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That's what c++20 coroutines paired with c++23 generators are for. It's easy to define whatever traversal you want and then expose it as generic code.

https://godbolt.org/z/fnGzszf3j

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Yeah, but is this really better than explicitly managing a stack?

Here is how I one could write in C: https://godbolt.org/z/P3ErP4T4d or possibly with the setup code moved into the helper function for convenience: https://godbolt.org/z/soz7W5z1G

Sure you can do it any way but the OP was about adding language features for iteration of a specific data structure. Iteration in C++ is state machine based and coroutines give you a neat way to implicitly model them without having to manage the boiler plate yourself.

Yeah, but what I do not get is why you need all this. What problem does this solve? The C function that advances to the next tree node does exactly the same. And what is neat about it? It just seems a lot more complicated. Or in other words: A new language feature should make things simpler and not more complicated to express. But maybe I am just missing a realistic examples where it actually makes things simpler?

Great example!

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