setEnabled(false);

When I click cancel the lambda is called twice.

However I don't understand why the action is triggered even when I just select the menu and not the menu item. Perhaps this merits another question on this forum.

Is it neccessary to use a mutex and flag that the algorithm has been cancelled?

I am setting up the dialog like this now:

dialog = new QProgressDialog(tr("Performing task") + "...", tr("Cancel"), 0, 4, this);
dialog->setLabelText("Updating...");
dialog->setWindowModality(Qt::WindowModal);
connect(task, SIGNAL(advance()), this, SLOT(advance()));
connect(dialog, &QProgressDialog::finished, dialog, &QProgressDialog::deleteLater);
connect(dialog, &QProgressDialog::canceled, this, [=] ()
{
    thread->quit();
    thread->wait();
    dialog->close();
});

I'm showing the dialog like this:

    dialog->setValue(0);
    dialog->show();

I'm advancing the progress bar like this:

void MainWindow::advance()
{
    dialog->setValue(dialog->value() + 1);
    QApplication::processEvents();
}

Another strange problem which I am encountering is that the progress bar / task is sometimes started even when I click on the menu but not the menu item. I am connecting the menu item to the function using:

connect(ui->actionTask, &QAction::triggered, this, &MainWindow::runTask);

signals and slot often a way better choice as it plays nice with the event loop.

Very good! Don't forget to Mark as Solved in Topic Tools.

I got your example to run... nice! How would I adapt it so that I can update the progress bar from within an algorithm (assuming that is possible):

void myAlgorithm(int n)
{
    int i;
    for (i = 0; i < n; i++)
    {
        someProcessingFunction(i);
        // update the progress bar here!
    }
}

void Dialog::on_showProgress1_clicked()
{
    QProgressDialog *dlg = new QProgressDialog(this);
    connect(dlg, &QProgressDialog::finished, dlg, &QProgressDialog::deleteLater);
    connect(dlg, &QProgressDialog::canceled, this, [=](){
        m_progressVal = 99;  // kind of cheesy, let the timer continue and cleanup
    });

    dlg->setRange(0,100);
    dlg->setWindowTitle("Test");
    dlg->setLabelText("Updating...");
    m_progressVal = 0;

    auto conn = std::make_shared<QMetaObject::Connection>();
    *conn = connect(&m_timer, &QTimer::timeout, this, [=]() {
        m_progressVal++;
        dlg->setValue(m_progressVal);

        if (m_progressVal == dlg->maximum()) {
            m_timer.stop();
            disconnect(*conn);
        }
    });

    dlg->show();

    m_timer.setInterval(100);
    m_timer.start();
}

Yeah. I'm using the version installed with the Mint software manager (5.5), so it's the same version as I'd get with "apt-get install". I don't know how to access Qt via Anaconda as it is not on the Anaconda menu.

In any case, you should first try to build your application with your distribution provided Qt.

But Qt Creator says version 5.5.

Did you install it yourself ?
If not, do you have the same problem with the Qt version provided by Mint ?
What window manager are you using ?

I tried adding more "QApplication::processEvents()", but it didn't make any difference.

I think I will have to create a separate thread for the progress dialogue.

What version of Qt are you using ?
On what platform ?
If Linux:
What distribution ?
What desktop environment ?