On Wed, 27 Oct 2021 at 20:36, <language.lawyer_at_[hidden]> wrote:

> On 27/10/2021 15:22, Yongwei Wu via Std-Discussion wrote:
> > 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.
>
> According to the current C++ model, access only happens to the object
> denoted by a glvalue. Access to a subobject doesn't count as access to the
> containing object.
> And the evaluation of the object expression of a class member access
> expression *itself* (i.e. ignoring what happens in its subexpressions) is
> not considered access to the class object it denotes.
>

Let me understand it better by an example:

struct A { uint32_t v1; uint32_t v2; };
struct B { uint32_t v1; uint16_t v2; };
A a;
B* pB = reinterpret_cast<B*>(&a);

Question: Is reading pB->v1 undefined behaviour?

I believe it was the case in C++17. Does the new definition of "access" (in
3.1) imply that pB->v1 is no longer undefined?

Best regards,

Yongwei

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