#include <QtGui>

class MyWidget: public QWidget
{
public:
    MyWidget(QWidget *parent = 0) : QWidget(parent) {}

    void paintEvent(QPaintEvent *)
    {
        QPainter p(this);

        QPen pen(Qt::black);
        pen.setCapStyle(Qt::FlatCap);
        p.setPen(pen);

        int y = rect().center().y();
        p.drawLine(QPointF(0, y), QPointF(width(), y));
    }
};

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

    MyWidget w;
    w.resize(100, 100);
    w.show();

    return app.exec();
}