H5Iobject_verify returns a pointer to the memory referenced by id after verifying that id is of type id_type. This function is analogous to dereferencing a pointer in C with type checking.
H5Iregister(H5I_type_ttype, void *object) takes an H5I_type_t and a void pointer to an object, returning an hid_t of that type. This hid_t can then be passed to H5Iobject_verify along with its type to retrieve the object.
H5Iobject_verify does not change the ID it is called on in any way (as opposed to H5Iremove_verify, which removes the ID from its type’s hash table).
Pointer to the object referenced by id on success, NULL on failure.