Totally deleting the QApplication Instance and recreating it in another thread

Question

How can I totally remove the QApplication instance, so it can be possible to recreate it again in a sheared library. If I have such a following code:

int main(int argc, char ** argv) {
QApplication *app = new QApplication(argc, argv);
MyWindow dialog;
dialog.show();
app->exec();
return 0
}

I want to be able to delete the instance after quiting the app (closing the application after app.exec()) I have tried to do the following:

app->setQuitOnLastWindowClosed(false);
app->quit();
delete app;

But non of them could be worked. I want to remove the QApplication instance like I did not create before. The QApplication in my shared library always works if I did not define the QApplication in the main.

1. The library (lib) and the main application (app) are using different copy of Qt.
2. I am developing the app, the lib is imported to my app.
3. After closing the first dialog, I am using an event to trigger the thread in the library which it is waiting for this event.

Answer 1

You don't need to do that, because QApplication can only ever be created in the main thread. If the library and the main executable share the same Qt library, presumably the main application already created the QApplication object and will run the event loop for you. So the only thing you need to do in the library is to create some objects and the event loop will process their events.

If the main executable doesn't use Qt, then you should create the application in the main thread, prime its event loop, and let the native event loop dispatch events for you.

Alas, to directly answer your question: presumably you want to do some stuff after app.exec() returns. Your code could simply be, then:

int main(int argc, char ** argv) {
  {
     QApplication app{argc, argv};
     MyWindow dialog;
     dialog.show();
     app.exec();
   }
   doSomething();
}

If I close the first dialog, the event loop will be left. Is it possible to keep the event loop?

Yes. You could clear the Qt::WA_QuitOnClose attribute of the first dialog. Or, preferably, set it on your dialog - the application won't quit as long as there are any dialogs with QT::WA_QuitOnClose still open.

Thus, for the second dialog, call:

secondDialog->setAttribute(Qt::WA_QuitOnClose);

Alternatively, if you don't have a pointer to the dialog, you can iterate all top-level widgets and clear (or set) the attribute on all of them:

for (auto widget : qApp->topLevelWidgets())
  widget->setAttribute(Qt::QA_QuitOnClose, false);