Re: ctype.h macros may conflict with C++11 UDL

[Craig Howland <howland at LGSInnovations dot com>]

On 10/10/2018 06:59 AM, Corinna Vinschen wrote:
> On Oct  9 18:03, Glenn Elliott wrote:
> > Hello newlib maintainers,
> >
> > ctype.h “#define"s the following values: _U, _L, _N, _S, _P, _C, _X, and _B.
> >
> > These macros may conflict with C++11 user defined literals (UDL) (https://en.cppreference.com/w/cpp/language/user_literal <https://en.cppreference.com/w/cpp/language/user_literal>).  It’s easy to see how C++11 UDL suffixes might conflict it the macros defined in ctype.h.  For instance, one may define a UDL function of “_N” to instantiate a C++ type that represents newtons of force.  Indeed, this is done by this units library: https://github.com/nholthaus/units <https://github.com/nholthaus/units>
> >
> > Must these macros leak from ctype.h?
> Yes, they have to, otherwise the isXXX ctype macros can't be resolved
> successfully.  Am I the only one thinkiong it was a bit non-considerate
> of the C++ standarization commitee to use the underscore in a way
> potentially colliding with implementation-defined symbols.
>
> Probably the most feasible workaround is to change those macros to
> double underscores throughout.
>
>
> Corinna
It should be OK.  From the examples on the quoted page:

double  operator""  _Z(long  double);  // error: all names that begin with underscore
                                   // followed by uppercase letter are reserved
double  operator""_Z(long  double);  // OK: even though _Z is reserved ""_Z is allowed

Notice that it is saying _[A-Z].* is still reserved (as in C).  (The second line 
works because of the preprocessor maximal-munch rule, given that the ""_Z 
construct is defined as a possible token.)  They comment on it in the definition 
for user-defined-string-literal:

"the character sequence "" followed, without a space, by the character sequence 
that becomes the ud-suffix. This special syntax makes it possible to use 
language keywords and reserved identifiers as ud-suffixes, and is used by the 
declaration of operator ""if from the header <complex>. Note that using this 
form does not change the rules that user-defined literal operators must begin 
with an underscore: declarations such as operator ""if may only appear as part 
of a standard library header. However, it allows the use of an underscore 
followed by a capital letter (which is otherwise a reserved identifier)"

The user-defined literals are specifically called out in the definition for 
preprocessing tokens 
(https://en.cppreference.com/w/cpp/language/translation_phases Phase 3, point 
1d).  That is, the standard took special care to not clobber the reserved 
identifiers.  So the worst thing that might happen for Newlib would be that 
maybe we'd need to add some spaces where they are used, but that could be easily 
checked.

Craig