Is it possible to use a macro defined value in C/C ?-CodePudding

In the LLVM codebase, I see this lines:

class LLVM_EXTERNAL_VISIBILITY Function : public GlobalObject,
                                          public ilist_node<Function> {

My LSP (clangd) tells me that LLVM_EXTERNAL_VISIBILITY refers to

/// LLVM_LIBRARY_VISIBILITY - If a class marked with this attribute is linked
/// into a shared library, then the class should be private to the library and
/// not accessible from outside it.  Can also be used to mark variables and
/// functions, making them private to any shared library they are linked into.
/// On PE/COFF targets, library visibility is the default, so this isn't needed.
///
/// LLVM_EXTERNAL_VISIBILITY - classes, functions, and variables marked with
/// this attribute will be made public and visible outside of any shared library
/// they are linked in to.
#if __has_attribute(visibility) &&                                             \
    (!(defined(_WIN32) || defined(__CYGWIN__)) ||                              \
     (defined(__MINGW32__) && defined(__clang__)))
#define LLVM_LIBRARY_VISIBILITY __attribute__ ((visibility("hidden")))
#if defined(LLVM_BUILD_LLVM_DYLIB) || defined(LLVM_BUILD_SHARED_LIBS)
#define LLVM_EXTERNAL_VISIBILITY __attribute__((visibility("default")))
#else
#define LLVM_EXTERNAL_VISIBILITY     // <-------------- THIS
#endif
#else
#define LLVM_LIBRARY_VISIBILITY
#define LLVM_EXTERNAL_VISIBILITY
#endif

I can't figure out the usefulness: why should I #define an identifier without a value, but using that as if it had one?

One idea is that it acts as a conditional: if something happens, then the preprocessor substitutes some text; if not, it simply doesn't substitute anything, but prevents the compiler from complaining about some undefined LLVM_LIBRARY_VISIBILITY token. Is this correct?

Moreover, what kind of attributes can be specified there? Something about the linkage of the class?

Thanks!

CodePudding user response：

One idea is that it acts as a conditional: if something happens, then the preprocessor substitutes some text; if not, it simply doesn't substitute anything, but prevents the compiler from complaining about some undefined LLVM_LIBRARY_VISIBILITY token. Is this correct?

Pretty much. Some build environments, it'll have a value. Others not. Seems mainly a matter of portability.

__attribute__((visibility("default"))) refers to the visibility of the symbol it is attached to. The default visibility can be changed at compile time.