[boost] Re: What is Double Dispatch ?

From: Andreas Huber (ah2003_at_[hidden])
Date: 2003-06-05 17:05:41

Bohdan,

> Ooops, sorry ... i was terribly unclear.
>
> Andreas Huber  wrote:
> "boost::fsm no longer uses acyclic visitor! The current double
> dispatch implementation (which was inspired by Aleksey's fsm lib)
> uses one virtual call followed by a linear search for a suitable
> reaction, using one RTTI comparison per visited reaction. Although
> you cannot change the double dispatch implementation itself, you can
> change one aspect of it, i.e. RTTI."
>
> I mean :
>  Where can i find something about cool "double dispatch" used in
> recently submitted FSM code ?
> Or Aleksey's fsm lib sources ?

You can find Aleksey's fsm lib here:

http://www.mywikinet.com/mpl/fsm_example_25_jul_02.zip

In my library, you can look for simple_state::react_impl() which is the
override for the abstract state_base::react_impl(). The implementation is
quite simple: all reactions are contained in an mpl::list, which is passed
to a function. The function then calls the front reaction. If the reaction
returns that there was a match the result is returned. Otherwise, the
function calls itself again with the front of the list removed. Of course,
there is a specialisation for an empty list which does nothing.

BTW, this double dispatch variant is only "cool" ;-) for cases where the
second stage of the double dispatch has to choose from only a few different
possibilities. That's because the second stage performs a linear search. In
contrast to GOF-visitor, which is essentially constant-time, this variant
becomes slower the more choices it has in the second stage. But it scales
much better in terms of dependencies.

Regards,

Andreas