* Unions, common-initial subsequence and UB
@ 2022-10-20 11:46 Wilhelm Meier
2022-10-20 13:10 ` Wilhelm Meier
0 siblings, 1 reply; 2+ messages in thread
From: Wilhelm Meier @ 2022-10-20 11:46 UTC (permalink / raw)
To: gcc-help
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
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Unions, common-initial subsequence and UB
2022-10-20 11:46 Unions, common-initial subsequence and UB Wilhelm Meier
@ 2022-10-20 13:10 ` Wilhelm Meier
0 siblings, 0 replies; 2+ messages in thread
From: Wilhelm Meier @ 2022-10-20 13:10 UTC (permalink / raw)
To: gcc-help
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
}
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