Added support for structs and strings in unions.

(C++ only for now).
Also fixed vector of union support in the object API.

Bug: 36902939
Change-Id: I935f4cc2c303a4728e26c7916a8ec0adcd6f84cb
Tested: on Linux.

include/flatbuffers/flatbuffers.h

@@ -360,6 +360,8 @@ private:
   const uint8_t *data_;
 };
 
+struct String;
+
 // This is used as a helper type for accessing vectors.
 // Vector::data() assumes the vector elements start after the length field.
 template<typename T> class Vector {
@@ -391,6 +393,18 @@ public:
     return static_cast<E>(Get(i));
   }
 
+  // If this a vector of unions, this does the cast for you. There's no check
+  // to make sure this is the right type!
+  template<typename U> const U *GetAs(uoffset_t i) const {
+    return reinterpret_cast<const U *>(Get(i));
+  }
+
+  // If this a vector of unions, this does the cast for you. There's no check
+  // to make sure this is actually a string!
+  const String *GetAsString(uoffset_t i) const {
+    return reinterpret_cast<const String *>(Get(i));
+  }
+
   const void *GetStructFromOffset(size_t o) const {
     return reinterpret_cast<const void *>(Data() + o);
   }
@@ -1327,6 +1341,13 @@ FLATBUFFERS_FINAL_CLASS
                                      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) {
+    Align(AlignOf<T>());
+    buf_.push_small(structobj);
+    return Offset<const T *>(GetSize());
+  }
+
   /// @brief The length of a FlatBuffer file header.
   static const size_t kFileIdentifierLength = 4;