[Development] HEADS UP: Don't use QList, use Q_DECLARE_TYPEINFO

[Иван Комиссаров]

QImage is a "bad" class for a QList (it has a vtable)

2015-07-10 14:37 GMT+03:00 Andreas Aardal Hanssen <andreas at hanssen.name>:

>
> > On 10 Jul 2015, at 14:24, Marc Mutz <marc.mutz at kdab.com> wrote:
> > That just goes to show how bad of an influence QList has. But it never
> hid its
> > design, and, indeed, the "default container" in Qt 3 was QValueList,
> which
> > actually *was* a doubly-linked list. Why Qt always nominates a list as
> the
> > default container while the STL recommends std::vector is one of the
> mysteries
> > to which only Trolls know the answer :)
>
>
> QList is the perfect default container for applications that value
> convenience and intuitiveness over best performance (a typical feature for
> most APIs in Qt), but it still outperforms arrays and linked lists for the
> most common operations for complex types, such as insertion and removal.
> The two most useful examples of QList usage in Qt applications are
> QList<QWidget *>, and QList<QImage>. To cater for the problem of custom
> complex types, all types in Qt were made shallow (the shallowness of
> containers and smartness of QList when used with shallow types go hand in
> hand, which is another typical feature for APIs in Qt). It should be no
> surprise to users that creating a QList of 100 QImages does *not* lead to
> 100 news or mallocs.
>
> Qt APIs also help programmers avoid making mistakes. QList has the
> undisputable advantage of allowing lists of dynamic sized elements. It’s a
> common programmer’s mistake to run into trouble with lists of dynamic
> elements when those elements are stored in arrays. Trouble leads to hacks
> leads to bugs. It doesn’t take too much imagination to understand how much
> of a pain you can run into, and what horrors could be applied to work
> around, creating a QVector of QVectors. The problem is a bit hard to grasp
> for some developers, and using QList you just don’t run into those problems.
>
> So, I dispute the general -1 of any use of QList. Use yours brains, and -1
> where brain has not been applied. ;-)
>
> Andreas
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