Qt emit signal after widget is closed

[jsulm]

@pmanc said in Qt emit signal after widget is closed:

The deleteLater will make the program crash

Did you analyse the crash? What happens? What are you doing in the destructor?

[pmanc]

In the destuctor of AdvancedDialog there's only

delete ui;

The error is:

Exception triggered
the inferior stopped because it triggered an exception.
Stopped in thread 18 by: Exception at 0x7fff5b5fd49 code 0xc0000005
read access violation at 0xffffffffffffff  flags=0x0 (first chance)

The debugger is stuck at

AdvancedDialog::errorTcpSignal

Thanks!

Edit: I think the problem is that errorTcpSignal is declared in the header of AdvancedDialog, so when AdvancedDialog is deleted the signal cannot be emitted.

[Christian Ehrlicher]

We need to see the backtrace.

[pmanc]

I hope this is good

Thanks!

[jsulm]

@pmanc Looks like the signal is still emitted. Can you try to disconnect manually before calling deleteLater() to see whether it still crashes?

[pmanc]

@jsulm I added

disconnect(ad, nullptr, nullptr, nullptr);

before deleteLater, but still crashes, plus at live 295 I have a semantic error:

moc_advanceddialog.cpp:295:5: error: no matching function for call to 'activate'
qobjectdefs.h:397:17: note: candidate function not viable: no known conversion from 'AdvancedDialog *' to 'QObject *' for 1st argument
qobjectdefs.h:398:17: note: candidate function not viable: no known conversion from 'AdvancedDialog *' to 'QObject *' for 1st argument
qobjectdefs.h:396:17: note: candidate function not viable: requires 3 arguments, but 4 were provided

[J.Hilk]

@pmanc QObject::Connect returns a QMetaObject::Connection you're supposed to store that and call disconnect on that object

[pmanc]

@J-Hilk
Sorry, i didn't know.
Now I have

AdvancedDialog *ad = new AdvancedDialog(this);
QMetaObject::Connection c = connect(ad, SIGNAL(errorTcpSignal(QString)), this, SLOT(displayTcpError(QString)));
ad->exec();
disconnect(c);
ad->deleteLater();

But still crashes, this SIGNAL is emitted.

[Bonnie]

disconnect won't stop the signal emitting, just stop calling the slot.
I think you should disconnect the other connection, from that SocketError.
That connection doesn't have a receiver context, so the lambda will be called even after this is destroyed.
How about add this as the receiver context:

connect(&t, QOverload<QAbstractSocket::SocketError>::of(&QAbstractSocket::error), this, [=](QAbstractSocket::SocketError err){
            emit errorTcpSignal(...);
        });

[pmanc]

@Bonnie
Thank you very much for your reply.
The problem is that I want the signal to be emitted when I close the QDialog too.
I have MainWindow with the slot for the error, and the QDialog that emits the signal.
The core of the problem is that I want to receive the signal from QDialog 's QtConcurrent thread also after QDialog has been closed.

[Bonnie]

From your screen capture, the errorTcpSignal is a member from AdvancedDialog, right?
Where do you emit it then? I'm not quite understanding your code since I don't know what is this in those connections.
If you want the dialog to emit the signal, then you can't delete it.

[pmanc]

@Bonnie
Yes, errorTcpSignal is a member from AdvancedDialog.
I emit it from a QtConcurrent thread in AdvancedDialog when an AdvancedDialog 's button is pressed.
If I don't delete it everything works, but I'll have a memory leak.

[Bonnie]

@pmanc said in Qt emit signal after widget is closed:

If I don't delete it everything works, but I'll have a memory leak.

No, you won't.
It have a parent widget (in your code when you new it), so it will be deleted after the parent is destoyed.
But I still don't think it is a right design since you want to delete the dialog but also need it to send signals.
Though I don't fully understand your logic, I think you should handle the QtConcurrent related code in your QMainWindow.

[pmanc]

@Bonnie
I see. But every time I open a new AdvancedDialog I see my memory use to grow in the task manager.
Should I initialize AdvancedDialog once in the MainWindow constructor and show / hide it when I need?

By the way, great community, everyone!

[jsulm]

@pmanc said in Qt emit signal after widget is closed:

But every time I open a new AdvancedDialog

If you need this dialog often then don't delete it and don't recreate it each time you need it. Create it once.

[Bonnie]

@pmanc said in Qt emit signal after widget is closed:

Should I initialize AdvancedDialog once in the MainWindow constructor and show / hide it when I need?

If you want to keep handling QtConcurrent related things in the dialog , I think you should.

[pmanc]

Ok thank you very much for your time!