I do not understand when to use QAbstractItemModel::createIndex()

JonB

This question inspired by my answer at https://forum.qt.io/post/817523. Although I believe my answer to the OP there is correct, I (apparently like him too) do not understand when one needs to use createIndex(), as opposed to just index() itself.

In have written proxy models where I override map{From,To}Source(), and my code works, but it doesn't mean I understand!

In, say, https://codebrowser.dev/qt6/qtbase/src/corelib/itemmodels/qidentityproxymodel.cpp.html#_ZNK19QIdentityProxyModel11mapToSourceERK11QModelIndex we see map{From,To}Source() using createIndex/createSourceIndex() respectively.

But in, say, https://stackoverflow.com/a/18130955/489865 we see those methods using index/sourceModel()->index() instead.

Could someone explain what calling createIndex() versus index() is all about? Preferably in terms I can understand :)

Christian Ehrlicher

In short - index() is a public function which can and should be called from outside to get a proper index for the given row, column and parent.
createIndex() is the internal function which actually creates a new QModelIndex() with the given row, column and some internal data to e.g. build up the parent-child tree structure for a QTreeView. Some models also store a pointer (or similar) to the actual data structure behind this model. E.g. the id to the database row so when calling data() one could do a lookup in a QMap by this id.
so index() normally calls createIndex() after some sanity checks (see e.g. QStandardItemModel

JonB

@Christian-Ehrlicher
Fair enough. It's just that in those two links, both doing a proxy model derived from a base, one calls index() in map{From,To}Source() while the other calls createIndex(). Not clear which then is the best or right one.

GrecKo

The mapToSource in the StackOverflow answer is incorrect. Not because it calls index but because it returns an index from the proxy model and not from the source model.
It should be sourceModel->index(...) instead of index(...).

createIndex is potentially cheaper to call, but for that you'd have to now what should the internal pointer be, so it is not always possible to do so. index would set the internal pointer correctly.

Call createIndex when you can (or need), call index instead.
In a tree model the internal pointer is needed to retrieve where you are in the tree hierarchy from an index, in a flat table or list model it is not needed but might be used for optimization.