QtConcurrent::run with objects that are deleted

Vadi2

I have the following code:

auto future = QtConcurrent::run(this, &XMLexport::saveXml, fileName);
auto watcher = new QFutureWatcher<bool>;
QObject::connect(watcher, &QFutureWatcher<bool>::finished, [=]() { mpHost->xmlSaved(fileName); });
watcher->setFuture(future);

If my QFutureWatcher goes off when mpHost (a QPointer) is already gone, bad things happen. What would be the best way of dealing with this? I don't care about calling xmlSaved() if the mpHost is already gone.

sierdzio

@Vadi2 said in QtConcurrent::run with objects that are deleted:

[=]() { if (mpHost) mpHost->xmlSaved(fileName); }

Vadi2

First thing I tried! It didn't work; I get a crash deep inside Qt then even though debugger says host is 0x0.

sierdzio

Yeah, thought that would be too easy... :/

sierdzio

Ah, try capturing by ref:

[&]() { if (mpHost) mpHost->xmlSaved(fileName); }

Since = captures by value, it may be that it copies the pointer value at the time you call connect() (so when object is still alive), which is no longer true later. Capturing by reference should work better.

Christian Ehrlicher

@Vadi2 said in QtConcurrent::run with objects that are deleted:

QObject::connect(watcher, &QFutureWatcher<bool>::finished, [=]() { mpHost->xmlSaved(fileName); });

That's the reason why you should avoid the connect() with three params...

QObject::connect(watcher, &QFutureWatcher<bool>::finished, mpHost, [=]() { mpHost->xmlSaved(fileName); });

Vadi2

That worked, but I'm not sure why and what does it mean for the mphost. Is there documentation I can read on this?

I've already read https://github.com/KDE/clazy/blob/master/docs/checks/README-connect-3arg-lambda.md but it does not explain it well

SGaist

Use the doc Luke ;-)

Short version: the ~~fourth~~ third parameter is a "context" if the context gets delete, then then the disconnection follows and therefore in your case the lambda will not get executed.

[Typo fixed ~kshegunov]