[Development] Qt6 source changes

[Иван Комиссаров]

Hello, Thiago wrote in the "buildsystem thread" that Qt 6 will be source compatible with Qt5. As long as I fully support source-compatibility with both hands, I would like to ask several questions about possible source-breaking changes.

1) QAbstractItemModel. I want to introduce change that adds QModelIndex parameter to  headerData(). For now, QHeaderView ignores the rootIndex() property which prevents creating models with sub tables in each index (only tree models are supported now). The example - you need to display a struct (or, protobuf message) as a table. Each struct can have sub structs (or, arrays of structs) so the tree is not enough to display it. I wrote a working prototype back in 2015 which required subclassing QAIM and copying the whole QHeaderView into the project. In this case source compatibility can be saved by having 2 virtual headerData methods, the one without modelIndex market deprecated.

2) QVariantList/QList<QImage> and friends. I would like to to see most of QList usages in Qt eliminated in favor of QVector. I won't repeat Marc's arguments against QList. Instead, I want to ask the other thing - is breaking user code that uses "QList<T> list = someQtApiFunc();" is forbidden too? If Assumption that QVector is API-compatible with QList (which is not true, I suppose, but can be mostly fixed) that should not be a big deal to replace the usages of a QList with "auto list = someQtApiFunc();". Qt should really evolve and stucking with QList for the next 10 years is a bad idea IMO.

Иван Комиссаров