Re: [boost] [msm]Event convert at exit point

Subject: Re: [boost] [msm]Event convert at exit point
From: Takatoshi Kondo (redboltz_at_[hidden])
Date: 2011-10-12 09:39:49

Hi Christophe,

>> Hi Takatoshi,
>>
>> this question has already been asked some time ago in a slightly different
>> context (http://lists.boost.org/Archives/boost/2010/09/170506.php). This is
>> unfortunately one of the points where I had to leave the compile-time world
>> earlier than I would like (but found no better solution yet). While Event1
>> will not be considered for forwarding, the compiler does not know because
>> it's only known at run-time, and it therefore complains that you cannot
>> create Event1 from Event2.
>>
>> The surprise to me is that the compiler is happy with only your second
>> constructor, I'd expect it to require a constructor taking Event1. Should it
>> happen, you will need to use the enable_if / disable_if workaround presented
>> in the above thread.
>>
>> I'll update the doc so that this current limitation is documented until I
>> find a better solution.

Thanks for the reply. I understood current situation.

> Or simplier:
>
> template <class T> Event3(T const& ):val(0) {} // not used
> Event3(Event2 const& e):val(e.val) {} //conversion constructor

Correct. It works well.
In my usecase, I think it's typical, the outer event(Event3) shouldn't
know about
inner event(Event2). In other words, outer event is an interface of a
sub state machine.

When I use the SFINAE solution, the predicate is important.
If I choose the is_same, it still needs inner event information.
template <class T> OuterEvent(T const& t, typename
enable_if<is_same<T, InnerEvent> >::type* = 0);

I considered some approaches.
The first approach is the InnerEvent inherits the OuterEvent and use
the is_base_of.

struct InnerEvent:OuterEvent {};
is_base_of<T, OuterEvent>

And next, I realized that if the InnerEvent inherits the OuterEvent,
we can construct
the OuterEvent from the InnerEvent automatically. It is the slicing.
In my usecase,
I think it works well.

struct OuterEvent {
    int val;
};
struct InnerEvent:OuterEvent {};
struct Exit:msm::front::exit_pseudo_state<OuterEvent> {};
//          Start      Event       Next       Action      Guard
msmf::Row < SubState1, InnerEvent, Exit,      msmf::none, msmf::none >

Regards,
Takatoshi