Re: [boost] [msm]omitting region id at the entry pseudo state

Subject: Re: [boost] [msm]omitting region id at the entry pseudo state
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2012-03-04 16:25:27

Hi Takatoshi,

> Hello,
>
> Now, I'm using Boost.Msm (Version1.49.0).
> I have a question about a transition behavior when I omit a region id
> at the entry pseudo state.
>
> The document said that when we use the explicit entry, if the state
> machine satisfied the conditions below, we can omit the region id.
> http://www.boost.org/doc/libs/1_49_0/libs/msm/doc/HTML/ch03s02.html#d0e852
> " Thanks to the mpl_graph library you can also omit to provide the
> region index and let MSM find out for you. The are however two points
> to note:
> - MSM can only find out the region index if the explicit entry state
> is somehow connected to an initial state through a transition, no
> matter the direction.
> - There is a compile-time cost for this feature. "
>
> Because of the discussion below, I believe that this rule can also be
> applied to entry pseudo states.
> http://boost.2283326.n4.nabble.com/msm-how-to-distinguish-region-ids-tt3612968.html#a3622072

It does, I just forgot to mention it in the doc.

> However, when I omitted the region id, the state machine transitioned
> to the SubState1.
> struct Entry1:msmf::entry_pseudo_state<> {};
>
> If I provided the region id, the state machine transitioned to the 
> SubState2.
> struct Entry1:msmf::entry_pseudo_state<0> {};
>
> See entry_pt_omit_rid.cpp (Attached file)
>
> As far as I can understand, Entry1 and SubState1, initial_state, are
> connected. (See fig.png)
>
> Is there any reason that I can't omit the region id in this case?

Looks like a bug. It's a bit late to fix it tonight, but if I change your 
initial state declaration to:

typedef mpl::vector<SubState1> initial_state;

Then it works. Must be some silly mistake and not too hard to fix, but you 
can use the workaround in the meantime.

Cheers,
Christophe