Simplify templates

docs/source/languages/cpp.md

@@ -242,14 +242,19 @@ provide the following functions to aide in the serialization process:
     }
 ```
 
--   `native_type_template("type<{{T}}>")` (on a struct) together with
+-   `native_type_template("type")` (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:
+    struct declaration (and `native_type`) per instantiation.
+    `native_type_template_arg` is angle-bracket-appended to
+    `native_type_template` to form the field's native type, so
+    `native_type_template: "Native::Box"` with
+    `native_type_template_arg: "Native::TypeA"` produces
+    `Native::Box<Native::TypeA>`. For example:
 
 ```cpp
-    struct Box (native_type_template: "Native::Box<{{T}}>") {
+    struct Box (native_type_template: "Native::Box") {
       value: int32;
     }
 
@@ -271,10 +276,10 @@ is equivalent to writing, on each field itself:
 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:
+commas:
 
 ```cpp
-    struct Pair (native_type_template: "Native::Pair<{{T0}}, {{T1}}>") {
+    struct Pair (native_type_template: "Native::Pair") {
       value: int32;
     }
 
@@ -283,6 +288,7 @@ commas and using indexed placeholders:
     }
 ```
 
+which produces `Native::Pair<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`.
 

src/idl_parser.cpp

@@ -1337,35 +1337,16 @@ CheckedError Parser::ParseField(StructDef& struct_def) {
       }
     }
 
-    // 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;
+    // The native type is `native_type_template`, the bare template name,
+    // with the arguments appended as a trailing, angle-bracketed template
+    // argument list, e.g. "Native::Box" + ["Native::TypeA"] ->
+    // "Native::Box<Native::TypeA>".
+    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 (i) native_type_str += ", ";
+      native_type_str += 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'");
+    native_type_str += ">";
 
     auto native_type_val = new Value();
     native_type_val->type = native_type_template_arg->type;

tests/native_type_test.fbs

@@ -24,12 +24,12 @@ table Matrix (native_type:"Native::Matrix") {
 // 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}}>") {
+struct Tagged (native_type_template: "Native::Tagged") {
   value:int32;
 }
 
 // A native type template that takes more than one argument.
-struct Pair (native_type_template: "Native::Pair<{{T0}}, {{T1}}>") {
+struct Pair (native_type_template: "Native::Pair") {
   value:int32;
 }
 

tests/parser_test.cpp

@@ -106,14 +106,6 @@ void ErrorTest() {
       "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");