Cannot delete items of a QVBoxLayout

Niagarer

Hi!
this is in the constructor:

myGroupBox->setLayout(myBoxLayout);
myBoxLayout->setAlignment(Qt::AlingTop);

and then I call this:

if(myBoxLayout->takeAt(0) != NULL){
    QLayoutItem *item;
    while((item = myBoxLayout->takeQt(0)) != NULL){
        delete item->widget();
        delete item;
    }
}

to remove all widgets from the layout.
The program just crashes. What did I miss?
This code came from this thread, but I can't find a answer for my problem there:
https://stackoverflow.com/questions/4272196/qt-remove-all-widgets-from-layout
Thanks for answers

Niagarer

Ahh about 1300 lines, unfortunately not small.
It works now... aaand yes, I think the deleting of the builddirectory worked.
What happened:
I deleted the build directory and then the program crashed at an other point before the setting up of the layout. I found a thing that I would not call a mistake, but it surely is in some way, because of my endless function calling, the function, where I set up the layouts got called twice. I don't really know, why this is a mistake, but after I fixed it, it worked and then the program was able to say that it is a nullpointer. So I changed it back to new QVBoxLayout and everything works.
The weird thing is, that I really don't know why it was a problem to setting up the layout as a nullpointer twice.
But, anyway, thanks for your help!

mrjj

Hi
Seems ok but there some notes


if(myBoxLayout->takeAt(0) != NULL){ << you take first out first here ( and do not delete it)
    QLayoutItem *item;
    while((item = myBoxLayout->takeQt(0)) != NULL){
        delete item->widget();
        delete item;
    }
}

IF you single step , what lines does it crash on ?

Niagarer

@mrjj
on the line with the if statement

mrjj

Hi
It crashes at
if(myBoxLayout->takeAt(0) .. ?

did you forget to new myBoxLayout ?

Niagarer

@mrjj
Nope, this is a class attribute and in the constructor (all in the same class) I do

myBoxLayout = new QVBoxLayout;

and there is also no error, that works.

mrjj

@Niagarer
Cant guess then.

try
qDebug() << "items:" << myBoxLayout->count();
just before the If.

Niagarer

@mrjj
Thanks, I tried and it crashes at exactely this line you gave me.
I can not do anything with the layout
Seems to be a nasty one...

mrjj

@Niagarer
If it crashes there, something must be wrong with the pointer
myBoxLayout->count();

You did show real code, yes ?

A classic is to define in .h
and then do
QVBoxLayout *myBoxLayout = new QVBoxLayout; ( in ctror)
and make local variable instead.

set it to null where u define it
QVBoxLayout *myBoxLayout=nullptr;

and check if its NON null before using it on the count()

Niagarer

@mrjj
Ok, I tried it.
I did not make the classic mistake but if I define it with a nullpointer

myBoxLayout = nullptr;

and then asking if it's nullptr where I try to delete all content, it says, it is not null
But I only defined it once...
And if I ask that this way:

myBoxLayout = nullptr;
if(myBoxLayout != nullptr){
    qDebug() << "layout is not null";
}

it says, it is not null...

mrjj

@Niagarer
what ??
myBoxLayout = nullptr;
if(myBoxLayout != nullptr){
qDebug() << "layout is not null";
}

that code should say nothing. :)
I assume that
myBoxLayout = nullptr; <<< in .h yes ?

Sorry, i have really no idea.
Its does work that way and i have used takeAt many times.

Niagarer

@mrjj
In the .h file:

    QVBoxLayout *myBoxLayout;

and in the .cpp file:

myBoxLayout = nullptr;
if(myBoxLayout == nullptr){
    qDebug() << "layout is null";
}

and it prints nothing...
I really don't understand that too.
I just hope that I am not doing a totally dumb mistake here...

mrjj

@Niagarer
Hi, no worries, we all do stupid mistakes sometimes.
Its a sign of a productive programmer ;)

first
QVBoxLayout *myBoxLayout; -> QVBoxLayout *myBoxLayout=nullptr.
in .h
then do NOT
myBoxLayout = nullptr; before the check. ( you kill it )

also , it must print something ???
myBoxLayout = nullptr;
if(myBoxLayout == nullptr){
qDebug() << "layout is null";
}
else something is really, really wrong.

Can you test this small sample ?
https://www.dropbox.com/s/tkl1hdik6q0qwl2/myreuselayout.zip?dl=0
Uses your code to remove buttons from layout.

Niagarer

@mrjj
Now, it does not print any qDebugs() anymore... what the hell!?

? Offline

Close Creator, delete the build directory and start Creator again. Sometime strange things happen.

Niagarer

Ok, I did and now I have a mistake at a point in the code before the setting up of the layout.
I add a Tab of a tabWidget and there it crashes.,.. This was never a problem, this line works sonce weeks, I did not change anything there.
I deleted the build directory, opened creator again, ran qmake, rebuilt and that's it, no errors, it is just crashing

Niagarer

@mrjj
Yep, this project works. So it has to be my bad.

? Offline

I say "unitialized memory somewhere".

mrjj

I agree @Wieland , the big question is just Where :)

@Niagarer
If its a small project, can you zip and link it here ?

Niagarer

Ahh about 1300 lines, unfortunately not small.
It works now... aaand yes, I think the deleting of the builddirectory worked.
What happened:
I deleted the build directory and then the program crashed at an other point before the setting up of the layout. I found a thing that I would not call a mistake, but it surely is in some way, because of my endless function calling, the function, where I set up the layouts got called twice. I don't really know, why this is a mistake, but after I fixed it, it worked and then the program was able to say that it is a nullpointer. So I changed it back to new QVBoxLayout and everything works.
The weird thing is, that I really don't know why it was a problem to setting up the layout as a nullpointer twice.
But, anyway, thanks for your help!

mrjj

Super :)
Im not sure what calling the function twice could be issue but it did do something it seems.