// In header: <boost/log/utility/type_dispatch/type_dispatcher.hpp>

class type_dispatcher {
public:
  // member classes/structs/unions
  template<typename T> 
  class callback {
  public:
    // public member functions
    void operator()(T const &) const;
    explicit operator bool() const noexcept;
    bool operator!() const noexcept;
  };
  // construct/copy/destruct
  explicit type_dispatcher(get_callback_impl_type) noexcept;
  type_dispatcher(type_dispatcher const &) = default;
  type_dispatcher & operator=(type_dispatcher const &) = default;
  ~type_dispatcher();
  // public member functions
  template<typename T> callback< T > get_callback();
};