Am 06.05.2019 um 12:04 schrieb Lars Knoll:
On 6 May 2019, at 10:27, Konstantin Shegunov <kshegu...@gmail.com
<mailto:kshegu...@gmail.com>> wrote:
On Mon, May 6, 2019 at 10:42 AM Lars Knoll <lars.kn...@qt.io
<mailto:lars.kn...@qt.io>> wrote:
Not sure whether it’s most projects, but there certainly are
users doing it. And we’ve been using the pattern in our examples
in some cases as well. But I can relate to Allan that creating
widgets on the stack is bad style (as it conflicts with the way
we memory manage them), and if I’d have a choice today, I would
probably prefer to enforce them to be created on the heap by some
means.
Fine, there seems to be somewhat of an argument in that regard. I'd
raise the question then, what should be done for QObject derived
classes, and I don't mean the ones that come from Qt (i.e. QFile,
which Thiago mentioned), but the ones the user defines. In the end
QObject is for us to derive from - get signals and slots and all
those goodies. But then if one restricts QObject to be on the heap
how do we treat the user classes? What I mean is - you can control
how allocations happen in the library, say you provide custom
new/delete for QObject and/or QWidget and restrict the stack
(presumably through some trickery that disables the constructor),
does this mean we are to force the user to write their own allocation
operators for their classes? This doesn't really sound practical to
say the least. Moreover there's really little reason (from the user's
point of view) to have something in the heap if it can go on the
stack. What do we do with QCoreApplicaiton, force the heap on it?
It’s somewhat of a philosophical discussion now, as we are where we
are and users can and are creating QObject’s on the stack. I don’t
think we can change that now (even for Qt 6), and restricting it would
lead to rather ugly syntax in C++ (as the language doesn’t really help
enforcing something like this). So that means we’ll have to continue
to support it.
That's something that needs to be taken into account in any change in
how memory management could work in Qt6. The patch I propose does not
take that into account and is indeed almost entirely source compatible.
(No changes in Qt Creator required, and two minor changes in tests.)
And the constructors/apis that are marked with QT_UNSAFE_API by default
don't even emit any warning. The user can opt into those warnings.
One thing I forgot to mention, which is something I only have a mediocre
solution for is, modal QDialogs (or any other top level widgets) on the
stack. For those the parent pointer is both the owner and indicates to
which parent the dialog is modal. In my opinion, the best solution is to
disentangle this. E.g. introduce a new method ::setModalTo(QWidget *),
which if used overwrites to which parent a dialog is modal. (That is by
default a dialog is modal to its parent, unless something else is set
via setModalTo). This means code that uses modal widgets on the stack
would need some conversion work, but that's as far as I can see the best
solution.
The main benefits I see here that this scheme can co-exist with current
code, i.e. code bases could slowly opt-in and migrate step by step, and
it does not add the need to sprinkle user side code with the typical
smart pointer line noise.
The same is true for what I'm proposing. It can and does allow users to
convert at they choosing, and since the vast majority of QObjects are
created with parents, most QObjects will be created via makeChild. Using
unique pointers is the exception not the norm with what I've proposed.
And indeed due to the ergonomics of makeChild via make_unique, the user
is steared towards using the better way to construct QObjects.
Now, what André is proposing is very similar to just having a owning
smart pointer that remembers the pointed at object after it has lost
ownership. I see no reason why that (or more fuller proxy classes)
couldn't work. But,
* The class/smart pointer would need to be move only, which limits how
useful it is.
* The semantics feel strange to me.
And while I'd encourage someone to try it out, I'm of the firm opinion
that Qt should try to get over its NIH attitude. One of the issues with
Qt in the real world is that it's just that tiny bit differently than
what non Qt C++ users are familiar with for. Sometimes that's for a good
reason. but there's a cost there that is imho unappreciated inside the
Qt bubble.
Also nowadays C++ is changing quite fast and the way to profit from the
advancements in standard c++ is by making sure that Qt works great with
new standard c++ idioms. It's not by investing time into inventing a
slightly different idiom.
For example, Microsoft is still researching whether static analysers
could detect dangling pointers [2]. That tool obviously will understand
unique_ptr and is unlikely to understand our custom solution.
daniel
[1]
https://codereview.qt-project.org/#/c/260618/1/tests/auto/opengl/qglbuffer/tst_qglbuffer.cpp
and
https://codereview.qt-project.org/#/c/260618/1/tests/auto/opengl/qglbuffer/tst_qglbuffer.cpp
[2]
https://devblogs.microsoft.com/cppblog/lifetime-profile-update-in-visual-studio-2019-preview-2/
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