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Subject: Re: [std-proposals] Allowing access to object representations
From: language.lawyer_at_[hidden]
Date: 2019-08-21 15:03:55


Indeed, the code has UB to my reading.
It is funny that in C++20 we have "2's complement" integers and placing very strict restrictions on how the values of congruent values are stored and yet the behavior of such code is undefined.

On 21/08/2019 23:00, sdkrystian via Std-Proposals wrote:
> No, the range of representable values for `unsigned` does not include negative numbers.
>
> http://eel.is/c++draft/basic.types#basic.fundamental-2.sentence-5
>
> Sent from my Samsung Galaxy smartphone.
>
> -------- Original message --------
> From: "language.lawyer--- via Std-Proposals" <std-proposals_at_[hidden]>
> Date: 8/21/19 15:56 (GMT-05:00)
> To: std-proposals_at_[hidden]
> Cc: language.lawyer_at_[hidden]
> Subject: Re: [std-proposals] Allowing access to object representations
>
> BTW, does the following code has defined behavior?
>
> int x = -1;
> std::cout << *reinterpret_cast<unsigned*>(&x);
>
> 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!
> >
> > Cheers,
> > Timur
> >
> > On 21 Aug 2019, at 21:33, sdkrystian via Std-Proposals <std-proposals_at_[hidden] <mailto:std-proposals_at_[hidden]>> wrote:
> >
> >> Ah, ignore my example then.
> >>
> >>
> >>
> >> Sent from my Samsung Galaxy smartphone.
> >>
> >> -------- Original message --------
> >> From: "language.lawyer--- via Std-Proposals" <std-proposals_at_[hidden] <mailto:std-proposals_at_[hidden]>>
> >> Date: 8/21/19 15:31 (GMT-05:00)
> >> To: std-proposals_at_[hidden] <mailto:std-proposals_at_[hidden]>
> >> Cc: language.lawyer_at_[hidden] <mailto:language.lawyer_at_[hidden]>
> >> Subject: Re: [std-proposals] Allowing access to object representations
> >>
> >> On 21/08/2019 22:11, Timur Doumler wrote:
> >> > This doesn't make sense to me.
> >>
> >> That's why some people think this should be fixed.
> >>
> >> > If I have an object of type int (or, say, a struct containing an int), then the value has 4 bytes. If I access that value by dereferencing a char pointer that aliases that object, all I can ever get from that char is a single byte. How can this be "the object"?
> >>
> >> The standard just says what result you get, it is an implementation's problem to answer the "how" question.
> >> However, http://eel.is/c++draft/expr.pre#4 could help some implementations:
> >> > If during the evaluation of an expression, the result is not mathematically defined or not in the range of representable values for its type, the behavior is undefined.
> >>
> >> If you have an int object and try to access its value through a char lvalue and the value stored in the int object is not in the range representable by the char type, the behavior is undefined.
> >>
> >> [expr.pre]/4 helps little-endian machine implementations, because they just can fetch the first byte of the int object and this would give the right value when there is no UB.
> >> IDK how a big-endian maching implementation can achieve the standard-conforming behavior :-)
> >>
> >> > Cheers,
> >> > Timur
> >> >
> >> >> On 21 Aug 2019, at 20:59, language.lawyer--- via Std-Proposals <std-proposals_at_[hidden] <mailto:std-proposals_at_[hidden]>> wrote:
> >> >>
> >> >>> On 21/08/2019 21:55, Krystian Stasiowski via Std-Proposals wrote:
> >> >>> Yes, you access the value of the object of the object, however, since its not specified what exactly you get
> >> >>
> >> >> It is specified: you get the value contained in the object.
> >> >>
> >> >>> http://eel.is/c++draft/conv.lval#3.4
> >> >>
> >> >> Indeed, it is specified here.
> >> >> --
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> >> >
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