An extremely noob question about QtQuick viewer implementation???

[Zingam]

I am looking at how the qml viewer (for 4.8 and 5.0) is implemented. And I wonder why is it so.

In the .h(eader) we have:

class QtQuick2ApplicationViewer : public QQuickView
{
    Q_OBJECT
 
...
 
private:
    class QtQuick2ApplicationViewerPrivate *d;
};

class QtQuick2ApplicationViewer : public QQuickView
{
    Q_OBJECT

...

private:
    class QtQuick2ApplicationViewerPrivate *d;
};

To copy to clipboard, switch view to plain text mode 

Then in the .CPP file:

class QtQuick2ApplicationViewerPrivate
{
    QString mainQmlFile;
    friend class QtQuick2ApplicationViewer;
    static QString adjustPath(const QString &path);
};
 
QtQuick2ApplicationViewer::QtQuick2ApplicationViewer(QWindow *parent)
    : QQuickView(parent)
    , d(new QtQuick2ApplicationViewerPrivate())
{
    connect(engine(), SIGNAL(quit()), SLOT(close()));
    setResizeMode(QQuickView::SizeRootObjectToView);
 
#ifdef Q_OS_ANDROID
    engine()->setBaseUrl(QUrl::fromLocalFile("/"));
#endif
}

class QtQuick2ApplicationViewerPrivate
{
    QString mainQmlFile;
    friend class QtQuick2ApplicationViewer;
    static QString adjustPath(const QString &path);
};

QtQuick2ApplicationViewer::QtQuick2ApplicationViewer(QWindow *parent)
    : QQuickView(parent)
    , d(new QtQuick2ApplicationViewerPrivate())
{
    connect(engine(), SIGNAL(quit()), SLOT(close()));
    setResizeMode(QQuickView::SizeRootObjectToView);

#ifdef Q_OS_ANDROID
    engine()->setBaseUrl(QUrl::fromLocalFile("/"));
#endif
}

To copy to clipboard, switch view to plain text mode 

So I have basically two questions.
(1) The first one about the friend class. Why is it necessary. I don't see any reason why would anybody use a friend class. Is there any real use for friend classes (except for exotics that anybody could live without)?

(2) The second one is: connect(engine(), SIGNAL(quit()), SLOT(close())); Isn't quit() defined as slot? How is it a signal here? Is it wrong to use quit() instead of close()? connect(engine(), SIGNAL(quit()), SLOT(quit()));

[Le_B]

1 - i can't really answer this but this implementation is done in all QT class i think.

2 - the quit signal is from the engine not the viewer, so if the engine says i m off the view cannot do much and has to quit to avoid problems