Merge pull request #274 from zarvox/msvc2015-exception-count

Support extracting the current exception count on MSVC2015.

c++/src/kj/exception.c++

@@ -550,23 +550,31 @@ uint uncaughtExceptionCount() {
 
 #elif _MSC_VER
 
-#if 0
-// TODO(msvc): The below was copied from:
-//     https://github.com/panaseleus/stack_unwinding/blob/master/boost/exception/uncaught_exception_count.hpp
-//   Alas, it doesn not appera to work on MSVC2015. The linker claims _getptd() doesn't exist.
+#if _MSC_VER >= 1900
+// MSVC14 has a refactored CRT which now provides a direct accessor for this value.
+// See https://svn.boost.org/trac/boost/ticket/10158 for a brief discussion.
+extern "C" int *__cdecl __processing_throw();
+
+uint uncaughtExceptionCount() {
+  return static_cast<uint>(*__processing_throw());
+}
+
+#elif _MSC_VER >= 1400
+// The below was copied from:
+// https://github.com/panaseleus/stack_unwinding/blob/master/boost/exception/uncaught_exception_count.hpp
 
 extern "C" char *__cdecl _getptd();
 
 uint uncaughtExceptionCount() {
   return *reinterpret_cast<uint*>(_getptd() + (sizeof(void*) == 8 ? 0x100 : 0x90));
 }
-#endif
-
+#else
 uint uncaughtExceptionCount() {
   // Since the above doesn't work, fall back to uncaught_exception(). This will produce incorrect
   // results in very obscure cases that Cap'n Proto doesn't really rely on anyway.
   return std::uncaught_exception();
 }
+#endif
 
 #else
 #error "This needs to be ported to your compiler / C++ ABI."