A little memory leak in a simple Qt program

mrjj

Hi
after you open the cmd
call
"C:\Qt\5.14.1\msvc2017_64\bin\qtenv2.bat"
or
"C:\Qt\5.14.1\mingw73_64\bin\qtenv2.bat"
adjust path to math your compiler / installation
then its setup.

qcoderpro

Thanks.
About the first post. Why have we a little leak there, please? Is it because the label won't get deleted until the program terminates? When was is it supposed to be deleted to avoid leaking?

And if we want to have a proper code for that example based on Qt standards, we need to firstly have a parent, like a QMainWindow or QWidget or even the QApplication itself, and then set it as the parent to the label. True?

mrjj

@qcoderpro said in A little memory leak in a simple Qt program:

Thanks.
About the first post. Why have we a little leak there, please? Is it because the label won't get deleted until the program terminates? When was is it supposed to be deleted to avoid leaking?

When you use new on a Qt type, and do not give it a parent, you must manually delete it.
so in fist code, its been newed so it won't be cleanup as such when the program exits. ( well it will but on OS level)

And if we want to have a proper code for that example based on Qt standards, we need to firstly have a parent, like a QMainWindow or QWidget or even the QApplication itself, and then set it as the parent to the label. True?

True. the parent must exist to assign it but in many cases,
the labels are put inside a form or similar so it makes sense.

JonB

@qcoderpro said in A little memory leak in a simple Qt program:

And if we want to have a proper code for that example based on Qt standards, we need to firstly have a parent, like a QMainWindow or QWidget or even the QApplication itself, and then set it as the parent to the label. True?

QLabel (any QWidget) has to have a QWidget *parent, not a QObject *parent. You will not be able to use QApplication * as the parent....

Hence if you were to stick with your original create QLabel with no QWidget available to be its parent, to have it deleted you would need to use one of the two approaches: @mrjj's make it a stack variable or @SGaist's do an explicit delete. The same will (presumably, unless a Qt expert tells me otherwise) apply if you introduce, say, a QMainWindow as the top-level window.

qcoderpro

@JonB

(any QWidget) has to have a QWidget *parent

Does it mean that every widget's parent must only be a QWidegt or any other widget which has inherited QWidget?

Bonnie

@qcoderpro

Does it mean that every widget's parent must only be a QWidegt or any other widget which has inherited QWidget?

Yes, or nullptr, which means it has no parent. Usually a top-level widget (a window) does not have a parent.
If you must create a parentless widget on the heap and hope it be automatically deleted when it is closed, you can use:

setAttribute(Qt::WA_DeleteOnClose)

qcoderpro

@Bonnie
cool hint! :)

That actually raised this question in my mind. We say widgets built on the heap (using new) must be either explicitly deleted using the "delete" keyword or child of a parent. Assume we've 10 widgets created on the heap. Then they're all children of a top-level widget, for example, a QMainWindow. Actually we should be certain that QMainWindow is deleted, since it comprises many heap widgets, before the program terminates. One solution is the one you said. Another solution is to firstly create the top-level widget on the stack (although its children are created on the heap). Do you know of a third solution?

Bonnie

@qcoderpro
We usually just create the top-level widget on the stack or delete it manually.
If you do want to create the widget on the heap and do not want to delete it manually, you need Qt to delete it for you.
For example, what the "Qt::WA_DeleteOnClose" attribute do is, as the doc says, "Makes Qt delete this widget when the widget has accepted the close event".
So Qt delete it for you after it is closed.
But if the window is created but never shows, then it won't be deleted (because it never closes).
Another solution I can think of would be using QSharedPointer, it will "delete the pointer it is holding when it goes out of scope".

JonB

@qcoderpro
How many different solutions do you want? :)

There are only really two: if you new it you must delete it --- which in the case of setAttribute(Qt::WA_DeleteOnClose) is done for you by the Qt framework when the window is closed --- or if you put it on the stack it gets deleted by the run-time code when it goes out of scope.

In the case of your one QMainWindow it is usual to allocate this on the stack in the method where you call QApplication::exec(). You could new and delete it explicitly if you wish. I have seen debates about whether setAttribute(Qt::WA_DeleteOnClose) on your top-level main window (assuming you new it!) works correctly or not, simplest is to put it on the stack.

qcoderpro

@JonB

So the best habit is to create the top level widget on the stack and parent the children, all, to it, regardless of them being created on heap or stack as well. So this way there is almost no class which needs to implement the destructor!

One other thing I didn't understand well I think is the termination time of a top-level widget, actually our program, the time when it goes out of scope and Qt deletes it from the stack. This will be done when we close the program, not? But "setAttribute(Qt::WA_DeleteOnClose)" seemingly works the same way too!