How does QDebug() << stuff; add a newline automatically?

Qt uses a method similar to @Evan. See a version of qdebug.h for the implementation details, but they stream everything to an underlying text stream, and then flush the stream and an end-line on destruction of the temporary QDebug object returned by qDebug().

Something like this will do:

struct debug {
    debug() {
    }

    ~debug() {
        std::cerr << m_SS.str() << std::endl;
    }

public:
    // accepts just about anything
    template<class T>
    debug &operator<<(const T &x) {
        m_SS << x;
        return *this;
    }
private:
    std::ostringstream m_SS;
};

Which should let you do things like this:

debug() << "hello world";

I've used a pattern like this combined with a lock to provide a stream like logging system which can guarantee that log entries are written atomically.

NOTE: untested code, but should work :-)

When you write that this is the typical usage:

debug() << "stuff" << "more stuff" << std::endl;

are you definitely planning to construct a debug object each time you use it? If so, you should be able to get the behavior you want by having the debug destructor add the newline:

~debug()
{
    *this << std::endl;

    ... the rest of your destructor ...
}

That does mean you cannot do something like this:

// this won't output "line1" and "line2" on separate lines
debug d;
d << "line1";
d << "line2";