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<title>Issue 3937: I/O manipulators should be specified in terms of base classes</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#New">New</a> status.</em></p>
<h3 id="3937"><a href="lwg-active.html#3937">3937</a>. I/O manipulators should be specified in terms of base classes</h3>
<p><b>Section:</b> 31.7.7 <a href="https://wg21.link/std.manip">[std.manip]</a>, 31.7.8 <a href="https://wg21.link/ext.manip">[ext.manip]</a>, 31.7.9 <a href="https://wg21.link/quoted.manip">[quoted.manip]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2023-05-25 <b>Last modified:</b> 2023-06-01</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#std.manip">active issues</a> in [std.manip].</p>
<p><b>View all other</b> <a href="lwg-index.html#std.manip">issues</a> in [std.manip].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
All the manipulators in 31.7.7 <a href="https://wg21.link/std.manip">[std.manip]</a> and 31.7.8 <a href="https://wg21.link/ext.manip">[ext.manip]</a>
use language of the form:
</p>
<blockquote>
An object of unspecified type such that if <code>out</code>
is an object of type <code>basic_ostream&lt;charT, traits&gt;</code>
then the expression <code>out &lt;&lt; resetiosflags(mask)</code> behaves as if
[...]
</blockquote>
<p>
This needs to work for any object with a public and unambiguous base class
of type <code>basic_ostream&lt;charT, traits&gt;</code>, not only objects
of exactly that type.
</p>
<p>
In C++98 and C++03, <code>setfill</code> did actually get this right
("if <code>out</code> is (or is derived from)
<code>basic_ostream&lt;charT,traits&gt;</code>"),
but that was "fixed" for C++11 to be consistent with the other manipulators.
</p>
<p>
31.7.9 <a href="https://wg21.link/quoted.manip">[quoted.manip]</a> is slightly different, talking about
"an instance of <code>basic_ostream</code>", but that's not right either.
</p>

<p><i>[2023-06-01; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>



<p id="res-3937"><b>Proposed resolution:</b></p>
<p>
</p>





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