[C++] Allow using symbol visibility annotations also on non-Windows platforms

In GCC and clang the symbol visibility behavior of MSVC can be mirror using `-fvisibility=hidden`.
This allows to more easily test that symbol visbility annotations work correctly and can potentially lead to smaller binaries.
The default behavior on non-Windows platforms is not changed with this commit.

See also https://gcc.gnu.org/wiki/Visibility

Also fixes a tiny mistake where the _WIN32 macro was used to silence a MSVC warning where the _MSC_VER macro should have been used instead.

Author: stephanlachnit

lang/c++/impl/json/JsonDom.hh

@@ -31,7 +31,7 @@
 
 namespace avro {
 
-class AVRO_DECL InputStream;
+class InputStream;
 
 namespace json {
 class Entity;
@@ -59,7 +59,7 @@ enum class EntityType {
     Obj
 };
 
-const char *typeToString(EntityType t);
+AVRO_DECL const char *typeToString(EntityType t);
 
 inline std::ostream &operator<<(std::ostream &os, EntityType et) {
     return os << typeToString(et);

lang/c++/include/avro/Compiler.hh

@@ -25,14 +25,14 @@
 
 namespace avro {
 
-class AVRO_DECL InputStream;
+class InputStream;
 
 /// This class is used to implement an avro spec parser using a flex/bison
 /// compiler.  In order for the lexer to be reentrant, this class provides a
 /// lexer object for each parse.  The bison parser also uses this class to
 /// build up an avro parse tree as the avro spec is parsed.
 
-class AVRO_DECL ValidSchema;
+class ValidSchema;
 
 /// Given a stream containing a JSON schema, compiles the schema to a
 /// ValidSchema object.  Throws if the schema cannot be compiled to a valid

lang/c++/include/avro/Config.hh

@@ -19,25 +19,45 @@
 #ifndef avro_Config_hh
 #define avro_Config_hh
 
-// Windows DLL support
-
-#ifdef _WIN32
+#ifdef _MSC_VER
 #pragma warning(disable : 4275 4251)
+#endif // _MSC_VER
+
+/*
+ * Symbol visibility macros:
+ *  - AVRO_DLL_EXPORT annotation for exporting symbols
+ *  - AVRO_DLL_IMPORT annotation for importing symbols
+ *  - AVRO_DLL_HIDDEN annotation for hiding symbols
+ *  - AVRO_DYN_LINK needs to be defined when compiling / linking avro as dynamic library
+ *  - AVRO_SOURCE needs to be defined when compiling avro as library
+ *  - AVRO_DECL contains the correct symbol visibility annotation depending on the two macros above
+ */
+
+#if defined _WIN32 || defined __CYGWIN__
+#define AVRO_DLL_EXPORT __declspec(dllexport)
+#define AVRO_DLL_IMPORT __declspec(dllimport)
+#define AVRO_DLL_HIDDEN
+#else
+#define AVRO_DLL_EXPORT [[gnu::visibility("default")]]
+#define AVRO_DLL_IMPORT [[gnu::visibility("default")]]
+#define AVRO_DLL_HIDDEN [[gnu::visibility("hidden")]]
+#endif // _WIN32 || __CYGWIN__
 
-#if defined(AVRO_DYN_LINK)
+#ifdef AVRO_DYN_LINK
 #ifdef AVRO_SOURCE
-#define AVRO_DECL __declspec(dllexport)
+#define AVRO_DECL AVRO_DLL_EXPORT
 #else
-#define AVRO_DECL __declspec(dllimport)
+#define AVRO_DECL AVRO_DLL_IMPORT
 #endif // AVRO_SOURCE
 #endif // AVRO_DYN_LINK
 
-#include <intsafe.h>
-using ssize_t = SSIZE_T;
-#endif // _WIN32
-
 #ifndef AVRO_DECL
 #define AVRO_DECL
 #endif
 
+#ifdef _WIN32
+#include <intsafe.h>
+using ssize_t = SSIZE_T;
+#endif // _WIN32
+
 #endif

lang/c++/include/avro/buffer/BufferStream.hh

@@ -35,7 +35,7 @@ namespace avro {
  *
  **/
 
-class AVRO_DECL ostream : public std::ostream {
+class ostream : public std::ostream {
 
 public:
     /// Default constructor, creates a new OutputBuffer.
@@ -65,7 +65,7 @@ protected:
  *
  **/
 
-class AVRO_DECL istream : public std::istream {
+class istream : public std::istream {
 
 public:
     /// Constructor, requires an InputBuffer to read from.

lang/c++/include/avro/buffer/BufferStreambuf.hh

@@ -41,7 +41,7 @@ namespace avro {
  * but we have no need since all writes are immediately stored in the buffer.
  **/
 
-class AVRO_DECL ostreambuf : public std::streambuf {
+class ostreambuf : public std::streambuf {
 
 public:
     /// Default constructor creates a new OutputBuffer.
@@ -86,7 +86,7 @@ private:
  *
  **/
 
-class AVRO_DECL istreambuf : public std::streambuf {
+class istreambuf : public std::streambuf {
 
 public:
     /// Default constructor requires an InputBuffer to read from.