[RFC] Characters per line: from punch card (80) to line printer (132) (was: [Patch][OpenMP/OpenACC/Fortran] Fix mapping of optional (present|absent) arguments)

[Jonathan Wakely]

On Thu, 5 Dec 2019 at 22:19, Segher Boessenkool wrote:
>
> On Thu, Dec 05, 2019 at 08:56:35PM +0000, Jonathan Wakely wrote:
> > On Thu, 5 Dec 2019 at 20:07, Segher Boessenkool
> > <segher@kernel.crashing.org> wrote:
> > > On Thu, Dec 05, 2019 at 05:03:43PM +0000, Jonathan Wakely wrote:
> > > > C++17 introduces a nice feature, with rationale similar to declaring
> > > > variables in a for-loop init-statement:
>
> > > > Unfortunately nearly every time I've tried to use this recently, I've
> > > > found it's impossible in 80 columns, e.g. this from yesterday:
> > > >
> > > >     if (auto __c = __builtin_memcmp(&*__first1, &*__first2, __len) <=>
> > > > 0; __c != 0)
> > > >       return __c;
> > > >
> > > > When you're forced to uglify every variable with a leading __ you run
> > > > out of characters pretty damn quickly.
> > >
> > > If using this "nice feature" forces you to uglify your code, then maybe
> > > it is not such a nice feature, and you should not use it.
> >
> > The uglification has absolutely nothing to do with the 'if'
> > init-statement feature, all code in libstdc++ headers has to be
> > uglified, always. Blame the C preprocessor for that, not C++ features.
> >
> > My point is that 80 characters runs out quicker when 10% of it goes on
> > visual noise that's only needed because the C preprocessor means we
> > can't have nice names.
>
> (Not sure where the preprocessor comes in, these underscores are just to
> satisfy language rules afaics, but maybe you mean something else?)

The language rules are only necessary because of the preprocessor.
Users can define macros with names like "pred" and "cmp" before
including any standard library header, and because macros don't
respect lexical scope, that would break any standard library code
using "pred" and "cmp". So the std::lib has to use reserved names.

It's entirely due to the preprocessor that we can't use sane names for
local variables, or for any implementation detail in a header (unlike
in C, our helper types and functions are in a namespace so won't
collide with users' types and functions, but we still have to use
reserved names because the preprocessor doesn't respect namespaces).