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Subject: Re: [std-proposals] Allowing access to object representations
From: language.lawyer_at_[hidden]
Date: 2019-08-24 18:53:56


On 25/08/2019 02:47, sdkrystian via Std-Proposals wrote:
> > 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.

I like interpreting it as applying only to prvalues.
Because prvalues has values as their _result_ (http://eel.is/c++draft/basic.lval#def:result,prvalue)
  
> -------- 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`.
> --
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