Re: gnulib and C++

[John W. Eaton]

On 18-Feb-2010, Bruno Haible wrote:

| Ralf proposes:
| > I suggest you could
| > produce a helper header to #undef them again; you could even do that
| > automatically during bootstrap with above.  Then only a portability
| > layer would need to avoid using these symbols in another namespace.
| 
| This would only be a makeshift. Your example
| 
| >   class foo
| >   {
| >   public:
| >     ...
| >     int open (some, args);
| >     ...
| >   };
| 
| >   int foo::open (some, args) { ... }
| 
| clearly indicates that in C++, one should minimize the use of
| preprocessor defines for lowercase identifiers.
| 
| C++ has its own, idiosyncratic, ways of aliasing and overriding
| functions. If we were to change gnulib's fcntl.h like this:
| 
| *** lib/fcntl.in.h.orig       Thu Feb 18 23:44:21 2010
| --- lib/fcntl.in.h    Thu Feb 18 23:31:02 2010
| ***************
| *** 79,87 ****
|   
|   #if @GNULIB_OPEN@
|   # if @REPLACE_OPEN@
| ! #  undef open
| ! #  define open rpl_open
| ! extern int open (const char *filename, int flags, ...) _GL_ARG_NONNULL 
((1));
|   # endif
|   #elif defined GNULIB_POSIXCHECK
|   # undef open
| --- 79,89 ----
|   
|   #if @GNULIB_OPEN@
|   # if @REPLACE_OPEN@
| ! #  ifndef __cplusplus
| ! #   undef open
| ! #   define open rpl_open
| ! #  endif
| ! extern int rpl_open (const char *filename, int flags, ...) _GL_ARG_NONNULL 
((1));
|   # endif
|   #elif defined GNULIB_POSIXCHECK
|   # undef open
| ***************
| *** 111,116 ****
| --- 113,132 ----
|   }
|   #endif
|   
| + #ifdef __cplusplus
| + namespace gnulib
| + {
| + # if @GNULIB_OPEN@
| +   int (*const open) (const char *filename, int flags, ...) =
| + #  if @REPLACE_OPEN@
| +     rpl_open;
| + #  else
| +     open;
| + #  endif
| + # endif
| + }
| + #endif
| + 
|   /* Fix up the FD_* macros, only known to be missing on mingw.  */
|   
|   #ifndef FD_CLOEXEC
| 
| 
| Then you can use the identifier 'gnulib::open' on any platform.
| On platforms where REPLACE_OPEN = 1, calls to 'gnulib::open'
| will be automatically inlined to calls to 'rpl_open', on the
| other platforms to calls to the C function 'open'.

Maybe I'm missing something, but what happens if @GNULIB_OPEN@ is 0?
It doesn't look like we get the symbol inside the gnulib namespace in
that case.

To handle all the cases, I find that I need to write

  #ifdef __cplusplus
  namespace gnulib
  {
    int (*const open) (const char *filename, int flags, ...) =
  # if @GNULIB_OPEN@
  #  if @REPLACE_OPEN@
      ::rpl_open;
  #  else
      ::open;
  #  endif
  # else
     ::open;
  # endif
  }
  #endif


I would much prefer to use a solution like this instead of having to
try to dodge the possible symbol redefinitions in my code.

If we can agree on a pattern to use for putting these symbols in a
namespace, I am willing to help make the changes in gnulib.

jwe