#include <QtGui>

class Operation
    : public QObject
{
    Q_OBJECT;
public:
    Operation(QObject *parent); 
    public slots:
        void perform();
        void cancel();
private: 
    int steps;
    QProgressDialog* pd;
    QTimer* t;
};
Operation::Operation(QObject *parent)
    : QObject(parent)
    , steps(1) //NOTE 1, and no dialog is ever shown, 0 and it's ok
{
    pd = new QProgressDialog("Operation in progress.", "Cancel",0,100);
    connect(pd, SIGNAL(canceled()), this, SLOT(cancel()));
    t = new QTimer(this);
    connect(t, SIGNAL(timeout()), this, SLOT(perform()));;
    t->start(100);
}


void Operation::perform()
{
    pd->setValue(steps);
    //... perform one percent of the operation
    steps++;
    if (steps > pd->maximum())
        t->stop();
}

void Operation::cancel()
{
    t->stop();
    //... cleanup
}

class MyWidget : public QWidget
{
    Q_OBJECT
public:
    MyWidget(QWidget *parent = 0) : QWidget(parent)
    {
        QPushButton *pb = new QPushButton("Start", this);
        connect(pb, SIGNAL(clicked()), this, SLOT(start()));
    }
    public slots:
        void start()
        {
            new Operation(this);
        }
};

#include "main.moc"

int main(int argc, char **argv)
{
    QApplication a(argc, argv);
    MyWidget w;
    w.show();
    return a.exec();
}