__attribute__((visibility("hidden"))) - "type attributes are honored only at type definition"

[Christian Parpart <trapni at gentoo dot org>]

Hi all,

I'm writing (together with a friend) a cross platform library. however, I 
recently introduced the machanism to export only those *wanted* symbols.
On the GCC/linux side, it's quite clear (for me) how to do this. But for the 
windows/MSVC side, I'm really not familar in any way.
Now, to get the code together, we introduced a #macro that declares those 
symbols to be exportable like the following:

// general API export/import macros
#if defined(SW_GCC)
#   if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
        // XXX visibility is available in GCC 3.4 and higher
#       define SW_EXPORT __attribute__((visibility("default")))
#       define SW_NO_EXPORT __attribute__((visibility("hidden")))
#       define SW_IMPORT /*!*/
#   else
#       define SW_EXPORT /*!*/
#       define SW_NO_EXPORT /*!*/
#       define SW_IMPORT /*!*/
#   endif
#elif SW_WIN32
#   define SW_EXPORT __declspec(dllexport)
#   define SW_NO_EXPORT /*!*/
#   define SW_IMPORT /*__declspec(dllimport)*/
#else
#   define SW_EXPORT /*!*/
#   define SW_NO_EXPORT /*!*/
#   define SW_IMPORT /*!*/
#endif

// SWL API export/import
#if defined(MAKE_SWL)
#   define SWL_API SW_EXPORT
#else
#   define SWL_API SW_IMPORT
#endif

Now, defining a symbol with this SW_EXPORT modifier is quite okay, but on the 
windows side, my friend says, he *NEEDS* this to be done for forward 
declarations as well. *ANYWHERE* he forward declares symbol "foo" to be 
exportable/importable, that symbol declaration has to have that SWL_API macro 
between as well.

This doesn't like GCC (obviousely) and complains about like this:

module.h:line: warning: type attributes are honored only at type definition

in a line like this:

template<class T> class SWL_API TSharedPtr;


Now... in how far is this right and/or how could we find a Windows/Linux 
compatible macro-way to make each compiler/linker happy here?

Thanks in advance,
Christian Parpart.

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