[Development] Qt 5 types under consideration for deprecation / removal in Qt 6

Jean-Michaël Celerier jeanmichael.celerier at gmail.com
Wed May 29 14:22:45 CEST 2019


> We probably need to keep QRWL around a while longer, since C++ added
shared_mutex only in C++17.

Wasn't it said at the world summit that Qt 6 would base itself on c++17 ?
https://blog.qt.io/blog/2018/06/13/qt-contributors-summit-2018-wrap/

Best,
Jean-Michaël


On Wed, May 29, 2019 at 12:55 PM Mutz, Marc via Development <
development at qt-project.org> wrote:

> Hi,
>
> Here's a list of stuff I consider has served it's purpose and is no
> longer needed, with respective replacements:
>
> = Priority 1 =
>
> == QSharedDataPointer / QExplicitlySharedDataPointer ==
>
> These are basically Qt-internals, and should never have been public in
> the first place. It's _because_ they are public that we have two of
> them, and soon a third one (properly non-public):
> https://codereview.qt-project.org/c/qt/qtbase/+/115213 That commit's
> message also explains what's wrong with the QSDP and QESDP.
>
>
> == Q_FOREACH -> ranged for loops
> (https://codereview.qt-project.org/c/qt/qtbase/+/147363) ==
>
> Q_FOREACH is less readable (because it allows mutating the container
> under iteration), and produces _a lot_ of code. It also is intrinsically
> tied to just a few Qt containers (excluding QVLA). Ranged for loops work
> on anything, incl. a C array. Also, as a macro, it will continue to
> cause problems down the road (e.g. with modules). Being a macro also
> means that it cannot deal with value_types that contain a comma.
>
> === related: Q_FOREVER -> for(;;) ===
>
> Suggested by Lars in ibid. Basically because it's a macro.
>
>
> == Java-style iteration
> (https://codereview.qt-project.org/c/qt/qtbase/+/262344) ==
>
> It's very easy to write quadratic loops with it.remove(), and a review
> of Qt code has shown that some users still use container.remove(), which
> is just as unsafe as with STL iterators. I also noted between 100b/loop
> and 5KiB for four loops of text size savings.
>
>
> == QScopedPointer -> std::unique_ptr ==
>
> Suggested by Thiago on
> https://codereview.qt-project.org/c/qt/qtbase/+/261553
>
> I agree. We now have std::unique_ptr, and it's movable. QScopedPointer
> had the problem that it didn't know what it wanted to be:
> boost::scoped_ptr or std::unique_ptr. A real scoped pointer would not
> offer release(), a unique_ptr would need to provide move semantics.
> QScopedPointer has release(), but no moves (were proposed, but not
> accepted).
>
>
> == QLinkedList -> std::list
> (https://codereview.qt-project.org/c/qt/qtbase/+/261421) ==
>
> Of the Qt containers, this one is the most-unused (in Qt code), as well
> as the one that's furthest behind it's STL counter-part. Whenever I use
> std::list, I use it for splice(), a function that just cannot be added
> to a CoW container. Qt is already almost completely QLinkedList-free.
> Let's bury QLinkedList.
>
>
> == qHash() -> std::hash ==
>
> Suggested by Lars in
> https://codereview.qt-project.org/c/qt/qtbase/+/261819. To be precise,
> he's suggesting to specialise std::hash and have qHash() call std::hash.
>
> Only problem I see so far is that std doesn't provide us with a tool to
> hash composites. E.g. there's no std::hash for std::tuple (which would
> mean we can std::tie the members and hash the result), and only C++17
> adds some kind of raw bits hashing (via std::string_view). We'd need to
> provide these building blocks ourselves, which can be done, but it means
> we'll have at least _some_ qHash()-like functions we need for
> std::hash<> implementations.
>
> Actual problem?
>
>
> == QPaintDevice ==
>
> I'd like this to become a static interface. In very shortened terms:
> everything that has a QPaintEngine *paintEngine() method is a paint
> device. QPainter's ctor would become a template and do the virtual
> dispatch internally, just like Sean Parent's document type in C++
> Seasoning.
>
> This would solve a lot of problems: QWidget would no longer need to use
> multiple inheritance, and QImage and QPixmap would become proper value
> types, without virtual functions that create problems with move
> semantics and swapping.
>
>
> == QRegExp ==
>
> Is QRegularExpression good enough these days? :)
>
>
> == MT plumbing ==
>
> Let's make use of the std facilities to enable checkers such as TSAN to
> work without hacks. It's not Qt's business to reimplement threading
> primitives.
>
> Probably need to keep some stuff around, like QThread, because of the
> interaction with event processing, but before we add a lot of time
> making the following classes play nice with the std, let's perspectively
> remove them:
>
> === QAtomic -> std::atomic ===
>
> It already is just a thin wrapper around std::atomic, so there's not
> much point keeping it.
>
> === QMutex / QReadWriteLock -> std::*mutex* ===
>
> It has too many responsibilities. Where the std knows many different
> mutex classes, Qt folds everything into just two.
>
> We probably need to keep QRWL around a while longer, since C++ added
> shared_mutex only in C++17.
>
> === QMutexLocker -> std::unique_lock ===
>
> 1:1 replacement in the vast majority of cases. unique_lock has a lot
> more features (movable, adopting a locked mutex, not tied to any
> particular mutex class, ...)
>
> === QWaitCondition -> std::condition_variable(_any) ===
>
> Plumbing that std::condition_variable can do better.
>
>
>
>
>
> = Priority 2 =
>
> == QQueue / QStack -> std::queue, std::stack ==
>
> These classes publicly inherit QList and QVector, resp., and are both
> very inflexible (due to the fixed underlying container), as well as too
> flexible (they offer non-queue, non-stack behaviour, such as iteration).
>
> For QQueue, we have the additional problem that QList is going to be
> deprecated/removed in Qt 6 (see previous discussion).
>
>
> == QSharedPointer / QWeakPointer -> std::shared_ptr/weak_ptr ==
>
> Once they are stripped of their magic QObject handling and QObject
> handling returned to QPointer proper, they don't do much other than
> std::shared_ptr, except being less flexible and largely untested for
> exception-safety.
>
>
>
>
>
> = Priority 3 =
>
> == QSet / QHash -> std::unordered_set/map ==
>
> I'd really like to see these gone. Mainly to free up their names for OA
> hash containers, something that the STL doesn't, yet, have.
>
>
> == QMap -> std::map ==
>
> These classes have taken some design decisions that make them very badly
> integrated into modern C++. One is that QMultiMap is-a QMap. The first
> and foremost is, though, that *it returns value_type, making ranged-for
> useless for maps. If we're going to change the return value of *it,
> though, we might as well kick the whole class out, too, since the code
> adjustments on the user's parts would be roughly the same.
>
>
> ... to be continued.
>
> Flame away :)
>
> Thanks,
> Marc
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> Development at qt-project.org
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>
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