QWidget: Must construct a QApplication before a QPaintDevice crash error

[eric_vi]

i am having 2 different plug-ins that work inside a non QT application... as long as i stay in debug mode all fine... when i go to release and deploy them... as long as i use the same plug in all fine...
when i start switching from one of the plug-in to the other i start to get this QWidget: Must construct a QApplication before QPaintDevice ... i am sure my QApplication has been created before i checked it out

it sounds more like the frameworks which are exactly the same (but dynamic, deployed and independent for each plug-in) are not unloaded and create a conflict

I taht possible? how can you make sure when you quit one plug-in that use QT frameworks without quitting the application that has launched the plug-in that there cannot be conflict of frameworks with a subsequent plug-in that is launched in the same session

[ChrisW67]

I have no idea what, "I taht possible?" is referring to or, indeed, much of what your post is trying to say.

What do you mean by "plug-ins that work inside a non QT application"? Are you talking about ActiveX controls on Windows or something else?
What do you mean by , "switching from one of the plug-in to the other"? Do you mean moving focus between widgets presented by the "plug-ins", loading and unloading the "plug-ins", or something else?

Where and how is the (singular) QApplication instance created? Where and how is that QApplication instance destroyed?

[high_flyer]

I think what he is trying to say is, that he has a non Qt application, which uses plugins which link to Qt.
This will also account to the sort of the problem he (probably) refers to regarding the construction of a QApplication.

@OP
Do you need to use Qt GUI facilities or services that need an event loop?
Consider very good what you relay need in your Qt plugins, it might be that you can get rid of all the event loop dependent code, which will free you from the need to have an instantiated QApplication/QCoreApplication in your plugins.

[eric_vi]

Hello again, thank you,

yes i have non Qt application (not made by me so i have no control over it. This application can call plugins following a user interaction. So a user can decide to launch a plug-in... Those plug-in use QT and do have a user interface and multiple QT frameworks are needed ( i do not use static QT frameworks)

i have sub classed QApplication

class myQApplication : public QApplication
{
public:
    myQApplication(int &argc, char **argv);
 
protected:
        bool    event(QEvent *);
 
};

class myQApplication : public QApplication
{
public:
    myQApplication(int &argc, char **argv);

protected:
        bool    event(QEvent *);

};

To copy to clipboard, switch view to plain text mode 

and as soon as the plug-in starts before any event loop i call
my_qapp = new myQApplication(argc, argv);


then my event loop

when i quit the plug-in i do
delete my_qapp;


All this works perfectly well in debug mode..

when i go to release i need to deploy it so each plug-in has its own copy of the frameworks.

In release mode, as long as i use one plug-in, ii can do multiple launch and quit, all works fine. The problem occurs when i quit one of the plug-in and launch the second one... i got this error message
and in console i can see some message that says that the framework of plug-in number 1 or the framework or plug-in number 2 might be used and that is where i am getting really confused.

[high_flyer]

There are some things you are not saying - for example, where/when (if at all?) you start the the QApplication event loop? (exec())


Try calling quit() on your QApplication before you delete it in your DLL, see if this helps.

[eric_vi]

the application uses only one plug-in at a time..


there are not many steps.. the crash occurs during the initialisation of my QMainWindow
as i am giving a minimum and a maximum size (setMinimumSize...)

so i have not reached the even loop yet

{
    MainWindow *w;
 
    my_qapp = new myQApplication(argc, argv);
 
    // MainWindow is a sub class of QMainWindow
    w = new MainWindow();
 
    my_qapp->exec();
 
 
}

{
    MainWindow *w;

    my_qapp = new myQApplication(argc, argv);
	
    // MainWindow is a sub class of QMainWindow
    w = new MainWindow();
	
    my_qapp->exec();


}

To copy to clipboard, switch view to plain text mode 

i do have any quit() when i delete my QApplication at exit time i will try that

[amleto]

{
    MainWindow *w;
 
    my_qapp = new myQApplication(argc, argv);
 
    // MainWindow is a sub class of QMainWindow
    w = new MainWindow();
 
    my_qapp->exec();
}

{
    MainWindow *w;

    my_qapp = new myQApplication(argc, argv);
	
    // MainWindow is a sub class of QMainWindow
    w = new MainWindow();
	
    my_qapp->exec();
}

To copy to clipboard, switch view to plain text mode 

what is this I dont even...

correct:

int main(int argc, char** argv) // please include method name etc so we have some context!!
{ 
    myQApplication my_qapp(argc, argv);
 
    // MainWindow is a sub class of QMainWindow
    MainWindow w;
    w.show()
 
    my_qapp.exec();
}

int main(int argc, char** argv) // please include method name etc so we have some context!!
{ 
    myQApplication my_qapp(argc, argv);
 
    // MainWindow is a sub class of QMainWindow
    MainWindow w;
    w.show()
 
    my_qapp.exec();
}

To copy to clipboard, switch view to plain text mode 

This is not java, dear.

[eric_vi]

i don't know about java... i had it this way before but because of this bug i was not exactly sure where the objects would be created so i tried with a 'new' allocator to see if it was making any difference

[eric_vi]

i tried to put a quit() when deleting the QApplication... this does not make any change...

when within the same session i quit one plug-in to launch another (each of those plug-in are deployed and have their own set of frameworks) I get this QWidget: Must construct a QApplication before a QPaintDevice crash error... i am running on Mac OS and if i examine the console log i can see that during the launch of plugin 2 i get multiple warnings such as

objc[722]: Class QNSStatusItem is implemented in both /Library/Application Support/plugin1/Frameworks/QtGui.framework/Versions/4/QtGui and /Library/Application Support/plugin2/Frameworks/QtGui.framework/Versions/4/QtGui. One of the two will be used. Which one is undefined.


i really have no clue why the frameworks from the previous plug-in launch can still be used!

[high_flyer]

Without seeing the real code its hard to tell.

[eric_vi]

i have described what the code is doing there is not much more to it

but let me describe again my issue, hopefully someone can give me some hints, because i am totally stuck...


i am in a deployed configuration (in debug mode i have no problem at all, since i am always using a unique framework).

i have an application (non QT), that launch thru user interaction PlugIn1 (QTmade that has been deployed and point to his own set of QT frameworks), all is fine so far...
the user quits PlugIn1 ( doing so the QApplication from PlugIn1 is deleted and i added a quit()), the Plugin1 process is gone and the user returns to the main application then launch plugIn2 (which is similar to PlugIn 1, deployed with its own set of QT frameworks)...
When launching PlugIn2... i can see in console mode a warning that QT might still use the frameworks of PlugIn1 (!!!!), and of course if it does so i got the QApplication before a QPaintDevice error, since PlugIn1 is no longer there and has been quitted before.

so my question... how can i make sure the qt framework binaries are not going to be used anymore when i quit a plug-in working on QT frameworks that is used within an application, because so far it does not seem to happen for me.