote>
</blockquote>


<p id="res-1502"><b>Proposed resolution:</b></p><p>
Resolved 2011-03 Madrid meeting by paper <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3278">N3278</a>
</p>




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