Fix reverse_iterator in Vector and tests (#5344)

Before this commit tests for iterators passed, even if the code inside
the loop has not been executed.

include/flatbuffers/flatbuffers.h

@@ -201,14 +201,16 @@ template<typename T, typename IT> struct VectorIterator {
 template<typename Iterator> struct VectorReverseIterator :
   public std::reverse_iterator<Iterator> {
 
-  explicit VectorReverseIterator(Iterator iter) : iter_(iter) {}
+  explicit VectorReverseIterator(Iterator iter) :
+    std::reverse_iterator<Iterator>(iter) {}
 
-  typename Iterator::value_type operator*() const { return *(iter_ - 1); }
-
-  typename Iterator::value_type operator->() const { return *(iter_ - 1); }
+  typename Iterator::value_type operator*() const {
+    return *(std::reverse_iterator<Iterator>::current);
+  }
 
- private:
-  Iterator iter_;
+  typename Iterator::value_type operator->() const {
+    return *(std::reverse_iterator<Iterator>::current);
+  }
 };
 
 struct String;
@@ -269,11 +271,11 @@ template<typename T> class Vector {
   iterator end() { return iterator(Data(), size()); }
   const_iterator end() const { return const_iterator(Data(), size()); }
 
-  reverse_iterator rbegin() { return reverse_iterator(end()); }
-  const_reverse_iterator rbegin() const { return const_reverse_iterator(end()); }
+  reverse_iterator rbegin() { return reverse_iterator(end() - 1); }
+  const_reverse_iterator rbegin() const { return const_reverse_iterator(end() - 1); }
 
-  reverse_iterator rend() { return reverse_iterator(end()); }
-  const_reverse_iterator rend() const { return const_reverse_iterator(end()); }
+  reverse_iterator rend() { return reverse_iterator(begin() - 1); }
+  const_reverse_iterator rend() const { return const_reverse_iterator(begin() - 1); }
 
   const_iterator cbegin() const { return begin(); }
 

tests/test.cpp

@@ -253,29 +253,37 @@ void AccessFlatBufferTest(const uint8_t *flatbuf, size_t length,
   unsigned char inv_data[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
   // Check compatibilty of iterators with STL.
   std::vector<unsigned char> inv_vec(inventory->begin(), inventory->end());
-  for (auto it = inventory->begin(); it != inventory->end(); ++it) {
+  int n = 0;
+  for (auto it = inventory->begin(); it != inventory->end(); ++it, ++n) {
     auto indx = it - inventory->begin();
     TEST_EQ(*it, inv_vec.at(indx));  // Use bounds-check.
     TEST_EQ(*it, inv_data[indx]);
   }
+  TEST_EQ(n, inv_vec.size());
 
-  for (auto it = inventory->cbegin(); it != inventory->cend(); ++it) {
+  n = 0;
+  for (auto it = inventory->cbegin(); it != inventory->cend(); ++it, ++n) {
     auto indx = it - inventory->cbegin();
     TEST_EQ(*it, inv_vec.at(indx));  // Use bounds-check.
     TEST_EQ(*it, inv_data[indx]);
   }
+  TEST_EQ(n, inv_vec.size());
 
-  for (auto it = inventory->rbegin(); it != inventory->rend(); ++it) {
-    auto indx = inventory->rend() - it;
+  n = 0;
+  for (auto it = inventory->rbegin(); it != inventory->rend(); ++it, ++n) {
+    auto indx = inventory->rend() - it - 1;
     TEST_EQ(*it, inv_vec.at(indx));  // Use bounds-check.
     TEST_EQ(*it, inv_data[indx]);
   }
+  TEST_EQ(n, inv_vec.size());
 
-  for (auto it = inventory->crbegin(); it != inventory->crend(); ++it) {
-    auto indx = inventory->crend() - it;
+  n = 0;
+  for (auto it = inventory->crbegin(); it != inventory->crend(); ++it, ++n) {
+    auto indx = inventory->crend() - it - 1;
     TEST_EQ(*it, inv_vec.at(indx));  // Use bounds-check.
     TEST_EQ(*it, inv_data[indx]);
   }
+  TEST_EQ(n, inv_vec.size());
 
   TEST_EQ(monster->color(), Color_Blue);