void MainWindow::draw()
{
    painter->begin(ui->pushButton);
    painter->setPen(Qt::blue);
    painter->drawLine(1,1,20,20);
    painter->end();
}

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

    return a.exec();
}

painter=new QPainter;