[Development] Proposal: QVariant/std::any QModelIndex::internalData() for Qt 7

[Igor Khanin]

Hi Bogdan,

Perhaps I misunderstood, but isn't `QPersistentModelIndex` Qt's current 
solution to your motivational example of needing to keep a model index 
beyond the current event loop tick? How does it currently fall short?

My understanding is that `QModelIndex` is intended as a lightweight and 
cheap to copy reference into a model as it exists at the moment it was 
created, very much akin to a STL iterator. I think that having it hold 
arbitrary data, or participate in the ownership of the model data, goes 
against that.

Best,
Igor

On 01/07/2026 16:02, Bogdan Vatra via Development wrote:
> Hello there,
>
> I'd like to propose a new accessor on QModelIndex for Qt 7 and get feedback
> from the list:
>
> QVariant QModelIndex::internalData() const;
> or
> std::any QModelIndex::internalData() const; // as std::any is 1/2 of
> sizeof(QVariant).
>
> Why?
> QModelIndex currently exposes two ways for a model to attach opaque context to
> an index:
>
>    quintptr QModelIndex::internalId() const;
>    void *QModelIndex::internalPointer() const;
>
> Both work well for *static* model content, where the model's backing data
> lives for as long as the model itself.
> They fall short for *dynamic* models, where individual items can be created,
> moved, or destroyed independently of any QModelIndex that currently references
> them.
>
> Let me give you an example which will randomly crash a QSortFilterProxyModel:
>
> // main loop has a QAbstractItemModel::dataChanged (or any other QAIM signal
> which will reset the idx internalPointer) event posted, e.g by another thread
> // get an idx
> myView->setCurrentIndex(idx);
> myView->scrollTo(idx); // At this point there is no way to know if idx is
> still valid. It depends on what `myView->setCurrentIndex(idx);` is doing and
> there is no way to know if internalPointer is still valid or not. If inside of
> that method QCoreApplication::processEvents is called, myView->scrollTo(idx)
> gets an invalid idx...
>
> How my proposal fixes this problem?
> In a QVariant/std::any we can store a smart pointer and this way the data is
> valid all the time. It can still hold a plain quintptr for models that don't
> need this, so internalId()/internalPointer() could become thin wrappers around
> the same underlying storage instead of needing a member variable of their own.
>
>
> Cheers,
> BogDan.
>
> P.S. It can not go in Qt6 as it will break the ABI ...
>
>