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<title>Issue 828: Static initialization for std::mutex?</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="828"><a href="lwg-defects.html#828">828</a>. Static initialization for <code>std::mutex</code>?</h3>
<p><b>Section:</b> 32.6.4.2.2 <a href="https://wg21.link/thread.mutex.class">[thread.mutex.class]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Peter Dimov <b>Opened:</b> 2008-04-18 <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#thread.mutex.class">issues</a> in [thread.mutex.class].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
[Note: I'm assuming here that  [basic.start.init]/1 will be fixed.]
</p>
<p>
Currently <code>std::mutex</code> doesn't support static initialization. This is a
regression with respect to <code>pthread_mutex_t</code>, which does. I believe that
we should strive to eliminate such regressions in expressive power where
possible, both to ease migration and to not provide incentives to (or
force) people to forego the C++ primitives in favor of pthreads.
</p>

<p><i>[
Sophia Antipolis:
]</i></p>


<blockquote>
<p>
We believe this is implementable on POSIX, because the initializer-list
feature and the constexpr feature make this work. Double-check core
language about static initialization for this case. Ask core for a core
issue about order of destruction of statically-initialized objects wrt.
dynamically-initialized objects (should come afterwards). Check
non-POSIX systems for implementability.
</p>
<p>
If ubiquitous implementability cannot be assured, plan B is to introduce
another constructor, make this constexpr, which is
conditionally-supported. To avoid ambiguities, this new constructor needs
to have an additional parameter.
</p>
</blockquote>

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


<blockquote>
<p>
Jens: constant initialization seems to be ok core-language wise
</p>
<p>
Consensus: Defer to threading experts, in particular a Microsoft platform expert.
</p>
<p>
Lawrence to send e-mail to Herb Sutter, Jonathan Caves, Anthony Wiliams,
Paul McKenney, Martin Tasker, Hans Boehm, Bill Plauger, Pete Becker,
Peter Dimov to alert them of this issue.
</p>
<p>
Lawrence: What about header file shared with C? The initialization syntax is different in C and C++.
</p>
<p>
Recommend Keep in Review
</p>
</blockquote>

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

<blockquote><p>
Keep in Review status pending feedback from members of the Concurrency subgroup.
</p></blockquote>

<p><i>[
See related comments from Alisdair and Daniel in <a href="lwg-defects.html#827" title="constexpr shared_ptr::shared_ptr()? (Status: Resolved)">827</a><sup><a href="https://cplusplus.github.io/LWG/issue827" title="Latest snapshot">(i)</a></sup>.
]</i></p>


<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/n2994.htm">N2994</a>.
</p></blockquote>



<p id="res-828"><b>Proposed resolution:</b></p>
<p>
Change 32.6.4.2.2 <a href="https://wg21.link/thread.mutex.class">[thread.mutex.class]</a>:
</p>

<blockquote><pre>
class mutex {
public:
  <ins>constexpr</ins> mutex();
  ...
</pre></blockquote>





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