From 3cc2daa78f6d8ea7edfd64482626b3549df1339d Mon Sep 17 00:00:00 2001
From: Stefan F <32997632+stefan301@users.noreply.github.com>
Date: Sun, 14 Aug 2022 21:21:55 +0200
Subject: [PATCH] make_span overloads for pointer to vector (#7374) (#7435)

* make_span overloads for pointer to vector (#7374)

* findings from the review

Co-authored-by: Derek Bailey <derekbailey@google.com>
---
 include/flatbuffers/vector.h | 18 +++++++++
 tests/test.cpp               | 72 ++++++++++++++++++++++++++++++++++++
 2 files changed, 90 insertions(+)

diff --git a/include/flatbuffers/vector.h b/include/flatbuffers/vector.h
index c227ddd7a..6bcdfe263 100644
--- a/include/flatbuffers/vector.h
+++ b/include/flatbuffers/vector.h
@@ -327,6 +327,24 @@ FLATBUFFERS_CONSTEXPR_CPP11 flatbuffers::span<const uint8_t> make_bytes_span(
   return span<const uint8_t>(vec.Data(), vec.size() * sizeof(U));
 }
 
+// Convenient helper functions to get a span of any vector, regardless
+// of whether it is null or not (the field is not set).
+template<class U>
+FLATBUFFERS_CONSTEXPR_CPP11 flatbuffers::span<U> make_span(Vector<U> *ptr)
+    FLATBUFFERS_NOEXCEPT {
+  static_assert(Vector<U>::is_span_observable,
+                "wrong type U, only LE-scalar, or byte types are allowed");
+  return ptr ? make_span(*ptr) : span<U>();
+}
+
+template<class U>
+FLATBUFFERS_CONSTEXPR_CPP11 flatbuffers::span<const U> make_span(
+    const Vector<U> *ptr) FLATBUFFERS_NOEXCEPT {
+  static_assert(Vector<U>::is_span_observable,
+                "wrong type U, only LE-scalar, or byte types are allowed");
+  return ptr ? make_span(*ptr) : span<const U>();
+}
+
 // Represent a vector much like the template above, but in this case we
 // don't know what the element types are (used with reflection.h).
 class VectorOfAny {
diff --git a/tests/test.cpp b/tests/test.cpp
index fb3925605..e7a906e77 100644
--- a/tests/test.cpp
+++ b/tests/test.cpp
@@ -4572,6 +4572,77 @@ void JsonUnsortedArrayTest()
   TEST_NOTNULL(monster->testarrayoftables()->LookupByKey("ccc"));
 }
 
+void VectorSpanTest() {
+  flatbuffers::FlatBufferBuilder builder;
+
+  auto mloc =
+      CreateMonster(builder, nullptr, 0, 0, builder.CreateString("Monster"),
+      builder.CreateVector<uint8_t>({ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }));
+
+  FinishMonsterBuffer(builder, mloc);
+
+  auto monster = GetMonster(builder.GetBufferPointer());
+  auto mutable_monster = GetMutableMonster(builder.GetBufferPointer());
+
+  { // using references
+    TEST_NOTNULL(monster->inventory());
+
+    flatbuffers::span<const uint8_t> const_inventory =
+        flatbuffers::make_span(*monster->inventory());
+    TEST_EQ(const_inventory.size(), 10);
+    TEST_EQ(const_inventory[0], 0);
+    TEST_EQ(const_inventory[9], 9);
+
+    flatbuffers::span<uint8_t> mutable_inventory =
+        flatbuffers::make_span(*mutable_monster->mutable_inventory());
+    TEST_EQ(mutable_inventory.size(), 10);
+    TEST_EQ(mutable_inventory[0], 0);
+    TEST_EQ(mutable_inventory[9], 9);
+
+    mutable_inventory[0] = 42;
+    TEST_EQ(mutable_inventory[0], 42);
+
+    mutable_inventory[0] = 0;
+    TEST_EQ(mutable_inventory[0], 0);
+  }
+
+  { // using pointers
+    TEST_EQ(flatbuffers::VectorLength(monster->inventory()), 10);
+
+    flatbuffers::span<const uint8_t> const_inventory =
+        flatbuffers::make_span(monster->inventory());
+    TEST_EQ(const_inventory.size(), 10);
+    TEST_EQ(const_inventory[0], 0);
+    TEST_EQ(const_inventory[9], 9);
+
+    flatbuffers::span<uint8_t> mutable_inventory =
+        flatbuffers::make_span(mutable_monster->mutable_inventory());
+    TEST_EQ(mutable_inventory.size(), 10);
+    TEST_EQ(mutable_inventory[0], 0);
+    TEST_EQ(mutable_inventory[9], 9);
+
+    mutable_inventory[0] = 42;
+    TEST_EQ(mutable_inventory[0], 42);
+
+    mutable_inventory[0] = 0;
+    TEST_EQ(mutable_inventory[0], 0);
+  }
+
+  {
+    TEST_ASSERT(nullptr == monster->testnestedflatbuffer());
+
+    TEST_EQ(flatbuffers::VectorLength(monster->testnestedflatbuffer()), 0);
+
+    flatbuffers::span<const uint8_t> const_nested =
+        flatbuffers::make_span(monster->testnestedflatbuffer());
+    TEST_ASSERT(const_nested.empty());
+
+    flatbuffers::span<uint8_t> mutable_nested =
+        flatbuffers::make_span(mutable_monster->mutable_testnestedflatbuffer());
+    TEST_ASSERT(mutable_nested.empty());
+  }
+}
+
 int FlatBufferTests() {
   // clang-format off
 
@@ -4678,6 +4749,7 @@ int FlatBufferTests() {
   NestedVerifierTest();
   PrivateAnnotationsLeaks();
   JsonUnsortedArrayTest();
+  VectorSpanTest();
   return 0;
 }