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<title>Issue 3658: basic_streambuf::sputn is both overspecified and underspecified</title>
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<h3 id="3658"><a href="lwg-active.html#3658">3658</a>. <code>basic_streambuf::sputn</code> is both overspecified and underspecified</h3>
<p><b>Section:</b> 31.6.3.3.5 <a href="https://wg21.link/streambuf.pub.put">[streambuf.pub.put]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2022-01-17 <b>Last modified:</b> 2022-01-30</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The specification for <code>basic_streambuf::sputn</code> is:
</p>
<blockquote><p>
<i>Returns:</i> <code>xsputn(s, n)</code>.
</p></blockquote>
<p>
One interpretation of this implies that <code>sputn</code> can't insert characters directly into the 
put area if there is space for them, it has to make a virtual call. That has significant overhead 
for repeated calls, such as inserting many small strings/string_views in a loop.
<p/>
But another interpretation is that it doesn't use "<i>Effects:</i> Equivalent to" or anything like 
that, so doesn't say an actual call to <code>xsputn</code> happens. Strictly speaking, it only says it 
returns the value that <code>xsputn</code> would return, and doesn't even have to produce any of its effects!
<p/>
We should describe the effects, not the return value, and we should do so in a way that does not 
overconstrain the implementation. It should not be necessary to make a virtual call to <code>xsputn</code> 
if the put area has capacity for the characters. On the other hand, if it doesn't have capacity, then 
calling <code>xsputn</code> is the best option; it allows the derived <code>streambuf</code> to decide how 
best to handle large writes.
<p/>
The proposed resolution replaces the <i>Returns:</i> element with an <i>Effects:</i> element, so that 
we specify that those effects actually occur. A normative remark is added to give the implementation 
leeway to avoid the virtual call when it isn't needed.
</p>

<p><i>[2022-01-30; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
Jonathan to revise P/R to also cover <code>sgetn</code>.
</p>



<p id="res-3658"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>. 
</p>

<ol>
<li><p>Modify 31.6.3.3.5 <a href="https://wg21.link/streambuf.pub.put">[streambuf.pub.put]</a> as indicated:</p>

<blockquote>
<pre>
streamsize sputn(const char_type* s, streamsize n);
</pre>
<blockquote>
<p>
<ins>-?- <i>Preconditions:</i> <code>[s, s+n)</code> is a valid range.</ins>
<p/>
<ins>-?- <i>Effects:</i> <code>return xsputn(s, n)</code>.</ins>
<p/>
<ins>-?- <i>Remarks:</i> When <code>(epptr() - pptr()) &gt;= n</code>, it is unspecified whether the characters 
are written directly to the output sequence without calling <code>xsputn</code>.</ins>
<p/>
<del>-2- <i>Returns:</i> <code>xsputn(s, n)</code>.</del>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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