How does qDebug() print a line break?

[d_stranz]

This sounds like a "How long is a piece of string?" question

qDebug() returns an instance of the QDebug class. Each of the operator<< calls return a reference to the QDebug instance, which allows you to chain together << calls. When the code reaches the end of the statement (";"), the QDebug instance goes out of scope. The destructor is called, it pushes the CRLF onto the stream, flushes the stream, and closes it. That's how it "knows".