net+util: fix build with gcc-7
Introduce an NDN_CXX_FALLTHROUGH macro to simplify
annotating switch cases that fall through.
Update code-style to recommend using NDN_CXX_FALLTHROUGH.
Change-Id: I80e4ff8d16a09f6ec9343f0480bc57ae0fcd1be4
diff --git a/src/util/backports.hpp b/src/util/backports.hpp
index 1d0a92a..df1d775 100644
--- a/src/util/backports.hpp
+++ b/src/util/backports.hpp
@@ -1,5 +1,5 @@
/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
-/**
+/*
* Copyright (c) 2015-2017 Regents of the University of California.
*
* This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
@@ -28,6 +28,30 @@
#include <boost/lexical_cast.hpp>
#endif
+#ifdef __has_cpp_attribute
+# define NDN_CXX_HAS_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
+#else
+# define NDN_CXX_HAS_CPP_ATTRIBUTE(x) 0
+#endif
+
+#ifdef __has_include
+# define NDN_CXX_HAS_INCLUDE(x) __has_include(x)
+#else
+# define NDN_CXX_HAS_INCLUDE(x) 0
+#endif
+
+#if NDN_CXX_HAS_CPP_ATTRIBUTE(fallthrough)
+# define NDN_CXX_FALLTHROUGH [[fallthrough]]
+#elif NDN_CXX_HAS_CPP_ATTRIBUTE(clang::fallthrough)
+# define NDN_CXX_FALLTHROUGH [[clang::fallthrough]]
+#elif NDN_CXX_HAS_CPP_ATTRIBUTE(gnu::fallthrough)
+# define NDN_CXX_FALLTHROUGH [[gnu::fallthrough]]
+#elif defined(__GNUC__) && (__GNUC__ >= 7)
+# define NDN_CXX_FALLTHROUGH __attribute__((fallthrough))
+#else
+# define NDN_CXX_FALLTHROUGH ((void)0)
+#endif
+
namespace ndn {
#if __cpp_lib_make_unique >= 201304