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. The replacement of undefined identifiers with 0 in "#if " directives is, IMHO, a particularly bad case of historical baggage. Compilers can't change that now, but I think it is a good idea to use -Wundef for your own code.
