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

Subject: Re: [boost] [msm]Event convert at exit point
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2011-10-10 15:54:44

> Hello,
>
> I have a question about Boost.Msm event conversion at exit pseudo state.
> (Version 1.47.0)
>
> http://www.boost.org/doc/libs/1_47_0/libs/msm/doc/HTML/ch03s02.html#d0e942
>
> Exit pseudo states
>
> struct event6
> {
>    event6(){}
>    template <class Event>
>    event6(Event const&){}
> }; //convertible from any event
>
> Above document said that we can convert the event using the template
> constructor. This exmaple doesn't access the constructor's parameter.
>
> I want to propagate some value of the event. Event2 is a source event.
> And Event3 is a target event like this.(see attached file)
>
>    struct Event2 {
>        Event2(int val_):val(val_) {}
>        int val;
>    };
>
>    struct Event3 {
>        Event3(int val_):val(val_) {}
>        template <class T> Event3(T const& e):val(e.val) {} // error
>        Event3(Event2 const& e):val(e.val) {} // OK
>        int val;
>    };
>
>    // Transition table
>    struct transition_table:mpl::vector<
>        //          Start      Event       Next       Action      Guard
>        msmf::Row < SubState1, Event2,     Exit,      msmf::none, 
> msmf::none >
>    > {};
>
>    struct Exit :msm::front::exit_pseudo_state<Event3> {};
>
> But compile error occurred. The compiler said Event1 doesn't have a member 
> 'val'.
> I'm not sure why Event3's constructor instantiate from Event1. This is the 
> question.
> If I define the constructor that has a parameter Event2 instead of 
> template
> parameter, compile error doesn't occur.
>
> exit_pt_convert.cpp(attached file) is a source code that reproduces this 
> error.
>
> Thanks,
> Takatoshi
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.

Regards,

Christophe