29 Input/output library [input.output]

29.11 Synchronized output streams [syncstream]

29.11.2 Class template basic_­syncbuf [syncstream.syncbuf]

29.11.2.5 Overridden virtual functions [syncstream.syncbuf.virtuals]

int sync() override;
Effects: Records that the wrapped stream buffer is to be flushed.
Then, if emit_­on_­sync is true, calls emit().
[Note 1:
If emit_­on_­sync is false, the actual flush is delayed until a call to emit().
— end note]
Returns: If emit() was called and returned false, returns -1; otherwise 0.