#include <QApplication>
#include <QtWidgets>

class CustomWid : public QWidget {
public:
    CustomWid()
    {
        setFixedSize(100,100);
    }
protected:
    void paintEvent(QPaintEvent* evt){
        QPainter painter(this);

        painter.setCompositionMode(QPainter::CompositionMode_SourceOut);

        painter.setRenderHint(QPainter::Antialiasing, false);

        painter.drawLine(50, 50, 20, 20);

    }
};


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
//    QMessageBox b;
//    b.exec();
    QWidget w;
    auto lay = new QVBoxLayout(&w);
    lay->addWidget(new QPushButton("test"));
    lay->addWidget(new CustomWid);
    w.show();
    return a.exec();
}