Question

I need to verify that I can use unions a certain way.

For C99, this answer is adequate: Union element alignment

C99 – section 6.7.2.1 Structure and
union specifiers (paragraph 14):

A pointer to a union object, suitably
converted, points to each of its
members (or if a member is a bitfield,
then to the unit in which it resides),
and vice versa.

Can someone please help me find the equivalent guarantee for C++?

More generally, is there a good source for finding section/paragraph citations for C++ standards which are equivalent to a known feature in C?

Thanks a lot.

Answer 1

In C++ (14882:2003), this is split between two paragraphs:

9.5[class.union]/1

Each data member is allocated as if it
were the sole member of a struct.

9.2[class.mem]/17

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.

I haven’t heard of a cross-reference between C and C++ like that, it may be non-trivial, as this example shows.