Add a builder option to enable/disable vtables deduplication. (#4132)

* Add a builder option to enable/disable vtables deduplication.

* address code review comments

include/flatbuffers/flatbuffers.h

@@ -647,7 +647,7 @@ FLATBUFFERS_FINAL_CLASS
                              const simple_allocator *allocator = nullptr)
       : buf_(initial_size, allocator ? *allocator : default_allocator),
         nested(false), finished(false), minalign_(1), force_defaults_(false),
-        string_pool(nullptr) {
+        dedup_vtables_(true), string_pool(nullptr) {
     offsetbuf_.reserve(16);  // Avoid first few reallocs.
     vtables_.reserve(16);
     EndianCheck();
@@ -724,6 +724,10 @@ FLATBUFFERS_FINAL_CLASS
   /// @param[in] bool fd When set to `true`, always serializes default values.
   void ForceDefaults(bool fd) { force_defaults_ = fd; }
 
+  /// @brief By default vtables are deduped in order to save space.
+  /// @param[in] bool dedup When set to `true`, dedup vtables.
+  void DedupVtables(bool dedup) { dedup_vtables_ = dedup; }
+
   /// @cond FLATBUFFERS_INTERNAL
   void Pad(size_t num_bytes) { buf_.fill(num_bytes); }
 
@@ -861,6 +865,7 @@ FLATBUFFERS_FINAL_CLASS
     auto vt_use = GetSize();
     // See if we already have generated a vtable with this exact same
     // layout before. If so, make it point to the old one, remove this one.
+    if (dedup_vtables_) {
       for (auto it = vtables_.begin(); it != vtables_.end(); ++it) {
         auto vt2 = reinterpret_cast<voffset_t *>(buf_.data_at(*it));
         auto vt2_size = *vt2;
@@ -869,6 +874,7 @@ FLATBUFFERS_FINAL_CLASS
         buf_.pop(GetSize() - vtableoffsetloc);
         break;
       }
+    }
     // If this is a new vtable, remember it.
     if (vt_use == GetSize()) {
       vtables_.push_back(vt_use);
@@ -1267,6 +1273,8 @@ FLATBUFFERS_FINAL_CLASS
 
   bool force_defaults_;  // Serialize values equal to their defaults anyway.
 
+  bool dedup_vtables_;
+
   struct StringOffsetCompare {
     StringOffsetCompare(const vector_downward &buf) : buf_(&buf) {}
     bool operator() (const Offset<String> &a, const Offset<String> &b) const {