Using __STDC_VERSION__ macro

[Vincent Lefevre]

On 2017-02-01 16:44:31 +0100, David Brown wrote:
> On 01/02/17 15:13, Mason wrote:
> > On 01/02/2017 14:52, Jonathan Wakely wrote:
> >> On 1 February 2017 at 13:49, Mason wrote:
> >>> Hello,
> >>>
> >>> https://gcc.gnu.org/gcc-5/porting_to.html states
> >>>
> >>> "Typically, this macro is used as in the following:
> >>>
> >>>   #if !defined __STDC_VERSION__ || __STDC_VERSION__ < 199901L
> >>>     /* ... */
> >>>   #else
> >>>   # include <stdint.h>
> >>>   #endif
> >>> "
> >>>
> >>> I think it is unnecessary to test for the existence of __STDC_VERSION__
> >>> because N1256 6.10.1p4 states
> >>> "all remaining identifiers [...] are replaced with the pp-number 0"
> >>>
> >>> Therefore
> >>> #if __STDC_VERSION__ < 199901L
> >>> would work as expected, even when __STDC_VERSION__ is undefined.
> >>> (As the constant-expression would be false.)
> >>
> >> But you'd get a warning with -Wundef
> > 
> > Right.
> > 
> > I'm not convinced by the usefulness of -Wundef, though.
> > 
> > I suppose its only use is catching typos such as
> > 
> > #define FOO 42
> > #if GOO
> > #endif
> > 
> > which would silently discard the conditional code.
> > 
> 
> -Wundef is useful /precisely/ because it catches such mistakes.

However, it yields false positives on code that doesn't contain any
ambiguity (and is easier to read than lengthier code), such as:

#if __STDC_VERSION__ < 199901L

Since __STDC_VERSION__ is a standard macro, it is quite obvious that
even if __STDC_VERSION__ is not defined, this is not a typo. So, IMHO,
-Wundef should whitelist some common macros (and/or perhaps let the
user provide his own whitelist).

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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