Date: Sat, 24 Aug 2019 19:47:02 -0400
> However, there is no analog of [expr.pre]/4 for assignment.It depends on how you interpret it. Assignment yields a glvalue, which will be evaluated (unless in an unevaluated context) and its result is an object - whether or not you interpret "[...] result is not mathematically defined or not in the range of representable values [...]" as applying uniformly to all value categories (prvalues, or in the case of an glvalue, the value of the result) or if it only applies to prvalues will change the meaning.
-------- Original message --------From: "language.lawyer--- via Std-Proposals" <std-proposals_at_[hidden]> Date: 8/24/19 17:35 (GMT-05:00) To: std-proposals_at_[hidden] Cc: language.lawyer_at_[hidden] Subject: Re: [std-proposals] Allowing access to object representations On 21/08/2019 22:31, language.lawyer_at_[hidden] wrote:> 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.However, there is no analog of [expr.pre]/4 for assignment.In the code int x = WHATEVER; *reinterpret_cast<char*>(&x) = 0;the value of `x` shall become `0`.-- Std-Proposals mailing listStd-Proposals_at_[hidden]://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
-------- Original message --------From: "language.lawyer--- via Std-Proposals" <std-proposals_at_[hidden]> Date: 8/24/19 17:35 (GMT-05:00) To: std-proposals_at_[hidden] Cc: language.lawyer_at_[hidden] Subject: Re: [std-proposals] Allowing access to object representations On 21/08/2019 22:31, language.lawyer_at_[hidden] wrote:> 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.However, there is no analog of [expr.pre]/4 for assignment.In the code int x = WHATEVER; *reinterpret_cast<char*>(&x) = 0;the value of `x` shall become `0`.-- Std-Proposals mailing listStd-Proposals_at_[hidden]://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
Received on 2019-08-24 18:49:19