On Wed, 27 Oct 2021 at 20:36, <language.lawyer@gmail.com> wrote:
On 27/10/2021 15:22, Yongwei Wu via Std-Discussion wrote:
> On Wed, 27 Oct 2021 at 19:57, <language.lawyer@gmail.com> 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 Wu
URL: http://wyw.dcweb.cn/