Solved Why doesn't QApplication offer a function mainWindow() ?

[Joachim W]

Since more than 10 years, the question has been discussed in many threads, here, at Stackoverflow, and elsewhere, how to get from qApp to the application's QMainWindow. Proposed solutions are horrible workarounds. Would the Qt developers consider adding a function

QMainWindow* QApplication::mainWindow()

or are there unsurmountable reasons why such a function cannot or should not exist?

[JonB]

@Joachim-W
I do understand you are asking about why Qt does not provide it. I use my own (Python/PyQt)

def findMainWindow() -> typing.Union[QMainWindow, None]:
    # Global function to find the (open) QMainWindow in application
    app = QApplication.instance()
    for widget in app.topLevelWidgets():
        if isinstance(widget, QMainWindow):
            return widget
    return None

which is not too bad, just checking you know it can be as simple as that?

[J.Hilk]

@Joachim-W said in Why doesn't QApplication offer a function mainWindow() ?:

or are there unsurmountable reasons why such a function cannot or should not exist?

because having a QMainWindow is not mandatory for a QApplication ? Or even that there's only 1 QMainWindow instance

[Joachim W]

@J.Hilk well, then a function QList<QMainWindow*> QApplication::mainWindows() would be nice to have ...

[J.Hilk]

@Joachim-W there is
like @JonB said
https://doc.qt.io/qt-5/qapplication.html#topLevelWidgets

[JonB]

@Joachim-W
I suspect that Qt does not need to keep a list of main windows for itself, so it's not going to offer one to you. It simply has a bunch of topLevelWidgets, main windows and others (e.g. closing the last of whatever these are induces program exit). Hence that code is all there is to detect which ones are QMainWindow instances.

You & I may think there is something special about QMainWindow, and even that there should be only one of them, but to Qt they are just one more instance of a QWidget, with some furniture :) I see that the solution to https://stackoverflow.com/questions/318641/multiple-qmainwindow-instances discusses this.

[J.Hilk]

@Joachim-W here's a simple c++ adaptation of @JonB function,

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    const QWidgetList &list = QApplication::topLevelWidgets();

    for(QWidget * w : list){
        QMainWindow *mainWindow = qobject_cast<QMainWindow*>(w);
        if(mainWindow)
            qDebug() << "MainWindow found" << w;
    }

    return a.exec();
}