Re: [Boost-users] [Statechart] casting event type inunconsumed_event

From: Andreas Huber (ahd6974-spamboostorgtrap_at_[hidden])
Date: 2007-04-12 15:05:33

JD wrote:
>   Thanks for your quick answer.
>   It works for this particular problem, but still I have a more
> general problem with Boost.Statechart that bother me. In the faq I
> read:
>
> Q. Does Boost.Statechart support polymorphic events?
> A. No. Although events can be derived from each other to write common
> code only once
>
>   Ok but polymorphism is more than just factorizing code.

In my experience with events you rarely need true polymorphism. If you 
do, you can often use the trick demonstrated in the FAQ item you refer 
to. True, from a purely functional perspective this is a limitation I'd 
rather not have in the library. However, this limitation makes the 
implementation of the internally used RTTI mechanism much easier and 
much faster.

> The way an
> event has still to define a "MostDerived" template prevent from using
> an event as a generic object.

It depends. It is true that there is currently no way to use a common 
custom base class for events and states as long as you restrict yourself 
to single inheritance. That is, if event_base does not offer what you 
look for then you need to multi-inherit your events from your custom 
base and statechart::event<> and use cross casts to gain access to the 
custom base. This is demonstrated with events here:

<http://www.boost-consulting.com/boost/libs/statechart/doc/tutorial.html#GettingStateInformationOutOfTheMachine>

(see the call to state_cast<>, which uses dynamic_cast under the hood)

[snip]
> //snippet on
> getline(is, line);
> theGenericEvent = the_function_in_question(line);
> MyMachine.post_event(theGenericEvent);
> //snippet off
>
>   I simply can't do that. Or I'm missing something.
>   What is the rational for events not supporting polymorphism?

Have a look at the signature of post_event:

<http://www.boost-consulting.com/boost/libs/statechart/doc/reference.html#post_event3>

What is stopping you to declare theGenericEvent variable above as 
event_base type?

HTH,

-- 
Andreas Huber
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