Deleting an object created with “new”

Question

Objects from my class Note are stored by keeping a pointer in a list called m_noteList :

QList< QSharedPointer<Note> > m_noteList;   

I am creating an object of that class by:

QSharedPointer<Note> note(new Note(this));
m_noteList << note;

Deleting goes by (after getting the right index idx to identify the object):

delete m_noteList[idx].data();

Error message in QtCreator (Debug-mode)

HEAP[MyApp.exe]: 
HEAP: Free Heap block 024FC240 modified at 02500330 after it was freed

And dissambler:

0x777349ab  <+0x0000>         cmpb   $0x0,0x2(%eax)
0x777349af  <+0x0000>         je     0x777349c6
0x777349b1  <+0x0000>         movb   $0x1,0x7774d640
0x777349b8  <+0x0000>         mov    %ecx,0x7774d644
0x777349be  <+0x0000>         int3
0x777349bf  <+0x0000>         movb   $0x0,0x7774d640   //<-- Here is a stop mark
0x777349c6  <+0x0000>         ret
0x777349c7  <+0x0000>         nop
...

I have also tried this:

m_noteList.removeAt(idx);

Which does not work either. SIGSEGV signal emitted by operating system stopped the process (Segmentation fault)

So I really don't know what I do wrong and how to get around it

My solution is quite dirty: Setting some kind of an deleted -tag to my objects, but I need to check this in every other function to ignore that object...

EDIT:

I realised one thing: If i delete one note (with removeAt() ), it is deleted (i see it by watching the console print outputs), but its gui does not disappear. Then if I click again on that object's specific button that is to delete the note, the application crashes (because it does not really exist any more, but only the gui). But if I write this deleteLater before, after deleting it, the gui also disappears and everything seems fine. Can someone please explain, why?

m_noteList.at(idx)->deleteLater();
m_noteList.removeAt(idx);

Answer 1

You are mixing up three memory management strategies:

1. QObject parent-child memory management: when a parent QObject is deleted, it automatically deletes all child QObjects . Explained here . Deleting the object is done automatically, so explicit deletion is not necessary.

2. QSharedPointer memory management: when the last QSharedPointer referring to an object is deleted, the object is deleted. Explained here . The object is deleted automatically, so explicit deletion is not necessary.

3. "Manual" memory management: new an object and delete an object at the right time. Explained in the C++ standard. The object must be deleted manually.

You should select only one of the above to make sure you don't double-delete objects. I suggest choosing between #1 and #2 depending on how you plan to use the objects. If you plan to share the objects across "parents", use #2, otherwise use #1.

Answer 2

Given a QList<QSharedPointer<Note>> m_noteList :

The deletion of the Note object is accomplished by m_noteList.removeAt(idx) , assuming that there are no other extant copies of the pointer. You simply destruct the last shared pointer to a given note, and that's it. Invoking delete on a shared pointer's data is always an error .

I cannot reproduce your issue with removeAt - it will only happen if you delete the Note first, which you're not supposed to do.