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<title>Issue 1277: std::thread::id should be trivially copyable</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#C++11">C++11</a> status.</em></p>
<h3 id="1277"><a href="lwg-defects.html#1277">1277</a>. <code>std::thread::id</code> should be trivially copyable</h3>
<p><b>Section:</b> 32.4.3.2 <a href="https://wg21.link/thread.thread.id">[thread.thread.id]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Anthony Williams <b>Opened:</b> 2009-11-24 <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.thread.id">issues</a> in [thread.thread.id].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The class definition of <code>std::thread::id</code> in
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n3000.pdf">N3000</a>
is:
</p>

<blockquote><pre>
class thread::id {
public:
  id();
};
</pre></blockquote>

<p>
Typically, I expect that the internal data members will either be
pointers or integers, so that in practice the class will be trivially
copyable. However, I don't think the current wording guarantees it, and
I think it would be useful. In particular, I can see a use for
<code>std::atomic&lt;std::thread::id&gt;</code> to allow a <code>thread</code>
to claim ownership of a data structure atomicly, and
<code>std::atomic&lt;T&gt;</code> requires that <code>T</code> is trivially
copyable.
</p>

<p><i>[
2010-02-12 Moved to Tentatively Ready after 7 positive votes on c++std-lib.
]</i></p>




<p id="res-1277"><b>Proposed resolution:</b></p>
<p>
Add a new sentence to 32.4.3.2 <a href="https://wg21.link/thread.thread.id">[thread.thread.id]</a> p1:
</p>

<blockquote><p>
1 An object of type <code>thread::id</code> provides a unique identifier for
each thread of execution and a single distinct value for all
<code>thread</code> objects that do not represent a thread of execution
(32.4.3 <a href="https://wg21.link/thread.thread.class">[thread.thread.class]</a>). Each thread of execution has an
associated <code>thread::id</code> object that is not equal to the
<code>thread::id</code> object of any other thread of execution and that is
not equal to the <code>thread::id</code> object of any <code>std::thread</code>
object that does not represent threads of execution. The library may
reuse the value of a <code>thread::id</code> of a terminated thread that can
no longer be joined. <ins><code>thread::id</code> shall be a trivially
copyable class (11 <a href="https://wg21.link/class">[class]</a>).</ins>
</p></blockquote>






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