Qt: How do I handle the event of the user pressing the ‘X'(close) button ?

Try this solutions:

The QMainWindow  can be  override  the closeEvent method.

#include <QCloseEvent>
void MainWindow::closeEvent (QCloseEvent *event)
{
QMessageBox::StandardButton resBtn = QMessageBox::question( this, APP_NAME,
                                                      tr("Are you sure?\n"),
                                                      QMessageBox::Cancel | QMessageBox::No | QMessageBox::Yes,
                                                      QMessageBox::Yes);
  if (resBtn != QMessageBox::Yes) {
       event->ignore();
} else {
      event->accept();
   }
}

By using  subclassing a QDialog,  closeEvent will be callend and   to override reject():

void MyDialog::reject()
{
QMessageBox::StandardButton resBtn = QMessageBox::Yes;
if (changes) {
resBtn = QMessageBox::question( this, APP_NAME,
                                  tr("Are you sure?\n"),
                                  QMessageBox::Cancel | QMessageBox::No | QMessageBox::Yes,
                                  QMessageBox::Yes);
}
if (resBtn == QMessageBox::Yes) {
    QDialog::reject();
    }
}

Use this:

To override the QWidget::closeEvent(QCloseEvent *event) in class definition and add the code into the function.

class foo : public QMainWindow
{
    Q_OBJECT
private:
    void closeEvent(QCloseEvent *bar);
    // ...
};
 
 
void foo::closeEvent(QCloseEvent *bar)
{
   
      // Do something
      bar->accept();
}

The given simple code is:

void aboutToQuit();

signal of your QApplication. This signal should be raised just before app closes.