QCache and QSharedPointer

Question

The problem is, that I have a QVector of QSharedPointer and I want to put part of them to my QCache . How can I insert a shared pointer to QCache ? What happens if I set the pointer to my cache but destroy the QVector and the shared pointer in it? Will the memory be released and my cache points to nowhere?

Answer 1

It is a bug if you put just a pointer to your item to QChache and at the same time such pointer is managed by QSharedPointer . The item object can be destroyed by QSharedPointer destructor, so QChache will have invalid pointer. It is a generic issue that you cannot have different owners of a pointer that do not know each other. If you manage pointers by QSharedPointer<Item> then QChache should not work directly with Item it should work only with QSharedPointer<Item> , for example:

// wrong
QCache<int, Item> cache;
QVector<QSharedPointer<Item> > vec;
Item *item = new Item;
vec.push_back(QSharedPointer<Item>(item));
cache.insert(1, item);
// or
cache.insert(1, vec.at(0).data());

// also wrong
QCache<int, <QSharedPointer<Item> > cache;
vec.push_back(QSharedPointer<Item>(item));
cache.insert(1, new QSharedPointer<Item>(item));
// here two different instances of QSharedPointer are initialized
// by the same pointer 'item'

// correct
QCache<int, <QSharedPointer<Item> > cache;
cache.insert(1, new QSharedPointer<Item>(vec.at(0)));

So, QCache should have its own copy of dynamically allocated QSharedPointer object and that QSharedPointer object should be correctly initialized by other QSharedPointer that already has ownership of the Item instance.