[How to delete a QList]

[appdev]

Hello guys, I hope you're doing very well.

So I have a QList which includes 9 elements which type is QString and I want to delete it.

Some people suggested doing this:

qDeleteAll(list);

list.clear();

So I tried it and got an error.

That solution was suggested to someone who wanted to delete a QList of pointers to objects.

So I was wondering how I can delete the whole list and not only its items.

I've searched on the internet and all I found was how to delete the list's items.

I also took a look on the documentation of Qt6 and didn't find any function that could ensure the Qlist deletion.

Please Help me.

Any little help would be so appreciated.

Thank you all.

[Christian Ehrlicher]

@appdev said in [How to delete a QList]:

Some people suggested doing this:
qDeleteAll(list);

Only when you store pointers to objects which you don't when you use QList<QString>

list.clear();

That removes all elements from the list and is what you want.

[appdev]

@Christian-Ehrlicher First of all thank you.

Well I used only list.clear() but I got an empty QList.

In fact here is the real thing;

I'm reading a csv file and storing all the lines in QList.

So I have about 54000 QLists since the file includes 54000 lines.

But my goal is to delete some lines that I won't be needing later in my program.

I thought after storing the lines in QLists, I would only keep the QLists that correponsd to these lines so I had to delete them completely.

After using list.clear() and using qDebug to see the output;

I get the lists i need and other lists that are empty.

[Christian Ehrlicher]

@appdev said in [How to delete a QList]:

I get the lists i need and other lists that are empty.

Then don't use clear but only remove the indexes you don't need or copy all needed into a new container and clear the old one.

[appdev]

I think the remove functions, only remove the item in the QList and not the QList itself.

Please correct me if I'm wrong.

[Christian Ehrlicher]

@appdev said in [How to delete a QList]:

I think the remove functions, only remove the item in the QList and not the QList itself.

?

You said you've a QList<QList<Item>> - so why should removeAt() on the outer container remove a single Item?

[appdev]

yes, I mean I have many QLists, about 54000.

QList [ value_1; value_2; value_3; value_4; value_5]
QList [ value_1; value_2; value_3; value_4; value_5]
QList [ value_1; value_2; value_3; value_4; value_5]
QList [ value_1; value_2; value_3; value_4; value_5]
QList [ value_1; value_2; value_3; value_4; value_5]...... until 54000.

I want to delete all the QLists which values at the third position or index are different from 05 and I want to keep only those which values at that position are equal to 5.
At the end I sohuld only have 24 QLists.

[Christian Ehrlicher]

And what's the problem - I already told you how to remove an element of a QList:
Iterate over your list (from the back) and remove every element you don't want
or
Iterator over your list and copy all elements you want to a new list.

[appdev]

Okay Now I see, it does make sense but for QList which includes another QList.

Earlier I said that I have QList<QList<Item>> but that's not actually the case.

I have a QList <QString>.

So when I use the clear function, it deletes all of the QString elements.

When I use the QDebug to see if the list is gone or not, As an output I get an empty list.

But logically if I delete the list and use Qdebug(), I'm not supposed to see anything in the output of the application, not even an empty list.

I think there is a difference between deleting a list and deleting its items right ?

[J.Hilk]

@appdev can you please show your actual code ?

[Christian Ehrlicher]

@appdev said in [How to delete a QList]:

I have a QList <QString>.

So I have about 54000 QLists since the file includes 54000 lines.

What now?

[JonB]

@appdev said in [How to delete a QList]:

But logically if I delete the list and use Qdebug(), I'm not supposed to see anything in the output of the application, not even an empty list.

Not true.

• If you have a QList variable, you will always have a list, which may be empty or have items. But you still have a QList.

• If instead you go for a QList * variable, which you new, you can then delete theQList when you're done with it, and you won't have an empty list, you will have no list. (You will still have a QList * variable, but it won't point to an (allocated) QList.)

This is not particular to QList, it applies to C++ generally.

Note, btw, that this is not advocating using a QList * over a QList. A QList * is just 4/8 bytes (plus the allocation overhead when you new it), but an empty QList may be not much bigger than that anyway, so why bother with a pointer, allocating etc. etc. unless you have a specific need for that.

[appdev]

Hello guys,
I got it, thank you for your precious help, I really appreciate it.

[SGaist]

Hi,

In addition to the good points made by my fellow, deleting a pointer to a QList does not delete the content of the list if they are also pointers.

[appdev]

Hello again,
Thank you for all the precious information.