When do I need Q_OBJECT?

[Morea]

Hi.
As I understand it, I need Q_OBJECT macro if I declare a class that should use signals and slots, and it should then also be derived from QObject?
Like

class Foo:public QObject{
Q_OBJECT
// etc.
}

class Foo:public QObject{
Q_OBJECT
// etc.
}

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But if I derive from a class that already is derived from a QObject, like

class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
   MainWindow(QWidget * parent = 0);
private:
    Ui::MainWindow ui;
};

class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
   MainWindow(QWidget * parent = 0);
private:
    Ui::MainWindow ui;
};

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Is Q_OBJECT really needed here? I mean, there is nothing that uses signals/slots, the MainWindow constructor contains no signals emits or connects.
I mean, the QMainWindow class is derived from QObject, doesn't it get the Q_OBJECT for that part?