tst.cpp

#include <QApplication>
#include <QFileDialog>

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

    QFileDialog dialog(0, "Test Dialog");
    dialog.setConfirmOverwrite(true);
    dialog.setAcceptMode(QFileDialog::AcceptSave);

    // As soon as I uncomment this, to use the Qt build-in dialog,
    // the confirmation dialog is shown when picking an existing file.
    //dialog.setOption(QFileDialog::DontUseNativeDialog);

    dialog.exec();

    return app.exec();
}