From Lionel Lagarde, "When a function do:

OSG_DEBUG << "Hello world!\n";

the underlying stream is not automatically flushed. It is flushed
if endl is queued instead of \n:

OSG_DEBUG << "Hello world!" << std::endl;


The notify macros do:
stream->setCurrentSeverity(severity);
return *stream;

So, if a function do:

OSG_DEBUG << "This is a debug message\n";
OSG_NOTICE << "This is a notice message" << std::endl;

the debug message will be classified as a notice message.

It is a problem when the application uses a NotifyHandler. The notify
method of the handler is called with:

severity = NOTICE
message = "This is a debug message\nThis is a notice message"

The attached Notify.cpp contains an automatic flush of the stream when
the current severity changes.

"