....
QQmlContext * context = new QQmlContext(engine.rootContext());
context->setContextProperty("data", &data);
QQmlComponent component(&engine, QUrl(strQmlPath));
....

Item {
    Connections {
        target: data
        onNewMessagePosted: window.visible = false
    }
}

class MessageBoard : public QObject
{
    Q_OBJECT
    public: void callMe () {
        emit newMessagePosted ();
    } ;

    signals: void newMessagePosted ();
};

...
engine.rootContext()->setContextProperty("data", &data);
...