Sean Parent developed ‘forest’ for C++ that automatically maintains the invariant that its structure is always a hierarchy. It includes full-order iteration through its default iterator: https://stlab.cc/2020/12/01/forest-introduction.html
Don't need to go that far, std::map is a tree (the standard does not dictate it, but it is in all ilmplementations), and you could always traverse it. The whole idea of iterators was popularized by C++ and its STL.
std::map uses a tree as an implementation detail to achieve certain performance guarantees. It is not a tree from the user’s perspective, however. That is, there are no parent/child relationships between the elements in a std::map.
