QWidget: Must construct a QApplication before a QWidget

[jronald]

Env

OS archlinux
Qt 6.10.1
Qwt 6.3.0

Problem

After recent updates of Qt, an error occurs, error message:

QWidget: Must construct a QApplication before a QWidget

And formerly no error.

Sample codes

https://github.com/user-attachments/files/24138817/TestQwt.zip

More Info

Similar post: https://forum.qt.io/topic/115273/qwidget-must-construct-a-qapplication-before-a-qwidget, but no solution.

[JonB]

@jronald said in QWidget: Must construct a QApplication before a QWidget:

But obviously Qwt is built with an older version of Qt.

I see it now says there

So libqwt.so links against qt5, so trying to use it with qt6 won't relly work.

If your repo release of Qwt is Qt5 you really aren't going to get anywhere if you are using Qt 6! They may both come from a repo but they are not intended to be used together. Surprised you didn't get a linker error or something.

[Christian Ehrlicher]

Use a debugger and e.g. QT_FATAL_WARNINGS to see where it comes from and fix it.

[jronald]

@Christian-Ehrlicher The test project works a few days ago, it doesn't work only after some updates of Qt, not sure it's caused by Qwt or Qt.

[JonB]

@jronald Still try what @Christian-Ehrlicher has suggested.

[jronald]

simplied the test codes in one file as below:
main.cpp

#include <qwt_plot_curve.h>
#include <qwt_plot.h>
#include <qapplication.h>

int main(int argc, char **argv)
{
    QApplication a(argc, argv);
    double x[101];
    double y[101];
    
    for ( int i = 0; i < 101; i++ ) {
        x[i] =  i / 10.0;
        y[i] = sin(x[i]);
    }
        
    QwtPlot plot;
    QwtPlotCurve *curve = new QwtPlotCurve();
    curve->setRawSamples(x, y, 101);
    curve->attach(&plot);
    plot.show();
    return a.exec();
}

when set env QT_FATAL_WARNINGS, the only output is still the same, i.e.

QWidget: Must construct a QApplication before a QWidget

When debug, it aborts at line QwtPlot plot;, message as below:

[JonB]

@jronald
So first simplify it further: sounds like you need no more than

    QApplication a(argc, argv);
    QwtPlot plot;

Right? Or is there more to it?

Then in case it helps please show the full stack trace when it errors/crashes.

[jronald]

@JonB

int main(int argc, char **argv)
{
    QApplication a(argc, argv);
    QwtPlot plot;
    return 0;
}

same error

[SGaist]

@jronald hi,

As @JonB requested, please provide the full stack trace of the crash.

[jronald]

@SGaist said in QWidget: Must construct a QApplication before a QWidget:

please provide the full stack trace of the crash.

1 ??                                                            0x7ffff5c9890c 
2 raise                                                         0x7ffff5c3e3a0 
3 abort                                                         0x7ffff5c2557a 
4 QMessageLogger::fatal(const char *, ...) const                0x7ffff46982e0 
5 ??                                                            0x7ffff55455b5 
6 QFramePrivate::QFramePrivate()                                0x7ffff5644f3b 
7 QFrame::QFrame(QWidget *, QFlags<Qt::WindowType>)             0x7ffff5644fbf 
8 QwtPlot::QwtPlot(QWidget *)                                   0x7ffff773d122 
9 main                                              main.cpp 12 0x5555555588d9

[JonB]

@jronald
Hmm, does not tell us much more than you have said --- it is crashing inside something QFramePrivate called during your QwtPlot plot; constructor.

I don't know anything about Qwt. Is it something where you get source code and you compile it yourself --- I imagine so? Do you compile it separately from your own code, so you have a created perhaps some sort of "library" which you link against? Especially since you say "After recent updates of Qt, an error occurs", is there any chance your code and Qwt are not compiled identically? Are you sure they use the same version of Qt --- e.g. if it has changed for your source code has changed equally for Qwt --- and you are compiling both for debug or both for release? Have you done a complete, clean rebuild? Does the problem occur both when you compile everything for debug and everything for release? Compiling both your code & Qwt for debug might help track something down. If you also build Qt yourself and so have sources you could step though in debugger right to the point where it errors down in QFramePrivate and probably thereby figure what exactly is happening.

Otherwise, let's check it's only a Qwt issue: we know the stack trace dies in QFrame: try creating a QFrame in a standalone program with no Qwt anywhere, does that work OK?

[jronald]

Qwt if a famous chart lib for Qt.
It is installed from the official repo of archlinux (https://archlinux.org/packages/extra/x86_64/qwt/).
It's last update time is 2024-06-29 21:30 UTC.
So it must uses an older version of Qt, I've reported it on the archlinux forum. (https://bbs.archlinux.org/viewtopic.php?pid=2277935)

[JonB]

@jronald
If you are not using the same version of Qt to compile your code versus the version of Qt used to compile any libraries, Qwt or otherwise, which you use things will obviously go wrong.

In general, if you use a Qt package (such as Qwt) from a repo then you must use the Qt itself package from that repo. You should not use an existing repo-package with a more up-to-date Qt you might have obtained for yourself. If, perhaps, you have fetched/built Qt 6.10.1 for your own development purposes then you should compile any libraries, such as Qwt, for yourself with that same Qt version.

[jronald]

@JonB

Both Qt and Qwt are installed from the official repo of archlinux.
But obviously Qwt is built with an older version of Qt.
Asking for help from archlinux (https://bbs.archlinux.org/viewtopic.php?pid=2277935).

[JonB]

@jronald said in QWidget: Must construct a QApplication before a QWidget:

But obviously Qwt is built with an older version of Qt.

I see it now says there

So libqwt.so links against qt5, so trying to use it with qt6 won't relly work.

If your repo release of Qwt is Qt5 you really aren't going to get anywhere if you are using Qt 6! They may both come from a repo but they are not intended to be used together. Surprised you didn't get a linker error or something.

[jronald]

@JonB said in QWidget: Must construct a QApplication before a QWidget:

If your repo release of Qwt is Qt5 you really aren't going to get anywhere if you are using Qt 6! They may both come from a repo but they are not intended to be used together. Surprised you didn't get a linker error or something.

More suprised, it even runs normally serveral days ago, before some updates of Qt6.

I've tested qwt-qt6, it works.