On Wed, 27 Oct 2021 at 19:57, <language.lawyer_at_[hidden]> wrote:

> On 27/10/2021 14:44, Yongwei Wu via Std-Discussion wrote:
> > When looking up the exact wording of aliasing rules in C++, I am
> surprised
> > to find that much content seems to be gone in the C++20 (draft, N4860)
> > under 7.2.1 p11. Specifically, there is no mention of the validity of
> > aliasing with "a type that is a (possibly cv-qualified) base class type
> of
> > the dynamic type of the object", which existed in earlier C++ standards.
> >
> > Also the online link: https://eel.is/c++draft/basic.lval#11
> >
> > I do not think the standard has changed the rule, so I must have missed
> > something. Can anyone point to me how to read the rules, maybe pointing
> me
> > to a discussion of the wording change?
>
> https://wg21.cmeerw.net/cwg/issue2051
>
> https://stackoverflow.com/questions/56878519/what-happened-to-the-aggregate-or-union-type-that-includes-one-of-the-aforement
>
> TL;DR the removed rules have never been applicable.
>

Thanks for the info. But the issue is not completely resolved for me.

Say:

struct A { … };
struct B : A { … };
A* pA = new B{…};

Is using pA->… to access the base-class fields undefined behaviour?

Interpreting the rules literally, it seems to be the case … but that seems
very surprising to me.

-- 
Yongwei Wu
URL: http://wyw.dcweb.cn/