Qt in other class

[Morea]

I wish to write a totaly simple C++ class. It should contain a pointer to a QMainWidget (or another suitable Qt object) and if a certain bool variable is set to false the pointer should be 0 or else it should point to a qtobject that contains three buttons and a big area where I can draw stuff.

Like this

class Box{
private:
    SomeQtclass*  theQtObject;
    bool use_gui; 
    void drawstuff(){if (use_gui) /*do stuff*/}
    void do_random_thing_not_directly_affecting_the_gui();
};

class Box{
private:
    SomeQtclass*  theQtObject;
    bool use_gui; 
    void drawstuff(){if (use_gui) /*do stuff*/}
    void do_random_thing_not_directly_affecting_the_gui();
};

To copy to clipboard, switch view to plain text mode 

In the main application I would then have

int main()
{
   Box mybox;
   cin>>mybox.use_gui;
   while (true){
       mybox.do_random_thing_not_directly_affecting_the_gui();
       mybox.drawstuff();
   }
}

int main()
{
   Box mybox;
   cin>>mybox.use_gui;
   while (true){
       mybox.do_random_thing_not_directly_affecting_the_gui();
       mybox.drawstuff();
   }
}

To copy to clipboard, switch view to plain text mode 

The Box class should be a kind of class that could be included in many other programs, like std::string.

How do I write this kind of code? I need some kind of QApplication, right? Where do I put it?
Anyone with idas, please speak up!