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Re: UB, unions, common initial sequence and constexpr

From: Lénárd Szolnoki <cpp_at_[hidden]>
Date: Fri, 21 Oct 2022 14:00:06 +0100

It's explicitly not allowed in constexpr:



On 21 October 2022 12:36:57 BST, Wilhelm Meier via Std-Discussion <std-discussion_at_[hidden]> wrote:
>HI all,
>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 active
> constexpr char t1 = u.b.e0; // should be ok, because read via a struct with common initial sequence, but also in a constexpr context?
>Std-Discussion mailing list

Received on 2022-10-21 13:00:12