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 following can be used to have a boost::variant which can hold an (SQL) NULL value for example.

class Null { };

typedef boost::variant Value;

To make it more useful things like operator== and operator<< are handy. For example:

std::ostream& operator<<(std::ostream &lhs, const Null &rhs)
{
     lhs << "*NULL*";
     return lhs;
}

int main()
{
    Variant v("hello");
    std::cout << v << std::endl;
    v = Null();
    std::cout << v << std::endl;
    ...
}

Will give:

hello
*NULL*