QList children - struct or custom classes derived from QObject?

Question

I'm currently working on Qt application on Symbian platform. Application has an sqlite database and initial data is populated from txt files.

I'm implementing online update from data that comes in json format. So i would like to create generic functions in my db update class that takes QList of classes/structs and updated db from them. QList would be populated with objects either from txt, or json.

I already have the parsing in place, just considering what would be better in terms for performance:

1. Creating c++ structs and passing them (as objects hold only simple data) wrapped in QList
2. Creating custom classes derived from QObject and passing them as pointers in QList and then deleting everything with qDeleteAll
3. Any other way...

Answer 1

That depends on whether your classes are carrying behaviour or only state.

1. They carry behaviour.

   Then, a polymorphic class is in order, yes. Whether it needs to inherit from QObject is another question. Inherit from QObject only if you need its services (introspection, signals/slots, event handling). Otherwise, don't.

   As for qDeleteAll(): I wouldn't go there. Instead of naked pointers, use smart pointers, e.g. QSharedPointer. They keep track of the number of references to their payload, deleting it when the refcount falls to zero.

   In this case, don't use QList, but a more efficient container such as QVector.

2. They carry only state.

   Then, a "dumb" struct may suffice. In this case, don't use QList as a container, but something more efficient, like QVector (don't forget to make good use of the reserve() method).

In general, try to avoid QList<T> for types T where sizeof(T)>sizeof(void*) and non-buildin/non-Qt-types, because QList performance is degraded for these.