C / C++ / MFC

Neil Gaiman - Cold Colours

• To define exceptions, as Rama pointed out.
  
• To provide alternate constructors. Consider this example:

  struct chess_king{
    struct white{};
    struct black{};
    chess_king(const white&); //construct a white king
    chess_king(const black&); //construct a black king
    ...
  }

  We could have used bool to indicate wheter the piece is white or black, but defining the constructors with the empty classes white and black allow the compiler to optimize them away in release mode, which matters if you're writting highly speed critical code (this is not an academic example, I've actually used it for writing a game engine in the past.)