Unions, common-initial subsequence and UB

Unions, common-initial subsequence and UB

[Wilhelm Meier]

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.

Thanks for hints,
  Wilhelm

Re: Unions, common-initial subsequence and UB

[Wilhelm Meier]

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
}