In a C++ designated initialization, members may be omitted. (You were told wrong.)

There is probably no reason to enforce order unless unmentioned elements have a non-trivial destructor, so that could be relaxed in a future Standard. Members that need destruction would need to be destroyed in the opposite order; enforcing order allows reusing code already generated for the containing-object destructor, but in many interesting cases (e.g. C structs) there are no destructors to run anyway. In the others, there is no reason why it would need to re-use the class destructor.