double free or corruption (out) when closing a QDialog with the WA_DeleteOnClose attribute set
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@LRDPRDX you're right.
it's more sinister.
the QDialog implementation of delete on close is a
delete thiscall. 😱 very naughty, even if allocated on the heap.so its not double free call, it's corruption.
class MyPDialog : public QDialog { public: MyPDialog(QWidget *parent = nullptr) : QDialog(parent) {} void deleteOnClose() {delete this;} ~MyPDialog() { qInfo() << "~MyPDialog()"; } }; int main(int argc, char *argv[]) { QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling); QApplication app(argc, argv); MyPDialog d; d.deleteOnClose(); // d.setAttribute( Qt::WA_DeleteOnClose, true ); // int result = d.exec(); qInfo() << "After exec" /*<< result*/; return app.exec(); } -
@J-Hilk Cool -_-
the QDialog implementation of delete on close is a delete this call
Just to ensure : do you know this from this (line 613) ?
@LRDPRDX yes, its exactly where my debugger stopped.
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@kshegunov
A "typo" is a misspelling, not a complete reversal of a comment's intention! :DAnd I agree it's about time somebody said you just must not delete a stack variable, period. [Whether
WA_DeleteOnClosedocs might mention that is what this does, and so should only be used on heap instances, is another matter. I was never clear just what exactlyWA_DeleteOnClosedid, though I used it.@JonB said in double free or corruption (out) when closing a QDialog with the WA_DeleteOnClose attribute set:
A "typo" is a misspelling, not a complete reversal of a comment's intention! :D
I added the comma after the fact, which introduced the missing negation, so *shrug* happens.
@J-Hilk said in double free or corruption (out) when closing a QDialog with the WA_DeleteOnClose attribute set:
no matter what, telling a class to commit suicide is sinister in my books :P
Your books are wrong. We are a free-haven! ;)
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@JonB said in double free or corruption (out) when closing a QDialog with the WA_DeleteOnClose attribute set:
A "typo" is a misspelling, not a complete reversal of a comment's intention! :D
I added the comma after the fact, which introduced the missing negation, so *shrug* happens.
@J-Hilk said in double free or corruption (out) when closing a QDialog with the WA_DeleteOnClose attribute set:
no matter what, telling a class to commit suicide is sinister in my books :P
Your books are wrong. We are a free-haven! ;)
wrote on 8 Nov 2021, 12:54 last edited by@kshegunov said in double free or corruption (out) when closing a QDialog with the WA_DeleteOnClose attribute set:
I added the comma
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wrote on 8 Nov 2021, 12:56 last edited by mpergand 11 Aug 2021, 13:04
At the end of the QDialog:exec there are:
if (deleteOnClose) delete this; return res;On Mac i got:
malloc: *** error for object 0x7ffeefbffa60: pointer being freed was not allocated
so it crashes at the delete instruction and never returns. -
@J-Hilk said in double free or corruption (out) when closing a QDialog with the WA_DeleteOnClose attribute set:
it's more sinister.
There's nothing sinister, the OP already provided the answer.
@LRDPRDX said in double free or corruption (out) when closing a QDialog with the WA_DeleteOnClose attribute set:
I suspect that the deleteLater() function is not allowed to be called on a stack variable but to confirm I (probably) need to go through the QCoreApplication::postEvent() function which is too complicated to me as a user and not a Qt-developer.
No, delete is not allowed on a stack variable. You don't need to be a Qt developer to know this, and it has nothing to do with how you call the
delete-- from the outside, from the inside (i.e. deleting this), through a custom event or without.wrote on 8 Nov 2021, 13:02 last edited by@kshegunov said in double free or corruption (out) when closing a QDialog with the WA_DeleteOnClose attribute set:
You don't need to be a Qt developer to know this, and it has nothing to do with how you call the delete -- from the outside, from the inside (i.e. deleting this), through a custom event or without.
Agreed, but how do I know what the
Qt::WA_DeleteOnCloseactually does? One can guess it causes the call ofdeleteon the variable but that was just not enough to me to be sure (and obviously I was looking for an answer in the wrong place in the source code). Anyway, now I've got the answer. Thank you. -
@kshegunov said in double free or corruption (out) when closing a QDialog with the WA_DeleteOnClose attribute set:
You don't need to be a Qt developer to know this, and it has nothing to do with how you call the delete -- from the outside, from the inside (i.e. deleting this), through a custom event or without.
Agreed, but how do I know what the
Qt::WA_DeleteOnCloseactually does? One can guess it causes the call ofdeleteon the variable but that was just not enough to me to be sure (and obviously I was looking for an answer in the wrong place in the source code). Anyway, now I've got the answer. Thank you.Moderatorswrote on 8 Nov 2021, 13:09 last edited by kshegunov 11 Aug 2021, 13:16@LRDPRDX said in double free or corruption (out) when closing a QDialog with the WA_DeleteOnClose attribute set:
Agreed, but how do I know what the Qt::WA_DeleteOnClose actually does?
If you feel the name doesn't give it up, or isn't clear enough, I'd've suggested looking up in the documentation:
https://doc.qt.io/qt-5/qt.html#WidgetAttribute-enumIt's sort of the point of having documentation - so you don't need to go browsing through the code, and thankfully Qt has the best one I've ever seen.
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@LRDPRDX said in double free or corruption (out) when closing a QDialog with the WA_DeleteOnClose attribute set:
Agreed, but how do I know what the Qt::WA_DeleteOnClose actually does?
If you feel the name doesn't give it up, or isn't clear enough, I'd've suggested looking up in the documentation:
https://doc.qt.io/qt-5/qt.html#WidgetAttribute-enumIt's sort of the point of having documentation - so you don't need to go browsing through the code, and thankfully Qt has the best one I've ever seen.
wrote on 8 Nov 2021, 13:17 last edited by@kshegunov Yes, ofc, I read that. For some reason I interpret the word "delete" (when it's not formatted :) ) as a general way to destroy an object. -_-
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@jsulm I don't think so. As you see the output "After exec" is not present. So the error occurs before leaving the scope..
wrote on 10 Feb 2023, 00:26 last edited by habeebo 2 Oct 2023, 01:12In terminal, "double free or corruption (out)" message is printed after the destructor has been called.
Update: Sorry for bothering you. I was having similar issue. I saw your post in stackoverflow and then my problem solved when I allocated my QMainWindow object in heap instead of the stack.
Many thanks!