FlatBuffers 64 for C++ (#7935)

* First working hack of adding 64-bit. Don't judge :)

* Made vector_downward work on 64 bit types

* vector_downward uses size_t, added offset64 to reflection

* cleaned up adding offset64 in parser

* Add C++ testing skeleton for 64-bit

* working test for CreateVector64

* working >2 GiB buffers

* support for large strings

* simplified CreateString<> to just provide the offset type

* generalize CreateVector template

* update test_64.afb due to upstream format change

* Added Vector64 type, which is just an alias for vector ATM

* Switch to Offset64 for Vector64

* Update for reflection bfbs output change

* Starting to add support for vector64 type in C++

* made a generic CreateVector that can handle different offsets and vector types

* Support for 32-vector with 64-addressing

* Vector64 basic builder + tests working

* basic support for json vector64 support

* renamed fields in test_64bit.fbs to better reflect their use

* working C++ vector64 builder

* Apply --annotate-sparse-vector to 64-bit tests

* Enable Vector64 for --annotate-sparse-vectors

* Merged from upstream

* Add `near_string` field for testing 32-bit offsets alongside

* keep track of where the 32-bit and 64-bit regions are for flatbufferbuilder

* move template<> outside class body for GCC

* update run.sh to build and run tests

* basic assertion for adding 64-bit offset at the wrong time

* started to separate `FlatBufferBuilder` into two classes, 1 64-bit aware, the other not

* add test for nested flatbuffer vector64, fix bug in alignment of big vectors

* fixed CreateDirect method by iterating by Offset64 first

* internal refactoring of flatbufferbuilder

* block not supported languages in the parser from using 64-bit

* evolution tests for adding a vector64 field

* conformity tests for adding/removing offset64 attributes

* ensure test is for a big buffer

* add parser error tests for `offset64` and `vector64` attributes

* add missing static that GCC only complains about

* remove stdint-uintn.h header that gets automatically added

* move 64-bit CalculateOffset internal

* fixed return size of EndVector

* various fixes on windows

* add SizeT to vector_downward

* minimze range of size changes in vector and builder

* reworked how tracking if 64-offsets are added

* Add ReturnT to EndVector

* small cleanups

* remove need for second Array definition

* combine IndirectHelpers into one definition

* started support for vector of struct

* Support for 32/64-vectors of structs + Offset64

* small cleanups

* add verification for vector64

* add sized prefix for 64-bit buffers

* add fuzzer for 64-bit

* add example of adding many vectors using a wrapper table

* run the new -bfbs-gen-embed logic on the 64-bit tests

* remove run.sh and fix cmakelist issue

* fixed bazel rules

* fixed some PR comments

* add 64-bit tests to cmakelist

include/flatbuffers/table.h

@@ -47,14 +47,24 @@ class Table {
     return field_offset ? ReadScalar<T>(data_ + field_offset) : defaultval;
   }
 
-  template<typename P> P GetPointer(voffset_t field) {
+  template<typename P, typename OffsetSize = uoffset_t>
+  P GetPointer(voffset_t field) {
     auto field_offset = GetOptionalFieldOffset(field);
     auto p = data_ + field_offset;
-    return field_offset ? reinterpret_cast<P>(p + ReadScalar<uoffset_t>(p))
+    return field_offset ? reinterpret_cast<P>(p + ReadScalar<OffsetSize>(p))
                         : nullptr;
   }
-  template<typename P> P GetPointer(voffset_t field) const {
-    return const_cast<Table *>(this)->GetPointer<P>(field);
+  template<typename P, typename OffsetSize = uoffset_t>
+  P GetPointer(voffset_t field) const {
+    return const_cast<Table *>(this)->GetPointer<P, OffsetSize>(field);
+  }
+
+  template<typename P> P GetPointer64(voffset_t field) {
+    return GetPointer<P, uoffset64_t>(field);
+  }
+
+  template<typename P> P GetPointer64(voffset_t field) const {
+    return GetPointer<P, uoffset64_t>(field);
   }
 
   template<typename P> P GetStruct(voffset_t field) const {
@@ -131,15 +141,25 @@ class Table {
   }
 
   // Versions for offsets.
+  template<typename OffsetT = uoffset_t>
   bool VerifyOffset(const Verifier &verifier, voffset_t field) const {
     auto field_offset = GetOptionalFieldOffset(field);
-    return !field_offset || verifier.VerifyOffset(data_, field_offset);
+    return !field_offset || verifier.VerifyOffset<OffsetT>(data_, field_offset);
   }
 
+  template<typename OffsetT = uoffset_t>
   bool VerifyOffsetRequired(const Verifier &verifier, voffset_t field) const {
     auto field_offset = GetOptionalFieldOffset(field);
     return verifier.Check(field_offset != 0) &&
-           verifier.VerifyOffset(data_, field_offset);
+           verifier.VerifyOffset<OffsetT>(data_, field_offset);
+  }
+ 
+  bool VerifyOffset64(const Verifier &verifier, voffset_t field) const {
+    return VerifyOffset<uoffset64_t>(verifier, field);
+  }
+
+  bool VerifyOffset64Required(const Verifier &verifier, voffset_t field) const {
+    return VerifyOffsetRequired<uoffset64_t>(verifier, field);
   }
 
  private: