templated type

docs/source/languages/cpp.md

@@ -242,6 +242,50 @@ provide the following functions to aide in the serialization process:
     }
 ```
 
+-   `native_type_template("type<{{T}}>")` (on a struct) together with
+    `native_type_template_arg("type")` (on a field of that struct type):
+    lets a single struct declaration back a native type that is a C++
+    template, instantiated differently per field, instead of requiring one
+    struct declaration (and `native_type`) per instantiation. For example:
+
+```cpp
+    struct Box (native_type_template: "Native::Box<{{T}}>") {
+      value: int32;
+    }
+
+    table Example {
+      a: Box (native_inline, native_type_template_arg: "Native::TypeA");
+      b: Box (native_inline, native_type_template_arg: "Native::TypeB");
+    }
+```
+
+is equivalent to writing, on each field itself:
+
+```cpp
+      a: Box (native_inline, native_type: "Native::Box<Native::TypeA>", native_type_pack_name: "BoxTypeA");
+      b: Box (native_inline, native_type: "Native::Box<Native::TypeB>", native_type_pack_name: "BoxTypeB");
+```
+
+`native_type_template_arg` also auto-derives a `native_type_pack_name` of
+`<StructName><ArgShortName>` (here, `BoxTypeA`/`BoxTypeB`) so the Pack/UnPack
+functions stay unique across instantiations without spelling it out
+yourself; an explicit `native_type_pack_name` on the field still overrides
+this. More than one template parameter is supported by separating them with
+commas and using indexed placeholders:
+
+```cpp
+    struct Pair (native_type_template: "Native::Pair<{{T0}}, {{T1}}>") {
+      value: int32;
+    }
+
+    table Example2 {
+      p: Pair (native_inline, native_type_template_arg: "Native::Key, Native::Value");
+    }
+```
+
+`native_type_template_arg` is only valid on fields whose type is a struct
+(or vector of structs) that declares `native_type_template`.
+
 -   `native_type("type")` (on a table): Tables can also be represented with
 native types. For example, the following schema:
 

include/flatbuffers/idl.h

@@ -1018,6 +1018,8 @@ class Parser : public ParserState {
     known_attributes_["native_custom_alloc"] = true;
     known_attributes_["native_type"] = true;
     known_attributes_["native_type_pack_name"] = true;
+    known_attributes_["native_type_template"] = true;
+    known_attributes_["native_type_template_arg"] = true;
     known_attributes_["native_default"] = true;
     known_attributes_["flexbuffer"] = true;
     known_attributes_["private"] = true;

src/idl_gen_cpp.cpp

@@ -892,6 +892,24 @@ class CppGenerator : public BaseGenerator {
     return opts_.gen_nullable ? " _Nullable " : "";
   }
 
+  // A field's `native_type`/`native_type_pack_name` normally come from the
+  // referenced struct's own declaration. A field can override both (see
+  // `native_type_template_arg` in idl_parser.cpp, which synthesizes these
+  // two attributes directly on the field) to instantiate a templated
+  // native type differently per field.
+  static const Value* EffectiveNativeType(const FieldDef& field,
+                                          const StructDef& struct_def) {
+    if (const auto v = field.attributes.Lookup("native_type")) return v;
+    return struct_def.attributes.Lookup("native_type");
+  }
+
+  static const Value* EffectiveNativeTypePackName(const FieldDef& field,
+                                                   const StructDef& struct_def) {
+    if (const auto v = field.attributes.Lookup("native_type_pack_name"))
+      return v;
+    return struct_def.attributes.Lookup("native_type_pack_name");
+  }
+
   static std::string NativeName(const std::string& name, const StructDef* sd,
                                 const IDLOptions& opts) {
     // If the table is a native_type, return the native_type name.
@@ -1005,7 +1023,7 @@ class CppGenerator : public BaseGenerator {
       case BASE_TYPE_STRUCT: {
         auto type_name = WrapInNameSpace(*type.struct_def);
         if (IsStruct(type)) {
-          auto native_type = type.struct_def->attributes.Lookup("native_type");
+          auto native_type = EffectiveNativeType(field, *type.struct_def);
           if (native_type) {
             type_name = native_type->constant;
           }
@@ -3533,11 +3551,11 @@ class CppGenerator : public BaseGenerator {
       }
       case BASE_TYPE_STRUCT: {
         if (IsStruct(type)) {
-          const auto& struct_attrs = type.struct_def->attributes;
-          const auto native_type = struct_attrs.Lookup("native_type");
+          const auto native_type = EffectiveNativeType(afield, *type.struct_def);
           if (native_type) {
             std::string unpack_call = "::flatbuffers::UnPack";
-            const auto pack_name = struct_attrs.Lookup("native_type_pack_name");
+            const auto pack_name =
+                EffectiveNativeTypePackName(afield, *type.struct_def);
             if (pack_name) {
               unpack_call += pack_name->constant;
             }
@@ -3820,9 +3838,8 @@ class CppGenerator : public BaseGenerator {
           }
           case BASE_TYPE_STRUCT: {
             if (IsStruct(vector_type)) {
-              const auto& struct_attrs =
-                  field.value.type.struct_def->attributes;
-              const auto native_type = struct_attrs.Lookup("native_type");
+              const auto native_type =
+                  EffectiveNativeType(field, *field.value.type.struct_def);
               if (native_type) {
                 code += "_fbb.CreateVectorOfNativeStructs<";
                 code += WrapInNameSpace(*vector_type.struct_def) + ", " +
@@ -3832,8 +3849,8 @@ class CppGenerator : public BaseGenerator {
                 } else {
                   code += "(" + value;
                 }
-                const auto pack_name =
-                    struct_attrs.Lookup("native_type_pack_name");
+                const auto pack_name = EffectiveNativeTypePackName(
+                    field, *field.value.type.struct_def);
                 if (pack_name) {
                   code += ", ::flatbuffers::Pack" + pack_name->constant;
                 }
@@ -3999,12 +4016,12 @@ class CppGenerator : public BaseGenerator {
       }
       case BASE_TYPE_STRUCT: {
         if (IsStruct(field.value.type)) {
-          const auto& struct_attribs = field.value.type.struct_def->attributes;
-          const auto native_type = struct_attribs.Lookup("native_type");
+          const auto native_type =
+              EffectiveNativeType(field, *field.value.type.struct_def);
           if (native_type && field.native_inline) {
             code += "::flatbuffers::Pack";
             const auto pack_name =
-                struct_attribs.Lookup("native_type_pack_name");
+                EffectiveNativeTypePackName(field, *field.value.type.struct_def);
             if (pack_name) {
               code += pack_name->constant;
             }
@@ -4169,8 +4186,7 @@ class CppGenerator : public BaseGenerator {
           if (field->value.type.base_type == BASE_TYPE_STRUCT) {
             if (IsStruct(field->value.type)) {
               auto native_type =
-                  field->value.type.struct_def->attributes.Lookup(
-                      "native_type");
+                  EffectiveNativeType(*field, *field->value.type.struct_def);
               auto native_inline = field->attributes.Lookup("native_inline");
               if (native_type) {
                 pass_by_address = true;

src/idl_parser.cpp

@@ -1286,6 +1286,106 @@ CheckedError Parser::ParseField(StructDef& struct_def) {
         "'native_inline' can only be defined on structs, vector of structs or "
         "vector of tables");
 
+  auto native_type_template_arg =
+      field->attributes.Lookup("native_type_template_arg");
+  if (native_type_template_arg) {
+    if (!IsStruct(field->value.type) && !IsVectorOfStruct(field->value.type))
+      return Error(
+          "'native_type_template_arg' can only be defined on struct-typed "
+          "fields or vectors of structs");
+    const auto* target_struct = field->value.type.struct_def;
+    const auto* native_type_template =
+        target_struct->attributes.Lookup("native_type_template");
+    if (!native_type_template)
+      return Error(
+          "'native_type_template_arg' requires the field's type ('" +
+          target_struct->name +
+          "') to declare a 'native_type_template' attribute");
+    if (field->attributes.Lookup("native_type"))
+      return Error(
+          "'native_type_template_arg' cannot be combined with an explicit "
+          "'native_type' on the same field");
+
+    // Split on top-level commas, so multiple template arguments can be
+    // given (e.g. "::Foo::Key, ::Foo::Value"). Commas nested inside a
+    // template argument that is itself templated (e.g.
+    // "std::pair<int, T>") are not treated as separators.
+    std::vector<std::string> args;
+    {
+      const auto& s = native_type_template_arg->constant;
+      int depth = 0;
+      size_t start = 0;
+      for (size_t i = 0; i < s.size(); ++i) {
+        if (s[i] == '<') {
+          ++depth;
+        } else if (s[i] == '>') {
+          --depth;
+        } else if (s[i] == ',' && depth == 0) {
+          args.push_back(s.substr(start, i - start));
+          start = i + 1;
+        }
+      }
+      args.push_back(s.substr(start));
+      for (auto& arg : args) {
+        const auto b = arg.find_first_not_of(" \t");
+        const auto e = arg.find_last_not_of(" \t");
+        arg = b == std::string::npos ? "" : arg.substr(b, e - b + 1);
+        if (arg.empty())
+          return Error(
+              "'native_type_template_arg' contains an empty template "
+              "argument");
+      }
+    }
+
+    // Substitute each argument for its placeholder: `{{T0}}`, `{{T1}}`, etc.
+    // For a single argument, the unindexed `{{T}}` spelling is also
+    // accepted (equivalent to `{{T0}}`).
+    std::string native_type_str = native_type_template->constant;
+    for (size_t i = 0; i < args.size(); ++i) {
+      std::vector<std::string> placeholders = { "{{T" + NumToString(i) +
+                                                 "}}" };
+      if (i == 0) placeholders.push_back("{{T}}");
+      size_t replaced = 0;
+      for (const auto& placeholder : placeholders) {
+        for (;;) {
+          const auto pos = native_type_str.find(placeholder);
+          if (pos == std::string::npos) break;
+          native_type_str.replace(pos, placeholder.length(), args[i]);
+          ++replaced;
+        }
+      }
+      if (!replaced)
+        return Error("'native_type_template' on '" + target_struct->name +
+                     "' does not reference the '{{T" + NumToString(i) +
+                     "}}' placeholder required for template argument " +
+                     NumToString(i + 1) + " ('" + args[i] + "')");
+    }
+    if (native_type_str.find("{{T") != std::string::npos)
+      return Error(
+          "'native_type_template' on '" + target_struct->name +
+          "' references a '{{T<n>}}' placeholder beyond the " +
+          NumToString(args.size()) +
+          " template argument(s) given in 'native_type_template_arg'");
+
+    auto native_type_val = new Value();
+    native_type_val->type = native_type_template_arg->type;
+    native_type_val->constant = native_type_str;
+    field->attributes.Add("native_type", native_type_val);
+
+    if (!field->attributes.Lookup("native_type_pack_name")) {
+      std::string pack_name = target_struct->name;
+      for (const auto& arg : args) {
+        const auto last_colon = arg.find_last_of(':');
+        pack_name +=
+            last_colon == std::string::npos ? arg : arg.substr(last_colon + 1);
+      }
+      auto pack_name_val = new Value();
+      pack_name_val->type = native_type_template_arg->type;
+      pack_name_val->constant = pack_name;
+      field->attributes.Add("native_type_pack_name", pack_name_val);
+    }
+  }
+
   auto nested = field->attributes.Lookup("nested_flatbuffer");
   if (nested) {
     if (nested->type.base_type != BASE_TYPE_STRING)

tests/native_type_test.fbs

@@ -20,6 +20,19 @@ table Matrix (native_type:"Native::Matrix") {
   values:[float];
 }
 
+// A struct whose native type is a template, instantiated differently per
+// field via `native_type_template_arg` instead of needing one flatbuffers
+// struct declaration per specialization (compare to Vector3D/Vector3DAlt
+// above).
+struct Tagged (native_type_template: "Native::Tagged<{{T}}>") {
+  value:int32;
+}
+
+// A native type template that takes more than one argument.
+struct Pair (native_type_template: "Native::Pair<{{T0}}, {{T1}}>") {
+  value:int32;
+}
+
 table ApplicationData {
   vectors:[Vector3D];
   vectors_alt:[Vector3DAlt];
@@ -27,6 +40,9 @@ table ApplicationData {
   position_inline:Vector3D (native_inline);
   matrix:Matrix;
   matrices:[Matrix];
+  tagged_a:Tagged (native_inline, native_type_template_arg: "Native::TagA");
+  tagged_b:Tagged (native_inline, native_type_template_arg: "Native::TagB");
+  pair_ab:Pair (native_inline, native_type_template_arg: "Native::TagA, Native::TagB");
 }
 
 root_type ApplicationData;

tests/native_type_test_generated.h

@@ -24,10 +24,18 @@ struct Vector3DAlt;
 struct Matrix;
 struct MatrixBuilder;
 
+struct Tagged;
+
+struct Pair;
+
 struct ApplicationData;
 struct ApplicationDataBuilder;
 struct ApplicationDataT;
 
+bool operator==(const Tagged &lhs, const Tagged &rhs);
+bool operator!=(const Tagged &lhs, const Tagged &rhs);
+bool operator==(const Pair &lhs, const Pair &rhs);
+bool operator!=(const Pair &lhs, const Pair &rhs);
 bool operator==(const ApplicationDataT &lhs, const ApplicationDataT &rhs);
 bool operator!=(const ApplicationDataT &lhs, const ApplicationDataT &rhs);
 
@@ -37,6 +45,10 @@ inline const ::flatbuffers::TypeTable *Vector3DAltTypeTable();
 
 inline const ::flatbuffers::TypeTable *MatrixTypeTable();
 
+inline const ::flatbuffers::TypeTable *TaggedTypeTable();
+
+inline const ::flatbuffers::TypeTable *PairTypeTable();
+
 inline const ::flatbuffers::TypeTable *ApplicationDataTypeTable();
 
 FLATBUFFERS_MANUALLY_ALIGNED_STRUCT(4) Vector3D FLATBUFFERS_FINAL_CLASS {
@@ -121,6 +133,72 @@ FLATBUFFERS_MANUALLY_ALIGNED_STRUCT(4) Vector3DAlt FLATBUFFERS_FINAL_CLASS {
 };
 FLATBUFFERS_STRUCT_END(Vector3DAlt, 12);
 
+FLATBUFFERS_MANUALLY_ALIGNED_STRUCT(4) Tagged FLATBUFFERS_FINAL_CLASS {
+ private:
+  int32_t value_;
+
+ public:
+  static const ::flatbuffers::TypeTable *MiniReflectTypeTable() {
+    return TaggedTypeTable();
+  }
+  Tagged()
+      : value_(0) {
+  }
+  Tagged(int32_t _value)
+      : value_(::flatbuffers::EndianScalar(_value)) {
+  }
+  int32_t value() const {
+    return ::flatbuffers::EndianScalar(value_);
+  }
+  void mutate_value(int32_t _value) {
+    ::flatbuffers::WriteScalar(&value_, _value);
+  }
+};
+FLATBUFFERS_STRUCT_END(Tagged, 4);
+
+inline bool operator==(const Tagged &lhs, const Tagged &rhs) {
+  return
+      (lhs.value() == rhs.value());
+}
+
+inline bool operator!=(const Tagged &lhs, const Tagged &rhs) {
+    return !(lhs == rhs);
+}
+
+
+FLATBUFFERS_MANUALLY_ALIGNED_STRUCT(4) Pair FLATBUFFERS_FINAL_CLASS {
+ private:
+  int32_t value_;
+
+ public:
+  static const ::flatbuffers::TypeTable *MiniReflectTypeTable() {
+    return PairTypeTable();
+  }
+  Pair()
+      : value_(0) {
+  }
+  Pair(int32_t _value)
+      : value_(::flatbuffers::EndianScalar(_value)) {
+  }
+  int32_t value() const {
+    return ::flatbuffers::EndianScalar(value_);
+  }
+  void mutate_value(int32_t _value) {
+    ::flatbuffers::WriteScalar(&value_, _value);
+  }
+};
+FLATBUFFERS_STRUCT_END(Pair, 4);
+
+inline bool operator==(const Pair &lhs, const Pair &rhs) {
+  return
+      (lhs.value() == rhs.value());
+}
+
+inline bool operator!=(const Pair &lhs, const Pair &rhs) {
+    return !(lhs == rhs);
+}
+
+
 struct Matrix FLATBUFFERS_FINAL_CLASS : private ::flatbuffers::Table {
   typedef Native::Matrix NativeTableType;
   typedef MatrixBuilder Builder;
@@ -223,6 +301,9 @@ struct ApplicationDataT : public ::flatbuffers::NativeTable {
   Native::Vector3D position_inline{};
   std::unique_ptr<Native::Matrix> matrix{};
   std::vector<std::unique_ptr<Native::Matrix>> matrices{};
+  Native::Tagged<Native::TagA> tagged_a{};
+  Native::Tagged<Native::TagB> tagged_b{};
+  Native::Pair<Native::TagA, Native::TagB> pair_ab{};
   ApplicationDataT() = default;
   ApplicationDataT(const ApplicationDataT &o);
   ApplicationDataT(ApplicationDataT&&) FLATBUFFERS_NOEXCEPT = default;
@@ -241,7 +322,10 @@ struct ApplicationData FLATBUFFERS_FINAL_CLASS : private ::flatbuffers::Table {
     VT_POSITION = 8,
     VT_POSITION_INLINE = 10,
     VT_MATRIX = 12,
-    VT_MATRICES = 14
+    VT_MATRICES = 14,
+    VT_TAGGED_A = 16,
+    VT_TAGGED_B = 18,
+    VT_PAIR_AB = 20
   };
   const ::flatbuffers::Vector<const Geometry::Vector3D *> *vectors() const {
     return GetPointer<const ::flatbuffers::Vector<const Geometry::Vector3D *> *>(VT_VECTORS);
@@ -279,6 +363,24 @@ struct ApplicationData FLATBUFFERS_FINAL_CLASS : private ::flatbuffers::Table {
   ::flatbuffers::Vector<::flatbuffers::Offset<Geometry::Matrix>> *mutable_matrices() {
     return GetPointer<::flatbuffers::Vector<::flatbuffers::Offset<Geometry::Matrix>> *>(VT_MATRICES);
   }
+  const Geometry::Tagged *tagged_a() const {
+    return GetStruct<const Geometry::Tagged *>(VT_TAGGED_A);
+  }
+  Geometry::Tagged *mutable_tagged_a() {
+    return GetStruct<Geometry::Tagged *>(VT_TAGGED_A);
+  }
+  const Geometry::Tagged *tagged_b() const {
+    return GetStruct<const Geometry::Tagged *>(VT_TAGGED_B);
+  }
+  Geometry::Tagged *mutable_tagged_b() {
+    return GetStruct<Geometry::Tagged *>(VT_TAGGED_B);
+  }
+  const Geometry::Pair *pair_ab() const {
+    return GetStruct<const Geometry::Pair *>(VT_PAIR_AB);
+  }
+  Geometry::Pair *mutable_pair_ab() {
+    return GetStruct<Geometry::Pair *>(VT_PAIR_AB);
+  }
   template <bool B = false>
   bool Verify(::flatbuffers::VerifierTemplate<B> &verifier) const {
     return VerifyTableStart(verifier) &&
@@ -293,6 +395,9 @@ struct ApplicationData FLATBUFFERS_FINAL_CLASS : private ::flatbuffers::Table {
            VerifyOffset(verifier, VT_MATRICES) &&
            verifier.VerifyVector(matrices()) &&
            verifier.VerifyVectorOfTables(matrices()) &&
+           VerifyField<Geometry::Tagged>(verifier, VT_TAGGED_A, 4) &&
+           VerifyField<Geometry::Tagged>(verifier, VT_TAGGED_B, 4) &&
+           VerifyField<Geometry::Pair>(verifier, VT_PAIR_AB, 4) &&
            verifier.EndTable();
   }
   ApplicationDataT *UnPack(const ::flatbuffers::resolver_function_t *_resolver = nullptr) const;
@@ -322,6 +427,15 @@ struct ApplicationDataBuilder {
   void add_matrices(::flatbuffers::Offset<::flatbuffers::Vector<::flatbuffers::Offset<Geometry::Matrix>>> matrices) {
     fbb_.AddOffset(ApplicationData::VT_MATRICES, matrices);
   }
+  void add_tagged_a(const Geometry::Tagged *tagged_a) {
+    fbb_.AddStruct(ApplicationData::VT_TAGGED_A, tagged_a);
+  }
+  void add_tagged_b(const Geometry::Tagged *tagged_b) {
+    fbb_.AddStruct(ApplicationData::VT_TAGGED_B, tagged_b);
+  }
+  void add_pair_ab(const Geometry::Pair *pair_ab) {
+    fbb_.AddStruct(ApplicationData::VT_PAIR_AB, pair_ab);
+  }
   explicit ApplicationDataBuilder(::flatbuffers::FlatBufferBuilder &_fbb)
         : fbb_(_fbb) {
     start_ = fbb_.StartTable();
@@ -340,8 +454,14 @@ inline ::flatbuffers::Offset<ApplicationData> CreateApplicationData(
     const Geometry::Vector3D *position = nullptr,
     const Geometry::Vector3D *position_inline = nullptr,
     ::flatbuffers::Offset<Geometry::Matrix> matrix = 0,
-    ::flatbuffers::Offset<::flatbuffers::Vector<::flatbuffers::Offset<Geometry::Matrix>>> matrices = 0) {
+    ::flatbuffers::Offset<::flatbuffers::Vector<::flatbuffers::Offset<Geometry::Matrix>>> matrices = 0,
+    const Geometry::Tagged *tagged_a = nullptr,
+    const Geometry::Tagged *tagged_b = nullptr,
+    const Geometry::Pair *pair_ab = nullptr) {
   ApplicationDataBuilder builder_(_fbb);
+  builder_.add_pair_ab(pair_ab);
+  builder_.add_tagged_b(tagged_b);
+  builder_.add_tagged_a(tagged_a);
   builder_.add_matrices(matrices);
   builder_.add_matrix(matrix);
   builder_.add_position_inline(position_inline);
@@ -358,7 +478,10 @@ inline ::flatbuffers::Offset<ApplicationData> CreateApplicationDataDirect(
     const Geometry::Vector3D *position = nullptr,
     const Geometry::Vector3D *position_inline = nullptr,
     ::flatbuffers::Offset<Geometry::Matrix> matrix = 0,
-    const std::vector<::flatbuffers::Offset<Geometry::Matrix>> *matrices = nullptr) {
+    const std::vector<::flatbuffers::Offset<Geometry::Matrix>> *matrices = nullptr,
+    const Geometry::Tagged *tagged_a = nullptr,
+    const Geometry::Tagged *tagged_b = nullptr,
+    const Geometry::Pair *pair_ab = nullptr) {
   auto vectors__ = vectors ? _fbb.CreateVectorOfStructs<Geometry::Vector3D>(*vectors) : 0;
   auto vectors_alt__ = vectors_alt ? _fbb.CreateVectorOfStructs<Geometry::Vector3DAlt>(*vectors_alt) : 0;
   auto matrices__ = matrices ? _fbb.CreateVector<::flatbuffers::Offset<Geometry::Matrix>>(*matrices) : 0;
@@ -369,7 +492,10 @@ inline ::flatbuffers::Offset<ApplicationData> CreateApplicationDataDirect(
       position,
       position_inline,
       matrix,
-      matrices__);
+      matrices__,
+      tagged_a,
+      tagged_b,
+      pair_ab);
 }
 
 ::flatbuffers::Offset<ApplicationData> CreateApplicationData(::flatbuffers::FlatBufferBuilder &_fbb, const ApplicationDataT *_o, const ::flatbuffers::rehasher_function_t *_rehasher = nullptr);
@@ -392,7 +518,10 @@ inline bool operator==(const ApplicationDataT &lhs, const ApplicationDataT &rhs)
       ((lhs.position == rhs.position) || (lhs.position && rhs.position && *lhs.position == *rhs.position)) &&
       (lhs.position_inline == rhs.position_inline) &&
       ((lhs.matrix == rhs.matrix) || (lhs.matrix && rhs.matrix && *lhs.matrix == *rhs.matrix)) &&
-      (lhs.matrices.size() == rhs.matrices.size() && std::equal(lhs.matrices.cbegin(), lhs.matrices.cend(), rhs.matrices.cbegin(), [](std::unique_ptr<Native::Matrix> const &a, std::unique_ptr<Native::Matrix> const &b) { return (a == b) || (a && b && *a == *b); }));
+      (lhs.matrices.size() == rhs.matrices.size() && std::equal(lhs.matrices.cbegin(), lhs.matrices.cend(), rhs.matrices.cbegin(), [](std::unique_ptr<Native::Matrix> const &a, std::unique_ptr<Native::Matrix> const &b) { return (a == b) || (a && b && *a == *b); })) &&
+      (lhs.tagged_a == rhs.tagged_a) &&
+      (lhs.tagged_b == rhs.tagged_b) &&
+      (lhs.pair_ab == rhs.pair_ab);
 }
 
 inline bool operator!=(const ApplicationDataT &lhs, const ApplicationDataT &rhs) {
@@ -405,7 +534,10 @@ inline ApplicationDataT::ApplicationDataT(const ApplicationDataT &o)
         vectors_alt(o.vectors_alt),
         position((o.position) ? new Native::Vector3D(*o.position) : nullptr),
         position_inline(o.position_inline),
-        matrix((o.matrix) ? new Native::Matrix(*o.matrix) : nullptr) {
+        matrix((o.matrix) ? new Native::Matrix(*o.matrix) : nullptr),
+        tagged_a(o.tagged_a),
+        tagged_b(o.tagged_b),
+        pair_ab(o.pair_ab) {
   matrices.reserve(o.matrices.size());
   for (const auto &matrices_ : o.matrices) { matrices.emplace_back((matrices_) ? new Native::Matrix(*matrices_) : nullptr); }
 }
@@ -417,6 +549,9 @@ inline ApplicationDataT &ApplicationDataT::operator=(ApplicationDataT o) FLATBUF
   std::swap(position_inline, o.position_inline);
   std::swap(matrix, o.matrix);
   std::swap(matrices, o.matrices);
+  std::swap(tagged_a, o.tagged_a);
+  std::swap(tagged_b, o.tagged_b);
+  std::swap(pair_ab, o.pair_ab);
   return *this;
 }
 
@@ -435,6 +570,9 @@ inline void ApplicationData::UnPackTo(ApplicationDataT *_o, const ::flatbuffers:
   { auto _e = position_inline(); if (_e) _o->position_inline = ::flatbuffers::UnPack(*_e); }
   { auto _e = matrix(); if (_e) { if(_o->matrix) { _e->UnPackTo(_o->matrix.get(), _resolver); } else { _o->matrix = std::unique_ptr<Native::Matrix>(_e->UnPack(_resolver)); } } else if (_o->matrix) { _o->matrix.reset(); } }
   { auto _e = matrices(); if (_e) { _o->matrices.resize(_e->size()); for (::flatbuffers::uoffset_t _i = 0; _i < _e->size(); _i++) { if(_o->matrices[_i]) { _e->Get(_i)->UnPackTo(_o->matrices[_i].get(), _resolver); } else { _o->matrices[_i] = std::unique_ptr<Native::Matrix>(_e->Get(_i)->UnPack(_resolver)); } } } else { _o->matrices.resize(0); } }
+  { auto _e = tagged_a(); if (_e) _o->tagged_a = ::flatbuffers::UnPackTaggedTagA(*_e); }
+  { auto _e = tagged_b(); if (_e) _o->tagged_b = ::flatbuffers::UnPackTaggedTagB(*_e); }
+  { auto _e = pair_ab(); if (_e) _o->pair_ab = ::flatbuffers::UnPackPairTagATagB(*_e); }
 }
 
 inline ::flatbuffers::Offset<ApplicationData> CreateApplicationData(::flatbuffers::FlatBufferBuilder &_fbb, const ApplicationDataT *_o, const ::flatbuffers::rehasher_function_t *_rehasher) {
@@ -451,6 +589,9 @@ inline ::flatbuffers::Offset<ApplicationData> ApplicationData::Pack(::flatbuffer
   auto _position_inline = ::flatbuffers::Pack(_o->position_inline);
   auto _matrix = _o->matrix ? CreateMatrix(_fbb, _o->matrix.get(), _rehasher) : 0;
   auto _matrices = _o->matrices.size() ? _fbb.CreateVector<::flatbuffers::Offset<Geometry::Matrix>> (_o->matrices.size(), [](size_t i, _VectorArgs *__va) { return CreateMatrix(*__va->__fbb, __va->__o->matrices[i].get(), __va->__rehasher); }, &_va ) : 0;
+  auto _tagged_a = ::flatbuffers::PackTaggedTagA(_o->tagged_a);
+  auto _tagged_b = ::flatbuffers::PackTaggedTagB(_o->tagged_b);
+  auto _pair_ab = ::flatbuffers::PackPairTagATagB(_o->pair_ab);
   return Geometry::CreateApplicationData(
       _fbb,
       _vectors,
@@ -458,7 +599,10 @@ inline ::flatbuffers::Offset<ApplicationData> ApplicationData::Pack(::flatbuffer
       _o->position ? &_position : nullptr,
       &_position_inline,
       _matrix,
-      _matrices);
+      _matrices,
+      &_tagged_a,
+      &_tagged_b,
+      &_pair_ab);
 }
 
 inline const ::flatbuffers::TypeTable *Vector3DTypeTable() {
@@ -514,6 +658,34 @@ inline const ::flatbuffers::TypeTable *MatrixTypeTable() {
   return &tt;
 }
 
+inline const ::flatbuffers::TypeTable *TaggedTypeTable() {
+  static const ::flatbuffers::TypeCode type_codes[] = {
+    { ::flatbuffers::ET_INT, 0, -1 }
+  };
+  static const int64_t values[] = { 0, 4 };
+  static const char * const names[] = {
+    "value"
+  };
+  static const ::flatbuffers::TypeTable tt = {
+    ::flatbuffers::ST_STRUCT, 1, type_codes, nullptr, nullptr, values, names
+  };
+  return &tt;
+}
+
+inline const ::flatbuffers::TypeTable *PairTypeTable() {
+  static const ::flatbuffers::TypeCode type_codes[] = {
+    { ::flatbuffers::ET_INT, 0, -1 }
+  };
+  static const int64_t values[] = { 0, 4 };
+  static const char * const names[] = {
+    "value"
+  };
+  static const ::flatbuffers::TypeTable tt = {
+    ::flatbuffers::ST_STRUCT, 1, type_codes, nullptr, nullptr, values, names
+  };
+  return &tt;
+}
+
 inline const ::flatbuffers::TypeTable *ApplicationDataTypeTable() {
   static const ::flatbuffers::TypeCode type_codes[] = {
     { ::flatbuffers::ET_SEQUENCE, 1, 0 },
@@ -521,12 +693,17 @@ inline const ::flatbuffers::TypeTable *ApplicationDataTypeTable() {
     { ::flatbuffers::ET_SEQUENCE, 0, 0 },
     { ::flatbuffers::ET_SEQUENCE, 0, 0 },
     { ::flatbuffers::ET_SEQUENCE, 0, 2 },
-    { ::flatbuffers::ET_SEQUENCE, 1, 2 }
+    { ::flatbuffers::ET_SEQUENCE, 1, 2 },
+    { ::flatbuffers::ET_SEQUENCE, 0, 3 },
+    { ::flatbuffers::ET_SEQUENCE, 0, 3 },
+    { ::flatbuffers::ET_SEQUENCE, 0, 4 }
   };
   static const ::flatbuffers::TypeFunction type_refs[] = {
     Geometry::Vector3DTypeTable,
     Geometry::Vector3DAltTypeTable,
-    Geometry::MatrixTypeTable
+    Geometry::MatrixTypeTable,
+    Geometry::TaggedTypeTable,
+    Geometry::PairTypeTable
   };
   static const char * const names[] = {
     "vectors",
@@ -534,10 +711,13 @@ inline const ::flatbuffers::TypeTable *ApplicationDataTypeTable() {
     "position",
     "position_inline",
     "matrix",
-    "matrices"
+    "matrices",
+    "tagged_a",
+    "tagged_b",
+    "pair_ab"
   };
   static const ::flatbuffers::TypeTable tt = {
-    ::flatbuffers::ST_TABLE, 6, type_codes, type_refs, nullptr, nullptr, names
+    ::flatbuffers::ST_TABLE, 9, type_codes, type_refs, nullptr, nullptr, names
   };
   return &tt;
 }

tests/native_type_test_impl.cpp

@@ -18,6 +18,34 @@ Geometry::Vector3DAlt PackVector3DAlt(const Native::Vector3D& obj) {
 const Native::Vector3D UnPackVector3DAlt(const Geometry::Vector3DAlt& obj) {
   return Native::Vector3D(obj.a(), obj.b(), obj.c());
 }
+
+Geometry::Tagged PackTaggedTagA(const Native::Tagged<Native::TagA>& obj) {
+  return Geometry::Tagged(obj.value);
+}
+
+const Native::Tagged<Native::TagA> UnPackTaggedTagA(
+    const Geometry::Tagged& obj) {
+  return Native::Tagged<Native::TagA>(obj.value());
+}
+
+Geometry::Tagged PackTaggedTagB(const Native::Tagged<Native::TagB>& obj) {
+  return Geometry::Tagged(obj.value);
+}
+
+const Native::Tagged<Native::TagB> UnPackTaggedTagB(
+    const Geometry::Tagged& obj) {
+  return Native::Tagged<Native::TagB>(obj.value());
+}
+
+Geometry::Pair PackPairTagATagB(
+    const Native::Pair<Native::TagA, Native::TagB>& obj) {
+  return Geometry::Pair(obj.value);
+}
+
+const Native::Pair<Native::TagA, Native::TagB> UnPackPairTagATagB(
+    const Geometry::Pair& obj) {
+  return Native::Pair<Native::TagA, Native::TagB>(obj.value());
+}
 }  // namespace flatbuffers
 
 namespace Geometry {

tests/native_type_test_impl.h

@@ -43,11 +43,42 @@ struct Matrix {
            (values == other.values);
   }
 };
+
+// Phantom tag types used purely to select a template specialization; they
+// carry no data of their own.
+struct TagA {};
+struct TagB {};
+
+// A native type template, instantiated per-field in the schema via
+// `native_type_template_arg` rather than needing one flatbuffers struct
+// declaration per specialization.
+template <typename T>
+struct Tagged {
+  int32_t value;
+
+  Tagged() : value(0) {}
+  explicit Tagged(int32_t _value) : value(_value) {}
+
+  bool operator==(const Tagged& other) const { return value == other.value; }
+};
+
+// A native type template taking more than one argument.
+template <typename T0, typename T1>
+struct Pair {
+  int32_t value;
+
+  Pair() : value(0) {}
+  explicit Pair(int32_t _value) : value(_value) {}
+
+  bool operator==(const Pair& other) const { return value == other.value; }
+};
 }  // namespace Native
 
 namespace Geometry {
 struct Vector3D;
 struct Vector3DAlt;
+struct Tagged;
+struct Pair;
 }  // namespace Geometry
 
 namespace flatbuffers {
@@ -55,6 +86,16 @@ Geometry::Vector3D Pack(const Native::Vector3D& obj);
 const Native::Vector3D UnPack(const Geometry::Vector3D& obj);
 Geometry::Vector3DAlt PackVector3DAlt(const Native::Vector3D& obj);
 const Native::Vector3D UnPackVector3DAlt(const Geometry::Vector3DAlt& obj);
+
+Geometry::Tagged PackTaggedTagA(const Native::Tagged<Native::TagA>& obj);
+const Native::Tagged<Native::TagA> UnPackTaggedTagA(const Geometry::Tagged& obj);
+Geometry::Tagged PackTaggedTagB(const Native::Tagged<Native::TagB>& obj);
+const Native::Tagged<Native::TagB> UnPackTaggedTagB(const Geometry::Tagged& obj);
+
+Geometry::Pair PackPairTagATagB(
+    const Native::Pair<Native::TagA, Native::TagB>& obj);
+const Native::Pair<Native::TagA, Native::TagB> UnPackPairTagATagB(
+    const Geometry::Pair& obj);
 }  // namespace flatbuffers
 
 #endif  // VECTOR3D_PACK_H

tests/parser_test.cpp

@@ -97,6 +97,25 @@ void ErrorTest() {
             "datatype already");
   TestError("struct X (force_align: 7) { Y:int; }", "force_align");
   TestError("struct X {}", "size 0");
+  TestError(
+      "struct X { Y:int; } table T { y:X (native_type_template_arg:\"int\"); "
+      "}",
+      "'native_type_template' attribute");
+  TestError(
+      "table T { y:int (native_type_template_arg:\"int\"); }",
+      "struct-typed fields");
+  TestError(
+      "struct X (native_type_template: \"Foo\") { Y:int; } "
+      "table T { y:X (native_type_template_arg:\"int\"); }",
+      "does not reference");
+  TestError(
+      "struct X (native_type_template: \"Foo<{{T0}}, {{T1}}>\") { Y:int; } "
+      "table T { y:X (native_type_template_arg:\"int\"); }",
+      "beyond the");
+  TestError(
+      "struct X (native_type_template: \"Foo<{{T}}>\") { Y:int; } "
+      "table T { y:X (native_type_template_arg:\"\"); }",
+      "empty template argument");
   TestError("{}", "no root");
   TestError("table X { Y:byte; } root_type X; { Y:1 } { Y:1 }", "end of file");
   TestError("table X { Y:byte; } root_type X; { Y:1 } table Y{ Z:int }",

tests/test.cpp

@@ -930,6 +930,10 @@ void NativeTypeTest() {
         Native::Vector3D(20 * i + 0.1f, 20 * i + 0.2f, 20 * i + 0.3f));
   }
 
+  src_data.tagged_a = Native::Tagged<Native::TagA>(7);
+  src_data.tagged_b = Native::Tagged<Native::TagB>(8);
+  src_data.pair_ab = Native::Pair<Native::TagA, Native::TagB>(9);
+
   src_data.matrix = std::unique_ptr<Native::Matrix>(new Native::Matrix(1, 2));
   src_data.matrix->values = {3, 4};
 
@@ -951,6 +955,9 @@ void NativeTypeTest() {
   TEST_EQ(dstDataT->position_inline.x, 4.0f);
   TEST_EQ(dstDataT->position_inline.y, 5.0f);
   TEST_EQ(dstDataT->position_inline.z, 6.0f);
+  TEST_EQ(dstDataT->tagged_a.value, 7);
+  TEST_EQ(dstDataT->tagged_b.value, 8);
+  TEST_EQ(dstDataT->pair_ab.value, 9);
 
   for (int i = 0; i < N; ++i) {
     const Native::Vector3D& v = dstDataT->vectors[i];