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<title>Issue 1106: Multiple exceptions from connected shared_future::get()?</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="1106"><a href="lwg-defects.html#1106">1106</a>. Multiple exceptions from connected <code>shared_future::get()</code>?</h3>
<p><b>Section:</b> 32.10.8 <a href="https://wg21.link/futures.shared.future">[futures.shared.future]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Thomas J. Gritzan <b>Opened:</b> 2009-04-03 <b>Last modified:</b> 2021-06-06</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#futures.shared.future">issues</a> in [futures.shared.future].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
It is not clear, if multiple threads are waiting in a
<code>shared_future::get()</code> call, if each will rethrow the stored exception.
</p>
<p>
Paragraph 9 reads:
</p>
<blockquote><p>
<i>Throws:</i> the stored exception, if an exception was stored and not
retrieved before.
</p></blockquote>
<p>
The "not retrieved before" suggests that only one exception is thrown,
but one exception for each call to <code>get()</code> is needed, and multiple calls
to <code>get()</code> even on the same <code>shared_future</code> object seem to be allowed.
</p>
<p>
I suggest removing "and not retrieved before" from the Throws paragraph.
I recommend adding a note that explains that multiple calls on <code>get()</code> are
allowed, and each call would result in an exception if an exception was
stored.
</p>

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

<blockquote>
<p>
We note there is a pending paper by Detlef
on such <code>future</code>-related issues;
we would like to wait for his paper before proceeding.
</p>
<p>
Alisdair suggests we may want language to clarify that this
<code>get()</code> function can be called from several threads
with no need for explicit locking.
</p>
<p>
Move to Open.
</p>
</blockquote>

<p><i>[
2010-01-23 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib.
]</i></p>




<p><b>Rationale:</b></p>
<p>
Resolved by paper
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2997.htm">N2997</a>.
</p>


<p id="res-1106"><b>Proposed resolution:</b></p>
<p>
Change  [futures.shared_future]:
</p>

<blockquote><pre>
const R&amp; shared_future::get() const;
R&amp; shared_future&lt;R&amp;&gt;::get() const;
void shared_future&lt;void&gt;::get() const;
</pre>
<blockquote>
<p>...</p>
<p>
-9- <i>Throws:</i> the stored exception, if an exception was stored<del> and not retrieved before</del>.
<ins>
[<i>Note:</i> Multiple calls on <code>get()</code> are allowed, and each call would result in an exception
if an exception was stored. &mdash; <i>end note</i>]
</ins>
</p>
</blockquote>
</blockquote>






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