That depends entirely on the implementation and use case. An append only arena, like is used in compilers, do not have ever invalid IDs by construction: you get

That depends entirely on the implementation and use case. An append only arena, like is used in compilers, do not have ever invalid IDs by construction: you get the ID back after inserting into the arena and have no way of removing the value after that. For a use case where arbitrary removal and editing of existing values is needed, generational arenas are used so the handle encodes the "generation" of the value. If on access the generation doesn't match, it means the same as a null pointer and you won't get a value back, in a language with sum types represented with a None sentinel. Disregarding entirely the memory safety aspects of turning a pointer access to an index access, there are benefits for using arenas (or struct of arrays) like better cache locality on access/avoiding memory fragmentation, and bunching values that don't outlast each other into a single free operation.