A::x as B::x where B is layout-compatible with A (assuming same member
names for this example).
Then you would be alone on your interpretation. I don't have a C spec
handy, but for C++03, that is not the case.
This is what I can find on the subject in the standard itself.
C++03 standard, 9.2 Class members / 16
If a POD-union contains two or more POD-structs that share a common
initial sequence, and if the PODunion
object currently contains one of these POD-structs, it is permitted to
inspect the common initial part
of any of them. Two POD-structs share a common initial sequence if
corresponding members have layoutcompatible
types (and, for bit-fields, the same widths) for a sequence of one or
more initial members.
<<<<
The rule above only applies to PODs in a union.
C++03 standard, 9.2 Class members / 167
A pointer to a POD-struct object, suitably converted using a
reinterpret_cast, points to its initial
member (or if that member is a bit-field, then to the unit in which it
resides) and vice versa. [Note: There
might therefore be unnamed padding within a POD-struct object, but not
at its beginning, as necessary to
achieve appropriate alignment. ]
<<<<
The section above, especially with the (non-binding) note, pretty
clearly states that the C-style hack of inheritance may not work in C+
+. There might be unnamed padding which differs between different POD
structs.
Frankly though, this entire thing is a mess. When you compare the
guarantees of the two quotes, /which appear right next to each other
in the standard/, I don't understand how you can reconcile them in a
sane implementation. So, when the POD types are members of a union,
there's no difference in padding bits, but when the same POD types are
not members of a union, there might be extra magical padding bits.
What?