[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.