There are many contexts in which it is useful to perform the same operation on each subobject of an object in sequence. These include serialization or [formatting](https://www.boost.org/doc/libs/1_85_0/libs/describe/doc/html/describe.html#example_print_function) or [hashing](https://www.boost.org/doc/libs/1_85_0/libs/describe/doc/html/describe.html#example_hash_value).