QSharedPointer is null with strongref existent

Redman

UaClient and PlcList both have a QList<QSharedPointer<PlcAddress>>. PlcList passes a Reference to UaClient of that list, and then UaClient does some things with that list. Test crashes when calling UaClient dtor bc QSharedPointer dtor can not free that memory.

1   QtSharedPointer::CustomDeleter<PlcAddress,QtSharedPointer::NormalDeleter>::execute                     qsharedpointer_impl.h 158  0x7ff66620a027 
2   QtSharedPointer::ExternalRefCountWithCustomDeleter<PlcAddress,QtSharedPointer::NormalDeleter>::deleter qsharedpointer_impl.h 180  0x7ff6661e9bc5 
3   QtSharedPointer::ExternalRefCountData::destroy                                                         qsharedpointer_impl.h 114  0x7ff6661e6c11 
4   QSharedPointer<PlcAddress>::deref                                                                      qsharedpointer_impl.h 458  0x7ff6661e6896 
5   QSharedPointer<PlcAddress>::deref                                                                      qsharedpointer_impl.h 451  0x7ff6661e6848 
6   QSharedPointer<PlcAddress>::~QSharedPointer<PlcAddress>                                                qsharedpointer_impl.h 281  0x7ff6661e21b4 
7   QSharedPointer<PlcAddress>::`scalar deleting destructor'                                               tst_Plc                    0x7ff6661e3238 
8   std::_Destroy_in_place<QSharedPointer<PlcAddress>>                                                     xmemory               310  0x7ff6661dd786 
9   std::_Destroy_range<QSharedPointer<PlcAddress> *,QSharedPointer<PlcAddress> *>                         xmemory               1098 0x7ff6661dd8f5 
10  std::destroy<QSharedPointer<PlcAddress> *>                                                             memory                574  0x7ff6661de82b 
11  QtPrivate::QGenericArrayOps<QSharedPointer<PlcAddress>>::destroyAll                                    qarraydataops.h       398  0x7ff6661e6ebb 
12  QArrayDataPointer<QSharedPointer<PlcAddress>>::~QArrayDataPointer<QSharedPointer<PlcAddress>>          qarraydatapointer.h   104  0x7ff6661e1fbd 
13  QList<QSharedPointer<PlcAddress>>::~QList<QSharedPointer<PlcAddress>>                                  tst_Plc                    0x7ff6661e2127 
14  UaClient::~UaClient                                                                                    UaClient.cpp          10   0x7ff6661fd066 
15  PlcList::~PlcList                                                                                      PlcList.h             21   0x7ff6661e24b2 
16  TestPlc::testUaClientMonitoringAndListModel                                                            tst_Plc.cpp           158  0x7ff6661db80e 
17  TestPlc::qt_static_metacall                                                                            tst_Plc.moc           196  0x7ff6661da54e 
18  QMetaMethodInvoker::invokeImpl                                                                         qmetaobject.cpp       2713 0x7ffeb56ae03e 
19  QMetaMethod::invokeImpl                                                                                qmetaobject.cpp       2551 0x7ffeb56aa383 
20  QMetaMethod::invoke<>                                                                                  qmetaobject.h         150  0x7fff0ca8dd77 
... <More>

Im running this in a test. After dtor of PlcList gets called at the end of the test the first two entries of the QList<QSharedPointer<PlcAddress>> in UaClient are null.

What I dont understand is, how can there still be a strongref to that pointer but it seems to be freed in the PlcList dtor.
~PlcList() = default; Is there something going on when using the default dtor?

jsulm

@Redman Hard to say without any code

Redman

This post is deleted!

Redman

PlcList.h

..
   QList<QSharedPointer<PlcAddress>> m_addressList;
..

PlcList.cpp


PlcList::~PlcList() {
   m_addressList.clear();
}

init(){
..
   m_uaClient.setAddressList(m_addressList);
..
}

UaClient.cpp

void
UaClient::setAddressList(const QList<QSharedPointer<PlcAddress>>& list) {
   for (const QSharedPointer<PlcAddress>& plcAddress : list) {
      m_addressList.append(plcAddress);
   }
}

That is basically everthing in regards to the communication betweend these two classes. When I explicitly clear the list, as I just posted in this answer, I can see that after the clear statement the strongref count goes down to 1, as expected. The next step in the stack trace is directly in the UaClient dtor, and there the pointer is suddenly null.

Christian Ehrlicher

@Redman said in QSharedPointer is null with strongref existent:

m_addressList

How is this defined in both classes? Do you explicitly delete the pointers somewhere?

Redman

@Christian-Ehrlicher Nope. But I found the solution just now. Doing

emit mysignal(QSharedPointer<MyClass>(this))

is not good. Through this signal, m_addressList got populated.
Unfortunately I can not understand why this results in this strange behaviour

Christian Ehrlicher

@Redman said in QSharedPointer is null with strongref existent:

Unfortunately I can not understand why this results in this strange behaviour

Because you delete this instance somewhere else - e.g. because it is derived from QObject and has a parent.

Redman

@Christian-Ehrlicher Please elaborate how emitting a newly created SharedPointer of this and then appending that to a List leads to that error. As I said in my previous answer, after removing that line everything works fine when shutting down the application.

Christian Ehrlicher

Since I don't know your code I can't say more. You are doing a double delete. We can't see where. So please provide a minimal compileable example of you want help.

JoeCFD

@Redman is PlcAddress a subclass of QObject? If yes, do not forget to call QObject in its constructor and register its reference count.