[ fromfile: singleton.xml id: singleton ]

As discussed earlier, the Singleton pattern is a specialized factory used in situations where we wish to limit the number or type of instances created.

The CustomerFactory::instance() method, defined in Example 16.7, is an example of a singleton factory. It creates an object if needed, but only the first time that the method is called. On subsequent calls it always returns a pointer to the same object.

[ . . . . ]

CustomerFactory* CustomerFactory::instance() {
    static CustomerFactory* retval = 0;
    if (retval == 0) retval = new CustomerFactory(qApp); 
    return retval;
}

It is important when dealing with heap objects that you do not leave memory leaks behind. You can use QObject's parent-child relationship to help in this regard.

As mentioned earlier, qApp is a pointer to a singleton instance of QApplication, which was presumably created in main(). The QApplication instance exists precisely as long as the application is running.

Why Not Use a static Parent?

If you make the parent of your heap objects a static QObject, children of this object will be destroyed after the QApplication is destroyed. Unless there is a compelling reason to the contrary, a program should not do anything with QObjects after the QApplication has been destroyed – including object cleanup. Static objects from different files of a multifile application are destroyed in a linker-dependent order. That order of destruction may cause unintended side-effects (e.g., segmentation faults at termination). See Section 8.6 for more details.