Add --force-defaults option to flatc [C++, parser] (#4729)

* Add --force-defaults option to flatc

To emit default values for fields which are not present or which are set
to the default value.

* flatc option --force-defaults should have a default value (false) and take action on the builder_ within the Parser constructor

* Add help text from flatc --force-defaults to Compiler.md doc

* Clarified docs for flatc --force-defaults, and imply that this behaviour is not normally needed.

* Updated docs and flatc help text for --force-defaults option

docs/source/Compiler.md

@@ -136,5 +136,7 @@ Additional options:
 
 -   `--root-type T` : Select or override the default root_type.
 
+-   `--force-defaults` : Emit default values in binary output from JSON.
+
 NOTE: short-form options for generators are deprecated, use the long form
 whenever possible.

include/flatbuffers/idl.h

@@ -392,6 +392,7 @@ struct IDLOptions {
   bool protobuf_ascii_alike;
   bool size_prefixed;
   std::string root_type;
+  bool force_defaults;
 
   // Possible options for the more general generator below.
   enum Language {
@@ -452,6 +453,7 @@ struct IDLOptions {
         reexport_ts_modules(true),
         protobuf_ascii_alike(false),
         size_prefixed(false),
+        force_defaults(false),
         lang(IDLOptions::kJava),
         mini_reflect(IDLOptions::kNone),
         lang_to_generate(0) {}
@@ -527,6 +529,9 @@ class Parser : public ParserState {
         source_(nullptr),
         anonymous_counter(0),
         recurse_protection_counter(0) {
+    if (opts.force_defaults) {
+      builder_.ForceDefaults(true);
+    }
     // Start out with the empty namespace being current.
     empty_namespace_ = new Namespace();
     namespaces_.push_back(empty_namespace_);

src/flatc.cpp

@@ -122,6 +122,7 @@ std::string FlatCompiler::GetUsageString(const char *program_name) const {
     "  --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"
+    "  --force-defaults   Emit default values in binary output from JSON\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"
@@ -277,6 +278,8 @@ int FlatCompiler::Compile(int argc, const char **argv) {
       } else if (arg == "--root-type") {
         if (++argi >= argc) Error("missing type following" + arg, true);
         opts.root_type = argv[argi];
+      } else if (arg == "--force-defaults") {
+        opts.force_defaults = true;
       } else {
         for (size_t i = 0; i < params_.num_generators; ++i) {
           if (arg == params_.generators[i].generator_opt_long ||