The dataChanged Signal Not Performing as Expected

[Corny]

Though this thread is rather old, I’m posting this in the event someone else comes across the same situation. Even though I indicated that I thought I had found the problem. It wasn’t until a bit later that I figured out the real problem.

It turns out that the QAbstractItemModel documentation for dataChanged(), states among other things that...

[signal] void QAbstractItemModel::dataChanged(const QModelIndex &topLeft, const QModelIndex &bottomRight, const QVector<int> &roles = QVector<int> ())
...
The optional roles argument can be used to specify which data roles have actually been modified. An empty vector in the roles argument means that all roles should be considered modified. The order of elements in the roles argument does not have any relevance.

Based on other Qt documentation that shows a value assigned to an argument apparently indicating that it is an optional argument, and that I wanted to address all roles, I assumed (erroneously), that if you wanted to address all of the rolls the role argument could be an empty vector, or the role argument was optional. However, emitting the signal without a role argument appeared to work but proved to be problematic. Then, when I came across the QAbstractItemView documentation for the dataChanged() signal, it stated in part...

[virtual protected slot] void QAbstractItemView::dataChanged(const QModelIndex &topLeft, const QModelIndex &bottomRight, const QVector<int> &roles = QVector<int> ())
...
The roles which have been changed can either be an empty container (meaning everything has changed), or a non-empty container with the subset of roles which have changed.

Which I took to mean that the role argument itself is not optional and a container must be used as the argument, but the contents of the container can be empty if you want to address all roles.

Sending an empty container QVector<int> solved the problem.

[ChristianEhrlicher]

Which I took to mean that the role argument itself is not optional and a container must be used as the argument, but the contents of the container can be empty if you want to address all roles.

This is nonsense - not passing the third argument will let the compiler add the default value for the third argument which is an empty QVector<int>() - so it's *exactly to same if you call dataChanged(idx, idx2, QVector<int>()) or dataChanged(idx, idx2) or dataChanged(idx, idx2, {})

[Corny]

I'm not sure how to respond to your reply other than that when emitting the signal without anything for the roles argument dataChanged(idx, idx2), I encounter the problems outlined above. When I actually emit using dataChanged(idx, idx2, QVector<int>()), I no longer experience those problems. The results have proven to be repeatable on multiple test runs, using both methods of emitting the signal.

I may have stated it in a manner that is not easily understood, but as noted, I have run multiple tests using both methods, and have repeatably achieved the exact same results for the equivalent method used. Is there something that I'm not seeing?

[ChristianEhrlicher]

There must be another reason - from the C++ side it doesn't matter - see https://en.cppreference.com/w/cpp/la...ault_arguments or a good C++ book.

[Corny]

I don't disagree, that in C++ that's the way it works. However I am at a loss understanding what exactly is happening here. I am simply attempting to let anyone else who may run into a similar circumstance, know how I resolved the situation.

[d_stranz]

I don't disagree, that in C++ that's the way it works.

As Christian points out, there is absolutely no difference between emitting dataChanged() with two arguments vs. with a third argument being an empty (but explicitly declared) QVector<int>(). Both versions should compile to exactly the same thing (with the compiler inserting the default argument in the first instance), so there is absolutely no way that a slot receiving the signal can tell how it was called in the source code. If you are seeing a difference in behavior if you build and run code written first one way and then the other with no other code changes and exactly the same runtime test conditions then I would also be at a loss to explain why.