[solved!]What's the proper way to delete/null a pointer to another dialog that has closed?

pwnstar23

So I have a mainwindow class, all it has right now is a button. This button opens another dialog, and sets a member pointer in mainwindow to that new dialog. The second dialog has a button that sends a signal to close itself. So the window is gone but my pointer in mainwindow is still pointing to it, it doesn't know it should delete/null the pointer.

@
class AnotherSubDialog;
class MainWindow
{
//stuff
AnotherSubDialog* mpSubDialog; // this object will call close() when it's "close" button is clicked. However the mainwindow class is not aware of this and doesn't delete/null the pointer.
};
@

What is the proper way to handle this behavior? I would image this kind of thing is done all the time.

noah

You can have the dialog delete itself by setting the Qt::WA_DeleteOnClose attribute.
You can use a QPointer instead of a standard pointer to have it clear itself when the memory its pointing to is deleted.

Alternately, you can create a slot in your MainWindow and connect the mpSubDialog finished(int) signal to that slot, then take care of deleting the pointer there.

twsimpson

You could always connect the signal emitted when your dialog closes to it's "deleteLater() slot": http://qt-project.org/doc/qt-4.8/qobject.html#deleteLater

pwnstar23

[quote author="Noah" date="1347989118"]You can have the dialog delete itself by setting the Qt::WA_DeleteOnClose attribute.
You can use a QPointer instead of a standard pointer to have it clear itself when the memory its pointing to is deleted.

Alternately, you can create a slot in your MainWindow and connect the mpSubDialog finished(int) signal to that slot, then take care of deleting the pointer there.[/quote]

I did the signal slot method you mentioned. When I had the slot delete the sub dialog pointer it would crash the app. When I only nulled the pointer it works fine. Is it crashing cause I am deleting the dialog for a second time? Meaning the sub dialog is deleting itself and any associated things I have added?

noah

[quote author="pwnstar23" date="1347992140"]
I did the signal slot method you mentioned. When I had the slot delete the sub dialog pointer it would crash the app. When I only nulled the pointer it works fine. Is it crashing cause I am deleting the dialog for a second time? Meaning the sub dialog is deleting itself and any associated things I have added? [/quote]

It's crashing because you're deleting the object emiting the signal. You should call mpSubDialog->deleteLater() instead, as Terence suggested.

andre

I think you are both right: you are deleting from a signal, but you are also deleting a second time. I recommend you use the QPointer method instead.

pwnstar23

If I use a QPointer then can I get away with just making the SubWindows finished signal, link to a custom slot that has deletelater() in it?

@
mpSubWindow = new SubWindow(this);
connect( mpSubWindow, SIGNAL( finished(int) ), mpSubWindow, SLOT( DeleteSlot( int )));
mpSubWindow->show();

private slots:
void DeleteSlot(int value)
{
this->deleteLater();
}
@

Or is there a better place for the deleteLater() call to be located?

noah

[quote author="pwnstar23" date="1348004498"]If I use a QPointer then can I get away with just making the SubWindows finished signal, link to a custom slot that has deletelater() in it?
[/quote]

You certainly can. In fact, you can connect it directly, since you don't need a perfect match in the parameters from the signal and slot you're connecting. Of course if there's anything else you want to do when the dialog closes your method is better.

@connect(mpSubWindow, SIGNAL(finished(int)), mpSubWindow, SLOT(deleteLater()))@

andre

Noah is right. Though, if there is anything else you would like to do on finish, you could also simply use two connects. I usually prefer that: have one function that does one thing, rather than one function that reacts to one signal.

Note that instead of using the deleteLater, you could also set the delete-on-close flag of the window and have Qt take care of deleting it. You do that by calling
@
mpSubWindow->setAttribute(Qt::WA_DeleteOnClose, true);
@
It doesn't matter which you use really.

pwnstar23

[quote author="Andre" date="1348034642"]

Note that instead of using the deleteLater, you could also set the delete-on-close flag of the window and have Qt take care of deleting it. You do that by calling
@
mpSubWindow->setAttribute(Qt::WA_DeleteOnClose, true);
@
It doesn't matter which you use really.
[/quote]

This seems like the simplest, less error prone way of doing it after reading all the responses.

Thanks guys, this thread is now solved!

Note for future people: mpSubWindow is a QPointer<SubWindowClass>.