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Subject: Re: [std-proposals] Allowing access to object representations
From: Jake Arkinstall (jake.arkinstall_at_[hidden])
Date: 2019-08-21 18:24:14


It's nothing to do with the value of the int object as a whole, but of the
leftmost byte. Why on earth would it *not* depend on endianness?

On Wed, Aug 21, 2019 at 11:49 PM sdkrystian via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> > On big-endian systems we'll end up with `ch==0`; on little-endian
> systems we'll have `ch==42`.
>
> I think not - the value of the int object does not truncate. It's value is
> 42.
>
> -------- Original message --------
> From: Arthur O'Dwyer via Std-Proposals <std-proposals_at_[hidden]>
> Date: 8/21/19 17:30 (GMT-05:00)
> To: std-proposals_at_[hidden]
> Cc: Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]>
> Subject: Re: [std-proposals] Allowing access to object representations
>
> On Wed, Aug 21, 2019 at 4:37 PM language.lawyer--- via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> On 21/08/2019 22:51, language.lawyer_at_[hidden] wrote:
>> > On 21/08/2019 22:44, Timur Doumler via Std-Proposals wrote:
>> >> So you're saying that, even without any pointer arithmetic, just this
>> code:
>> >>
>> >> int x = 100000;
>> >> std::cout << *reinterpret_cast<char*>(&x);
>> >>
>> >> has undefined behaviour?
>> >>
>> >> If that's the case then this is even more insane than I thought.
>> Please clarify whether this is really what you're saying here!
>> >
>> > I suspect you was trying to answer my mail, but anyways yes, I think
>> that your example has UB.
>>
>> UB if 100000 is outside of the range representable by char, ofc.
>> On a platform where char represents the same range of values as int the
>> code obviously won't have UB.
>>
>
> Well, regardless of your views on UB, we can all agree that the behavior of
> int x = 42;
> char ch = *reinterpret_cast<char*>(&x);
> is not portable. On big-endian systems we'll end up with `ch==0`; on
> little-endian systems we'll have `ch==42`. (This contradicts something
> sdkrystian said earlier; he seemed to be under the impression that `ch==42`
> always.)
>
> Whether the integer value of `x` happens to be less than CHAR_MAX doesn't
> matter at all; the behavior is implementation-defined at best — and UB at
> worst. I'm inclined to believe the people who say it's UB, because in my
> experience pretty much anything involving `reinterpret_cast` is UB.
>
> However, all is not lost — it'll still do what you expect, in practice! :)
>
> –Arthur
> --
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>



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