make flatbuffers::IsFieldPresent safer (#4988)

Give the vtable offset enum inside each table the name
"FlatBuffersVTableOffset" and base type voffset_t so it can be used as a
dependent type in IsFieldPresent. This makes that function slightly
safer since it prevents calling it with arbitrary, non-table types.
Now, the only way to use IsFieldPresent incorrectly is to create your
own type which does not inherit from flatbuffers::Table but has a
dependent voffset convertible type "FlatBuffersVTableOffset".

include/flatbuffers/base.h

@@ -122,9 +122,11 @@
     defined(__clang__)
   #define FLATBUFFERS_FINAL_CLASS final
   #define FLATBUFFERS_OVERRIDE override
+  #define FLATBUFFERS_VTABLE_UNDERLYING_TYPE : flatbuffers::voffset_t
 #else
   #define FLATBUFFERS_FINAL_CLASS
   #define FLATBUFFERS_OVERRIDE
+  #define FLATBUFFERS_VTABLE_UNDERLYING_TYPE
 #endif
 
 #if (!defined(_MSC_VER) || _MSC_VER >= 1900) && \

include/flatbuffers/flatbuffers.h

@@ -2370,9 +2370,11 @@ typedef uint64_t hash_value_t;
 // Note: this function will return false for fields equal to the default
 // value, since they're not stored in the buffer (unless force_defaults was
 // used).
-template<typename T> bool IsFieldPresent(const T *table, voffset_t field) {
+template<typename T>
+bool IsFieldPresent(const T *table, typename T::FlatBuffersVTableOffset field) {
   // Cast, since Table is a private baseclass of any table types.
-  return reinterpret_cast<const Table *>(table)->CheckField(field);
+  return reinterpret_cast<const Table *>(table)->CheckField(
+      static_cast<voffset_t>(field));
 }
 
 // Utility function for reverse lookups on the EnumNames*() functions

include/flatbuffers/reflection_generated.h

@@ -100,7 +100,7 @@ inline const char *EnumNameBaseType(BaseType e) {
 }
 
 struct Type FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_BASE_TYPE = 4,
     VT_ELEMENT = 6,
     VT_INDEX = 8
@@ -160,7 +160,7 @@ inline flatbuffers::Offset<Type> CreateType(
 }
 
 struct KeyValue FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_KEY = 4,
     VT_VALUE = 6
   };
@@ -229,7 +229,7 @@ inline flatbuffers::Offset<KeyValue> CreateKeyValueDirect(
 }
 
 struct EnumVal FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_NAME = 4,
     VT_VALUE = 6,
     VT_OBJECT = 8,
@@ -337,7 +337,7 @@ inline flatbuffers::Offset<EnumVal> CreateEnumValDirect(
 }
 
 struct Enum FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_NAME = 4,
     VT_VALUES = 6,
     VT_IS_UNION = 8,
@@ -462,7 +462,7 @@ inline flatbuffers::Offset<Enum> CreateEnumDirect(
 }
 
 struct Field FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_NAME = 4,
     VT_TYPE = 6,
     VT_ID = 8,
@@ -644,7 +644,7 @@ inline flatbuffers::Offset<Field> CreateFieldDirect(
 }
 
 struct Object FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_NAME = 4,
     VT_FIELDS = 6,
     VT_IS_STRUCT = 8,
@@ -779,7 +779,7 @@ inline flatbuffers::Offset<Object> CreateObjectDirect(
 }
 
 struct RPCCall FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_NAME = 4,
     VT_REQUEST = 6,
     VT_RESPONSE = 8,
@@ -891,7 +891,7 @@ inline flatbuffers::Offset<RPCCall> CreateRPCCallDirect(
 }
 
 struct Service FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_NAME = 4,
     VT_CALLS = 6,
     VT_ATTRIBUTES = 8,
@@ -989,7 +989,7 @@ inline flatbuffers::Offset<Service> CreateServiceDirect(
 }
 
 struct Schema FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_OBJECTS = 4,
     VT_ENUMS = 6,
     VT_FILE_IDENT = 8,

samples/monster_generated.h

@@ -235,7 +235,7 @@ struct Monster FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return MonsterTypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_POS = 4,
     VT_MANA = 6,
     VT_HP = 8,
@@ -443,7 +443,7 @@ struct Weapon FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return WeaponTypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_NAME = 4,
     VT_DAMAGE = 6
   };

src/idl_gen_cpp.cpp

@@ -1710,7 +1710,7 @@ class CppGenerator : public BaseGenerator {
       // We need to add a trailing comma to all elements except the last one as
       // older versions of gcc complain about this.
       code_.SetValue("SEP", "");
-      code_ += "  enum {";
+      code_ += "  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {";
       for (auto it = struct_def.fields.vec.begin();
            it != struct_def.fields.vec.end(); ++it) {
         const auto &field = **it;

tests/monster_test_generated.h

@@ -539,7 +539,7 @@ struct TestSimpleTableWithEnum FLATBUFFERS_FINAL_CLASS : private flatbuffers::Ta
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return TestSimpleTableWithEnumTypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_COLOR = 4
   };
   Color color() const {
@@ -609,7 +609,7 @@ struct Stat FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return StatTypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_ID = 4,
     VT_VAL = 6,
     VT_COUNT = 8
@@ -713,7 +713,7 @@ struct Referrable FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return ReferrableTypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_ID = 4
   };
   uint64_t id() const {
@@ -882,7 +882,7 @@ struct Monster FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return MonsterTypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_POS = 4,
     VT_MANA = 6,
     VT_HP = 8,
@@ -1665,7 +1665,7 @@ struct TypeAliases FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return TypeAliasesTypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_I8 = 4,
     VT_U8 = 6,
     VT_I16 = 8,

tests/namespace_test/namespace_test1_generated.h

@@ -82,7 +82,7 @@ struct TableInNestedNS FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return TableInNestedNSTypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_FOO = 4
   };
   int32_t foo() const {

tests/namespace_test/namespace_test2_generated.h

@@ -42,7 +42,7 @@ struct TableInFirstNS FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return TableInFirstNSTypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_FOO_TABLE = 4,
     VT_FOO_ENUM = 6,
     VT_FOO_STRUCT = 8
@@ -119,7 +119,7 @@ struct TableInC FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return TableInCTypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_REFER_TO_A1 = 4,
     VT_REFER_TO_A2 = 6
   };
@@ -184,7 +184,7 @@ struct SecondTableInA FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return SecondTableInATypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_REFER_TO_C = 4
   };
   const NamespaceC::TableInC *refer_to_c() const {

tests/union_vector/union_vector_generated.h

@@ -251,7 +251,7 @@ struct Attacker FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return AttackerTypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_SWORD_ATTACK_DAMAGE = 4
   };
   int32_t sword_attack_damage() const {
@@ -317,7 +317,7 @@ struct Movie FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static const flatbuffers::TypeTable *MiniReflectTypeTable() {
     return MovieTypeTable();
   }
-  enum {
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
     VT_MAIN_CHARACTER_TYPE = 4,
     VT_MAIN_CHARACTER = 6,
     VT_CHARACTERS_TYPE = 8,