Crash when add to many QWidgets to QVector

TomNow99

Hello,

My code ( whole ):

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    wek.reserve(90*90);

    for(int i=0;i<90*90;i++)
    {
        wek.append(new QLabel(this));
    }
}

wek is QVector<QLabel *>

When I change 90x90 to 80x80 everything is ok.

I check that sizeof(QLabel) is 48, so I need 48x90x90 bytes < 1 mb memory. My pc has 8 gb ram ( 3.7 gb is free ).

Using windows 10, Qt 5.14.0, MinGW 64 bit

jsulm

@TomNow99 What type of crash is it? Do you have stack trace after crash?

JonB

@TomNow99
After you have answered @jsulm .
If there is indeed a crash, if anything it's probably more likely for the number of widgets to show than putting them in a vector. What happens if you remove the parent: wek.append(new QLabel());?

P.S.
Don't create anything near 10,000 widgets, regardless!

TomNow99

@jsulm SIGSEGV @JonB

When I remove parent - everything is ok

jsulm

@TomNow99 Looks like it is crashing when drawing a widget. Probably Qt does not like many widgets to be added that fast :-) I suggest to file a bug in Qt bug tracker as there should not be any crashes just because you add many widgets in a loop.

TomNow99

@jsulm @JonB

I had a problem ( and still have with adding many widgets to grid / print them on the screen ):

https://forum.qt.io/topic/118875/is-there-a-maximum-number-of-qlabels-in-a-screen/2

In this topic I see that user J.Hilk doesn't have a problem with adding many labels very fast. Maybe because of OS ( my Win 10 vs his linux )?

EDIT: I try add many QLabels to QGridLayout ( 120 x 120 ) on linux ( VirtualBox - Fedora ) and it was no problem ( I had to wait a moment, but there are ). So do you think OS is a problem?

Can someone check and agree with me that in Win 10 I can't add many labels?

JonB

@TomNow99
I can't test for you, but:

I try add many QLabels to QGridLayout ( 120 x 120 )

That's 10,000+ labels, and totally the wrong thing to do, as I wrote earlier. Regardless of whether there is any bug on whatever OS, you really, really should not ignore this! You want to change something like this to use, say, a table and a QStyledItemDelegate with the labels/text drawn rather than as actual Qt widgets....

[EDIT I see my learned colleague @VRonin has up-voted this post. As a campaigner for QStyledItemDelegate instead of individual widgets, I'm sure he would be happy to help out if you have any questions about this approach ;-) ]

TomNow99

@JonB I agree with you, but... what if I would like to have 10000 signals and slots to this qlabels? Of course I can check mouse click position, but there is many math.

jsulm

@TomNow99 What is your use case? Why do you need that many labels?!

TomNow99

@jsulm For example: when I was in a College I had exercise with machine learning. I needed many labels ( they shows image ( 1 pixel = 1 label ) ), which I can click and change their color ( black - white ).

JonB

@TomNow99 said in Crash when add to many QWidgets to QVector:

I would like to have 10000 signals and slots to this qlabels? Of course I can check mouse click position, but there is many math.

Nobody wants 10,000 signals & slots. Honest! One signal, one slot, one parameter saying where the click is. Yes, if that's the choice, do the math (and btw it sounds like two divide operations) :)