Date: Fri, 25 Sep 2020 18:39:52 +0100 From: Jonathan Wakely <jwakely@...hat.com> To: Alejandro Colomar <colomar.6.4.3@...il.com> Cc: fweimer@...hat.com, linux-man@...r.kernel.org, libc-alpha@...rceware.org, gcc@....gnu.org, rusty@...tcorp.com.au, linux-kernel@...r.kernel.org, libstdc++@....gnu.org, libc-coord@...ts.openwall.com, enh@...gle.com Subject: Re: [PATCH v2] <sys/param.h>: Add nitems() and snitems() macros On 25/09/20 18:30 +0200, Alejandro Colomar via Libstdc++ wrote: >Hello Jonathan, > >On 2020-09-25 16:48, Jonathan Wakely wrote: >> Do you really need to provide snitems? >> >> Users can use (ptrdiff_t)nitems if needed, can't they? > >They can, but that adds casts in the code, >which makes longer lines that are somewhat harder to read. >To avoid that, users may sometimes omit the cast with possible UB. >BTW, I use > >IMO, array indices should be declared as 'ptrdiff_t' always, >and not 'size_t'. More generically, I use unsigned integer types for two >reasons: bitwise operations, and library functions that require me to >do so. > >I don't intend to force anyone with my opinion, of course, >but if I were to choose a type for 'nitems()', it would be 'ptrdiff_t'. > >However, for legacy reasons people will expect that macro to be unsigned, >so I'd have 'nitems()' unsigned, and then a signed version prefixed >with an 's'. > >Some very interesting links about this topic: > >Bjarne Stroustrup (and others) about signed and unsigned integers: >https://www.youtube.com/watch?v=Puio5dly9N8&t=12m56s >https://www.youtube.com/watch?v=Puio5dly9N8&t=42m41s > >The two links above are two interesting moments of the same video. > >I guess that might be the reason they added std::ssize, BTW. Yes, I'm aware of all the rationale. I already said that it makes sense in C++ where you have generic code. I am not convinced that it's necessary to add to <sys/param.h> when all it does is a cast from size_t to ptrdiff_t.
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