Question

The following code is a simplified version of what I use for event dispatching. The essential point is
that there is a static_cast<T*> on the argument of a template functor and another class makes
sure that the argument passed to the functor is what the static_cast casts to.

struct AbstractArg {
  virtual ~AbstractArg() { }
};

struct AbstractFunctor {
  virtual void operator()(AbstractArg*) const = 0;
  virtual ~AbstractFunctor() { }
};


namespace evil {

template<typename ArgT>
struct CastingFunctor :  AbstractFunctor {
  void operator()(AbstractArg* aarg) const
  {
    ArgT* arg = static_cast<ArgT*>(arg); // Danger!
    // ... do stuff with it
  }
};

} // namespace evil


class TypeidManager
{
public:
  typedef std::map<std::string, AbstractFunctor*> map_type;

  void dispatch(AbstractArg* arg) const
  {
    map_type::const_iterator it = typeid_map.find(std::string(typeid(*arg).name()));
    if (it != typeid_map.end())
    {
      AbstractFunctor* pFunctor = it->second;
      (*pFunctor)(arg);
    }
  };

  template<typename ArgT>
  void register_func( void (*fun_ptr)(ArgT*))
  {
    typeid_map.insert(std::make_pair(std::string(typeid(ArgT).name()),
                                     new evil::CastingFunctor<ArgT>));
  }

private:
  map_type typeid_map;
};

Is there a name for this pattern?

Thanks!

Answer 1

This isn’t an anti-pattern, it’s a really useful technique often used with type erasure.