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

Giuseppe D'Angelo giuseppe.dangelo at kdab.com
Mon Jun 10 12:13:26 CEST 2019

On 08/06/2019 20:31, André Pönitz wrote:
> On Sat, Jun 08, 2019 at 06:14:36PM +0200, Giuseppe D'Angelo via Development wrote:
>> On 05/06/2019 23:01, André Pönitz wrote:
>>> As a matter of fact, some of the previous deprecations, e.g. the removal
>>> of qalgorithm, triggered re-implementing the deprecated functionality
>>> downstream, effectively shifting the burden of doing (or, rather,
>>> *keeping*) them once centrally to all users who need it decentrally.
>> The huge impact that these deprecations have is also due to another reason:
>> the approach of systematically diverging from "upstream".
>> Qt really really fails at not over-extending (in the name of convenience, or
>> compatibility, or whatever). Especially for such low-level facilities there
>> is a hidden, slow trend at:
>> 1) taking something "upstream" lacks, or is available only in very recent
>> versions, or is not as widespread as it should be, or similar (limiting
>> ourselves to "upstream" == Standard Library, that used to be the case with
>> QtAlgorithms and -no-stl; one could say the same with
>> QSharedPointer/QWeakPointer, and so on.)
>> 2) reimplementing it so that our users have it (yay!)
>> 3) giving it easier APIs (yay!)
>> 4) *extending* it in incompatible ways with upstream
>> (QWeakPointer(QObject*); QSharedPointer in QVariants; etc.).
> I see that pattern, too. But now, instead putting the break between 3) and 4),
> the whole thing is killed, and everybody downstream has to do 1)-3) again,
> or put up with what the standard offers.
> And could prevent overextension by -x'ing the respective change on gerrit.
> So *that* sounds fixable.

It's not *that* simple:

* some of these APIs come from pre-gerrit era (actually, given thread is 
a QtCore fun fest anyhow, a good 90% of them)

* some of them simply slip in by accident;

* some are just unpredictable at code review time (e.g. suppose we 
decide to add a QOutcome or QExpected to Qt, based on things which are 
not standardized yet, what's going to happen when the std:: versions 
land and supposedly end up with an incompatible design?)

The decision of "killing everything" usually comes from a combination of 

* lack of manpower to maintain the 1-3 bits;

* very little sense at keeping the 1-3 parts into Qt if they're now 
widely available by other means, with more features, etc;

* the fact that the 3/4 cut is either going to be an API break (and 
then, API break for API break, just refactor to use the upstream thing 
in the first place, so pay the 1-3 anyhow); or a behaviour change, for 
which the compiler won't warn you, making it even worse. The only way 
for you to be aware is to force the whole kill.

>> * We could keep things where they are, supported, thus offering the easier
>> APIs; but simply reimplement them on top of the "upstream" equivalents.
>> (Ignore the possible ABI break.)
> The last one is the most reasonable.

When doable without API breaks, yes, I'd also agree. What about the NOT 
doable though?

>> Here's where the "extension" bites us: if the Qt equivalent offered
>> something that upstream is not offering, and we can't reimplement it, then
>> what do we do? Dropping support for it would be, at best, an API break; and
>> at worst, a _silent_ behavioural change.
> Proactively preventing a possible future API break by removing things
> immediately is a questionable approach. Taken to the extreme this means
> each and every bit of Qt should be removed as otherwise it might break API
> at some time.

I'm not sure what you mean by this. When is "immediately" here?

>> This is exactly the case of what happened with QtAlgorithms. I believe the
>> culprit back then was the usage of qSwap / qLess or so into the algorithms
>> themselves (don't remember all the details; also this was pre C++11, and
>> using detection idioms and SFINAE to work around this would've been another
>> major fun -- file it under "development investment").
>> Long story short, making qSort() call std::sort() might not have kept the
>> behaviour contract with the user.
> *shrug*
> And what if I, as a user, would not care?

Come on, this is not an excuse. Just because it doesn't break *your* 
code it doesn't mean it won't break others. (And we KNOW of the 
behavioural changes involved.)

> What about https://valdyas.org/fading/hacking/happy-porting/
>     "[...] none, not a single one of all of the reasons you want to deprecate
>     Q_FOREACH is a reason I care even a little bit about. It’s going to
>     be deprecated? Well, that’s a decision, and a dumb one. It doesn’t
>     work on std containers, QVarLengthArray or C arrays? I don’t use
>     it on those. It adds 100 bytes of text size? Piffle. It makes it hard
>     to reason about the loop for you? I don’t care.
>     What I do care is the 1559 places where we use Q_FOREACH in Krita.
>     Porting this will take weeks. [...]"
> ?

Isn't how to cover this kind of cases the whole point of the original 
post of the thread?

(I won't discuss Q_FOREACH deprecation *itself*; assume it's an API 
"XYZ" that got deprecated, and of course it's got users who are going to 
complain, and I keep asking how do we minimize that.)

>> Is that break acceptable? Is it not acceptable? It's debatable, as usual;
>> but please don't give me the "who in their minds overloads qSwap / qLess /
>> qFoobar anyhow?". When it comes to fundamental stuff in QtCore (containers,
>> algorithms, etc.) I'd like to stay on the conservative side; and therefore
>> the decision, in the end, was to deprecate QtAlgorithms rather than porting
>> them over stdlib ones.
>> (If you want, it's a corollary of Hyrum's law. The lower you are a software
>> stack, the bigger is your number of users => and Hyrum follows...)
>> Now, if everyone is unhappy and reimplements QtAlgorithms in their own
>> projects, I guess that was the wrong decision? Are we committing it all over
>> again when Qt 6 comes and we drop many deprecated APIs? Should we think
>> about a Qt5Support module, without API/ABI promises, where to dump things
>> without deprecation warnings, just a big bold writing on top "no guarantees
>> are given for these things"?
> We won't find consensus here.

... why not? Can at least discuss the preferred options before saying 
that there isn't a consensus? :)

> good starting point for all my personal use cases. For other it isn't.
> What I see happening is that projects simply stick for longer with older
> versions of Qt the more effort the upgrade is. These are practically
> cold forks, fragmenting the ecosystem. Needlessly, if you ask me.

Wait, what? Two seconds before you were advocating for those breaks (as 
they were not touching YOUR code), now you do care about them?

My 2 c,
Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
KDAB - The Qt, C++ and OpenGL Experts

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