The member functions of the provided stream buffer
can be called from emit() while a lock is held,
which might result in a deadlock if used incautiously.
Effects: Move constructs the base class
and sb from the corresponding subobjects of other,
and calls basic_ostream<charT, traits>::set_rdbuf(addressof(sb)).
A flush on a basic_osyncstream does not flush immediately:
{
osyncstream bout(cout);
bout <<"Hello,"<<'\n'; // no flush
bout.emit(); // characters transferred; cout not flushed
bout <<"World!"<< endl; // flush noted; cout not flushed
bout.emit(); // characters transferred; cout flushed
bout <<"Greetings."<<'\n'; // no flush}// characters transferred; cout not flushed