Refactor FieldDef to model presense as an enum rather than 2 bools.

include/flatbuffers/idl.h

@@ -291,12 +291,11 @@ struct Definition {
 struct FieldDef : public Definition {
   FieldDef()
       : deprecated(false),
-        required(false),
         key(false),
         shared(false),
         native_inline(false),
         flexbuffer(false),
-        optional(false),
+        presence(kDefault),
         nested_flatbuffer(NULL),
         padding(0) {}
 
@@ -306,21 +305,46 @@ struct FieldDef : public Definition {
   bool Deserialize(Parser &parser, const reflection::Field *field);
 
   bool IsScalarOptional() const {
-    return IsScalar(value.type.base_type) && optional;
+    return IsScalar(value.type.base_type) && IsOptional();
+  }
+  bool IsOptional() const {
+    return presence == kOptional;
+  }
+  bool IsRequired() const {
+    return presence == kRequired;
+  }
+  bool IsDefault() const {
+    return presence == kDefault;
   }
 
   Value value;
   bool deprecated;  // Field is allowed to be present in old data, but can't be.
                     // written in new data nor accessed in new code.
-  bool required;    // Field must always be present.
   bool key;         // Field functions as a key for creating sorted vectors.
   bool shared;  // Field will be using string pooling (i.e. CreateSharedString)
                 // as default serialization behavior if field is a string.
   bool native_inline;  // Field will be defined inline (instead of as a pointer)
                        // for native tables if field is a struct.
   bool flexbuffer;     // This field contains FlexBuffer data.
-  bool optional;       // If True, this field is Null (as opposed to default
-                       // valued).
+
+  enum Presence {
+    // Field must always be present.
+    kRequired,
+    // Non-presence should be signalled to and controlled by users.
+    kOptional,
+    // Non-presence is hidden from users.
+    // Implementations may omit writing default values.
+    kDefault,
+  };
+  Presence static MakeFieldPresence(bool optional, bool required) {
+    // clang-format off
+    return required ? FieldDef::kRequired
+         : optional ? FieldDef::kOptional
+                    : FieldDef::kDefault;
+    // clang-format on
+  }
+  Presence presence;
+
   StructDef *nested_flatbuffer;  // This field contains nested FlatBuffer data.
   size_t padding;                // Bytes to always pad after this field.
 };

src/idl_gen_cpp.cpp

@@ -1871,7 +1871,7 @@ class CppGenerator : public BaseGenerator {
   void GenVerifyCall(const FieldDef &field, const char *prefix) {
     code_.SetValue("PRE", prefix);
     code_.SetValue("NAME", Name(field));
-    code_.SetValue("REQUIRED", field.required ? "Required" : "");
+    code_.SetValue("REQUIRED", field.IsRequired() ? "Required" : "");
     code_.SetValue("SIZE", GenTypeSize(field.value.type));
     code_.SetValue("OFFSET", GenFieldOffsetName(field));
     if (IsScalar(field.value.type.base_type) || IsStruct(field.value.type)) {
@@ -2340,7 +2340,7 @@ class CppGenerator : public BaseGenerator {
     for (auto it = struct_def.fields.vec.begin();
          it != struct_def.fields.vec.end(); ++it) {
       const auto &field = **it;
-      if (!field.deprecated && field.required) {
+      if (!field.deprecated && field.IsRequired()) {
         code_.SetValue("FIELD_NAME", Name(field));
         code_.SetValue("OFFSET_NAME", GenFieldOffsetName(field));
         code_ += "    fbb_.Required(o, {{STRUCT_NAME}}::{{OFFSET_NAME}});";
@@ -2684,7 +2684,7 @@ class CppGenerator : public BaseGenerator {
         // in _o->field before attempting to access it. If there isn't,
         // depending on set_empty_strings_to_null either set it to 0 or an empty
         // string.
-        if (!field.required) {
+        if (!field.IsRequired()) {
           auto empty_value = opts_.set_empty_strings_to_null
                                  ? "0"
                                  : "_fbb.CreateSharedString(\"\")";
@@ -2793,7 +2793,7 @@ class CppGenerator : public BaseGenerator {
         // If set_empty_vectors_to_null option is enabled, for optional fields,
         // check to see if there actually is any data in _o->field before
         // attempting to access it.
-        if (opts_.set_empty_vectors_to_null && !field.required) {
+        if (opts_.set_empty_vectors_to_null && !field.IsRequired()) {
           code = value + ".size() ? " + code + " : 0";
         }
         break;

src/idl_gen_csharp.cpp

@@ -1142,7 +1142,7 @@ class CSharpGenerator : public BaseGenerator {
       for (auto it = struct_def.fields.vec.begin();
            it != struct_def.fields.vec.end(); ++it) {
         auto &field = **it;
-        if (!field.deprecated && field.required) {
+        if (!field.deprecated && field.IsRequired()) {
           code += "    builder.Required(o, ";
           code += NumToString(field.value.offset);
           code += ");  // " + field.name + "\n";

src/idl_gen_fbs.cpp

@@ -136,7 +136,7 @@ std::string GenerateFBS(const Parser &parser, const std::string &file_name) {
         GenComment(field.doc_comment, &schema, nullptr, "  ");
         schema += "  " + field.name + ":" + GenType(field.value.type);
         if (field.value.constant != "0") schema += " = " + field.value.constant;
-        if (field.required) schema += " (required)";
+        if (field.IsRequired()) schema += " (required)";
         schema += ";\n";
       }
     }

src/idl_gen_java.cpp

@@ -649,7 +649,7 @@ class JavaGenerator : public BaseGenerator {
       std::string src_cast = SourceCast(field.value.type);
       std::string method_start =
           "  public " +
-          (field.required ? "" : GenNullableAnnotation(field.value.type)) +
+          (field.IsRequired() ? "" : GenNullableAnnotation(field.value.type)) +
           GenPureAnnotation(field.value.type) + type_name_dest + optional +
           " " + MakeCamel(field.name, false);
       std::string obj = "obj";
@@ -1095,7 +1095,7 @@ class JavaGenerator : public BaseGenerator {
       for (auto it = struct_def.fields.vec.begin();
            it != struct_def.fields.vec.end(); ++it) {
         auto &field = **it;
-        if (!field.deprecated && field.required) {
+        if (!field.deprecated && field.IsRequired()) {
           code += "    builder.required(o, ";
           code += NumToString(field.value.offset);
           code += ");  // " + field.name + "\n";

src/idl_gen_json_schema.cpp

@@ -239,7 +239,7 @@ class JsonSchemaGenerator : public BaseGenerator {
       std::vector<FieldDef *> requiredProperties;
       std::copy_if(properties.begin(), properties.end(),
                    back_inserter(requiredProperties),
-                   [](FieldDef const *prop) { return prop->required; });
+                   [](FieldDef const *prop) { return prop->IsRequired(); });
       if (!requiredProperties.empty()) {
         auto required_string(Indent(3) + "\"required\" : [");
         for (auto req_prop = requiredProperties.cbegin();

src/idl_gen_kotlin.cpp

@@ -646,7 +646,7 @@ class KotlinGenerator : public BaseGenerator {
           writer.IncrementIdentLevel();
           for (auto it = field_vec.begin(); it != field_vec.end(); ++it) {
             auto &field = **it;
-            if (field.deprecated || !field.required) { continue; }
+            if (field.deprecated || !field.IsRequired()) { continue; }
             writer.SetValue("offset", NumToString(field.value.offset));
             writer += "builder.required(o, {{offset}})";
           }

src/idl_gen_lobster.cpp

@@ -110,13 +110,13 @@ class LobsterGenerator : public BaseGenerator {
               offsets + ")";
 
       } else {
-        auto defval = field.optional ? "0" : field.value.constant;
+        auto defval = field.IsOptional() ? "0" : field.value.constant;
         acc = "buf_.flatbuffers_field_" + GenTypeName(field.value.type) +
               "(pos_, " + offsets + ", " + defval + ")";
       }
       if (field.value.type.enum_def)
         acc = NormalizedName(*field.value.type.enum_def) + "(" + acc + ")";
-      if (field.optional)
+      if (field.IsOptional())
         acc += ", buf_.flatbuffers_field_present(pos_, " + offsets + ")";
       code += def + "():\n        return " + acc + "\n";
       return;
@@ -201,7 +201,7 @@ class LobsterGenerator : public BaseGenerator {
               NormalizedName(field) + ":" + LobsterType(field.value.type) +
               "):\n        b_.Prepend" + GenMethod(field.value.type) + "Slot(" +
               NumToString(offset) + ", " + NormalizedName(field);
-      if (IsScalar(field.value.type.base_type) && !field.optional)
+      if (IsScalar(field.value.type.base_type) && !field.IsOptional())
         code += ", " + field.value.constant;
       code += ")\n        return this\n";
     }

src/idl_gen_php.cpp

@@ -536,7 +536,7 @@ class PhpGenerator : public BaseGenerator {
     for (auto it = struct_def.fields.vec.begin();
          it != struct_def.fields.vec.end(); ++it) {
       auto &field = **it;
-      if (!field.deprecated && field.required) {
+      if (!field.deprecated && field.IsRequired()) {
         code += Indent + Indent + "$builder->required($o, ";
         code += NumToString(field.value.offset);
         code += ");  // " + field.name + "\n";
@@ -645,7 +645,7 @@ class PhpGenerator : public BaseGenerator {
     for (auto it = struct_def.fields.vec.begin();
          it != struct_def.fields.vec.end(); ++it) {
       auto &field = **it;
-      if (!field.deprecated && field.required) {
+      if (!field.deprecated && field.IsRequired()) {
         code += Indent + Indent + "$builder->required($o, ";
         code += NumToString(field.value.offset);
         code += ");  // " + field.name + "\n";