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Subject: Re: [std-proposals] Allowing access to object representations
From: sdkrystian (sdkrystian_at_[hidden])
Date: 2019-08-21 15:00:15


No, the range of representable values for `unsigned` does not include negative numbers.http://eel.is/c++draft/basic.types#basic.fundamental-2.sentence-5Sent 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.>> >> -->> >> Std-Proposals mailing list>> >> Std-Proposals_at_[hidden] <mailto:Std-Proposals_at_[hidden]>>> >> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals>> >>> -- >> Std-Proposals mailing list>> Std-Proposals_at_[hidden] <mailto:Std-Proposals_at_[hidden]>>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals>> -- >> Std-Proposals mailing list>> Std-Proposals_at_[hidden] <mailto:Std-Proposals_at_[hidden]>>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals> -- Std-Proposals mailing listStd-Proposals_at_[hidden]https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals



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