Listing 6

class Requisite
{
public:
  int resource_type;
  time_t range[2];
};
class Request
{
public:
  std::list<Requisite> requisites;
  int user_id;
  int event_type;
};
int schedule(const Request &request, std::list<Reservation> &reservations);

template<class Src, class Dst>
class Error
{
public:
  std::string description;
  Handle source;
  Chain<Src, Dst> chain;
};

try {
  schedule(req, res);
} catch (Error<Request, int> &e) {
  // ...
} catch (Error<Request, User> &e) {
  // ...
} catch (Error<Request, time_t[2]> &e) {
  // ...
}

template<class Src>

class Handle_Link : public Typed_Link<Src, Handle>
{
public:
  Handle &traverse(Src &id) 
  {
    static Handle h(id);
    return h = Handle(id);
  }
  //...
};

template<class Src>
class Error_Base
{
public:
  virtual Chain<Src,Handle> get_link(void) = 0;
  std::string description;
  Handle source;
  // ...
};
template<class Src, class Dst>
class Error : public Error_Base<Src>
{
public:
  Chain<Src,Handle> get_link(void)
  {
    return chain + Handle_Link<Dst>();
  }
  Chain<Src, Dst> chain;
  // ...
};

 ...
} catch (Error_Base<Request> &e) {
  Chain<Request, Handle> c = e.get_link();
  if (c.get_distance(some_user) <
      c.get_distance(some_requisite)) {
    // smells like trouble in the requisite!
  }
}