Add --root-type option to flatc [C++, parser, JSON] (#4728)

* Add --root-type option to flatc

To select or override a root_type

* Add help text from flatc --root-type to Compiler.md doc

docs/source/Compiler.md

@@ -128,5 +128,7 @@ Additional options:
 -   `--reflect-types` : Add minimal type reflection to code generation.
 -   `--reflect-names` : Add minimal type/name reflection.
 
+-   `--root-type T` : Select or override the default root_type.
+
 NOTE: short-form options for generators are deprecated, use the long form
 whenever possible.

include/flatbuffers/idl.h

@@ -390,6 +390,7 @@ struct IDLOptions {
   bool reexport_ts_modules;
   bool protobuf_ascii_alike;
   bool size_prefixed;
+  std::string root_type;
 
   // Possible options for the more general generator below.
   enum Language {

src/flatc.cpp

@@ -120,6 +120,7 @@ std::string FlatCompiler::GetUsageString(const char *program_name) const {
     "  --no-ts-reexport   Don't re-export imported dependencies for TypeScript.\n"
     "  --reflect-types    Add minimal type reflection to code generation.\n"
     "  --reflect-names    Add minimal type/name reflection.\n"
+    "  --root-type T      Select or override the default root_type\n"
     "FILEs may be schemas (must end in .fbs), or JSON files (conforming to preceding\n"
     "schema). FILEs after the -- must be binary flatbuffer format files.\n"
     "Output files are named using the base file name of the input,\n"
@@ -268,6 +269,9 @@ int FlatCompiler::Compile(int argc, const char **argv) {
         opts.mini_reflect = IDLOptions::kTypes;
       } else if (arg == "--reflect-names") {
         opts.mini_reflect = IDLOptions::kTypesAndNames;
+      } else if (arg == "--root-type") {
+        if (++argi >= argc) Error("missing type following" + arg, true);
+        opts.root_type = argv[argi];
       } else {
         for (size_t i = 0; i < params_.num_generators; ++i) {
           if (arg == params_.generators[i].generator_opt_long ||
@@ -407,6 +411,13 @@ int FlatCompiler::Compile(int argc, const char **argv) {
       }
     }
 
+    if (!opts.root_type.empty()) {
+      if (!parser->SetRootType(opts.root_type.c_str()))
+        Error("unknown root type: " + opts.root_type);
+      else if (parser->root_struct_def_->fixed)
+        Error("root type must be a table");
+    }
+
     if (opts.proto_mode) GenerateFBS(*parser.get(), output_path, filebase);
 
     // We do not want to generate code for the definitions in this file

src/idl_parser.cpp

@@ -2432,9 +2432,12 @@ CheckedError Parser::DoParse(const char *source, const char **include_paths,
       auto root_type = attribute_;
       EXPECT(kTokenIdentifier);
       ECHECK(ParseNamespacing(&root_type, nullptr));
-      if (!SetRootType(root_type.c_str()))
-        return Error("unknown root type: " + root_type);
-      if (root_struct_def_->fixed) return Error("root type must be a table");
+      if (opts.root_type.empty()) {
+        if (!SetRootType(root_type.c_str()))
+          return Error("unknown root type: " + root_type);
+        if (root_struct_def_->fixed)
+          return Error("root type must be a table");
+      }
       EXPECT(';');
     } else if (IsIdent("file_identifier")) {
       NEXT();