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QStandardItem and default data role

  • My app uses mostly QSql...Model models but also some QStandardItemModels.

    Whether I like the approach or not, I have gotten used to the SQL stuff where, coming from QAbstractItemModel, they all use:

    QVariant QAbstractItemModel::data(const QModelIndex &index, int role = Qt::DisplayRole) const
    bool QAbstractItemModel::setData(const QModelIndex &index, const QVariant &value, int role = Qt::EditRole)

    i.e. the default roles for data()/setData() are DisplayRole/EditRole respectively.

    While refactoring other people's code, to my surprise I found that in the case of QStandardItemModel it sticks to that, BUT for QStandardItem

    QVariant QStandardItem::data(int role = Qt::UserRole + 1) const
    void QStandardItem::setData(const QVariant &value, int role = Qt::UserRole + 1)

    i.e. both use a default of Qt::UserRole + 1.

    Correct me if I'm wrong, because my brain is beginning to hurt here, but this means that:, col))
    standardItemModel.item(row, col).data()

    i.e. QStandardItemModel::data()/setData() use different default roles from QStandardItem::data()/setData(). With the code I am looking at mixing these two legitimate ways of addressing QStandardItem items it's getting very hairy... :(

    What's going on here? This is a really bad idea :( Why is QStandardItem defaulting differently from everything else, is it just to confuse me?

  • Lifetime Qt Champion

    Because QStandardItemModel + QStandardItem is a convenience class and already provides access to Qt::DisplayRole and others via custom functions (setFont, setIcon, setText, setToolTip). There is no need to use QStandardItemModel::data() at all.

  • @Christian-Ehrlicher
    Before I comment on what the problems are, did you mis-type:

    There is no need to use QStandardItemModel::data() at all.

    Did you mean QStandardItem:data() instead of QStandardItemModel::data()? Or what?

  • Lifetime Qt Champion

    @JonB: It's from QAbstractItemModel::data().

  • @Christian-Ehrlicher
    I do not understand what you are talking about now. (Btw, yes, I know what is derived from what.) As I have said, there are two ways at getting at a standard model items' data:

    1., col))
    2. standardItemModel.item(row, col).data()

    The first is going via QStandardItemModel, the second via QStandardItem. Agreed? And you would assume they both return the same result, agreed? Well, they don't, because of the default role difference.

  • Lifetime Qt Champion

    @JonB said in QStandardItem and default data role:

    And you would assume they both return the same result, agreed?

    No, I don't agree here.
    Since QStandardItemModel is a convenience class you should also stick at the convenience way on accessing the data via the QStandardModelItem and it's functions I wrote above.

  • @Christian-Ehrlicher
    Right then.

    First of all I'm not interested in any of setFont, setIcon, setToolTip. What I am interested in is the different data which can be stored/returned via setData/data() using the two roles DisplayRole vs EditRole.

    And second, it's all very well saying what one "should stick at", but I am talking about reviewing code written by someone else. When storing/retrieving data in a QStandardItemModel, that person has sometimes used QStandardItemModel::set/Data() directly and sometimes used QStandardItemModel::item(...).set/Data(). Both of those address the same QStandardItem in the model. You (I) would have thought they would set/return the same value. But they do not, because of the difference in the default roles between the two seemingly-identical approaches. So the code I'm reviewing is not right. :( Which is what I am whinging about.

  • Lifetime Qt Champion

    Yes, relying on the default value isn't that good sometimes... even in the qt module tests there were some issues with the default role which only came up after a change in the convenience QTable/Tree/ListWidget classes to publish the modified roles instead no roles (3rd argument in dataChanged())

  • Lifetime Qt Champion

    Don't know if this works in Python but I C++ I would derive from QStandardItemModel, make data() private and replace all occurrences of QStandardItemModel with the new class. Then I would get a compiler warning as soon as DerivedClass::data() is called.

  • @Christian-Ehrlicher

    make data() private

    Nope, this is Python! Everything is public. Even if it's private it's still public. Unless Qt chose to rename the data/setData() methods to __data()/__setData(). And even then they'd still be public really.

    Then I would get a compiler warning as soon as DerivedClass::data() is called.

    This is Python, don't make me laugh ;-) Compiler? Warning?


  • @Christian-Ehrlicher
    But here's what I don't get even if I tried to adopt what you say. Again, please correct me if I'm wrong.

    All these models are heavily used by the code from QTableViews etc. The whole point is that uses the data()/setData() methods (with defaults DisplayRole/EditRole). So surely if you hide those two methods with privacy you won't be able to use your derived class from table views, no?

    Apparently unlike you, I do not see the problem in QStandardItemModel::data()'s behaviour. I see the problem in the non-consistent QStandardItem::data()'s behaviour. I'm not sure whether that is getting across.

    In the code, at line #10 data is being set via

    standardItemModel.setData(standardItemModel.index(0, 0), foo)

    Then at line #10,000 data is being read back from the same item via

    bar = standardItemModel.item(0, 0).data()

    I expect foo & bar to be the same value. But they are not.

  • Lifetime Qt Champion

    @JonB said in QStandardItem and default data role:

    But they are not.

    Correct, because you're mixing the convenience class with the underlying QAIM.

    --> standardItemModel.item(0,0)->setData(foo) is what you want if you want to use QStandardItemModel::data() without a parameter.
    Or, as I said before - don't rely on default parameters at all.

  • @Christian-Ehrlicher
    No! Unless yet again we are misunderstanding each other :)

    Let's be clear: foo was my data value, not a role. Then quoting from what you have just written:


    This is using QStandardItemModel::item()::setData(). QStandardItemModel::item() is a QStandardItem. QStandardItem::setData() has default parameter int role = Qt::UserRole + 1.


    This is using QStandardItemModel::data() (not QStandardItem::data()), which has default parameter int role = Qt::DisplayRole.

    For consistency --- and assuming you are not passing any role explicitly --- one must either stick to doing data operations via paired QStandardItemModel::set/Data(index(0, 0)) or via paired QStandardItemModel::item(0, 0)::set/Data(). Your example just broke that consistency, it's using one of each! The code I am looking at (not written by me, who now knows better) has equally broken that consistent pairing, and so is wrong. Hence I am now spending hours tracking down precisely where current code accesses data directly off QStandardItemModel vs via QStandardItemModel::item().

    My original question was: to avoid just this, why [there must be a reason] does QStandardItem::set/Data() use UserRole+1, instead of DisplayRole/EditRole as the respective role defaults just like say QStandardItemModel::set/Data() does and same for other models? Then I wouldn't be crying in a bath over this....

  • Lifetime Qt Champion

    QStandardItemModel is a convenience function which should be filled with QStandardItem::data()/setData(). If you mix it with QAbstractItemView::setData() then you also should use the corresponding QAIV::data() function.

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