allocate immediately required fields (#1)

Reviewed-on: BigfootDev/flatbuffers-bigfoot#1
Co-authored-by: Romain BOULLARD <romain.boullard@protonmail.com>

tests/required_table_default_test.cpp

@@ -0,0 +1,39 @@
+#include "required_table_default_test.h"
+
+#include "required_table_default_test_generated.h"
+#include "test_assert.h"
+
+namespace flatbuffers {
+namespace tests {
+
+// Exercises the (required) table-field default member initializer added to
+// GenMember() in idl_gen_cpp.cpp: a `(required)` table field can never
+// legally be missing once Pack()/Verify() succeed, so a plain native default
+// constructor must not leave it null - callers should never have to
+// remember to allocate it themselves before the first Pack(). The sibling
+// `optional_inner` field (same type, no `required`) is checked alongside it
+// to confirm the change is scoped to required fields only.
+void RequiredTableDefaultTest() {
+  using RequiredTableDefaultTestNS::Holder;
+  using RequiredTableDefaultTestNS::HolderT;
+
+  HolderT src;
+  TEST_NOTNULL(src.required_inner.get());
+  TEST_NULL(src.optional_inner.get());
+
+  // Never manually allocated required_inner - only relying on the default
+  // constructor above - yet Pack() must still succeed and round-trip it.
+  src.required_inner->value = 42;
+
+  flatbuffers::FlatBufferBuilder fbb;
+  fbb.Finish(Holder::Pack(fbb, &src));
+
+  auto dst = RequiredTableDefaultTestNS::UnPackHolder(fbb.GetBufferPointer());
+
+  TEST_NOTNULL(dst->required_inner.get());
+  TEST_EQ(dst->required_inner->value, 42u);
+  TEST_NULL(dst->optional_inner.get());
+}
+
+}  // namespace tests
+}  // namespace flatbuffers