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<title>Issue 397: ostream::sentry dtor throws exceptions</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#NAD_Editorial">NAD Editorial</a> status.</em></p>
<h3 id="397"><a href="lwg-closed.html#397">397</a>. <code>ostream::sentry</code> dtor throws exceptions</h3>
<p><b>Section:</b> 31.7.6.2.4 <a href="https://wg21.link/ostream.sentry">[ostream.sentry]</a> <b>Status:</b> <a href="lwg-active.html#NAD_Editorial">NAD Editorial</a>
 <b>Submitter:</b> Martin Sebor <b>Opened:</b> 2003-01-05 <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#ostream.sentry">issues</a> in [ostream.sentry].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD Editorial">NAD Editorial</a> status.</p>
<p><b>Discussion:</b></p>
    <p>
17.4.4.8, p3 prohibits library dtors from throwing exceptions.
    </p>
    <p>
27.6.2.3, p4 says this about the ostream::sentry dtor:
    </p>
    <pre>
    -4- If ((os.flags() &amp; ios_base::unitbuf) &amp;&amp; !uncaught_exception())
        is true, calls os.flush().
    </pre>
    <p>
27.6.2.6, p7 that describes ostream::flush() says:
    </p>
    <pre>
    -7- If rdbuf() is not a null pointer, calls rdbuf()->pubsync().
        If that function returns ?-1 calls setstate(badbit) (which
        may throw ios_base::failure (27.4.4.3)).
    </pre>
    <p>
That seems like a defect, since both pubsync() and setstate() can
throw an exception.
    </p>
<p><i>[
The contradiction is real.  Clause 17 says destructors may never
throw exceptions, and clause 27 specifies a destructor that does
throw.  In principle we might change either one.  We're leaning
toward changing clause 17: putting in an "unless otherwise specified"
clause, and then putting in a footnote saying the sentry destructor
is the only one that can throw.  PJP suggests specifying that
sentry::~sentry() should internally catch any exceptions it might cause.
]</i></p>


<p><i>[
See <a href="lwg-closed.html#418" title="exceptions thrown during iostream cleanup (Status: NAD)">418</a><sup><a href="https://cplusplus.github.io/LWG/issue418" title="Latest snapshot">(i)</a></sup> and <a href="lwg-defects.html#622" title="behavior of filebuf dtor and close on error (Status: CD1)">622</a><sup><a href="https://cplusplus.github.io/LWG/issue622" title="Latest snapshot">(i)</a></sup> for related issues.
]</i></p>


<p><i>[
2009-07 Frankfurt
]</i></p>


<blockquote>
<p>
Move to Review. Add "Throws: nothing" to the specification of <code>ostream::sentry::~sentry()</code>.
</p>
</blockquote>

<p><i>[
2009-10-13 Daniel adds:
]</i></p>


<blockquote><p>
The proposed resolution of <a href="lwg-defects.html#835" title="Tying two streams together (correction to DR 581) (Status: C++11)">835</a><sup><a href="https://cplusplus.github.io/LWG/issue835" title="Latest snapshot">(i)</a></sup> is written to match the outcome
of this issue.
</p></blockquote>

<p><i>[
2009 Santa Cruz:
]</i></p>


<blockquote><p>
Move to Open.  Our intent is to solve this issue with <a href="lwg-defects.html#835" title="Tying two streams together (correction to DR 581) (Status: C++11)">835</a><sup><a href="https://cplusplus.github.io/LWG/issue835" title="Latest snapshot">(i)</a></sup>.
</p></blockquote>

<p><i>[
2010-03-06 Martin updates wording.
]</i></p>


<p><i>[
2010 Pittsburgh:
]</i></p>


<blockquote><p>
Moved to NAD Editorial.
</p></blockquote>



<p><b>Rationale:</b></p>
<p>
Solved by <a href="lwg-defects.html#835" title="Tying two streams together (correction to DR 581) (Status: C++11)">835</a><sup><a href="https://cplusplus.github.io/LWG/issue835" title="Latest snapshot">(i)</a></sup>.
</p>


<p id="res-397"><b>Proposed resolution:</b></p>
<p>
Add after  [ostream::sentry] p17:
</p>

<blockquote>
<pre>
~sentry();
</pre>
<blockquote>
<p>
-17- If <code>(os.flags() &amp; ios_base::unitbuf)</code>
is <code>true</code>, calls <code>os.flush()</code>.
</p>

<p><ins>
<i>Throws:</i> Nothing.
</ins></p>
</blockquote>
</blockquote>





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