QPointer-QWeakPointer-Qt5-Debacle

[Asperamanca]

Just because the constructor taking a QObject is deprecated doesn't mean the functionality isn't still there.

If QWeakPointer actually lost it's QObject-tracking capabilities, I would report that as a bug, and a critical one, too.

[DerManu]

[quote author="Asperamanca" date="1367907884"]Just because the constructor taking a QObject is deprecated doesn't mean the functionality isn't still there.
[/quote]

in qsharedpointer_impl.h, they've got all the QObject-targeted interface of QWeakPointer excluded from compilation via
@#if QT_DEPRECATED_SINCE(5, 0)
...
#endif@
I'd expect things that were marked as deprecated in a version not to be directly excluded in that version, but maybe in Qt6... However, that directive clearly excludes it for the default defines. I do wonder whether that's intended or a bug. I'd need to tell all my clients to include some hack like "#define QT_DISABLE_DEPRECATED_BEFORE QT_VERSION_CHECK(0, 0, 0)" before including any Qt headers in their projects and probably even recompile Qt with that flag on some systems. Can't be serious.

[Asperamanca]

I don't have a working Qt5 environment. Could you check it with a small example? If they've really broken that, I could submit this as a bug via Qt commercial support. While we currently stick to Qt4, there's always the future to think of.

[DerManu]

Checked it, they really excluded the newly deprecated interface by default.
@#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QWeakPointer>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();

QWeakPointer<QWidget> mPointer;

private:
Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H@

@#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
mPointer(this),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
}

MainWindow::~MainWindow()
{
delete ui;
}@

Works with Qt4.x, fails with Qt5.0.x saying
@mainwindow.cpp: In constructor 'MainWindow::MainWindow(QWidget*)':
mainwindow.cpp:7:24: error: no matching function for call to 'QWeakPointer<QWidget>::QWeakPointer(MainWindow* const)'
mainwindow.cpp:7:24: note: candidates are: [blablabla]@

as expected when such a constructor isn't defined.

[Asperamanca]

I have compared the source codes 4.8.4. against 5.0.2, and sure enough, they've just defined it away. I have submitted a support request, and expect an official statement how application and library developers are expected to handle such back-and-forth tactics.

[JKSH]

Hi Asperamanca, can you please post a link to your support request here? So that others can follow it.

Anyway, there is a large backlog in the JIRA issue tracker, so the request might get buried under lots of other requests. If you don't get any response in a week, try posting the question to the Interest mailing list, which is a lot more active: http://lists.qt-project.org/mailman/listinfo/interest

[JKSH]

Actually, I found an official statement: http://www.macieira.org/blog/2012/07/continue-using-qpointer/

[Asperamanca]

Thanks for the link. It still doesn't explain why no way was implemented that would allow someone to specifically allow the QWeakPointer - QObject use case.

I don't know how to post a link to my support request - I can only access it via my Qt commercial user account. But I'll keep you posted about what's going on.

[JKSH]

Ah, I didn't realize you submitted a commercial support request. Yes, please keep us updated.

I'm not familiar with the intricacies of the different smart pointer classes, but I was under the impression that "deprecated" means "not recommended but still available", not "removed from the library". It could be an error.

[ChrisW67]

Yes, the flip-flopping over QWeakPointer is annoying to those who immediately adjusted all their code. It seems though that QPointer works in Qt4 programs although for a couple of dot releases it may generate a deprecation warning at compile time (not in 4.8), and it works in Qt 5.0. The solution would seem to be use QPointer and ignore the late unpleasantness. No version dependent #ifdef required.

I may be wrong, but this should only be a problem to clients of your library if the QWeakPointer is part of the API, which you have just changed only to change back, or the library has not been programmed to maintain binary compatibility (so changing private members could break things).

[Asperamanca]

The support answer suggests to use

@DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0@

[JKSH]

Thanks, Asperamanca. Not an ideal solution, but it does let code run.