C++ generated code now has constants for vtable offsets.

Besides making the generated code looking a lot more readable,
it also allows you to use these offsets in calls to
Table::CheckField, to see if a field is present in a table.

Change-Id: I1b4cc350c4f27c4e474c31add40c701ef4ae63b2
Tested: On Linux.

Author: Wouter van Oortmerssen

samples/monster_generated.h

@@ -59,41 +59,49 @@ MANUALLY_ALIGNED_STRUCT(4) Vec3 FLATBUFFERS_FINAL_CLASS {
 STRUCT_END(Vec3, 12);
 
 struct Monster FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
-  const Vec3 *pos() const { return GetStruct<const Vec3 *>(4); }
-  Vec3 *mutable_pos() { return GetStruct<Vec3 *>(4); }
-  int16_t mana() const { return GetField<int16_t>(6, 150); }
-  bool mutate_mana(int16_t _mana) { return SetField(6, _mana); }
-  int16_t hp() const { return GetField<int16_t>(8, 100); }
-  bool mutate_hp(int16_t _hp) { return SetField(8, _hp); }
-  const flatbuffers::String *name() const { return GetPointer<const flatbuffers::String *>(10); }
-  flatbuffers::String *mutable_name() { return GetPointer<flatbuffers::String *>(10); }
-  const flatbuffers::Vector<uint8_t> *inventory() const { return GetPointer<const flatbuffers::Vector<uint8_t> *>(14); }
-  flatbuffers::Vector<uint8_t> *mutable_inventory() { return GetPointer<flatbuffers::Vector<uint8_t> *>(14); }
-  Color color() const { return static_cast<Color>(GetField<int8_t>(16, 2)); }
-  bool mutate_color(Color _color) { return SetField(16, static_cast<int8_t>(_color)); }
+  enum {
+    VT_POS = 4,
+    VT_MANA = 6,
+    VT_HP = 8,
+    VT_NAME = 10,
+    VT_INVENTORY = 14,
+    VT_COLOR = 16,
+  };
+  const Vec3 *pos() const { return GetStruct<const Vec3 *>(VT_POS); }
+  Vec3 *mutable_pos() { return GetStruct<Vec3 *>(VT_POS); }
+  int16_t mana() const { return GetField<int16_t>(VT_MANA, 150); }
+  bool mutate_mana(int16_t _mana) { return SetField(VT_MANA, _mana); }
+  int16_t hp() const { return GetField<int16_t>(VT_HP, 100); }
+  bool mutate_hp(int16_t _hp) { return SetField(VT_HP, _hp); }
+  const flatbuffers::String *name() const { return GetPointer<const flatbuffers::String *>(VT_NAME); }
+  flatbuffers::String *mutable_name() { return GetPointer<flatbuffers::String *>(VT_NAME); }
+  const flatbuffers::Vector<uint8_t> *inventory() const { return GetPointer<const flatbuffers::Vector<uint8_t> *>(VT_INVENTORY); }
+  flatbuffers::Vector<uint8_t> *mutable_inventory() { return GetPointer<flatbuffers::Vector<uint8_t> *>(VT_INVENTORY); }
+  Color color() const { return static_cast<Color>(GetField<int8_t>(VT_COLOR, 2)); }
+  bool mutate_color(Color _color) { return SetField(VT_COLOR, static_cast<int8_t>(_color)); }
   bool Verify(flatbuffers::Verifier &verifier) const {
     return VerifyTableStart(verifier) &&
-           VerifyField<Vec3>(verifier, 4 /* pos */) &&
-           VerifyField<int16_t>(verifier, 6 /* mana */) &&
-           VerifyField<int16_t>(verifier, 8 /* hp */) &&
-           VerifyField<flatbuffers::uoffset_t>(verifier, 10 /* name */) &&
+           VerifyField<Vec3>(verifier, VT_POS) &&
+           VerifyField<int16_t>(verifier, VT_MANA) &&
+           VerifyField<int16_t>(verifier, VT_HP) &&
+           VerifyField<flatbuffers::uoffset_t>(verifier, VT_NAME) &&
            verifier.Verify(name()) &&
-           VerifyField<flatbuffers::uoffset_t>(verifier, 14 /* inventory */) &&
+           VerifyField<flatbuffers::uoffset_t>(verifier, VT_INVENTORY) &&
            verifier.Verify(inventory()) &&
-           VerifyField<int8_t>(verifier, 16 /* color */) &&
+           VerifyField<int8_t>(verifier, VT_COLOR) &&
            verifier.EndTable();
   }
 };
 
 struct MonsterBuilder {
   flatbuffers::FlatBufferBuilder &fbb_;
   flatbuffers::uoffset_t start_;
-  void add_pos(const Vec3 *pos) { fbb_.AddStruct(4, pos); }
-  void add_mana(int16_t mana) { fbb_.AddElement<int16_t>(6, mana, 150); }
-  void add_hp(int16_t hp) { fbb_.AddElement<int16_t>(8, hp, 100); }
-  void add_name(flatbuffers::Offset<flatbuffers::String> name) { fbb_.AddOffset(10, name); }
-  void add_inventory(flatbuffers::Offset<flatbuffers::Vector<uint8_t>> inventory) { fbb_.AddOffset(14, inventory); }
-  void add_color(Color color) { fbb_.AddElement<int8_t>(16, static_cast<int8_t>(color), 2); }
+  void add_pos(const Vec3 *pos) { fbb_.AddStruct(Monster::VT_POS, pos); }
+  void add_mana(int16_t mana) { fbb_.AddElement<int16_t>(Monster::VT_MANA, mana, 150); }
+  void add_hp(int16_t hp) { fbb_.AddElement<int16_t>(Monster::VT_HP, hp, 100); }
+  void add_name(flatbuffers::Offset<flatbuffers::String> name) { fbb_.AddOffset(Monster::VT_NAME, name); }
+  void add_inventory(flatbuffers::Offset<flatbuffers::Vector<uint8_t>> inventory) { fbb_.AddOffset(Monster::VT_INVENTORY, inventory); }
+  void add_color(Color color) { fbb_.AddElement<int8_t>(Monster::VT_COLOR, static_cast<int8_t>(color), 2); }
   MonsterBuilder(flatbuffers::FlatBufferBuilder &_fbb) : fbb_(_fbb) { start_ = fbb_.StartTable(); }
   MonsterBuilder &operator=(const MonsterBuilder &);
   flatbuffers::Offset<Monster> Finish() {

src/idl_gen_cpp.cpp

@@ -242,6 +242,12 @@ std::string GenUnderlyingCast(const Parser &parser, const FieldDef &field,
       : val;
 }
 
+std::string GenFieldOffsetName(const FieldDef &field) {
+  std::string uname = field.name;
+  std::transform(uname.begin(), uname.end(), uname.begin(), ::toupper);
+  return "VT_" + uname;
+}
+
 // Generate an accessor struct, builder structs & function for a table.
 static void GenTable(const Parser &parser, StructDef &struct_def,
                      const GeneratorOptions &opts, std::string *code_ptr) {
@@ -254,6 +260,19 @@ static void GenTable(const Parser &parser, StructDef &struct_def,
   code += "struct " + struct_def.name;
   code += " FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table";
   code += " {\n";
+  // Generate field id constants.
+  code += "  enum {\n";
+  for (auto it = struct_def.fields.vec.begin();
+       it != struct_def.fields.vec.end();
+       ++it) {
+    auto &field = **it;
+    if (!field.deprecated) {  // Deprecated fields won't be accessible.
+      code += "    " + GenFieldOffsetName(field) + " = ";
+      code += NumToString(field.value.offset) + ",\n";
+    }
+  }
+  code += "  };\n";
+  // Generate the accessors.
   for (auto it = struct_def.fields.vec.begin();
        it != struct_def.fields.vec.end();
        ++it) {
@@ -268,7 +287,7 @@ static void GenTable(const Parser &parser, StructDef &struct_def,
       auto accessor = is_scalar
         ? "GetField<"
         : (IsStruct(field.value.type) ? "GetStruct<" : "GetPointer<");
-      auto offsetstr = NumToString(field.value.offset);
+      auto offsetstr = GenFieldOffsetName(field);
       auto call =
           accessor +
           GenTypeGet(parser, field.value.type, "", "const ", " *", false) +
@@ -347,8 +366,7 @@ static void GenTable(const Parser &parser, StructDef &struct_def,
       code += prefix + "VerifyField";
       if (field.required) code += "Required";
       code += "<" + GenTypeSize(parser, field.value.type);
-      code += ">(verifier, " + NumToString(field.value.offset);
-      code += " /* " + field.name + " */)";
+      code += ">(verifier, " + GenFieldOffsetName(field) + ")";
       switch (field.value.type.base_type) {
         case BASE_TYPE_UNION:
           code += prefix + "Verify" + field.value.type.enum_def->name;
@@ -412,7 +430,7 @@ static void GenTable(const Parser &parser, StructDef &struct_def,
       } else {
         code += "Offset";
       }
-      code += "(" + NumToString(field.value.offset) + ", ";
+      code += "(" + struct_def.name + "::" + GenFieldOffsetName(field) + ", ";
       code += GenUnderlyingCast(parser, field, false, field.name);
       if (IsScalar(field.value.type.base_type))
         code += ", " + field.value.constant;
@@ -433,7 +451,8 @@ static void GenTable(const Parser &parser, StructDef &struct_def,
        ++it) {
     auto &field = **it;
     if (!field.deprecated && field.required) {
-      code += "    fbb_.Required(o, " + NumToString(field.value.offset);
+      code += "    fbb_.Required(o, ";
+      code += struct_def.name + "::" + GenFieldOffsetName(field);
       code += ");  // " + field.name + "\n";
     }
   }