#include <QtWidgets>

class Widget : public QWidget
{
    Q_OBJECT
public:
    explicit Widget(QWidget *parent = 0)
        : QWidget(parent)
    {
        resize(60, 60);
    }

protected:
    void paintEvent(QPaintEvent *e) {
        QPainter p(this);
        p.fillRect(rect(), Qt::red);
        QPainterPath fill;
        fill.addRect(rect());
        QPainterPath excluded;
        excluded.addRect(20, 20, 20, 20);

        p.setClipPath(fill.subtracted(excluded));
        p.fillRect(10, 10, 40, 40, Qt::blue);
    }
};

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

    Widget widget;
    widget.show();

    return app.exec();
}

#include "main.moc"