On Fri, Jun 11, 2021 at 11:52:41AM -0400, Tom Honermann via Gcc-patches wrote:
> On 6/7/21 5:11 PM, Joseph Myers wrote:
> > On Sun, 6 Jun 2021, Tom Honermann via Gcc-patches wrote:
> >
> > > When -fchar8_t support is enabled for non-C++ modes, the _CHAR8_T_SOURCE
> > > macro
> > > is predefined. This is the mechanism proposed to glibc to opt-in to
> > > declarations of the char8_t typedef and c8rtomb and mbrtoc8 functions
> > > proposed
> > > in N2653. See [2].
> > I don't think glibc should have such a feature test macro, and I don't
> > think GCC should define such feature test macros either - _*_SOURCE macros
> > are generally for the *user* to define to decide what namespace they want
> > visible, not for the compiler to define. Without proliferating new
> > language dialects, __STDC_VERSION__ ought to be sufficient to communicate
> > from the compiler to the library (including to GCC's own headers such as
> > stdatomic.h).
> >
> In general I agree, but I think an exception is warranted in this case for a
> few reasons:
>
> 1. The feature includes both core language changes (the change of type
> for u8 string literals) and library changes. The library changes
> are not actually dependent on the core language change, but they are
> intended to be used together.
> 2. Existing use of the char8_t identifier can be found in existing open
> source projects and likely exists in some closed source projects as
> well. An opt-in approach avoids conflict and the need to
> conditionalize code based on gcc version.
> 3. An opt-in approach enables evaluation of the feature prior to any
> WG14 approval.
But calling it _CHAR8_T_SOURCE is weird and inconsistent with everything
else.
In C++, there is __cpp_char8_t 201811L predefined macro for char8_t.
Using that in C is not right, sure.
Often we use __SIZEOF_type__ macros not just for sizeof(), but also for
presence check of the types, like
#ifdef __SIZEOF_INT128__
__int128 i;
#else
long long i;
#endif
etc., while char8_t has sizeof (char8_t) == 1, perhaps predefining
__SIZEOF_CHAR8_T__ 1
instead of _CHAR8_T_SOURCE would be better?
Jakub
