I am writing a program that uses QtConcurrent
to start threads. In my case, I use it to render a QGraphicsView when I use the mouse scroll.
I am starting the threads using the following code:
if (future.isRunning()) {
future.cancel();
}
future = QtConcurrent::run(this,&AeVectorLayer::render, renderparams, pos);
watcher.setFuture(future);
When the thread is finished I catch the signal finished
with a QfutureWatcher
.
This is my render function:
QList<AeGraphicsItem*> AeVectorLayer::render(Ae::renderParams renderparams, int pos)
{
AeVectorHandler *coso = new AeVectorHandler();
coso->openDataset(AeLayer::absoluteFilePath);
coso->setTransformCoordinates(myEPSG);
QList<AeGraphicsItem*> bla = coso->getItems(renderparams.sceneRect.x(),
renderparams.sceneRect.y(), renderparams.sceneRect.width(),
renderparams.sceneRect.height(), renderparams.zoom, color, this);
for (int i = 0; i < bla.size(); i++)
bla.at(i)->setZValue((qreal)pos);
delete coso;
return bla;
}
As you can see, I have a QList<QGraphicsItem*>
in my render function. How can I destroy this list when the future is cancelled? I undestand that in my code I am redefining the future
variable but I do not know how to avoid it.
Stop trying to manually manage memory and instead use a smart pointer that fits your use case. Because you use move-unaware QFuture
, you will need an std::shared_ptr
. Once the QFuture
/QFutureWatcher
s go out of scope, and you hold no more shared_ptr
instances, the resource will be deleted. So in your case your render
function should return a QList<std::shared_ptr<AeGraphicsItem>>
. Be careful when you transfer ownership from the shared_ptr
s to e.g. a QGraphicsScene
: you must release
from the shared_ptr
on ownership transfer.
Note that your isRunning
check followed by cancel
is fundamentally flawed: the future could be running when you call isRunning
but be finished by the time you call cancel
. If you want to cancel it, just call cancel
. Also note that you cannot meaningfully cancel QFuture
s returned by QtConcurrent::run
so what you're doing is very very wrong in and of itself.