A bug in the official libsigc++ 2.0 example?

I was following the official documentation for libsigc++-2.0 and I found this example:

class AlienDetector
{
public:
    AlienDetector();

    void run();

    sigc::signal<void> signal_detected;
};

void warn_people()
{
    cout << "There are aliens in the carpark!" << endl;
}

int main()
{
    AlienDetector mydetector;
    mydetector.signal_detected.connect( sigc::ptr_fun(warn_people) );

    mydetector.run();

    return 0;
}

As you can see both the run() function and the constructor for the AlienDetector class are not defined and therefore this code shouldn't compile ( but the doc takes for granted the fact that this code will work ).

Even more strange is the fact that if I define both run() and the constructor for the class, I can't see the effect of the library anyway, apparently the signal doesn't work and when run is called in the main no slot is activated.

I am forgetting about something here ? How this thing should be re-written ?

The documentation seems to be incomplete.

The most basic version of the code should look like:

AlienDetector::AlienDetector() {}

void AlienDetector::run() {
    sleep(3); // wait for aliens
    signal_detected.emit(); // panic
}

I've posted a fully functional make-based example on github, example1.cpp is the first example, example2.cpp is one using a member function.