Re: [Boost-users] [msm] how to write a generic class for all statemachine back-end.

Subject: Re: [Boost-users] [msm] how to write a generic class for all statemachine back-end.
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2011-06-23 16:04:06

>In our project, we need a generic interface to refer to different state machine (back-end). For example, in the following codes, we want

>to use SMBase* to refer to any concrete state machine. SMBase should have similar interface as state machine.

>struct User {
>  A(SMBase* s) : sm(s) ();
>  SMBase* sm;
>  ...
>  // User can use sm->start(), sm->process_event() etc
>}

>struct SMBase 
>{
>  virtual void start() = 0;
>  ... // how to design the rest ???
>}

>The problem of designing SMBase is the function "process_event". We want it to to be virtual function so SMWrapper can implement it. >But it also needs to be a template according to state machine interface. But we cannot have a virtual template function.

>How can we achieve/design this generic state machine interface?

Hi Jerry,

Yes, virtual template methods are not possible in C++ :(

(Note to C++ Standard Comittee: I'd consider donating a kidney for this one :) )

This leaves you with few possibilies. I see 2 at a first glance (suggestions welcome):

- virtual void process_event_event1(); virtual void process_event_event2(); ...etc (not fun but quite in the OO spirit)

- virtual void process_event(boost::any) followed by any_casts until you find the correct one (not fun either) 

All considered, solution 1 looks better. If you can define a common interface for state machines, then they are bound to expect common events.

HTH,

Christophe