QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget"

[jronald]

int main(int argc, char *argv[])
{
    MyApp a(argc, argv);
    
    QtTreePropertyBrowser * p = new QtTreePropertyBrowser;
    
    return 0;
}

built successfully, run time error: QWidget: Must construct a QApplication before a QWidget
tried both debug & release versions of QtPropertyBrowser

btw, the QtPropertyBrowser examples with the lib work

version info:
Qt 5.12.0
QtCreator 4.8.0
QtPropertyBrowser head at https://github.com/qtproject/qt-solutions.git
kit MSVC2017 64bit

[dheerendra]

What is MyApp object ? Looks like it is not inherited from QApplication ? Before constructing any QWidget based object, you must have QApplication created. In your case it not. Hence you have this run time error & application peacefully crashes.

[jronald]

@dheerendra said in QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget":

What is MyApp object ?

class MyApp : public QApplication
{
    Q_OBJECT
public:
    explicit MyApp (int &argc, char **argv);
    
signals:
    
public slots:
};

[dheerendra]

This seems to be ok. Event loop is not started. Can you add it ? a.exec() instead of return 0.

[jronald]

@dheerendra said in QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget":

This seems to be ok. Event loop is not started. Can you add it ? a.exec() instead of return 0.

added return a.exe();, crashed before reaching here

[dheerendra]

Instead of MyApp can you try creating the QApplication object ? Do you have any global object of QtTreePropertyBrowser somewhere else ?

[jronald]

@dheerendra said in QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget":

Instead of MyApp can you try creating the QApplication object ? Do you have any global object of QtTreePropertyBrowser somewhere else ?

tried, can't solve
when debugging, it can execute until the line of code to create QtTreePropertyBrowser

[dheerendra]

Strange. Can u just create qpushbutton object & see it works ? Comment ur treebrowser code.

[jronald]

@dheerendra said in QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget":

Strange. Can u just create qpushbutton object & see it works ? Comment ur treebrowser code.

everything else works

[SGaist]

Hi,

Do you have any static QWiget object ?

[jronald]

@SGaist said in QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget":

Do you have any static QWiget object ?

no

[dheerendra]

If possible can you share your project bat pastebin or somewhere else ?

[Maaz Momin]

@jronald I hope in the constructor definition you are passing the parameter to QApplication.

MyApp (int &argc, char **argv) : QApplication(argc, argv)
{
...
}

[jronald]

@dheerendra said in QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget":

If possible can you share your project bat pastebin or somewhere else ?

1. create a new Qt Widget Application
2. modify .pro to add INCLUDEPATH and LIBS for QtPropertyBrowser
3. edit MainWindow.cpp

#include "MainWindow.h"
#include "ui_MainWindow.h"
#include <QtTreePropertyBrowser>

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

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

Application Output Window shows: QWidget: Must construct a QApplication before a QWidget

[jronald]

@Maaz-Momin said in QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget":

@jronald I hope in the constructor definition you are passing the parameter to QApplication.

MyApp (int &argc, char **argv) : QApplication(argc, argv)
{
...
}

sure, it's by default

[Maaz Momin]

@jronald Can you paste your MyApp.h and MyApp.cpp here. Also If possible your .pro file.

[jronald]

@Maaz-Momin said in QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget":

@jronald Can you paste your MyApp.h and MyApp.cpp here. Also If possible your .pro file.

I've created a new Qt Widget Application for testing.

[SGaist]

@jronald said in QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget":

QtTreePropertyBrowser

Where did you get that class from ?

[jronald]

@SGaist said in QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget":

Where did you get that class from ?

QtPropertyBrowser head at https://github.com/qtproject/qt-solutions.git

[jronald]

@SGaist said in QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget":

@jronald said in QtTreePropertyBrowser runtime error: "QWidget: Must construct a QApplication before a QWidget":

QtTreePropertyBrowser

Where did you get that class from ?

Tried https://code.qt.io/qt-solutions/qt-solutions.git, to my surprise it works.
Thank you very much

2 points to mention:

• debug/release must be matched for app and the lib
• clean before build (rebuild may be ok, not tried), or delete the build dir manully