Crashing when accessing class within lambda

G_ka

Hello,
I would need some help to understand lambda functions. I get inconsistent SegFaults : when ran in GDB, my program always crashes. When ran outside of GDB, it sometimes works.
Here is my code :

connect(pathButton, &QPushButton::pressed, this,
[=, &optimiser](){
        QString dir = QFileDialog::getExistingDirectory(this, "Open Directory", optimiser.getmodPath(), QFileDialog::ShowDirsOnly | QFileDialog::DontResolveSymlinks);
        modpathTextEdit->setPlainText(dir);
        optimiser.setModPath(dir);
    });

If I remove optimiser.getmodPath(), it doesn't crash.

QString Optimiser::getmodPath() const
{
    return modPath;
}

It shouldn't be necessary, but here is my full code.

Thanks.

mrjj

Hi and welcome to the forums
I could not spot same code in gitlab ?

I was wondering if the optimiser you capture by reference will stay in scope ?

G_ka

Sorry, I linked the wrong branch. It is here.

The optimiser is declared in mainWindow, so it should stay in scope as far as I know.

mrjj

@G_ka
Hi
Aha :)
It seems to be defined as a local variable in constructor

MainWindow::MainWindow()
{
Optimiser optimiser("C:/", mw_log); // local variable

so since you capture it with reference, it will point to a deleted variable as
constructor has run and finished once you press button.

Put it as a class member of MainWindow (in .h )
and it will work fine.

J.Hilk

@G_ka
as an additional note,

you can give your connect statement an 4th parameter, for example optimiser, if it's an QObject-derived class -

Thatway the connection will get resolved when either the sender object(pathButton) or the reference object becomes invalid (optimiser).

connect(pathButton, &QPushButton::pressed,  &optimiser,
[=, &optimiser](){
        QString dir = QFileDialog::getExistingDirectory(this, "Open Directory", optimiser.getmodPath(), QFileDialog::ShowDirsOnly | QFileDialog::DontResolveSymlinks);
        modpathTextEdit->setPlainText(dir);
        optimiser.setModPath(dir);
    });

G_ka

Thank you, it worked. @J-Hilk I'll remember it.

Now, I have another issue. When I append anything to the log, like here, I crash.

It isn't related to Qt tho, and I probably should be able to fix it by myself. Thanks for the help already.

Edit : I've fixed it, I was giving the argument before it was initialized.