[flac] Add FlexBuffers option for generating data (#5519)

Alongside --binary and --json, an additional
switch (--flexbuffers) can be passed to convert
data using FlexBuffers

include/flatbuffers/flexbuffers.h

@@ -349,6 +349,12 @@ void AppendToString(std::string &s, T &&v, bool keys_quoted) {
 
 class Reference {
  public:
+  Reference()
+      : data_(nullptr),
+        parent_width_(0),
+        byte_width_(BIT_WIDTH_8),
+        type_(FBT_NULL) {}
+
   Reference(const uint8_t *data, uint8_t parent_width, uint8_t byte_width,
             Type type)
       : data_(data),

include/flatbuffers/idl.h

@@ -491,6 +491,8 @@ struct ServiceDef : public Definition {
 
 // Container of options that may apply to any of the source/text generators.
 struct IDLOptions {
+  // Use flexbuffers instead for binary and text generation
+  bool use_flexbuffers;
   bool strict_json;
   bool skip_js_exports;
   bool use_goog_js_export_format;
@@ -573,7 +575,8 @@ struct IDLOptions {
   bool set_empty_to_null;
 
   IDLOptions()
-      : strict_json(false),
+      : use_flexbuffers(false),
+        strict_json(false),
         skip_js_exports(false),
         use_goog_js_export_format(false),
         use_ES6_js_export_format(false),
@@ -707,6 +710,7 @@ class Parser : public ParserState {
   explicit Parser(const IDLOptions &options = IDLOptions())
       : current_namespace_(nullptr),
         empty_namespace_(nullptr),
+        flex_builder_(256, flexbuffers::BUILDER_FLAG_SHARE_ALL),
         root_struct_def_(nullptr),
         opts(options),
         uses_flexbuffers_(false),
@@ -908,6 +912,8 @@ class Parser : public ParserState {
   std::string error_;         // User readable error_ if Parse() == false
 
   FlatBufferBuilder builder_;  // any data contained in the file
+  flexbuffers::Builder flex_builder_;
+  flexbuffers::Reference flex_root_;
   StructDef *root_struct_def_;
   std::string file_identifier_;
   std::string file_extension_;