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#include <QtGui/QApplication>
#include <QtGui/QMainWindow>
#include <QtGui/QDialog>
#include <QtGui/QPushButton>

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow()
    {
        QPushButton *button = new QPushButton( "Show dialog", this );
        connect( button, SIGNAL(clicked()), SLOT(onShowDialog()) );
    }

protected slots:
    void MainWindow::onShowDialog()
    {
        QDialog *pDialog = new QDialog( this );
        pDialog->setWindowModality( Qt::WindowModal );
        pDialog->show();
        pDialog->activateWindow();
    }
};

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