Qwt 6.3.0 app calling Qwt 6.3.0 craches

[buster]

I have compiled Qwt 6.3.0 and see that the headers and the libs have been placed in
/usr/local/qwt-6.3.0/include/
and
/usr/local/qwt-6.3.0/lib/
respectively.

I have build a Qt 6.7.0 app. using the following CMakeLists file:
set(QWT_INCLUDE_DIR "/usr/local/qwt-6.3.0/include")
set(QWT_LIBRARY "/usr/local/qwt-6.3.0/lib/libqwt.so")
set(qwt_libraries ${QWT_LIBRARY}})
set(test_SRC
src/main.cpp
src/test.cpp
src/Plot.cpp
)
add_executable(test ${test_SRC})
target_link_libraries(test Qt6::Widgets ${QWT_LIBRARY})

Compiling the program and running it in the debugegr it craches with the error message:
QWidget: Must construct a QApplication before a QWidget
*** Program received signal SIGABRT (Aborted) ***

In main I have
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
test w;
w.show();

return app.exec();
}
which I think is correct.

In MainWindow (i,e, test):

test::test(QWidget *parent) :
QMainWindow(parent),
m_ui(new Ui::test)
{
qDebug() << "test::test 0";
qDebug() << qApp;

qDebug() << "test::test 1";
m_ui->setupUi(this);
qDebug() << "test::test 2";

m_plot = new Plot(this);
qDebug() << "test::test 3";

Starting the debugger yields:
test::test 0
QApplication(0x7ffde3f14770)
test::test 1
test::test 2
QWidget: Must construct a QApplication before a QWidget
*** Program received signal SIGABRT (Aborted) ***

test::test 3 is never printed out, so it seems the crash happens in the Plot constructor:
Plot::Plot(QWidget *parent)
: QwtPlot( parent )
{
qDebug() << "Plot::Plot 1";

Again, Plot::Plot 1 is not printed out.
Can it be that Qt 6.7.0 and Qwt 6.3.0 are incompatible?

[Uwe]

The warning is coming from the constructor of QWidget - what is a base class of QwtPlot - not from Qwt code.
For some reason the qApp pointer is broken - at least this is the check, that leads to the message in QWidgetPrivate::QWidgetPrivate ( see qwidget.cpp ).

HTH,
Uwe

[buster]

Yes, but just before instantiating qwt the qApp pointer appears to be OK:
qDebug() << qApp; // Check
QApplication(0x7ffde3f14770) // Prints out

So it is definitely Qwt that's messing up things.

At the top of the autogenerated README file for my test app I have this:

CMake Qt5 GUI Template
----------------------

You need Qt5 and CMake 2.8.11 or higher

Can it be that there somehow is a Qt5-Qt6 mix-up? Kubuntu 24.04 use Plasma 5.27.11, which use Qt 5.15.13.

But the contents of the file "Qt5Qwt6.pc", under /usr/local/qwt-6.3.0/lib/pkgconfig, reads:

prefix=/usr
exec_prefix=${prefix}
libdir=${prefix}/local/qwt-6.3.0/lib
includedir=${prefix}/local/qwt-6.3.0/include

Name: Qwt6
Description: Qt Widgets for Technical Applications
Version: 6.3.0
Libs: -L${libdir} -lqwt
Cflags: -I${includedir}
Requires: Qt5Widgets Qt5Concurrent Qt5PrintSupport Qt5OpenGL

The way I interpret this is that "Qt5Widgets" refers to Qt5 used by Plasma. So as far as I can tell everything seems right. But my app craches as soon as I instantiates Qwt...

It is pointed out here (
https://stackoverflow.com/questions/...e-a-qwidg?rq=3) that using different compiler versions can case trubble, but I built Qt 6.7 and Qwt 6.3.0 with the same compiler (gcc 13.2.0), so it cannot be that.

[Uwe]

Maybe using ldd to find out what libraries/dependencies your application has ?

HTH,
Uwe