Causes output to be processed when the buffer is not empty.
A reference to an object of type ios_base, or to a type that inherits from ios_base.
A reference to the object from which _Str is derived.
Note that endl also flushes the buffer.
nounitbuf is in effect by default.
The manipulator effectively calls _Str.setf(ios_base::unitbuf), and then returns _Str.