Unsolved Destructor called in foreach loop iteration in QList derived class
-
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).
-
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)
-
@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.