[Solved] QPainter and dynamic list of items



  • I use a class inherited from QQuickPaintedItem to draw data curves in Qml.
    The number of elements is unknown. And after 5s all data becomes invalid and a new data set starts.

    Therefore I have:
    @QVector<QLineF*> m_curve;@

    and a slot to receive data:
    @void Diagram::addDataPoint(int value1)
    {
    // Calculate line points
    ...
    QLineF* m_curveElement = new QLine(startPoint, endPoint);
    m_curve.append(m_curveElement)

    update(); // trigger redraw
    

    }@

    and paint:
    @void Diagram::paint(QPainter painter)
    {
    for (QLineF
    line : m_curve)
    {
    painter->drawLine(*line);
    }
    // qDeleteAll(m_curve); --> Exception

    }@

    • But this creates a memory leak, because there is no delete at all.
    • But if I delete the lines in paint() I get an exception.

    Question: When can I delete the created lines?



  • I found the following solution in my resetData method:

    @void Diagram::resetData()
    {
    QVector<CMyLine*> m_tmpCurveFlow(m_curve);
    m_curve.clear();
    this->update();
    qDeleteAll(m_tmpCurve);
    }@

    Any comments?


  • Lifetime Qt Champion

    Hi,

    Why not just use QVector<QLine> ?



  • Because this made a copy of every QLine, doesn't it?



  • And why is that a problem to copy two ints or two doubles?

    Edit: and in the paint method, you can use references....



  • Just to avoid to do an unnecessary step.
    But I tried it out and performance is the same.
    Thanks!



  • Don't try to optimize if it is not 100% needed. Copying a QLine (just 4? ints) is not expensive.


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