[C++] Fix for printing of enum in case output_enum_identifiers=1. (#5706)

* Add test-case for testing of the future Color in json (output_enum_identifiers = true)

* Refactoring of idl_gen_text.cpp. Fix for printing of bit-enum with active output_enum_identifiers=1.

* Move  GenerateText implementation into class

* Remove unnecessary code from flatbuffers.h

include/flatbuffers/flatbuffers.h

@@ -443,6 +443,13 @@ template<typename T, uint16_t length> class Array {
 
   return_type operator[](uoffset_t i) const { return Get(i); }
 
+  // If this is a Vector of enums, T will be its storage type, not the enum
+  // type. This function makes it convenient to retrieve value with enum
+  // type E.
+  template<typename E> E GetEnum(uoffset_t i) const {
+    return static_cast<E>(Get(i));
+  }
+
   const_iterator begin() const { return const_iterator(Data(), 0); }
   const_iterator end() const { return const_iterator(Data(), size()); }
 
@@ -518,10 +525,12 @@ template<typename T, uint16_t length> class Array<Offset<T>, length> {
   static_assert(flatbuffers::is_same<T, void>::value, "unexpected type T");
 
  public:
+  typedef const void* return_type;
+
   const uint8_t *Data() const { return data_; }
 
   // Make idl_gen_text.cpp::PrintContainer happy.
-  const void *operator[](uoffset_t) const {
+  return_type operator[](uoffset_t) const {
     FLATBUFFERS_ASSERT(false);
     return nullptr;
   }

tests/test.cpp

@@ -650,6 +650,19 @@ void JsonEnumsTest() {
   auto result = GenerateText(parser, builder.GetBufferPointer(), &jsongen);
   TEST_EQ(result, true);
   TEST_EQ(std::string::npos != jsongen.find("color: \"Red Blue\""), true);
+  // Test forward compatibility with 'output_enum_identifiers = true'.
+  // Current Color doesn't have '(1u << 2)' field, let's add it.
+  builder.Clear();
+  std::string future_json;
+  auto future_name = builder.CreateString("future bitflag_enum");
+  MonsterBuilder future_color(builder);
+  future_color.add_name(future_name);
+  future_color.add_color(
+      static_cast<Color>((1u << 2) | Color_Blue | Color_Red));
+  FinishMonsterBuffer(builder, future_color.Finish());
+  result = GenerateText(parser, builder.GetBufferPointer(), &future_json);
+  TEST_EQ(result, true);
+  TEST_EQ(std::string::npos != future_json.find("color: 13"), true);
 }
 
 #if defined(FLATBUFFERS_HAS_NEW_STRTOD) && (FLATBUFFERS_HAS_NEW_STRTOD > 0)