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

Philippe philwave at gmail.com
Wed May 29 13:48:55 CEST 2019


> == QSharedDataPointer / QExplicitlySharedDataPointer ==

No.
Very useful to create COW classes. Why removing this capability for Qt
users, while this is a common and useful Qt idiom?

>> === QAtomic -> std::atomic ===

There would be subbtle traps for code conversion:
eg. std::atomic::load() uses std::memory_order_seq_cst
while QAtomic::load() uses relaxed ordering

But this current difference is not good on the 1st place.

>> === QMutex / QReadWriteLock -> std::*mutex* ===

No..
On Windows I recently did some benchmarks for the fast code paths (no
lock), and QMutex is almost twice as fast than std::mutex. This is much
appreciated for performing code.

>== QSet / QHash -> std::unordered_set/map ==
>== QMap -> std::map ==

No. Many find these Qt classes easier to use.

Philippe


On Wed, 29 May 2019 12:53:35 +0200
"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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