Callback Wrapper (Functor)

The definition of a Callback Wrapper (functor) is shown below:

template <typename Signature>
class functor : public detail::get_functor_impl <
    boost::function_traits<Signature>::arity, 
    Signature
>::type {
  ...
}

This is an implementation detail, and usually not required to use the dispatcher library.

detail::get_functor_impl< int Arity, Signature > is a template facility which allows the functor client to define which base class it derives from. There are six (6) different functor implementations, dependent on the number of arguments provided in the signature. This technique is called parametric base type declaraion, which allows the compiler to choose the appropriate base class depending on template parameters.

The functor can be used in the following example:

boost::dispatch::functor<void()> f;
boost::dispatch::functor<int (std::string, double)> g;
boost::dispatch::functor<double (my_type, my_other_type, int)> h;

The functor wraps a Boost.Function function which by default is bound to functor::default_function that throws a boost::dispatch::unregistered_handler in case a functor is not initialized but invoked.

See discussion on referencing indexes without associated callbacks that throw an boost::dispatch::unregistered_handler.