How to deal with "QPixmap: Must construct a QGuiApplication before a QPixmap"

laozeng1982

Hi experts,

I am using some 3rd part dlls, wll I start the module which using these dlls, it say "QPixmap: Must construct a QGuiApplication before a QPixmap" and the program crashed.

Enviorment:
OS: Widows 11.
Qt version: 5.15.2
Complier: Qt MSVC 2019 x64

I did some digging and reading https://forum.qt.io/topic/130352/how-stop-the-qpixmap-must-construct-a-qguiapplication-before-a-qpixmap-in-a-dll, I will provide some information I think it might be useful.

main.cpp

#include <QApplication>
#include <QDesktopServices>
#include <QGuiApplication>
#include <QDesktopWidget>
#include <QScreen>
#include <QTextCodec>
#include <QDebug>
#include <DesignAndDebugger.h>
#include <QLibrary>

// this is application I worte
#include "EnhancedTools.h"

int main(int argc, char* argv[])
{
    // for function test!
    bool design_and_test = false;
    //    bool design_and_test = true;

    if(design_and_test) {
        DesignAndDebugger::runTest();
        return 0;
    }

    QTextCodec* codec = QTextCodec::codecForName("UTF-8");
    QTextCodec::setCodecForLocale(codec);

    qDebug() << "Main application, before qApp:" << QCoreApplication::instance();

    QApplication app(argc, argv);

    qDebug() << "Main application, after qApp:" << QCoreApplication::instance();
    qDebug() << "DLL, before loading icons:" << QCoreApplication::instance();
    
    // for test load libray, with or without it, same error
    QLibrary mylib("CarnacPlotQt5.dll");
    if(mylib.load()) {
        qDebug() << "CarnacPlotQt5.dll loaded";
    }

    qDebug() << "DLL, after loading icons:" << QCoreApplication::instance();

    EnhancedTools window;
    // get the screen number where window create.
    int currentScreen = app.desktop()->screenNumber(&window);

    // get the geometry of the screen
    QRect rect = QGuiApplication::screens().at(currentScreen)->geometry();

    // move to the center and top
    window.move((rect.width() - window.width()) / 2, 0);

    window.show();
    return app.exec();
}

EnhancedTools.pro, these are 3rd part libs I used.

INCLUDEPATH += C:/INT/Carnac/include
DEPENDPATH += C:/INT/Carnac/include

INCLUDEPATH += C:/INT/Carnac/src

LIBS += -LC:/INT/Carnac/lib/ -lCarnac
LIBS += -LC:/INT/Carnac/lib/ -lCarnacCgm
LIBS += -LC:/INT/Carnac/lib/ -lCarnacChartQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacContour
LIBS += -LC:/INT/Carnac/lib/ -lCarnacContourChartQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacContourQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacDeviatedWellQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacDriverQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacGauges
LIBS += -LC:/INT/Carnac/lib/ -lCarnacGaugesQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacMapQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacMultiWellQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacPlot
LIBS += -LC:/INT/Carnac/lib/ -lCarnacPlotQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacPluginQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacPs
LIBS += -LC:/INT/Carnac/lib/ -lCarnacQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacSeisLogQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacSeismic
LIBS += -LC:/INT/Carnac/lib/ -lCarnacSeismicQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacStylesQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacW32
LIBS += -LC:/INT/Carnac/lib/ -lCarnacWellBoreDataQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacWellLog
LIBS += -LC:/INT/Carnac/lib/ -lCarnacWellLogQt5
LIBS += -LC:/INT/Carnac/lib/ -lCarnacWellSchematic
LIBS += -LC:/INT/Carnac/lib/ -lCarnacWellSchematicQt5

I use debug model and get message like.

console output:

Any help will be appreciated!

jsulm

@laozeng1982 said in How to deal with "QPixmap: Must construct a QGuiApplication before a QPixmap":

QLibrary mylib("CarnacPlotQt5.dll");
if(mylib.load())

Why are you doing this?!
Your application already links against this DLL, there is no need to dynamically load it...

laozeng1982

@jsulm Yes, I just test it, but without this test, it still raise the same error.

J.Hilk

@laozeng1982 assuming you don't have any static QPixmaps or so,

move QApplication app(argc, argv); to the top of your main function

laozeng1982

@J-Hilk Hi, I move the "QApplication app(argc, argv);" to the top and remove QLibrary statement.

but It still the same error.

J.Hilk

@laozeng1982 than you have probably somewhere a static QObject

laozeng1982

@J-Hilk So, how Can I found where is the static Object?

If I don't start the module using the 3rd dlls, it will not raise the error, according to the debugger information, it was in the 3rd part dlls, even I found it, I can't modify it.

J.Hilk

@laozeng1982 its a dll, it's dynamically loaded -> load it after your QApplication instance is up and running.

laozeng1982

@J-Hilk Yes, I want to do this, but I am not familiar with QT, I don't know how to do this.

Second thing, do I need to modify my .pro file and remove static libs?

JonB

@J-Hilk said in How to deal with "QPixmap: Must construct a QGuiApplication before a QPixmap":

@laozeng1982 than you have probably somewhere a static QObject

That would have been my initial comment too. But when I looked at the first post in this thread I see

A stack trace showing me code called from some action/checkbox; and
Console output showing code running as far as the app.exec().

Assuming the OP means the error message about "no application" comes at this point how can it stem from a static QObject? That would happen literally at start up.

J.Hilk

@JonB you are correct, maybe its in a different thread

very strange everything hard to tell from afar