//! [0]
class MyHelpHandler : public QObject
{
    Q_OBJECT
public:
    ...
public slots:
    void showHelp(const QUrl &url);
};

QDesktopServices::setUrlHandler("help", helpInstance, "showHelp");
//! [0]

//! [1]
mailto:user@foo.com?subject=Test&body=Just a test
//! [1]

//! [2]
QDesktopServices::openUrl(QUrl("file:///C:/Documents and Settings/All Users/Desktop", QUrl::TolerantMode));
//! [2]