C++: What is the size of an object of an empty class?

Question

I was wondering what could be the size of an object of an empty class. It surely could not be 0 bytes since it should be possible to reference and

Answer 1

There is an exception: 0-length arrays

#include 

class CompletlyEmpty {
  char NO_DATA[0];
};

int main(int argc, const char** argv) {
  std::cout << sizeof(CompletlyEmpty) << '\n';
}