Re: [Boost-users] [MSM] Best place to execute an action with return value

Subject: Re: [Boost-users] [MSM] Best place to execute an action with return value
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2013-11-27 15:23:16

> Hi,
>
> I'm diving into this very useful piece of code which is MSM and I'm
> modelling a state machine for a gsm modem.
>
> When I have to perform a call I need to move from the IDLE state to
> WAITING_RESPONSE and, in the transaction action I have to call the
> MakeCall() method on my GSM object.
>
> This method has a return value and a non 0 value means it is failed.
> In this situation the SM don't have to move on to the next state but
> remains in the IDLE one, so...
>
> Since the Actions has no return value, I have to use a Guard instead or
> I have to add another state called MAKE_CALL?
>
> Thanks in advance,
> Daniele.

Hi,

it depends how your code is structured.
According to UML, guards should not execute any actions (just return a 
bool), but MSM will let you do.
The question is, is MakeCall a long-lasting, blocking function?
If yes then you're blocking the run-to-completion (the state machine stays 
long in an unstable state as it processes a transition), and it has bad 
side-effects (for example debugging and loggin). In such a case, yes you 
should have an extra state.

If MakeCall is fast, then a possible solution would be to use a guard as you 
suggested. The guard would execute MakeCall, then return true or false, 
false would stop the transition at this point. If more transitions with the 
same event and source state are found in the transition table above this 
one, they would be tried then, which allows you to continue processing, add 
error handling, etc.

HTH,
Christophe