Wrap a std::vector<uint8_t> with a flatbuffers::Allocator

FlatbuffersVectorAllocator.cpp

// Wraps an existing std::vector<uint8_t> as a flatbuffers::Allocator
class VectorAllocator : public flatbuffers::Allocator {
public:
  explicit VectorAllocator(std::vector<uint8_t>& buffer)
      : buffer_(buffer) {
    buffer_.clear();
  }

  uint8_t* allocate(size_t size) override {
    buffer_.resize(size);
    return buffer_.data();
  }

  void deallocate(uint8_t* p, size_t size) override {
    // No action needed; buffer is managed by std::vector
  }

  uint8_t* reallocate_downward(uint8_t* old_p, size_t old_size, size_t new_size, size_t in_use_back,
                               size_t in_use_front) override {
    FLATBUFFERS_ASSERT(new_size > old_size);  // vector_downward only grows
    buffer_.resize(new_size);
    memcpy(buffer_.data() + new_size - in_use_back, old_p + old_size - in_use_back, in_use_back);
    memcpy(buffer_.data(), old_p, in_use_front);
    return buffer_.data();
  }

private:
  std::vector<uint8_t>& buffer_;
};