C++ Logo


Advanced search

Re: [basic.life]/8.5 interpretation

From: language.lawyer_at <language.lawyer_at_[hidden]>
Date: Thu, 10 Sep 2020 13:30:47 +0300
I think, p1 p2 must be the old and the new objects, they can't be the same object.
I assume p1 and p2 are needed for the following code:

struct S { int i; };

S s {};
int& ri = s.i;

new (&s) S {};

where the wording guarantees that `ri` will refer to the new int subobject (o2) of the object of type S (p2)

On 10/09/2020 12:43, jim x via Std-Discussion wrote:
> 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

Received on 2020-09-10 05:34:20