HEDLEY_IMPORT with C++ classes

[mattyclarkson]

HEDLEY_IMPORT is specificed as extern which fails when used as so:

#ifdef PROJECT_COMPILATION
#define PROJECT_API HEDLEY_PUBLIC
#else
#define PROJECT_API HEDLEY_IMPORT
#endif

class PROJECT_API example {};

We would expect that example symbols are exported. However, when importing the result is class extern example {}; which is invalid.

A possible solution is:

#ifdef __cplusplus
#define HEDLEY_IMPORT
#else
#define HEDLEY_IMPORT extern
#endif

[mattyclarkson]

@nemequ do you have any thoughts on this?

[SFGrenade]

alternatively, since c++ can still have functions:

#define HEDLEY_IMPORT extern
#ifdef __cplusplus
#define HEDLEY_IMPORT_CLASS
#endif

[nemequ]

I've been digging into this a bit, and I think @SFGrenade's suggestion is on the right track; it doesn't seem like we can reuse the same macro for both cases. Having a second macro for classes would make sense to me.

That said, I'm a bit confused about what this new macro would be defined to if not nothing? I know in MSVC you can put __declspec(dllexport)/__declspec(dllimport), is that the behavior your want to achieve, or is there something else?

[SFGrenade]

my homecooked solution is https://github.com/SFGrenade/ZmqPb-Cpp/blob/master/include/zmqPb/_export.hpp, so probably something like

#if defined(HEDLEY_PRIVATE)
#  undef HEDLEY_PRIVATE
#endif
#if defined(HEDLEY_PUBLIC)
#  undef HEDLEY_PUBLIC
#endif
#if defined(HEDLEY_IMPORT)
#  undef HEDLEY_IMPORT
#endif
#if defined(HEDLEY_IMPORT_CLASS)
#  undef HEDLEY_IMPORT_CLASS
#endif
#if defined(_WIN32) || defined(__CYGWIN__)
#  define HEDLEY_PRIVATE
#  define HEDLEY_PUBLIC         __declspec(dllexport)
#  define HEDLEY_IMPORT         __declspec(dllimport)
#  ifdef __cplusplus
#    define HEDLEY_IMPORT_CLASS  HEDLEY_IMPORT
#  endif
#else
#  if \
    HEDLEY_HAS_ATTRIBUTE(visibility) || \
    HEDLEY_GCC_VERSION_CHECK(3,3,0) || \
    HEDLEY_SUNPRO_VERSION_CHECK(5,11,0) || \
    HEDLEY_INTEL_VERSION_CHECK(13,0,0) || \
    HEDLEY_ARM_VERSION_CHECK(4,1,0) || \
    HEDLEY_IBM_VERSION_CHECK(13,1,0) || \
    ( \
      defined(__TI_EABI__) && \
      ( \
        (HEDLEY_TI_CL6X_VERSION_CHECK(7,2,0) && defined(__TI_GNU_ATTRIBUTE_SUPPORT__)) || \
        HEDLEY_TI_CL6X_VERSION_CHECK(7,5,0) \
      ) \
    ) || \
    HEDLEY_MCST_LCC_VERSION_CHECK(1,25,10)
#    define HEDLEY_PRIVATE __attribute__((__visibility__("hidden")))
#    define HEDLEY_PUBLIC  __attribute__((__visibility__("default")))
#  else
#    define HEDLEY_PRIVATE
#    define HEDLEY_PUBLIC
#  endif
#  define HEDLEY_IMPORT    extern
#  ifdef __cplusplus
#    define HEDLEY_IMPORT_CLASS
#  endif
#endif

at https://github.com/nemequ/hedley/blob/master/hedley.h#L1641-L1677