Doh! 

I meant data members.

Wow the proposal examples was so misleading,  i was thinking about data members construction after reading the examples then i extrapolated to base classes.

Yes, as Jason pointed, base classes initialization order is well defined.

But data members of an object, as the proposal points to in its example, can be constructed in different order, at best you will have a compiler warning telling you that you initialized some data member before the other.

class PublicInterface {
    std::unique_ptr<class Impl> pimpl_;
};

"pimpl_", is data member not a base class (subobject)

Thanks Jason, nice catch, it was lucid 👌 

Nadir