Re: Unions, common-initial subsequence and UB

[Wilhelm Meier <wilhelm.meier@xxxxxxxx>]

On 20.10.22 13:46, Wilhelm Meier wrote:
> Hi all,
>
> this is more a general question instead of a g++ specific one:
>
> the standard explicitly makes this exception: accessing an inactive 
> union member is not UB if the members are structs with initial common 
> sequence.
>
> Is this also true in a constexpr context? I ask this because the chapter 
> for constexpr expression does not mention this exception accessing an 
> inactive memeber.

Here is some test-code:

struct A {
    char e0;
};
struct B {
    char e0;
};

template<typename A, typename B>
union U {
    A a;
    B b;
};

int main() {
    constexpr U<A, B> u{.a = {1}}; // sets member a to active
    constexpr char t1 = u.b.e0; // should be ok, because read via a 
struct with common initial sequence
}