[C++]Use noexcept in union type move ctor/Add move assingment (#4249)

* Use noexcept in union type move ctor/Add move assingment

* Add NOEXCEPT macro to deal with _MS_VER/Remove delegating ctor in union type class

* Add FLATBUFFERS_NOEXCEPT to generated union class

include/flatbuffers/flatbuffers.h

@@ -113,6 +113,13 @@
   #define FLATBUFFERS_CONSTEXPR
 #endif
 
+#if defined(__GXX_EXPERIMENTAL_CXX0X__) && __GNUC__ * 10 + __GNUC_MINOR__ >= 46 || \
+    defined(_MSC_FULL_VER) && _MSC_FULL_VER >= 190023026
+  #define FLATBUFFERS_NOEXCEPT noexcept
+#else
+  #define FLATBUFFERS_NOEXCEPT
+#endif
+
 #if defined(_MSC_VER)
 #pragma warning(push)
 #pragma warning(disable: 4127) // C4127: conditional expression is constant

samples/monster_generated.h

@@ -74,11 +74,13 @@ struct EquipmentUnion {
   flatbuffers::NativeTable *table;
 
   EquipmentUnion() : type(Equipment_NONE), table(nullptr) {}
-  EquipmentUnion(EquipmentUnion&& u):
+  EquipmentUnion(EquipmentUnion&& u) FLATBUFFERS_NOEXCEPT :
     type(Equipment_NONE), table(nullptr)
     { std::swap(type, u.type); std::swap(table, u.table); }
   EquipmentUnion(const EquipmentUnion &);
   EquipmentUnion &operator=(const EquipmentUnion &);
+  EquipmentUnion &operator=(EquipmentUnion &&u) FLATBUFFERS_NOEXCEPT
+    { std::swap(type, u.type); std::swap(table, u.table); return *this; }
   ~EquipmentUnion() { Reset(); }
 
   void Reset();

src/idl_gen_cpp.cpp

@@ -660,11 +660,13 @@ class CppGenerator : public BaseGenerator {
       code_ += "  flatbuffers::NativeTable *table;";
       code_ += "";
       code_ += "  {{NAME}}Union() : type({{NONE}}), table(nullptr) {}";
-      code_ += "  {{NAME}}Union({{NAME}}Union&& u):";
+      code_ += "  {{NAME}}Union({{NAME}}Union&& u) FLATBUFFERS_NOEXCEPT :";
       code_ += "    type({{NONE}}), table(nullptr)";
       code_ += "    { std::swap(type, u.type); std::swap(table, u.table); }";
       code_ += "  {{NAME}}Union(const {{NAME}}Union &);";
       code_ += "  {{NAME}}Union &operator=(const {{NAME}}Union &);";
+      code_ += "  {{NAME}}Union &operator=({{NAME}}Union &&u) FLATBUFFERS_NOEXCEPT";
+      code_ += "    { std::swap(type, u.type); std::swap(table, u.table); return *this; }";
       code_ += "  ~{{NAME}}Union() { Reset(); }";
       code_ += "";
       code_ += "  void Reset();";

tests/monster_test_generated.h

@@ -105,11 +105,13 @@ struct AnyUnion {
   flatbuffers::NativeTable *table;
 
   AnyUnion() : type(Any_NONE), table(nullptr) {}
-  AnyUnion(AnyUnion&& u):
+  AnyUnion(AnyUnion&& u) FLATBUFFERS_NOEXCEPT :
     type(Any_NONE), table(nullptr)
     { std::swap(type, u.type); std::swap(table, u.table); }
   AnyUnion(const AnyUnion &);
   AnyUnion &operator=(const AnyUnion &);
+  AnyUnion &operator=(AnyUnion &&u) FLATBUFFERS_NOEXCEPT
+    { std::swap(type, u.type); std::swap(table, u.table); return *this; }
   ~AnyUnion() { Reset(); }
 
   void Reset();