// In header: <boost/accumulators/framework/accumulators/droppable_accumulator.hpp>

template<typename Accumulator> 
struct with_cached_result {
  // types
  typedef Accumulator::result_type result_type;

  // construct/copy/destruct
  template<typename Args> with_cached_result(Args const &);
  with_cached_result(with_cached_result const &);
  with_cached_result& operator=(with_cached_result const &);
  ~with_cached_result();

  // public member functions
  template<typename Args> void on_drop(Args const &);
  template<typename Args> result_type result(Args const &) const;

  // private member functions
  void set(result_type const &);
  result_type const & get() const;
  bool has_result() const;
};