QPainter. Draw line

qt qpainter
Ufx picture Ufx · Jul 10, 2014 · Viewed 36.9k times · Source

I am trying to draw line.

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

    QPainter painter(&w);
    painter.setPen(QPen(Qt::black, 12, Qt::DashDotLine, Qt::RoundCap));
    painter.drawLine(0, 0, 200, 200);

    return a.exec();
}

But there is nothing painting on the window. What is wrong?

Answer

vahancho picture vahancho · Jul 10, 2014

You can not paint outside of the paintEvent() function, at least on Windows and Mac OS. However you can override your MainWindow class' paintEvent() function to draw the line there. For example:

class Widget : public QWidget
{
protected:
    void paintEvent(QPaintEvent *event)
    {
        QPainter painter(this);
        painter.setPen(QPen(Qt::black, 12, Qt::DashDotLine, Qt::RoundCap));
        painter.drawLine(0, 0, 200, 200);
    }
};

And the usage:

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

    Widget w;
    w.show();
    [..]

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