C++ Logo

STD-DISCUSSION

Advanced search

Subject: Re: [basic.life]/8.5 interpretation
From: jim x (xmh970252187_at_[hidden])
Date: 2020-09-10 04:43:31


I think your first opinion is the intent of the rule. Because, according
to the rule [intro.object#2] <https://eel.is/c++draft/intro.object#2>, the
new object has became the subobject of the object X, and the condition "o1
and o2 are direct subobjects of objects P1 and P2" is true. The standard
does not say `P1` and `P2` shall not be the same object, the object
`X` itself satisfies all rules listed in [basic.life#8]
<https://eel.is/c++draft/basic.life#8>. So, there's no necessary to use
`std::launder`.

John Mousseau via Std-Discussion <std-discussion_at_[hidden]>
于2020年9月9日周三 上午1:49写道:

> Dear list members,
>
> in draft N4860 <https://isocpp.org/files/papers/N4860.pdf>, which I
> assume is close to C++20, the wording of [basic.life]/8 differs from C++17
> in that the notion of "transparently replaceable" was involved. I am
> wondering whether condition 8.5 of that paragraph allows for p1 and p2 to
> be the same object.
>
> Consider the following most trivial example:
>
> struct X {int i; float f;};
> X x{3, 3.f};
> new(&x.i) int(5);
> // x.i == 5 without launder? True in C++17.
>
> According to C++17's wording this is surely the case as there is no
> const-qualification involved at all. However, in the newest wording, while
> conditions (8.1) through (8.4) are fulfilled (for o1 and o2 being the old
> and new int respectively), condition (8.5) might be unfulfilled, as neither
> are both objects complete, nor are they subobjects of (different) objects
> p1 and p2, unless, by [intro.object]/2, the new int becomes a subobject of
> x, and when then consider p1 and p2 of [basic.life]/8.5 to be identical. If
> we then consider x to be transparently-replaceable by itself, the condition
> would be fulfilled. Is that the intended interpretation or is the intention
> that the access in my example now requires std::launder even though no
> constness is involved?
>
> Sorry in case I am not seeing the obvious and thank you for your time.
>
> Best Regards
> John Mousseau
> --
> Std-Discussion mailing list
> Std-Discussion_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-discussion
>



STD-DISCUSSION list run by std-discussion-owner@lists.isocpp.org

Older Archives on Google Groups