[Solved] QPainter and dynamic list of items

th.thielemann

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?

th.thielemann

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?

SGaist

Hi,

Why not just use QVector<QLine> ?

th.thielemann

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

giesbert

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

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

th.thielemann

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

giesbert

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