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3571. flush_emit should set badbit if the emit call fails

Section: 31.7.6.5 [ostream.manip] Status: C++23 Submitter: Tim Song Opened: 2021-06-19 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

basic_osyncstream::emit is specified to set badbit if it fails (31.11.3.3 [syncstream.osyncstream.members] p1), but the emit part of the flush_emit manipulator isn't, even though the flush part does set badbit if it fails.

More generally, given an osyncstream s, s << flush_emit; should probably have the same behavior as s.flush(); s.emit().

The reference implementation linked in P0753R2 does set badbit on failure, so at least this part appears to be an oversight. As discussed in LWG 3570(i), basic_osyncstream::emit should probably be an unformatted output function, so the emit part of flush_emit should do so too.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

  1. Modify 31.7.6.5 [ostream.manip] as indicated:

    template<class charT, class traits>
  basic_ostream<charT, traits>& flush_emit(basic_ostream<charT, traits>& os);

    -12- Effects: Calls os.flush(). Then, if os.rdbuf() is a basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, behaves as an unformatted output function (31.7.6.4 [ostream.unformatted]) of os. After constructing a sentry object, calls buf->emit(). If that call returns false, calls os.setstate(ios_base::badbit).