PR feedback: update export macro documentation to match CMake

Author: andrurogerz

Docs/ExportMacroDefinitions.md

@@ -1,4 +1,4 @@
-## Export Macro Definitions
+# Export Macro Definitions
 
 A common approach for annotating project’s public interface is by defining a
 set of preprocessor macros that are used instead of adding annotations directly
@@ -7,38 +7,43 @@ to the source. This approach allows supporting visibility annotations
 DLL import/export annotations (`__declspec(dllimport)`,
 `__declspec(dllexport)`) when building for Windows.
 
-For example, the `PUBLIC_ABI` macro defined below can be added to symbols in a
+For example, the `PROJECT_EXPORT` macro defined below can be added to symbols in a
 project’s header files files that should be externally visible.
 
 ```cpp
-#ifdef _WIN32
-#ifdef EXPORTING_ABI
-// When building the Windows DLL, the public API must be annotated for exported.
-#define PUBLIC_ABI __declspec(dllexport)
+#if defined(PROJECT_EXPORT_STATIC)
+/* We are building Project as a static library (no exports) */
+#  define PROJECT_EXPORT
 #else
-// When building clients of the Windows DLL, the public API must be annotated for import.
-#define PUBLIC_ABI __declspec(dllimport)
-#endif
-#elif defined(__cplusplus) && defined(__has_cpp_attribute) && \
-      __has_cpp_attribute(gnu::visibility) && defined(__GNUC__) && !defined(__clang__)
-// When supported, use the gnu::visibility C++ attribute to set visibility to default.
-#define PUBLIC_API [[gnu::visibility("default")]]
-#elif defined(__has_attribute) && __has_attribute(visibility)
-// Otherwise, use the __attribute__ to set visibility to default.
-#define PUBLIC_ABI __attribute__((visibility("default")))
-#else
-#error "Required visibility attribute are not supported by compiler!"
+/* We are building Project as a shared library */
+#  if defined(_WIN32)
+#    if defined(Project_EXPORTS)
+       /* We are building this library */
+#      define PROJECT_EXPORT __declspec(dllexport)
+#    else
+       /* We are using this library */
+#      define PROJECT_EXPORT __declspec(dllimport)
+#    endif
+#  else
+#    define PROJECT_EXPORT __visibility__((visibility("default")))
+#  endif
 #endif
 ```
 
-A Windows project using these macros must be built with `EXPORTING_ABI` defined
-so that public symbols are annotated for export with `__declspec(dllexport)`.
-When building the client, however, `EXPORTING_ABI` must not be defined so the
-same set of public symbols will be properly annotated for import with
-`__declspec(dllimport)`.
+A Windows project using these macros must be built with `Project_EXPORTS`
+defined so that public symbols are annotated for export with
+`__declspec(dllexport)`. When building clients of the project, however,
+`Project_EXPORTS` must not be defined so the same set of public symbols will be
+properly annotated for import with `__declspec(dllimport)`.
 
 An ELF shared library or Mach-O dylib project will have all symbols publicly
 visible by default so annotations are not strictly required. However, when a
 project is built with default hidden symbol visibility using
 `-fvisibility-default=hidden`, individual symbols must be explicitly exported
 using a visibility annotations.
+
+## CMake Support
+When building with CMake, the `generate_export_header()` function can be used
+to auto-generate a header with appropriate export macro definitions. See [CMake
+documentation](https://cmake.org/cmake/help/latest/module/GenerateExportHeader.html)
+for details.