Implement native_shared attribute for C++.

Fixes #5141. See also #5145.

docs/source/CppUsage.md

@@ -88,6 +88,12 @@ convenient accessors for all fields, e.g. `hp()`, `mana()`, etc:
 
 *Note: That we never stored a `mana` value, so it will return the default.*
 
+The following attributes are supported:
+
+-   `native_shared` (on a field): For string fields, this enables the usage of
+    CreateSharedString() instead of CreateString() for generated
+    CreateXxxDirect(), and Pack() in object-based API if enabled.
+
 ## Object based API.  {#flatbuffers_cpp_object_based_api}
 
 FlatBuffers is all about memory efficiency, which is why its base API is written

include/flatbuffers/idl.h

@@ -259,6 +259,7 @@ struct FieldDef : public Definition {
         required(false),
         key(false),
         native_inline(false),
+        native_shared(false),
         flexbuffer(false),
         nested_flatbuffer(NULL),
         padding(0) {}
@@ -275,6 +276,8 @@ struct FieldDef : public Definition {
   bool key;         // Field functions as a key for creating sorted vectors.
   bool native_inline;  // Field will be defined inline (instead of as a pointer)
                        // for native tables if field is a struct.
+  bool native_shared;  // Field will be using CreateSharedString while packing
+                       // if field is a string.
   bool flexbuffer;     // This field contains FlexBuffer data.
   StructDef *nested_flatbuffer;  // This field contains nested FlatBuffer data.
   size_t padding;                // Bytes to always pad after this field.
@@ -623,6 +626,7 @@ class Parser : public ParserState {
     known_attributes_["cpp_str_type"] = true;
     known_attributes_["native_inline"] = true;
     known_attributes_["native_custom_alloc"] = true;
+    known_attributes_["native_shared"] = true;
     known_attributes_["native_type"] = true;
     known_attributes_["native_default"] = true;
     known_attributes_["flexbuffer"] = true;

src/idl_gen_cpp.cpp

@@ -2083,9 +2083,15 @@ class CppGenerator : public BaseGenerator {
         if (!field.deprecated) {
           code_.SetValue("FIELD_NAME", Name(field));
           if (field.value.type.base_type == BASE_TYPE_STRING) {
-            code_ +=
+            if (!field.native_shared) {
-                "  auto {{FIELD_NAME}}__ = {{FIELD_NAME}} ? "
+              code_ +=
-                "_fbb.CreateString({{FIELD_NAME}}) : 0;";
+                  "  auto {{FIELD_NAME}}__ = {{FIELD_NAME}} ? "
+                  "_fbb.CreateString({{FIELD_NAME}}) : 0;";
+            } else {
+              code_ +=
+                  "  auto {{FIELD_NAME}}__ = {{FIELD_NAME}} ? "
+                  "_fbb.CreateSharedString({{FIELD_NAME}}) : 0;";
+            }
           } else if (field.value.type.base_type == BASE_TYPE_VECTOR) {
             code_ += "  auto {{FIELD_NAME}}__ = {{FIELD_NAME}} ? \\";
             const auto vtype = field.value.type.VectorType();
@@ -2294,8 +2300,14 @@ class CppGenerator : public BaseGenerator {
     switch (field.value.type.base_type) {
       // String fields are of the form:
       //   _fbb.CreateString(_o->field)
+      // or
+      //   _fbb.CreateSharedString(_o->field)
       case BASE_TYPE_STRING: {
-        code += "_fbb.CreateString(" + value + ")";
+        if (!field.native_shared) {
+          code += "_fbb.CreateString(" + value + ")";
+        } else {
+          code += "_fbb.CreateSharedString(" + value + ")";
+        }
 
         // For optional fields, check to see if there actually is any data
         // in _o->field before attempting to access it. If there isn't,

src/idl_parser.cpp

@@ -777,7 +777,10 @@ CheckedError Parser::ParseField(StructDef &struct_def) {
 
   field->native_inline = field->attributes.Lookup("native_inline") != nullptr;
   if (field->native_inline && !IsStruct(field->value.type))
-    return Error("native_inline can only be defined on structs'");
+    return Error("native_inline can only be defined on structs");
+  field->native_shared = field->attributes.Lookup("native_shared") != nullptr;
+  if (field->native_shared && field->value.type.base_type != BASE_TYPE_STRING)
+    return Error("native_shared can only be defined on strings");
 
   auto nested = field->attributes.Lookup("nested_flatbuffer");
   if (nested) {