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<title>Issue 746: current_exception may fail with bad_alloc</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="746"><a href="lwg-defects.html#746">746</a>. <code>current_exception</code> may fail with <code>bad_alloc</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>
I understand that the attempt to copy an exception may run out of memory,
but I believe this is the only part of the standard that mandates failure
with specifically <code>bad_alloc</code>, as opposed to allowing an
implementation-defined type derived from <code>bad_alloc</code>.  For instance, the Core
language for a failed new expression is:
</p>
<blockquote>
<p>
Any other allocation function that fails to allocate storage shall indicate
failure only by throwing an exception of a type that would match a handler
(15.3) of type <code>std::bad_alloc</code> (18.5.2.1).
</p>
</blockquote>
<p>
I think we should allow similar freedom here (or add a blanket
compatible-exception freedom paragraph in 17)
</p>
<p>
I prefer the clause 17 approach myself, and maybe clean up any outstanding
wording that could also rely on it.
</p>
<p>
Although filed against a specific case, this issue is a problem throughout
the library. 
</p>

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


<blockquote>
<p>
Is issue bigger than library?
</p>
<p>
No - Core are already very clear about their wording, which is inspiration for the issue.
</p>
<p>
While not sold on the original 18.7.5 use case, the generalised 17.4.4.8 wording is the real issue.
</p>
<p>
Accept the broad view and move to ready
</p>
</blockquote>


<p id="res-746"><b>Proposed resolution:</b></p>
<p>
Add the following exemption clause to 16.4.6.14 <a href="https://wg21.link/res.on.exception.handling">[res.on.exception.handling]</a>:
</p>

<blockquote><p>
A function may throw a type not listed in its <i>Throws</i> clause so long as it is
derived from a class named in the <i>Throws</i> clause, and would be caught by an
exception handler for the base type.
</p></blockquote>





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