Default Vector Support C++ (#8870)

.gitignore

@@ -157,3 +157,6 @@ MODULE.bazel.lock
 
 # Ignore the generated docs
 docs/site
+
+# Ignore generated files
+*.fbs.h

CMakeLists.txt

@@ -218,6 +218,8 @@ set(FlatHash_SRCS
 set(FlatBuffers_Tests_SRCS
   ${FlatBuffers_Library_SRCS}
   src/idl_gen_fbs.cpp
+  tests/default_vectors_strings_test.cpp
+  tests/default_vectors_strings_test.h
   tests/evolution_test.cpp
   tests/flexbuffers_test.cpp
   tests/fuzz_test.cpp
@@ -496,13 +498,14 @@ if(FLATBUFFERS_BUILD_SHAREDLIB)
   endif()
 endif()
 
-function(compile_schema SRC_FBS OPT OUT_GEN_FILE) 
+function(compile_schema SRC_FBS OPT SUFFIX OUT_GEN_FILE) 
   get_filename_component(SRC_FBS_DIR ${SRC_FBS} PATH)
-  string(REGEX REPLACE "\\.fbs$" "_generated.h" GEN_HEADER ${SRC_FBS})
+  string(REGEX REPLACE "\\.fbs$" "${SUFFIX}.h" GEN_HEADER ${SRC_FBS})
   add_custom_command(
     OUTPUT ${GEN_HEADER}
     COMMAND "${FLATBUFFERS_FLATC_EXECUTABLE}"
       ${OPT}
+      --filename-suffix ${SUFFIX}
       -o "${SRC_FBS_DIR}"
       "${CMAKE_CURRENT_SOURCE_DIR}/${SRC_FBS}"
     DEPENDS flatc ${SRC_FBS}
@@ -512,12 +515,17 @@ function(compile_schema SRC_FBS OPT OUT_GEN_FILE)
 endfunction()
 
 function(compile_schema_for_test SRC_FBS OPT)
-  compile_schema("${SRC_FBS}" "${OPT}" GEN_FILE)
+  compile_schema("${SRC_FBS}" "${OPT}" "_generated" GEN_FILE)
+  target_sources(flattests PRIVATE ${GEN_FILE})
+endfunction()
+
+function(compile_schema_for_test_fbsh SRC_FBS OPT)
+  compile_schema("${SRC_FBS}" "${OPT}" ".fbs" GEN_FILE)
   target_sources(flattests PRIVATE ${GEN_FILE})
 endfunction()
 
 function(compile_schema_for_samples SRC_FBS OPT)
-  compile_schema("${SRC_FBS}" "${OPT}" GEN_FILE)
+  compile_schema("${SRC_FBS}" "${OPT}" "_generated" GEN_FILE)
   target_sources(flatsample PRIVATE ${GEN_FILE})
 endfunction()
 
@@ -542,6 +550,7 @@ if(FLATBUFFERS_BUILD_TESTS)
   SET(FLATC_OPT_SCOPED_ENUMS ${FLATC_OPT_COMP};--scoped-enums)
 
   compile_schema_for_test(tests/alignment_test.fbs "${FLATC_OPT_COMP}")
+  compile_schema_for_test_fbsh(tests/default_vectors_strings_test.fbs "${FLATC_OPT_COMP}")
   compile_schema_for_test(tests/arrays_test.fbs "${FLATC_OPT_SCOPED_ENUMS}")
   compile_schema_for_test(tests/native_inline_table_test.fbs "${FLATC_OPT_COMP}")
   compile_schema_for_test(tests/native_type_test.fbs "${FLATC_OPT_COMP}")

build_defs.bzl

@@ -187,6 +187,7 @@ def flatbuffer_cc_library(
         visibility = None,
         compatible_with = None,
         restricted_to = None,
+        filename_suffix = "_generated",
         target_compatible_with = None,
         srcs_filegroup_visibility = None,
         gen_reflections = False):
@@ -230,10 +231,13 @@ def flatbuffer_cc_library(
       Fileset([name]_reflection): (Optional) all generated reflection binaries.
       cc_library([name]): library with sources and flatbuffers deps.
     """
-    output_headers = [
+
-        (out_prefix + "%s_generated.h") % (s.replace(".fbs", "").split("/")[-1].split(":")[-1])
+    output_headers = []
-        for s in srcs
+    for s in srcs:
-    ]
+        base_name = s.split("/")[-1].split(":")[-1].replace(".fbs", "")
+        header = out_prefix + base_name + filename_suffix + ".h"
+        output_headers.append(header)
+
     if deps and includes:
         # There is no inherent reason we couldn't support both, but this discourages
         # use of includes without good reason.

include/flatbuffers/table.h

@@ -18,6 +18,7 @@
 #define FLATBUFFERS_TABLE_H_
 
 #include "flatbuffers/base.h"
+#include "flatbuffers/vector.h"
 #include "flatbuffers/verifier.h"
 
 namespace flatbuffers {
@@ -70,6 +71,32 @@ class Table {
     return GetPointer<P, uoffset64_t>(field);
   }
 
+  template <typename P, typename SizeT = uoffset_t,
+            typename OffsetSize = uoffset_t>
+  const Vector<P, SizeT>* GetVectorPointerOrEmpty(voffset_t field) const {
+    auto* ptr = GetPointer<const Vector<P, SizeT>*, OffsetSize>(field);
+    return ptr ? ptr : EmptyVector<P, SizeT>();
+  }
+
+  template <typename P, typename SizeT = uoffset_t>
+  const Vector<P, SizeT>* GetVectorPointer64OrEmpty(voffset_t field) const {
+    return GetVectorPointerOrEmpty<P, SizeT, uoffset64_t>(field);
+  }
+
+  template <typename P, typename SizeT = uoffset_t,
+            typename OffsetSize = uoffset_t>
+  Vector<P, SizeT>* GetMutableVectorPointerOrEmpty(voffset_t field) {
+    auto* ptr = GetPointer<Vector<P, SizeT>*, OffsetSize>(field);
+    // This is a const_cast, but safe, since all mutable operations on an
+    // empty vector are NOPs.
+    return ptr ? ptr : const_cast<Vector<P, SizeT>*>(EmptyVector<P, SizeT>());
+  }
+
+  template <typename P, typename SizeT = uoffset_t>
+  Vector<P, SizeT>* GetMutableVectorPointer64OrEmpty(voffset_t field) {
+    return GetMutableVectorPointerOrEmpty<P, SizeT, uoffset64_t>(field);
+  }
+
   template <typename P>
   P GetStruct(voffset_t field) const {
     auto field_offset = GetOptionalFieldOffset(field);
@@ -177,6 +204,39 @@ class Table {
     return VerifyOffsetRequired<uoffset64_t>(verifier, field);
   }
 
+  // Verify a string that may have a default value.
+  template <typename OffsetT = uoffset_t>
+  bool VerifyStringWithDefault(const Verifier& verifier,
+                               voffset_t field) const {
+    auto field_offset = GetOptionalFieldOffset(field);
+    return field_offset == 0 ||
+           verifier.VerifyString(GetPointer<const String*, OffsetT>(field));
+  }
+
+  // Verify a vector that has a default empty value.
+  template <typename P, typename SizeT = uoffset_t,
+            typename OffsetSize = uoffset_t>
+  bool VerifyVectorWithDefault(const Verifier& verifier,
+                               voffset_t field) const {
+    auto field_offset = GetOptionalFieldOffset(field);
+    return field_offset == 0 ||
+           verifier.VerifyVector(
+               GetPointer<const Vector<P, SizeT>*, OffsetSize>(field));
+  }
+
+  template <typename P, typename SizeT = uoffset_t>
+  bool VerifyVector64WithDefault(const Verifier& verifier,
+                                 voffset_t field) const {
+    return VerifyVectorWithDefault<P, SizeT, uoffset64_t>(verifier, field);
+  }
+
+ protected:
+  template <typename T, typename SizeT = uoffset_t>
+  static const Vector<T, SizeT>* EmptyVector() {
+    static const SizeT empty_vector_length = 0;
+    return reinterpret_cast<const Vector<T, SizeT>*>(&empty_vector_length);
+  }
+
  private:
   // private constructor & copy constructor: you obtain instances of this
   // class by pointing to existing data only

scripts/generate_code.py

@@ -400,6 +400,10 @@ flatc(
     schema="nested_union_test.fbs",
 )
 
+flatc(
+    NO_INCL_OPTS + CPP_OPTS,
+    schema="default_vectors_strings_test.fbs",
+)
 
 # Optional Scalars
 optional_scalars_schema = "optional_scalars.fbs"

src/idl_gen_cpp.cpp

@@ -1924,6 +1924,10 @@ class CppGenerator : public BaseGenerator {
       } else {
         return "0";
       }
+    } else if (IsVector(type) && field.value.constant == "[]") {
+      return "0";
+    } else if (IsString(type) && field.value.constant != "0") {
+      return "0";
     } else if (IsStruct(type) && (field.value.constant == "0")) {
       return "nullptr";
     } else {
@@ -2427,12 +2431,40 @@ class CppGenerator : public BaseGenerator {
         break;
       }
       case BASE_TYPE_STRING: {
+        if (field.value.constant != "0") {
+          if (field.offset64) {
+            code_ +=
+                "{{PRE}}VerifyStringWithDefault<::flatbuffers::uoffset64_t>("
+                "verifier, "
+                "{{OFFSET}})\\";
+          } else {
+            code_ += "{{PRE}}VerifyStringWithDefault(verifier, {{OFFSET}})\\";
+          }
+        } else {
           code_ += "{{PRE}}verifier.VerifyString({{NAME}}())\\";
+        }
         break;
       }
       case BASE_TYPE_VECTOR64:
       case BASE_TYPE_VECTOR: {
+        if (field.value.constant == "[]") {
+          const auto& vec_type = field.value.type.VectorType();
+          const std::string vtype_wire = GenTypeWire(
+              vec_type, "", VectorElementUserFacing(vec_type), field.offset64);
+          std::string verify_call;
+          if (field.offset64) {
+            verify_call = "{{PRE}}VerifyVector64WithDefault<" + vtype_wire;
+          } else {
+            verify_call = "{{PRE}}VerifyVectorWithDefault<" + vtype_wire;
+          }
+          if (field.value.type.base_type == BASE_TYPE_VECTOR64) {
+            verify_call += ", ::flatbuffers::uoffset64_t";
+          }
+          verify_call += ">(verifier, {{OFFSET}})\\";
+          code_ += verify_call;
+        } else {
           code_ += "{{PRE}}verifier.VerifyVector({{NAME}}())\\";
+        }
 
         switch (field.value.type.element) {
           case BASE_TYPE_STRING: {
@@ -2723,7 +2755,37 @@ class CppGenerator : public BaseGenerator {
       code_.SetValue("FIELD_VALUE", GenUnderlyingCast(field, true, call));
       code_.SetValue("NULLABLE_EXT", NullableExtension());
       code_ += "  {{FIELD_TYPE}}{{FIELD_NAME}}() const {";
+            if (IsVector(type) && field.value.constant == "[]") {
+        const auto& vec_type = type.VectorType();
+        const std::string vtype_wire = GenTypeWire(
+            vec_type, "", VectorElementUserFacing(vec_type), field.offset64);
+        std::string get_call;
+        if (field.offset64) {
+          get_call = "    return GetVectorPointer64OrEmpty<" + vtype_wire;
+        } else {
+          get_call = "    return GetVectorPointerOrEmpty<" + vtype_wire;
+        }
+        if (type.base_type == BASE_TYPE_VECTOR64) {
+          get_call += ", ::flatbuffers::uoffset64_t";
+        }
+        get_call += ">(" + offset_str + ");";
+        code_ += get_call;
+      } else if (IsString(type) && field.value.constant != "0") {
+        // TODO: Add logic to always convert the string to a valid C++ string
+        // literal by handling string escapes.
+        code_ += "    auto* ptr = {{FIELD_VALUE}};";
+        code_ += "    if (ptr) return ptr;";
+        code_ += "    static const struct { uint32_t len; const char s[" +
+                 NumToString(field.value.constant.length() + 1) +
+                 "]; } bfbs_string = { " +
+                 NumToString(field.value.constant.length()) + ", \"" +
+                 field.value.constant + "\" };";
+        code_ +=
+            "    return reinterpret_cast<const ::flatbuffers::String "
+            " *>(&bfbs_string);";
+      } else {
         code_ += "    return {{FIELD_VALUE}};";
+      }
       code_ += "  }";
     } else {
       auto wire_type = GenTypeBasic(type, false);
@@ -2910,6 +2972,27 @@ class CppGenerator : public BaseGenerator {
     } else {
       auto postptr = " *" + NullableExtension();
       auto wire_type = GenTypeGet(type, " ", "", postptr.c_str(), true);
+      code_.SetValue("FIELD_TYPE", wire_type);
+
+      if (IsVector(type) && field.value.constant == "[]") {
+        const auto& vec_type = type.VectorType();
+        const std::string vtype_wire = GenTypeWire(
+            vec_type, "", VectorElementUserFacing(vec_type), field.offset64);
+        code_ += "  {{FIELD_TYPE}}mutable_{{FIELD_NAME}}() {";
+        std::string get_call;
+        if (field.offset64) {
+          get_call =
+              "    return GetMutableVectorPointer64OrEmpty<" + vtype_wire;
+        } else {
+          get_call = "    return GetMutableVectorPointerOrEmpty<" + vtype_wire;
+        }
+        if (type.base_type == BASE_TYPE_VECTOR64) {
+          get_call += ", ::flatbuffers::uoffset64_t";
+        }
+        get_call += ">(" + offset_str + ");";
+        code_ += get_call;
+        code_ += "  }";
+      } else {
         const std::string accessor = [&]() {
           if (IsStruct(type)) {
             return "GetStruct<";
@@ -2920,14 +3003,14 @@ class CppGenerator : public BaseGenerator {
           return "GetPointer<";
         }();
         auto underlying = accessor + wire_type + ">(" + offset_str + ")";
-      code_.SetValue("FIELD_TYPE", wire_type);
+        code_.SetValue("FIELD_VALUE",
-      code_.SetValue("FIELD_VALUE", GenUnderlyingCast(field, true, underlying));
+                       GenUnderlyingCast(field, true, underlying));
-
         code_ += "  {{FIELD_TYPE}}mutable_{{FIELD_NAME}}() {";
         code_ += "    return {{FIELD_VALUE}};";
         code_ += "  }";
       }
     }
+  }
 
   std::string GetNestedFlatBufferName(const FieldDef& field) {
     auto nested = field.attributes.Lookup("nested_flatbuffer");
@@ -3305,9 +3388,17 @@ class CppGenerator : public BaseGenerator {
           } else {
             code_.SetValue("CREATE_STRING", "CreateSharedString");
           }
+          if (field->value.constant != "0") {
+            code_ +=
+                "  auto {{FIELD_NAME}}__ = {{FIELD_NAME}} ? "
+                "_fbb.{{CREATE_STRING}}({{FIELD_NAME}}) : "
+                "_fbb.{{CREATE_STRING}}(\"" +
+                field->value.constant + "\");";
+          } else {
             code_ +=
                 "  auto {{FIELD_NAME}}__ = {{FIELD_NAME}} ? "
                 "_fbb.{{CREATE_STRING}}({{FIELD_NAME}}) : 0;";
+          }
         } else if (IsVector(field->value.type)) {
           const std::string force_align_code =
               GenVectorForceAlign(*field, Name(*field) + "->size()");

src/idl_parser.cpp

@@ -2772,7 +2772,8 @@ bool Parser::SupportsOptionalScalars() const {
 
 bool Parser::SupportsDefaultVectorsAndStrings() const {
   static FLATBUFFERS_CONSTEXPR unsigned long supported_langs =
-      IDLOptions::kRust | IDLOptions::kSwift | IDLOptions::kNim;
+      IDLOptions::kRust | IDLOptions::kSwift | IDLOptions::kNim |
+      IDLOptions::kCpp | IDLOptions::kBinary | IDLOptions::kJson;
   return !(opts.lang_to_generate & ~supported_langs);
 }
 

tests/BUILD.bazel

@@ -33,6 +33,8 @@ cc_test(
         "alignment_test.cpp",
         "alignment_test.h",
         "alignment_test_generated.h",
+        "default_vectors_strings_test.cpp",
+        "default_vectors_strings_test.h",
         "evolution_test.cpp",
         "evolution_test.h",
         "evolution_test/evolution_v1_generated.h",
@@ -134,6 +136,7 @@ cc_test(
     deps = [
         ":alignment_test_cc_fbs",
         ":arrays_test_cc_fbs",
+        ":default_vectors_strings_test_cc_fbs",
         ":monster_extra_cc_fbs",
         ":monster_test_cc_fbs",
         ":native_type_test_cc_fbs",
@@ -272,3 +275,16 @@ flatbuffer_cc_library(
     name = "alignment_test_cc_fbs",
     srcs = ["alignment_test.fbs"],
 )
+
+flatbuffer_cc_library(
+    name = "default_vectors_strings_test_cc_fbs",
+    srcs = ["default_vectors_strings_test.fbs"],
+    flatc_args = [
+        "--gen-compare",
+        "--gen-mutable",
+        "--gen-object-api",
+        "--reflect-names",
+        "--filename-suffix .fbs",
+    ],
+    filename_suffix = ".fbs"
+)

tests/default_vectors_strings_test.cpp

@@ -0,0 +1,249 @@
+#include "tests/default_vectors_strings_test.h"
+
+#include <cstdint>
+#include <vector>
+
+#include "include/flatbuffers/buffer.h"
+#include "include/flatbuffers/flatbuffer_builder.h"
+#include "include/flatbuffers/string.h"
+#include "include/flatbuffers/vector.h"
+#include "include/flatbuffers/verifier.h"
+#include "tests/default_vectors_strings_test.fbs.h"
+#include "tests/test_assert.h"
+
+namespace flatbuffers {
+namespace tests {
+
+using flatbuffers::FlatBufferBuilder64;
+using flatbuffers::Offset;
+using flatbuffers::String;
+using flatbuffers::Verifier;
+
+void DefaultVectorsStringsTest_EmptyOnDefault_Const() {
+  FlatBufferBuilder64 builder;
+
+  // Create table without providing the fields with defaults.
+  DefaultVectorsStringsTest::TableWithDefaultVectorsBuilder tbl_builder(
+      builder);
+  tbl_builder.add_regular_int(100);
+  auto offset = tbl_builder.Finish();
+  builder.Finish(offset);
+
+  Verifier verifier(builder.GetBufferPointer(), builder.GetSize());
+  TEST_ASSERT(
+      DefaultVectorsStringsTest::VerifyTableWithDefaultVectorsBuffer(verifier));
+
+  const auto* table = DefaultVectorsStringsTest::GetTableWithDefaultVectors(
+      builder.GetBufferPointer());
+  TEST_NOTNULL(table);
+
+  // Verify default scalar vectors.
+  TEST_NOTNULL(table->int_vec());
+  TEST_EQ(table->int_vec()->size(), 0);
+  TEST_NOTNULL(table->bool_vec());
+  TEST_EQ(table->bool_vec()->size(), 0);
+  TEST_NOTNULL(table->char_vec());
+  TEST_EQ(table->char_vec()->size(), 0);
+  TEST_NOTNULL(table->uchar_vec());
+  TEST_EQ(table->uchar_vec()->size(), 0);
+  TEST_NOTNULL(table->short_vec());
+  TEST_EQ(table->short_vec()->size(), 0);
+  TEST_NOTNULL(table->ushort_vec());
+  TEST_EQ(table->ushort_vec()->size(), 0);
+  TEST_NOTNULL(table->uint_vec());
+  TEST_EQ(table->uint_vec()->size(), 0);
+  TEST_NOTNULL(table->long_vec());
+  TEST_EQ(table->long_vec()->size(), 0);
+  TEST_NOTNULL(table->ulong_vec());
+  TEST_EQ(table->ulong_vec()->size(), 0);
+  TEST_NOTNULL(table->float_vec());
+  TEST_EQ(table->float_vec()->size(), 0);
+  TEST_NOTNULL(table->double_vec());
+  TEST_EQ(table->double_vec()->size(), 0);
+
+  // Verify default string_vec.
+  TEST_NOTNULL(table->string_vec());
+  TEST_EQ(table->string_vec()->size(), 0);
+
+  // Verify default string fields.
+  TEST_NOTNULL(table->empty_string());
+  TEST_EQ_STR(table->empty_string()->c_str(), "");
+  TEST_NOTNULL(table->some_string());
+  TEST_EQ_STR(table->some_string()->c_str(), "some");
+
+  // Verify default struct_vec.
+  TEST_NOTNULL(table->struct_vec());
+  TEST_EQ(table->struct_vec()->size(), 0);
+
+  // Verify default table_vec.
+  TEST_NOTNULL(table->table_vec());
+  TEST_EQ(table->table_vec()->size(), 0);
+
+  // Verify default enum_vec.
+  TEST_NOTNULL(table->enum_vec());
+  TEST_EQ(table->enum_vec()->size(), 0);
+
+  // Verify non-default vector field.
+  TEST_NULL(table->regular_int_vec());
+
+  // Verify pointer and offset64 combinations.
+  TEST_NOTNULL(table->int_vec64());
+  TEST_EQ(table->int_vec64()->size(), 0);
+  TEST_NOTNULL(table->int_vec_offset64());
+  TEST_EQ(table->int_vec_offset64()->size(), 0);
+
+  // Verify non-default field.
+  TEST_EQ(table->regular_int(), 100);
+}
+
+void DefaultVectorsStringsTest_EmptyOnDefault_Mutable() {
+  FlatBufferBuilder64 builder;
+
+  // Create table without providing the fields with defaults.
+  DefaultVectorsStringsTest::TableWithDefaultVectorsBuilder tbl_builder(
+      builder);
+  tbl_builder.add_regular_int(100);
+  auto offset = tbl_builder.Finish();
+  builder.Finish(offset);
+
+  Verifier verifier(builder.GetBufferPointer(), builder.GetSize());
+  TEST_ASSERT(
+      DefaultVectorsStringsTest::VerifyTableWithDefaultVectorsBuffer(verifier));
+
+  auto* mutable_table =
+      DefaultVectorsStringsTest::GetMutableTableWithDefaultVectors(
+          builder.GetBufferPointer());
+  TEST_NOTNULL(mutable_table);
+
+  // Verify default scalar vectors.
+  TEST_NOTNULL(mutable_table->mutable_int_vec());
+  TEST_EQ(mutable_table->mutable_int_vec()->size(), 0);
+  TEST_NOTNULL(mutable_table->mutable_bool_vec());
+  TEST_EQ(mutable_table->mutable_bool_vec()->size(), 0);
+  TEST_NOTNULL(mutable_table->mutable_char_vec());
+  TEST_EQ(mutable_table->mutable_char_vec()->size(), 0);
+  TEST_NOTNULL(mutable_table->mutable_uchar_vec());
+  TEST_EQ(mutable_table->mutable_uchar_vec()->size(), 0);
+  TEST_NOTNULL(mutable_table->mutable_short_vec());
+  TEST_EQ(mutable_table->mutable_short_vec()->size(), 0);
+  TEST_NOTNULL(mutable_table->mutable_ushort_vec());
+  TEST_EQ(mutable_table->mutable_ushort_vec()->size(), 0);
+  TEST_NOTNULL(mutable_table->mutable_uint_vec());
+  TEST_EQ(mutable_table->mutable_uint_vec()->size(), 0);
+  TEST_NOTNULL(mutable_table->mutable_long_vec());
+  TEST_EQ(mutable_table->mutable_long_vec()->size(), 0);
+  TEST_NOTNULL(mutable_table->mutable_ulong_vec());
+  TEST_EQ(mutable_table->mutable_ulong_vec()->size(), 0);
+  TEST_NOTNULL(mutable_table->mutable_float_vec());
+  TEST_EQ(mutable_table->mutable_float_vec()->size(), 0);
+  TEST_NOTNULL(mutable_table->mutable_double_vec());
+  TEST_EQ(mutable_table->mutable_double_vec()->size(), 0);
+
+  // Verify default struct_vec.
+  TEST_NOTNULL(mutable_table->mutable_struct_vec());
+  TEST_EQ(mutable_table->mutable_struct_vec()->size(), 0);
+
+  // Verify default table_vec.
+  TEST_NOTNULL(mutable_table->mutable_table_vec());
+  TEST_EQ(mutable_table->mutable_table_vec()->size(), 0);
+
+  // Verify default enum_vec.
+  TEST_NOTNULL(mutable_table->mutable_enum_vec());
+  TEST_EQ(mutable_table->mutable_enum_vec()->size(), 0);
+
+  // Verify non-default vector field.
+  TEST_NULL(mutable_table->mutable_regular_int_vec());
+
+  // Verify pointer and offset64 combinations.
+  TEST_NOTNULL(mutable_table->mutable_int_vec64());
+  TEST_EQ(mutable_table->mutable_int_vec64()->size(), 0);
+  TEST_NOTNULL(mutable_table->mutable_int_vec_offset64());
+  TEST_EQ(mutable_table->mutable_int_vec_offset64()->size(), 0);
+}
+
+void DefaultVectorsStringsTest_WithValues() {
+  // Create a table with values for the defaulted vector fields.
+  FlatBufferBuilder64 builder;
+  auto int_vec64 = builder.CreateVector64(std::vector<int>({30, 40}));
+  auto int_vec_offset64 =
+      builder.CreateVector64<flatbuffers::Vector>(std::vector<int>({50, 60}));
+  auto int_vec = builder.CreateVector(std::vector<int>({1, 2}));
+  auto bool_vec = builder.CreateVector(std::vector<uint8_t>({true, false}));
+  auto string_vec = builder.CreateVector(std::vector<Offset<String>>(
+      {builder.CreateString("a"), builder.CreateString("b")}));
+  auto empty_string = builder.CreateString("not empty");
+  auto some_string = builder.CreateString("not some");
+  DefaultVectorsStringsTest::MyStruct structs[] = {
+      DefaultVectorsStringsTest::MyStruct(1, 2),
+      DefaultVectorsStringsTest::MyStruct(3, 4)};
+  auto struct_vec = builder.CreateVectorOfStructs(structs, 2);
+  auto regular_int_vec = builder.CreateVector(std::vector<int>({10, 20}));
+
+  DefaultVectorsStringsTest::TableWithDefaultVectorsBuilder tbl_builder(
+      builder);
+  tbl_builder.add_int_vec(int_vec);
+  tbl_builder.add_int_vec64(int_vec64);
+  tbl_builder.add_int_vec_offset64(int_vec_offset64);
+
+  tbl_builder.add_bool_vec(bool_vec);
+  tbl_builder.add_string_vec(string_vec);
+  tbl_builder.add_empty_string(empty_string);
+  tbl_builder.add_some_string(some_string);
+  tbl_builder.add_struct_vec(struct_vec);
+  tbl_builder.add_regular_int_vec(regular_int_vec);
+  auto offset = tbl_builder.Finish();
+  builder.Finish(offset);
+
+  Verifier verifier(builder.GetBufferPointer(), builder.GetSize());
+  TEST_ASSERT(
+      DefaultVectorsStringsTest::VerifyTableWithDefaultVectorsBuffer(verifier));
+
+  const auto* table = DefaultVectorsStringsTest::GetTableWithDefaultVectors(
+      builder.GetBufferPointer());
+
+  TEST_EQ(table->int_vec()->size(), 2);
+  TEST_EQ(table->int_vec()->Get(1), 2);
+  TEST_EQ(table->bool_vec()->size(), 2);
+  TEST_EQ(table->bool_vec()->Get(0), true);
+  TEST_EQ(table->string_vec()->size(), 2);
+  TEST_EQ_STR(table->string_vec()->Get(1)->c_str(), "b");
+  TEST_EQ_STR(table->empty_string()->c_str(), "not empty");
+  TEST_EQ_STR(table->some_string()->c_str(), "not some");
+  TEST_EQ(table->struct_vec()->size(), 2);
+  TEST_EQ(table->struct_vec()->Get(1)->b(), 4);
+  TEST_EQ(table->regular_int_vec()->size(), 2);
+  TEST_EQ(table->regular_int_vec()->Get(1), 20);
+
+  TEST_EQ(table->int_vec64()->size(), 2);
+  TEST_EQ(table->int_vec64()->Get(1), 40);
+  TEST_EQ(table->int_vec_offset64()->size(), 2);
+  TEST_EQ(table->int_vec_offset64()->Get(1), 60);
+}
+
+void DefaultVectorsStringsTest_BufferSize() {
+  FlatBufferBuilder64 builder;
+  // Create a table where all fields with default values are omitted.
+  DefaultVectorsStringsTest::TableWithDefaultVectorsBuilder tbl_builder(
+      builder);
+  auto offset = tbl_builder.Finish();
+  builder.Finish(offset);
+
+  Verifier verifier(builder.GetBufferPointer(), builder.GetSize());
+  TEST_ASSERT(
+      DefaultVectorsStringsTest::VerifyTableWithDefaultVectorsBuffer(verifier));
+
+  // The buffer should be small when only defaults are used.
+  // This value can be adjusted if the schema changes.
+  constexpr unsigned int buffer_size_threshold_in_bytes = 12;
+  TEST_ASSERT(builder.GetSize() <= buffer_size_threshold_in_bytes);
+}
+
+void DefaultVectorsStringsTest() {
+  DefaultVectorsStringsTest_EmptyOnDefault_Const();
+  DefaultVectorsStringsTest_EmptyOnDefault_Mutable();
+  DefaultVectorsStringsTest_WithValues();
+  DefaultVectorsStringsTest_BufferSize();
+}
+
+}  // namespace tests
+}  // namespace flatbuffers

tests/default_vectors_strings_test.fbs

@@ -0,0 +1,39 @@
+// C++ Only Schema for Default Vectors Test
+namespace DefaultVectorsStringsTest;
+
+struct MyStruct {
+  a:int;
+  b:int;
+}
+
+table MyTable {
+  a:int;
+}
+
+enum MyEnum:byte { A, B }
+
+table TableWithDefaultVectors {
+  int_vec:[int] = [];
+  bool_vec:[bool] = [];
+  char_vec:[byte] = [];
+  uchar_vec:[ubyte] = [];
+  short_vec:[short] = [];
+  ushort_vec:[ushort] = [];
+  uint_vec:[uint] = [];
+  long_vec:[long] = [];
+  ulong_vec:[ulong] = [];
+  float_vec:[float] = [];
+  double_vec:[double] = [];
+  string_vec:[string] = [];
+  empty_string:string = "";
+  some_string:string = "some";
+  struct_vec:[MyStruct] = [];
+  table_vec:[MyTable] = [];
+  enum_vec:[MyEnum] = [];
+  regular_int_vec:[int];
+  regular_int:int;
+  int_vec_offset64:[int] = [] (offset64);
+  int_vec64:[int]= [] (vector64);
+}
+
+root_type TableWithDefaultVectors;

tests/default_vectors_strings_test.h

@@ -0,0 +1,17 @@
+#ifndef THIRDPARTY_FLATBUFFERS_TESTS_DEFAULT_VECTORS_STRINGS_TEST_H_
+#define THIRDPARTY_FLATBUFFERS_TESTS_DEFAULT_VECTORS_STRINGS_TEST_H_
+
+namespace flatbuffers {
+namespace tests {
+
+void DefaultVectorsStringsTest_EmptyOnDefault_Const();
+void DefaultVectorsStringsTest_EmptyOnDefault_Mutable();
+void DefaultVectorsStringsTest_EmptyOnDefault();
+void DefaultVectorsStringsTest_WithValues();
+void DefaultVectorsStringsTest_BufferSize();
+void DefaultVectorsStringsTest();
+
+}  // namespace tests
+}  // namespace flatbuffers
+
+#endif  // THIRDPARTY_FLATBUFFERS_TESTS_DEFAULT_VECTORS_STRINGS_TEST_H_

tests/test.cpp

@@ -34,6 +34,7 @@
 #include "third_party/absl/container/flat_hash_set.h"
 #endif
 #include "alignment_test.h"
+#include "default_vectors_strings_test.h"
 #include "evolution_test.h"
 #include "flatbuffers/flatbuffers.h"
 #include "flatbuffers/idl.h"
@@ -1836,6 +1837,7 @@ int FlatBufferTests(const std::string& tests_data_path) {
   Offset64Tests();
   UnionUnderlyingTypeTest();
   StructsInHashTableTest();
+  DefaultVectorsStringsTest();
   return 0;
 }
 }  // namespace