draw.pro
<code>
QT      += core gui widgets
CONFIG  += c++11
SOURCES += mainwindow.cc
HEADERS +=
</code>

mainwindow.cc
<code C++>
#include <QApplication>
#include <QPainter>
#include <QWidget>

class Window : public QWidget
{
public:
    Window (QWidget * parent = nullptr);
    ~Window ();
    virtual void paintEvent (QPaintEvent *event);
};

void Window::paintEvent (QPaintEvent * event)
{
    QPainter painter;
    painter.begin (this);

    int w = this->width ();
    int h = height ();

    painter.setPen (QColor::fromRgb (255, 0, 0));
    painter.drawLine(0, 0, w-1, 0);

    painter.setPen (QColor::fromRgb (0, 255, 0));
    painter.drawLine(w-1, 0, w-1, h-1);

    painter.setPen (QColor::fromRgb (0, 0, 255));
    painter.drawLine (w-1, h-1, 0, h-1);

    painter.setPen (QColor::fromRgb (255, 200, 0));
    painter.drawLine (0, h-1, 0, 0);

    painter.end ();
}

Window::Window (QWidget * parent)
   :  QWidget (parent)
{
}

Window::~Window ()
{
}

int main (int argc, char *argv[])
{
    QApplication a (argc, argv);
    Window w;
    w.show ();
    return a.exec ();
}
</code>