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<title>Issue 744: What is the lifetime of an exception pointed to by an exception_ptr?</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#CD1">CD1</a> status.</em></p>
<h3 id="744"><a href="lwg-defects.html#744">744</a>. What is the lifetime of an exception pointed to by an <code>exception_ptr</code>?</h3>
<p><b>Section:</b> 17.9.7 <a href="https://wg21.link/propagation">[propagation]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2007-10-10 <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#propagation">issues</a> in [propagation].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Without some lifetime guarantee, it is hard to know how this type can be
used.  Very specifically, I don't see how the current wording would
guarantee and <code>exception_ptr</code> caught at the end of one thread could be safely
stored and rethrown in another thread - the original motivation for this
API.
</p>
<p>
(Peter Dimov agreed it should be clearer, maybe a non-normative note to
explain?)
</p>

<p><i>[
Bellevue:
]</i></p>


<blockquote>
<p>
Agree the issue is real.
</p>
<p>
Intent is lifetime is similar to a shared_ptr (and we might even want to
consider explicitly saying that it is a shared_ptr&lt; unspecified type &gt;).
</p>
<p>
We expect that most implementations will use shared_ptr, and the
standard should be clear that the exception_ptr type is intended to be
something whose semantics are smart-pointer-like so that the user does
not need to worry about lifetime management. We still need someone to
draught those words - suggest emailing Peter Dimov.
</p>
<p>
Move to Open.
</p>
</blockquote>


<p id="res-744"><b>Proposed resolution:</b></p>
<p>
Change 17.9.7 <a href="https://wg21.link/propagation">[propagation]</a>/7:
</p>

<blockquote><p>
-7- Returns: An <code>exception_ptr</code> object that refers to the currently
handled exception or a copy of the currently handled exception, or a
null <code>exception_ptr</code> object if no exception is being handled.
<ins>The referenced object remains valid at least as long as there is an
<code>exception_ptr</code> that refers to it.</ins>
If the function needs to allocate memory and the attempt
fails, it returns an <code>exception_ptr</code> object that refers to an instance of
<code>bad_alloc</code>. It is unspecified whether the return values of two successive
calls to <code>current_exception</code> refer to the same exception object. [<i>Note:</i>
that is, it is unspecified whether <code>current_exception</code> creates a new copy
each time it is called. <i>--end note</i>]
</p></blockquote>





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