RAII doesn't imply allocating.

My guess is that you're assuming all user defined types, and maybe even all non-trivial built-in types too, are boxed, meaning they're allocated on the heap when we create them.

That's not the case in C++ (the language in question here) and it's rarely the case in other modern languages because it has terrible performance qualities.

Open a file in the constructor, close it in the destructor. RAII with 0 allocations.

RAII doesn't necessarily require allocation?

Stack "allocations" are basically free.