class incrementer{
public:
  int addend;
  incrementer(const int _addend = 1){
    addend = _addend;
  }
// special method for using this class as a function
// i.e. functor:
  int operator()(const int arg) {
    return arg + addend;
  }
// non-special method i.e. member function:
  int backwards(const int arg){
    return arg - addend;
  }
};