31 Input/output library [input.output]

31.11 Synchronized output streams [syncstream]

31.11.2 Class template basic_syncbuf [syncstream.syncbuf] 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.