can't close app from dialog box (Qt app)

Question

Getting right to it I have a MainWindow and a dialog window which is executed if a condition is met but the problem is I can't get the app to quit if the cancel button from the dialog window is clicked. I've tried putting qApp->quit() in the slot function for the cancel button. I've tried connecting the cancel button slot to the predefined close slot for the MainWindow object via a clickme() signal from the dialog class. (as shown below)

qt application does not quit I read the answer to this question which I think got me close because it made me realize that I can't quit the app before showing the MainWindow but making that change didn't solve the problem. I even tried to explicitly emit the clickme() signal from cancel button slot but that actually caused the OS to throw a signal which threw an error at me saying "the inferior stopped because it received a signal from the operating system signal name: SIGSEGV signal meaning: segmentation fault"

Here's my code:

Notice warning; // this is the object for the dialog window also all of this code is in main.cpp
 warning.setModal(true);
    QObject::connect(&warning, SIGNAL(clickme()), &warning, SLOT(on_Cancel_button_clicked()));
    QObject::connect(&warning, SIGNAL(clickme()), &w, SLOT(close()));
    warning.exec();

Also before that code is

MainWindow w;
w.show();

Also while writing this question I tried this

QObject::connect(&warning, SIGNAL(clickme()), qApp, SLOT(quit()));

But that still didn't work. If you need more info just let me know.

Update: I'm starting to think that the reason I'm having so much trouble with this connect signal/slot function is because it's not designed to connect two windows of two different classes and I should rework my app to do everything from the MainWindow class which is a shame because when I picture a GUI program I picture multiple windows connected to each other regardless of whether or not the object representing each window is from the same class as the others yet I have such a hard time trying do that with the QT framework when it comes to trying to connect objects of different classes.

Update: please forgive me. I assume that the code that I originally thought was the answer would work and took a break from working on the program before actually testing out that code. Going back to it now I discovered that it doesn't work. The code I'm referring to is the following

QMessageBox msg;
    msg.setText("Continue?");
    msg.addButton(QMessageBox::Yes);
    msg.addButton(QMessageBox::No);
    QObject::connect(&msg, &QMessageBox::rejected,
                     &app, &QApplication::quit); // this line doesn't work for me and I don't know why
    QObject::connect(&msg, &QMessageBox::accepted, [&dlg]{
      (new QLabel("I'm running")).show();
    });

Answer 1

QApp->quit(); should work. Remove warning.setModal(true); The dialog becomes modal when you call exec(). SetModal(true) should be used with show() according to Qt docs. So this may be causing your problem.

Answer 2

I think I've found the problem.

Probably, you're calling exec() twice:

1. To enter the QApplicationevent loop
2. To execute the dialog.

Use show() instead of exec() for the dialog. You have an example below where you can check the signal/slot works well. In your application, you need the slot to close the window, but:

1. With the line of code dialog.exec();, the app keeps running. This is your issue.
2. With the line of code dialog.show();, the app stops.

By the way, I saw your last question update, but it is not correct. In fact, of course you can connect two different classes.

window.h

#ifndef WINDOW_H
#define WINDOW_H

#include <QApplication>
#include <QMainWindow>
#include <QAbstractButton>
#include <QDebug>
#include "dialog.h"

class Window : public QMainWindow
{
Q_OBJECT

public:
    Window()
    {
        dialog = new Dialog();
        dialog->setText("Continue?");
        dialog->addButton(QMessageBox::Yes);
        dialog->addButton(QMessageBox::No);
        auto onClick = [this]() {
            auto role = dialog->buttonRole(dialog->clickedButton());
            if (role == QMessageBox::NoRole) {
                qDebug() << "QMessageBox::NoRole";
                QApplication::quit();
            }
            if (role == QMessageBox::YesRole) {
                qDebug() << "QMessageBox::YesRole";
            }
        };

        QObject::connect(dialog, &QMessageBox::buttonClicked, onClick);

        dialog->show(); // this must be show(), not exec()
    }
    virtual ~Window() { delete dialog; }
private:
    Dialog *dialog;

public slots:
    void windowSlot() { qDebug() << Q_FUNC_INFO;
                        close();
                      }
};

#endif // WINDOW_H

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QMessageBox>

class Dialog : public QMessageBox
{
Q_OBJECT

public:
    Dialog() {}
    virtual ~Dialog() {}
};

#endif // DIALOG_H

main.cpp

#include <QApplication>
#include <QtGui>
#include "window.h"

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    Window window;
    window.setWindowTitle("window");
    window.show();

    return app.exec();
}

Update #1: a very interesting post.