[Interest] 64-bit internal ID for QModelIndex?

[Nikos Chantziaras]

On 22/08/12 00:54, Stephen Chu wrote:
> On 8/21/12 5:47 PM, Nikos Chantziaras wrote:
>> On 21/08/12 23:55, Stephen Chu wrote:
>>> On 8/21/12 4:26 PM, Nikos Chantziaras wrote:
>>>> On 21/08/12 22:45, Stephen Chu wrote:
>>>>> On 8/21/12 3:35 PM, Christoph Feck wrote:
>>>>>> On Tuesday 21 August 2012 20:55:00 Stephen Chu wrote:
>>>>>>> Is there a way to store a 64-bit internal ID in QModelIndex? The
>>>>>>> function QModelIndex::internalId() returns a 64-bit number but
>>>>>>> there's no way to put one in. The constructor takes only a 32-bit
>>>>>>> integer.
>>>>>>
>>>>>> Try using the overload that takes the void * pointer. The return value
>>>>>> is 64 bit, because pointers can also be 64 bit, and C++ doesn't allow
>>>>>> different overloads with different return values.
>>>>>
>>>>> Thanks for the help. But my app is currently 32-bit. :(
>>>>
>>>> Would a pointer to a qint64 work?
>>>
>>> Yes, it will. I was thinking about that but got stuck trying to figuring
>>> out how I can manage the memory since QModelIndedx does not own the
>>> object/memory pointed by the void*
>>>
>>> Say I allocate a QList<int64_t> and create indices with internal pointer
>>> pointing to elements in the list. When I need to update the model from
>>> external data, I will have to re-allocate the list and the existing
>>> pointers will be left dangling. What is the proper steps to update the
>>> model data in this case?
>>
>> Note that I didn't work with QAbstractItemModel before, so perhaps I'm
>> not the most suitable person here to give advice.  With that being said,
>> you might be able to allocate like this:
>>
>>      createIndex(row, column, new qint64(value));
>>
>> Access the value with:
>>
>>      *(qint64*)internalPointer();
>>
>> And when you don't need the QModelIndex anymore, you deallocate like this:
>>
>>      delete (qint64*)internalPointer();
>>
>> You should probably create wrappers for those operations.  Or work out a
>> subclass model.
>>
>> But again, I didn't work with that part of Qt before, so I might be
>> talking gibberish here ;-)
>>
>
> Actually it makes a lot of sense and I was trying to do the same. But I
> can not find out when a QModelIndex is no longer in use.

I see QAbstractItemModel has some signals:

   columnsAboutToBeRemoved()
   rowsAboutToBeRemoved()

It also has some virtual functions:

   removeRows()
   removeColumns()

They might be of some help in determining what's no longer needed.