Destructor called in foreach loop iteration in QList derived class

NickM

Hi All,
I'm doing a fairly basic QT excercise/tutorial the involves the following class:

class BookingList: public QList<Booking*>
{
public:
    BookingList();
    ~BookingList();
    int roomsAvailable(QDate d);
    bool vacancy(QDate a, QDate d);
    Booking* addBooking(Person c, QDate a, QDate d, Person* g1, Person* g2);
    void deleteAll();

    int NO_OF_ROOMS;
};

However something unexpected (at least to me) happens when I use a foreach loop in the roomsAvailable() function:

BookingList::~BookingList()
{
    qDeleteAll(*this);
    this->clear();
    qDebug()<<"destructor called...";
}

int BookingList::roomsAvailable(QDate d)
{
   int bookedRooms = 0;
    foreach(Booking* b, *this)
    {
        if(b->booked(d))
            bookedRooms++;
    }

    return NO_OF_ROOMS - bookedRooms;
}

Each iteration of this loop will call the destructor directly above, which is something I really didn't expect. Is it because, in dereferencing "this" I'm sending a copy to foreach(), which subsequently gets destroyed? Is this correct? Should I avoid this because it involves a lot of copying?

I replaced the foreach with a normal for loop and the destructor is not called (until the end of the program).

Christian Ehrlicher

Deriving from a container is for sure no good idea. Make it a member.

I replaced the foreach with a normal for loop and the destructor is not called (until the end of the program).

Take a look at the code of the macro and you will see why this happens: https://code.woboq.org/qt5/qtbase/src/corelib/global/qglobal.h.html#1044 (hint: your container will be detached/copied)

NickM

@Christian-Ehrlicher Thanks for the feedback and link. I've been working from a book called "An introduction to design patterns in C++ with QT" by Ezust and Ezust. They have a handful of examples where they derive from a QList<>. I'm personally not a fan of doing it that way, but its just surprising that they do it so liberally if it's bad practice.