QList: Convenient way to deep copy a pointer list?

Asperamanca

Let's say I have a list of pointers, like so:
QList<MyObject*> list;

MyObject has a valid copy constructor and assignment operator.
Is there a simple way to create a new list that is a deep copy of 'list'?

Something like this:
QList<MyObject*> copyOfList = qDeepCopy(list);

mrjj

Hi
What does using operator = yield?
http://doc.qt.io/qt-5/qlist.html#operator-eq

Asperamanca

@mrjj said in QList: Convenient way to deep copy a pointer list?:

Hi
What does using operator = yield?
http://doc.qt.io/qt-5/qlist.html#operator-eq

Depending on whether the type is marked as moveable or not, it either calls the copy constructor of T (the copy constructor of a pointer thus copying the pointer - not the content), or std::move or memcpy...the code gets a little complex there.

mrjj

Oh so in the other copy of the list,
it should not point to same T* but create new instances (of T) and change the new list to point to these?
Sorry, that im pretty sure you must implement yourself. :)

You could derive from QList and make overloaded = that does it for you but
might be overkill.

If MyObject is QObject based, you might get issues cloning them but I assume newing it / copy ctor is
already working ?

raven-worx

@mrjj said in QList: Convenient way to deep copy a pointer list?:

You could derive from QList and make overloaded = that does it for you but
might be overkill.

no need to. QList internally uses template methods. So implementing those methods directly for the custom type is already enough.

Unfortunately i can't tell you what the correct settings are, since i don't have time to check it in the sources right now.
But the first thing to change the behavior of QList is to generate QTypeInfo and the QList::node_construct():

template <>
class QTypeInfo<MyType>
{
public:
    enum {
        isPointer = true,
        isComplex = true,
        isStatic = false,
        isLarge = false,
        isDummy = false
    };
    static inline const char *name() { return "MyType"; }
};

template <>
Q_INLINE_TEMPLATE void QList<MyType>::node_construct(Node *n, const MyType &t)
{
    if (QTypeInfo<EyeOmsVariant>::isComplex)
            new (n) MyType(t);
    else
         ::memcpy(n, &t, sizeof(MyType));
}

Add something like this (implement it correctly) and the compiler will use them, instead creating them on-the-fly when needed.
(read on C++ templates for details).

Asperamanca

Thanks, but this is indeed overkill for my use case. I was hoping for an existing template method in the veins of qDeleteAll. If I come across this more often, I'll write it myself.

BTW I wouldn't do type-specific template overloading in this case. Who says that I always want to deep-copy a pointer list of a certain type? But it's an interesting approach, I didn't think of that. Thanks again!