[cfe-dev] libcxx: Platform independent printing using format constants

[Richard Smith]

On Thu, Oct 11, 2012 at 6:36 PM, Howard Hinnant <hhinnant at apple.com> wrote:

>
> On Oct 11, 2012, at 7:47 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>
> >> On Thu, Oct 11, 2012 at 10:56 AM, Howard Hinnant <hhinnant at apple.com>
> wrote:
> >>
> >> On Oct 11, 2012, at 12:35 AM, Richard Smith <richard at metafoo.co.uk>
> wrote:
> >>
> >> > On Wed, Oct 10, 2012 at 9:34 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
> >> > libc++ does this to pick up the macro definitions:
> >> >
> >> > #include <inttypes.h>
> >> >
> >> > However, glibc's inttypes.h only provides them in C++ mode if
> _STDC_FORMAT_MACROS is defined, and libc++ doesn't define it.
> >> >
> >> > Sorry, needs more underscore: __STDC_FORMAT_MACROS.
> >>
> >> Right.  This is C++11-conforming.  27.9.2 [c.files]/p3:
> >>
> >> > Table 135 describes header <cinttypes>. [ Note: The macros defined by
> <cinttypes> are provided unconditionally. In particular, the symbol
> __STDC_FORMAT_MACROS, mentioned in footnote 182 of the C standard, plays no
> role in C++. — end note ]
> >>
> >> Summary:  <inttypes.h> should not be protecting anything in C++ based
> on __STDC_FORMAT_MACROS, despite the non-normative footnote in C99's 7.8.1.
>  When the C++ committee looked at this issue, they told the C committee
> "thanks but no thanks.".
> >>
> > Right, but glibc follows the C99 standard, not the C++11 standard (nor
> the C11 standard, which removes this unwanted "protection"), so cinttypes
> needs to define __STDC_FORMAT_MACROS itself if it's going to include a C99
> <inttypes.h> header such as glibc's.
>
> That solution won't make a C++11 conforming std::lib.  C++11 also defines
> <inttypes.h> that *always* provides these macros.
>
> glibc could also follow the C99 standard by not guarding with
> __STDC_FORMAT_MACROS.  C99 has no normative statements that say it shall
> guard with __STDC_FORMAT_MACROS.  The only statements it has to such an
> effect are non-normative.
>
> Additionally the #define __STDC_FORMAT_MACROS in <cinttypes> won't
> necessarily be effective.  Consider the case that <inttypes.h> has
> traditional include guards:
>
> #include <inttypes.h>
> ...
> #include <cinttypes>
> ...
> // PRId64 defined here?
>
> PRId64 may or may not be defined here depending upon whether <inttypes.h>
> put it under an include guard.
>
> <inttypes.h> is the right place to fix this bug.


OK, but where do we work around the bug? Perhaps Clang should always define
 __STDC_FORMAT_MACROS in C++ mode.
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