How to control the exiting process of my application ?

[tonnot]

I have this line to control if the application is going to be finished,

connect(QApplication::instance(), SIGNAL(aboutToQuit()),this,SLOT(control_quit()));

connect(QApplication::instance(), SIGNAL(aboutToQuit()),this,SLOT(control_quit()));

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control_quit show a confirmation message.
I have two problems :
1.- The main window (where I have the connect...) dissapears during the proccess. How can avoid this ?
2.-How can I cancel the exiting ?
Thanks