In C++11 and above, C99/C11 features should be available even when using C++. For example, FLT_HAS_SUBNORM should be provided by <float.h>:
$ cat <<EOF | clang++ -xc++ -std=c++11 -
#include <float.h>
#ifndef FLT_HAS_SUBNORM
error “FLT_HAS_SUBNORM is missing”
#endif
EOF
This should also work when including <cfloat>. However, this currently:
- fails in -std=c++11 or above
- succeeds in -std=gnu++11 or above
- succeeds in -std=c11 or above
- succeeds in -std=gnu11 or above
If I understand correctly, the problem is that we currently have the following in clang/lib/Headers/float.h:
#include_next <float.h> // pick up whatever the system provides
…
#undef FLT_HAS_SUBNORM // undefine what the system provides
…
// redefine our own version of the macro
#if STDC_VERSION >= 201112L || !defined(STRICT_ANSI)
…
define FLT_HAS_SUBNORM FLT_HAS_DENORM
…
#endif
The #if block checks whether we’re compiling in C11 or above, or whether we’re compiling in a non-strict dialect (basically some any gnuXXX dialect). However, it doesn’t check for strictly conforming C++, and as a result we don’t get the definitions. I’d like to know what’s the right strategy for providing these definitions in C++, and whether there’s an established way of doing this already. Naively, changing the #if to
#if STDC_VERSION >= 201112L || !defined(STRICT_ANSI) || (defined(__cplusplus) && __cplusplus >= 201103L)
…
define FLT_HAS_SUBNORM FLT_HAS_DENORM
…
#endif
seems to do the job. But I’m not very familiar with how Clang’s headers are imbricked into libc++ and how they interact with per-platform headers, so I’m asking here.
Thanks,
Louis
