220 30248 article Path: news.gmane.org!.POSTED!not-for-mail From: Myriachan Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: P0298: A byte type definition: with undefined pointer arithmetic? 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B. wrote: > > I'm very interested in the resolution of that issue for the same reasons. > > Currently, by my understanding, the idea that any object can be read as a > "sequence" of ((un)signed) char* is not massively useful, when one > seemingly cannot iterate past the first element in that sequence... It then > means even memcpy cannot be implemented without special magic by the > compiler. > I think that the rule ought to be that char/unsigned char pointer arithmetic is valid so long as you don't leave the bounds (or 1 past end) of the dynamic type of the most-derived object. Arithmetic with pointers of other types would be sketchy at best. It ought to be legal to do this: struct Meow { int x; float y; int z; }; void Function(Meow *meow) { int *p = &meow.x; unsigned char *b = reinterpret_cast(p); b += offsetof(Meow, z) - (offsetof(Meow, x) + sizeof(meow->x)); int *q = reinterpret_cast(b); // OK because pointer is equal to a valid int * pointer *q = *p; // same as meow->z = meow->x; } Melissa -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/db74f34b-b0ab-46cb-b94a-eeb96ef10c2d%40isocpp.org. ------=_Part_2700_971713874.1483561636496 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Wednesday, January 4, 2017 at 2:54:36 AM UTC-8, D. B. w= rote:
I&#= 39;m very interested in the resolution of that issue for the same reasons.<= br>
Currently, by my understanding, the idea that any object can b= e read as a "sequence" of ((un)signed) char* is not massively use= ful, when one seemingly cannot iterate past the first element in that seque= nce... It then means even memcpy cannot be implemented without special magi= c by the compiler.

I think that the rule oug= ht to be that char/unsigned char pointer arithmetic is valid so long as you= don't leave the bounds (or 1 past end) of the dynamic type of the most= -derived object.=C2=A0 Arithmetic with pointers of other types would be ske= tchy at best.

It ought to be legal to do this:

struct Meow
{
=C2=A0 =C2=A0 int x;
=C2=A0 =C2=A0 float y;<= /span>
=C2=A0 = =C2=A0 int= z<= span style=3D"color: #660;" class=3D"styled-by-prettify">;
};

void Function(Meow= *<= span style=3D"color: #000;" class=3D"styled-by-prettify">meow)
{
=C2=A0 =C2=A0 int *p =3D &meow.x;
=C2=A0 =C2=A0 unsigned char *b <= /span>=3D<= span style=3D"color: #000;" class=3D"styled-by-prettify"> reinterpret_cast<unsigned char *>(p);
=C2=A0 =C2=A0 b +=3D offsetof(Meo= w,<= span style=3D"color: #000;" class=3D"styled-by-prettify"> z) - (offsetof(Meow<= /span>, x) + sizeof(me= ow->x));
=C2=A0 =C2=A0 int *q =3D reinterpret_cast<int *>(b); =C2=A0// OK because pointer is equal = to a valid int * pointer
=C2=A0 =C2=A0 *q =3D= *<= span style=3D"color: #000;" class=3D"styled-by-prettify">p; =C2=A0 // same as meow->z =3D meow->x;<= /span>
= }


Melissa

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+unsubscribe@isocpp.org.
To post to this group, send email to std-proposals@isocpp.org.
To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/db74f34b-b0ab-46cb-b94a-eeb96ef10c2d= %40isocpp.org.
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