[Interest] 64-bit internal ID for QModelIndex?

Andreas Pakulat apaku at gmx.de
Wed Aug 22 01:02:50 CEST 2012


On Tue, Aug 21, 2012 at 11:54 PM, Stephen Chu <stephen at ju-ju.com> wrote:
> On 8/21/12 5:47 PM, Nikos Chantziaras wrote:
>> 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.

Right, you cannot figure that out since QModelIndex are pod and
allocated/discarded whenever they go out of scope. So even though your
int64 is not used by one instance anymore, it might still be used by
another instance elsewhere.

You simply cannot do this due to a stupid API bug in the itemmodel
API. If you look at createIndex it takes a quint32 overload, but the
corresponding internalId in QModelIndex returns a 64bit int, thats
just broken API in Qt. So either you can cope with 32bit integers or
you'll have to build in 64bit mode and cast the void* into a qint64.
This bug also survived the api-changes in Qt5 as it seems - sadly.


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