Advantages of an empty class in C++

Question

What could be the possible advantages/uses of having an empty class?

P.S: This question might sound trivial to some of you but it is just for learning purpose and h

Answer 1

The answer by MartinStettner is fine though just to highlight an important point here: The concept of iterator tags or for that matter any tags in C++, is not strictly dependent on empty classes. The C++ tags, if stl writers would have wanted to, could well have been non-empty classes; that should work but then it won't add any additional value; at least for compile time acrobatics that it is usually reserved for.