CreateUnitializedVectorOfStructs and tests (#4781)

* CreateUnitializedVectorOfStructs and tests

* Incorporating review comments

* snake_case variable names

include/flatbuffers/flatbuffers.h

@@ -1545,6 +1545,12 @@ class FlatBufferBuilder {
                                      reinterpret_cast<uint8_t **>(buf));
   }
 
+  template<typename T>
+  Offset<Vector<const T*>> CreateUninitializedVectorOfStructs(size_t len, T **buf) {
+    return CreateUninitializedVector(len, sizeof(T),
+                                     reinterpret_cast<uint8_t **>(buf));
+  }
+
   /// @brief Write a struct by itself, typically to be part of a union.
   template<typename T> Offset<const T *> CreateStruct(const T &structobj) {
     NotNested();

tests/test.cpp

@@ -1950,6 +1950,34 @@ void EndianSwapTest() {
   TEST_EQ(flatbuffers::EndianSwap(flatbuffers::EndianSwap(3.14f)), 3.14f);
 }
 
+void UninitializedVectorTest() {
+  flatbuffers::FlatBufferBuilder builder;
+  
+  Test *buf = nullptr;
+  auto vector_offset = builder.CreateUninitializedVectorOfStructs<Test>(2, &buf);
+  TEST_NOTNULL(buf);
+  buf[0] = Test(10, 20);
+  buf[1] = Test(30, 40);
+  
+  auto required_name = builder.CreateString("myMonster");
+  auto monster_builder = MonsterBuilder(builder);
+  monster_builder.add_name(required_name); // required field mandated for monster.
+  monster_builder.add_test4(vector_offset);
+  builder.Finish(monster_builder.Finish());
+  
+  auto p = builder.GetBufferPointer();
+  auto uvt = flatbuffers::GetRoot<Monster>(p);
+  TEST_NOTNULL(uvt);
+  auto vec = uvt->test4();
+  TEST_NOTNULL(vec);
+  auto test_0 = vec->Get(0);
+  auto test_1 = vec->Get(1);
+  TEST_EQ(test_0->a(), 10);
+  TEST_EQ(test_0->b(), 20);
+  TEST_EQ(test_1->a(), 30);
+  TEST_EQ(test_1->b(), 40);
+}
+
 int main(int /*argc*/, const char * /*argv*/ []) {
   // clang-format off
   #if defined(FLATBUFFERS_MEMORY_LEAK_TRACKING) && \
@@ -2020,6 +2048,7 @@ int main(int /*argc*/, const char * /*argv*/ []) {
   JsonDefaultTest();
 
   FlexBuffersTest();
+  UninitializedVectorTest();
 
   if (!testing_fails) {
     TEST_OUTPUT_LINE("ALL TESTS PASSED");