[C++]Support reverse iterator in Vector (#5128)

* Add `const` keyword to the `operator-(const uoffset_t &)` function in
`VectorIterator`

* Support reverse iterator in Vector

Introduced a new VectorReverseIterator type. We cannot directly use
`std::reverse_iterator<VectorIterator>` because the signature of
`operator*` and `operator->` in the VectorIterator class are not
standard signatures.

Also added `rbegin()`, `rend()`, `cbegin()`, `cend()`, `crbegin()`
and `crend()` in the Vector class.

include/flatbuffers/flatbuffers.h

@@ -116,6 +116,7 @@ template<typename T, typename IT> struct VectorIterator {
   VectorIterator(const uint8_t *data, uoffset_t i)
       : data_(data + IndirectHelper<T>::element_stride * i) {}
   VectorIterator(const VectorIterator &other) : data_(other.data_) {}
+  VectorIterator() : data_(nullptr) {}
 
   VectorIterator &operator=(const VectorIterator &other) {
     data_ = other.data_;
@@ -183,7 +184,7 @@ template<typename T, typename IT> struct VectorIterator {
     return temp;
   }
 
-  VectorIterator operator-(const uoffset_t &offset) {
+  VectorIterator operator-(const uoffset_t &offset) const {
     return VectorIterator(data_ - offset * IndirectHelper<T>::element_stride,
                           0);
   }
@@ -197,6 +198,19 @@ template<typename T, typename IT> struct VectorIterator {
   const uint8_t *data_;
 };
 
+template<typename Iterator> struct VectorReverseIterator :
+  public std::reverse_iterator<Iterator> {
+
+  explicit VectorReverseIterator(Iterator iter) : iter_(iter) {}
+
+  typename Iterator::value_type operator*() const { return *(iter_ - 1); }
+
+  typename Iterator::value_type operator->() const { return *(iter_ - 1); }
+
+ private:
+  Iterator iter_;
+};
+
 struct String;
 
 // This is used as a helper type for accessing vectors.
@@ -207,6 +221,8 @@ template<typename T> class Vector {
       iterator;
   typedef VectorIterator<T, typename IndirectHelper<T>::return_type>
       const_iterator;
+  typedef VectorReverseIterator<iterator> reverse_iterator;
+  typedef VectorReverseIterator<const_iterator> const_reverse_iterator;
 
   uoffset_t size() const { return EndianScalar(length_); }
 
@@ -253,6 +269,20 @@ 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 rend() { return reverse_iterator(end()); }
+  const_reverse_iterator rend() const { return const_reverse_iterator(end()); }
+
+  const_iterator cbegin() const { return begin(); }
+
+  const_iterator cend() const { return end(); }
+
+  const_reverse_iterator crbegin() const { return rbegin(); }
+
+  const_reverse_iterator crend() const { return rend(); }
+
   // Change elements if you have a non-const pointer to this object.
   // Scalars only. See reflection.h, and the documentation.
   void Mutate(uoffset_t i, const T &val) {

tests/test.cpp

@@ -255,6 +255,24 @@ void AccessFlatBufferTest(const uint8_t *flatbuf, size_t length,
     TEST_EQ(*it, inv_data[indx]);
   }
 
+  for (auto it = inventory->cbegin(); it != inventory->cend(); ++it) {
+    auto indx = it - inventory->cbegin();
+    TEST_EQ(*it, inv_vec.at(indx));  // Use bounds-check.
+    TEST_EQ(*it, inv_data[indx]);
+  }
+
+  for (auto it = inventory->rbegin(); it != inventory->rend(); ++it) {
+    auto indx = inventory->rend() - it;
+    TEST_EQ(*it, inv_vec.at(indx));  // Use bounds-check.
+    TEST_EQ(*it, inv_data[indx]);
+  }
+
+  for (auto it = inventory->crbegin(); it != inventory->crend(); ++it) {
+    auto indx = inventory->crend() - it;
+    TEST_EQ(*it, inv_vec.at(indx));  // Use bounds-check.
+    TEST_EQ(*it, inv_data[indx]);
+  }
+
   TEST_EQ(monster->color(), Color_Blue);
 
   // Example of accessing a union: