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Section: 18.104.22.168 [ios.base.callback] Status: New Submitter: David Krauss Opened: 2016-03-14 Last modified: 2019-06-15
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register_callback allocates memory and so it can fail, but the case is unspecified. libc++ sets badbit, which is consistent with iword and pword. libstdc++ throws std::bad_alloc.
[2019-06-13; Billy comments]
Just as an additional data point: MSVC++ agrees with libstdc++ and also throws std::bad_alloc.