QWidget: Must construct a QApplication before a QWidget

JonB

@lakshmanGiri
In the debugger, put a breakpoint on the first line, QApplication a(argc, argv);. Do you get that far at run time, or does the error occur before you hit the breakpoint? Put qDebug() either side of that line too. If it comes before any debug output, it points to a global QWidget from another file, though I can't see one in what I looked at.

Christian Ehrlicher

To add @JonB suggestions - install a qt message handler, add a breakpoint there and see where it comes from

lakshmanGiri

@JonB
Heyy, Actually I am new to Qt and also came up with something like when the Build is in Debug, the following error occurs but when the build is in Release, the application works.

Why soo? Can someone please elaborate?.

J.Hilk

I can't really look into these files

#include <vtkSmartPointer.h>
#include <vtkImageViewer2.h>
#include <vtkDICOMImageReader.h>
#include <vtkRenderWindow.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkRenderer.h>

is there somewhere a static QWidget variable?

Why soo? Can someone please elaborate?.

luck

Christian Ehrlicher

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

Actually I am new to Qt

This is no excuse to not install the message handler and set a breakpoint there in the debugger.

mrjj

Hi
Using a message handler is the easiest way to find out what generates this.
Its not very complicated. Example here.

https://stackoverflow.com/questions/4954140/how-to-redirect-qdebug-qwarning-qcritical-etc-output

Please_Help_me_D

Hi,

I used to work with VTK 9.0.0 and everything was fine. Now I've built VTK 9.0.1 and I get the same error QWidget: Must construct a QApplication before a QWidget in my app.

More detailled.
VTK 9 has QVTKOpenGLNativeWidget wich is used as a widget where all the graphics is displayed.
I don't use QDesigner but instead I simply set QVTKOpenGLNativeWidget in layout and display it. But with new VTK 9.0.1 on the line where I create this private var widget vtkWidget = new QVTKOpenGLNativeWidget(); my program is stopped and the debugger leads me to the QVTKOpenGLNativeWidget.cxx file as shown on the pciture:

@J-Hilk in all the mentionned files the keyword static is found there:

I use Windows 10 x64, Qt 5.14.0, MSVC 2017

Christian Ehrlicher

@Christian-Ehrlicher said in QWidget: Must construct a QApplication before a QWidget:

To add @JonB suggestions - install a qt message handler, add a breakpoint there and see where it comes from

...

Please_Help_me_D

@Christian-Ehrlicher I havn't ever used qt message handler, but my main.cpp looks like:

#include <pybind11/embed.h> // everything needed for embedding
#include <pybind11/pybind11.h>
#include <pybind11/stl.h>
namespace py = pybind11;

#include "msvapplication.h"
#include "mainwindow/mainwindow.h"

#include <QSurfaceFormat>
#include <QVTKOpenGLStereoWidget.h>

void myMessageOutput(QtMsgType type, const QMessageLogContext &context, const QString &msg)
{
    QByteArray localMsg = msg.toLocal8Bit();
    const char *file = context.file ? context.file : "";
    const char *function = context.function ? context.function : "";
    switch (type) {
    case QtDebugMsg:
        fprintf(stderr, "Debug: %s (%s:%u, %s)\n", localMsg.constData(), file, context.line, function);
        break;
    case QtInfoMsg:
        fprintf(stderr, "Info: %s (%s:%u, %s)\n", localMsg.constData(), file, context.line, function);
        break;
    case QtWarningMsg:
        fprintf(stderr, "Warning: %s (%s:%u, %s)\n", localMsg.constData(), file, context.line, function);
        break;
    case QtCriticalMsg:
        fprintf(stderr, "Critical: %s (%s:%u, %s)\n", localMsg.constData(), file, context.line, function);
        break;
    case QtFatalMsg:
        fprintf(stderr, "Fatal: %s (%s:%u, %s)\n", localMsg.constData(), file, context.line, function);
        break;
    }
}

int main(int argc, char *argv[])
{
    qInstallMessageHandler(myMessageOutput);
    // needed to ensure appropriate OpenGL context is created for VTK rendering.
    QSurfaceFormat::setDefaultFormat(QVTKOpenGLNativeWidget::defaultFormat());

    std::wstring pythonHome( L"C:/Anaconda3/envs/MyEnv38" );
    Py_SetPythonHome(pythonHome.c_str());
    py::scoped_interpreter guard{}; // start the interpreter and keep it alive

    qDebug();
    MSVApplication a(argc, argv);
    qDebug();
    MainWindow w;

    w.show();

    return a.exec();
}

If set a breakpoint anywhere inside myMessageOutput then the app never reaches this function. Probably I dont understand the idea of message handler?

mrjj

@Please_Help_me_D
Hi
It looks good.

can you have a breakpoint and do
qDebug() << "Hello" and see if it then stop then?

The idea of the handler, is that it will then be our code for and used by qDebug function in any code.

So if the "Must construct a QApplication before a QWidget:"
is shown in the output panel, then we should be able to catch it.

Please_Help_me_D

@mrjj hi,

Yes, now debugger goes to myMessageOutput:

after I press F10 it goes there:

mrjj

Hi
Ok good than its working.
The trick is now to see if its stops on the error "Must construct a QApplication before a QWidget:"
(i hope before it crashes)

When it stops and msg is the right one, you can inspect the call stack
alt text

and see where it comes from.

Please_Help_me_D

@mrjj here are two screenshots
first screenshot - we haven't reach the error:

After that I press F10 and we are now in VTK source code. We still have not reached the error:

Then I press F10 once again and error appears:

Christian Ehrlicher

So do you actually create a QApplication before instantiating VolumeWidget? I would guess no.

Please_Help_me_D

@Christian-Ehrlicher I think so. Here is my main function:

int main(int argc, char *argv[])
{
    qInstallMessageHandler(myMessageOutput);
    // needed to ensure appropriate OpenGL context is created for VTK rendering.
    QSurfaceFormat::setDefaultFormat(QVTKOpenGLStereoWidget::defaultFormat());

    std::wstring pythonHome( L"C:/Anaconda3/envs/MyEnv38" );
    Py_SetPythonHome(pythonHome.c_str());
    py::scoped_interpreter guard{}; // start the interpreter and keep it alive

/* Here I create my application */
    qDebug() << "Hello";
    MSVApplication a(argc, argv); // This Class is inherited from QApplication
    qDebug() << "Bye bye :)";
    MainWindow w;


    w.show();
    return a.exec();
}

and VolumeWidget is instantiated inside MainWindow w or more precisely in some class inside MainWindow

Christian Ehrlicher

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

MSVApplication

Please show the ctor

Please_Help_me_D

@Christian-Ehrlicher I didn't understood what is the "ctor" but I think you wanted to see MSVApplication:

Header:

#ifndef MSVAPPLICATION_H
#define MSVAPPLICATION_H

#include "core/h5core.h"

#include "DockManager.h"
#include "DockWidget.h"
#include "DockWidgetTab.h"
#include "DockAreaWidget.h"
#include "DockAreaTitleBar.h"
#include "DockAreaTabBar.h"
#include "FloatingDockContainer.h"
#include "DockComponentsFactory.h"

#include <QApplication>

class AppSettings;

class MSVApplication : public QApplication
{
    Q_OBJECT

public:
    MSVApplication(int &argc, char **argv);
    ~MSVApplication() override;

    int findVisDockByName(QString dockName);

    void setDockManager(ads::CDockManager* dockManager);

    ads::CDockManager* getDockManager();
    ads::CDockWidget* getCurrentVisDock();
    QList<ads::CDockWidget*> getVisDockList();
    h5core::WindowType getCurrentWindowType();
    int getCurrentVisDockNum();
    int getVisDockListCount();

signals:
   void currentVisDockChanged(ads::CDockWidget* dockWidget, int num);
   void visDockAdded(ads::CDockWidget* dockWidget, int num);
   void visDockRemoved(ads::CDockWidget* dockWidget, int num);

private slots:
   void onFocusedDockChanged(
           ads::CDockWidget* old, ads::CDockWidget* now);
   void onDockAdded(ads::CDockWidget* visDock);
   void onDockAboutToBeRemovedRemoved(ads::CDockWidget* visDock);

private:
   ads::CDockManager* dockManager;
   int currentVisDockNumber = -1;
   QList<ads::CDockWidget*> visDockList;
   AppSettings* appSettings;
};

#endif // MSVAPPLICATION_H

Implementation:

#include "msvapplication.h"

#include "core/appsettings.h"

MSVApplication::MSVApplication(int &argc, char **argv):
    QApplication(argc, argv)
{
    appSettings = new AppSettings();
}

MSVApplication::~MSVApplication(){
    delete appSettings;
}

int MSVApplication::findVisDockByName(QString dockName){
    for (int i = 0; i < visDockList.count(); i++){
        if (visDockList[i]->windowTitle() == dockName)
            return i;
    }
    return -1;
}

void MSVApplication::setDockManager(ads::CDockManager* dockManager){
    this->dockManager = dockManager;

    connect(dockManager, &ads::CDockManager::focusedDockWidgetChanged,
            this, &MSVApplication::onFocusedDockChanged);
    connect(dockManager, &ads::CDockManager::dockWidgetAdded,
            this, &MSVApplication::onDockAdded);
    connect(dockManager, &ads::CDockManager::dockWidgetAboutToBeRemoved,
            this, &MSVApplication::onDockAboutToBeRemovedRemoved);
}

ads::CDockManager* MSVApplication::getDockManager(){
    return this->dockManager;
}

ads::CDockWidget* MSVApplication::getCurrentVisDock(){
    return dockManager->focusedDockWidget();
}

QList<ads::CDockWidget*> MSVApplication::getVisDockList(){
    return visDockList;
}

h5core::WindowType MSVApplication::getCurrentWindowType(){
    if (!getCurrentVisDock())
        return static_cast<h5core::WindowType>(0);

    int windowType = getCurrentVisDock()->property("WindowType").toInt();
    return static_cast<h5core::WindowType>(windowType);
}

int MSVApplication::getCurrentVisDockNum(){
    return currentVisDockNumber;
}

int MSVApplication::getVisDockListCount(){
    return visDockList.count();
}

void MSVApplication::onFocusedDockChanged(
        ads::CDockWidget* old, ads::CDockWidget* now){
    int num = -1;
    for (int i = 0; i < visDockList.count(); i++){
        if (now == visDockList[i]){
            num = i;
            break;
        }
    }

    if (num == -1)
        return;

    currentVisDockNumber = num;
    emit currentVisDockChanged(now, currentVisDockNumber);

    qDebug() << now->windowTitle();
}

void MSVApplication::onDockAdded(ads::CDockWidget* visDock){
    if (!visDock->features()
            .testFlag(ads::CDockWidget::DockWidgetFocusable))
        return;

    visDockList.push_back(visDock);
    emit visDockAdded(visDock, visDockList.count()-1);
}

void MSVApplication::onDockAboutToBeRemovedRemoved(ads::CDockWidget* visDock){
    int num = -1;
    for (int i = 0; i < visDockList.count(); i++){
        if (visDock == visDockList[i]){
            num = i;
            break;
        }
    }

    if (num == -1)
        return;

    visDockList.removeAt(num);
    emit visDockRemoved(visDock, num);
}

By the way my VolumeWidget is able to create QWidget without problem:

void VolumeWidget::init(){
    sortWidget = new SortWidget();

/* no preblem to create widget here */
    QWidget* myWidget = new QWidget();

/* but when creating VTK widget I get error */ 
    vtkWidget = new QVTKOpenGLNativeWidget();
    vtkWidget->setEnableHiDPI(true);

    QVBoxLayout* mainVLayout = new QVBoxLayout(this);
    mainVLayout->setContentsMargins(0, 0, 0, 0);
    mainVLayout->addWidget(sortWidget);
    mainVLayout->addWidget(vtkWidget);
}

IMPORTANT! I just remembered one thing from VTK documentation. There is said something about Qt restriction. Please look general info about these two classes (QVTKOpenGLStereoWidget and QVTKOpenGLNativeWidget each of those can be replaced by another) from these two links:
https://vtk.org/doc/nightly/html/classQVTKOpenGLStereoWidget.html#details
https://vtk.org/doc/nightly/html/classQVTKOpenGLNativeWidget.html#details

Cobra91151

@Please_Help_me_D

Hello! The ctor means constructor. I would suggest to add this code to your main.cpp.

#ifdef QT_DEBUG
    qputenv("QT_FATAL_WARNINGS", "1");
    qputenv("QT_MESSAGE_PATTERN",
            "Type: %{type}\nProduct Name: %{appname}\nFile: %{file}\nLine: %{line}\nMethod: %{function}\nThreadID: %{threadid}\nThreadPtr: %{qthreadptr}\nMessage: %{message}");
#endif

It will work only in the debug mode, so you can properly identify what causes that issue. I had the similar issue but with "QPixmap: Must construct a QGuiApplication before a QPixmap", this was really annoying issue, because I could not identify it properly, but when I have used the code above I have found out what causes it and fixed it. The actual problem with my issue was that QPixmap/QIcon was created in the QThread worker class and added using signals/slots to main thread. I have fixed it by sending only paths for those icons from the worker class and creating the actual icons in the main thread. Also, feel free to check out the Qt source code for this error (the above code will help you find the actual file), it will help you understand what causes it and how to fix it.

Please_Help_me_D

@Cobra91151 thank you for help.
I just set those few lines of code in my main(...) func:

int main(int argc, char *argv[])
{
#ifdef QT_DEBUG
    qputenv("QT_FATAL_WARNINGS", "1");
    qputenv("QT_MESSAGE_PATTERN",
            "Type: %{type}\nProduct Name: %{appname}\nFile: %{file}\nLine: %{line}\nMethod: %{function}\nThreadID: %{threadid}\nThreadPtr: %{qthreadptr}\nMessage: %{message}");
#endif

    qInstallMessageHandler(myMessageOutput);
    // needed to ensure appropriate OpenGL context is created for VTK rendering.
    QSurfaceFormat::setDefaultFormat(QVTKOpenGLNativeWidget::defaultFormat());

    std::wstring pythonHome( L"C:/Anaconda3/envs/MyEnv38" );
    Py_SetPythonHome(pythonHome.c_str());
    py::scoped_interpreter guard{}; // start the interpreter and keep it alive

    qDebug() << "Hello";
    MSVApplication a(argc, argv);
    qDebug() << "Bye bye :)";
    MainWindow w;

    w.show();
    return a.exec();
}

As I understood from the documentation qputenv() sets environment variable but I can't find those two variables (QT_FATAL_WARNINGS and QT_MESSAGE_PATTERN) neither in Qt project settings or Windows environment variables.
Could you explain how to get profit from those lines of code?

jsulm

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

I can't find those two variables

You have to set them.
"Qt project settings or Windows environment variables" - of course not as those are neither project settings nor Windows environment variables (Windows does not know anything about these two variables). Those are environment variable which you can set (either in QtCreator in "Run environment" or in the terminal where you start your app).