Re: [Boost-users] [MSM] multiple visitors / signatures, and visitors by ref in submachines

Subject: Re: [Boost-users] [MSM] multiple visitors / signatures, and visitors by ref in submachines
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2012-04-06 17:53:20

  Hi Albert, 

  >Hi,
  >
  >I think that the visitors and the flags ideas of the MSM are very generic and easy to use.
  >So, great!
  >
  >But, there is a way to define multiple visitors for a single state (base)?

  No.

  >I think that it is not possible because the visitors mechanism is based on a "simple/single typedef" and not on an "listed typedef"... isn't it?
  Yes.

  >So, why the visitors are not like flags?
  >I mean, why not to use an "accept_sig_list" instead of a single "accept_sig"?
  >Why is not possible a code like the following:
  >
  >struct my_visitable_state
  >{
  >  typedef mpl::vector< args<void, VisitorType1&>,  args<void, VisitorType2&> > accept_sig_list;
  >

  >  void accept(VisitorType1&) const {}
  >  void accept(VisitorType2&) const {}
  >};
  >

  It is possible. It has not been done because the interest in the visitor concept seems to have been (sadly) limited. This feature is dates a bit (about 3 years ago), and I got, if my memory is correct, exactly 2 questions about it (counting his one). This makes improvements get a low priority. Actually the concept could also do with a bit of redesign. For example args<void, SomeVisitor&,int> would look much better if defined as args<void (SomeVisitor&,int)>. If you would like to help, I'm interested ;-)

  >By the way... I think that the usage of boost::ref() is not propagated to the submachines...? 
  >I've found that when my state machine is also in a submachine, the visitor of the state machine is passed by reference, but the
  >submachine is passed always by value... is this a bug?
  >(just using a pointer solved my needs, so just to report...)
  I just fixed it in trunk (rev. 77793). Thanks for the report. 

  >
  >

  >Thanks,
  >
  >

  >Albert

  Cheers,
  Christophe