Re: [boost] [msm] Passing constructor arguments to submachine

Subject: Re: [boost] [msm] Passing constructor arguments to submachine
From: David Abrahams (dave_at_[hidden])
Date: 2010-06-02 18:16:33

At Wed, 2 Jun 2010 22:54:27 +0200,
Christophe Henry wrote:
>
> I didn't think about Parameter and I like the idea (I also see
> somewhere else where it could be useful) but I :
> 
> (a) don't have any parameter name to give to BOOST_PARAMETER_NAME
> (parameters being simply forwarded with BOOST_PP_ENUM_PARAMS)
> (b) don't know at preprocessor time in BOOST_PARAMETER_CONSTRUCTOR how
> many parameters the user will pass through the back-end to the
> front-end constructor or how many submachines/substates will be
> default-constructed by user's choice.
> 
> Concretely, the user can now do something like:
> player p(/* for the front-end*/ 3, "a");
> 
> And adding states for copy-construction could look like:
> player p(/* for the front-end*/ 3, "a",
>               /* some states copy-constructed*/  Paused("something") ,
> Playing("more data")  );
> 
> Of course I could provide some default names and create more
> constructor overloads with BOOST_PP_ENUM_PARAMS.
> 
> I was actually thinking of using a proto expression like:
> player p( (/* data for front-end*/params_ << 3 << "a" ),
>                 (/* some states copy-constructed*/ states_ <<
> Paused("something") << Playing("more data") ));
> 
> But I admit having not spent enough thinking time yet so I will look
> into it again.
> 
> Thanks for the idea, Dave. It will in any case be pretty cool to use
> Parameter to make back-end policies nicer.

Please take a close look at Boost.Accumulator; it has to solve
essentially the same problem IIUC, and it does so quite nicely.  You
can't use the high-level macros like BOOST_PARAMETER_CONSTRUCTOR in
the template that has to do the forwarding, though.  I think it might
even improve readability to do make your life easy and accept a single
argument pack (backend_args in the example below) for forwarding to
submachines

  player p(/* for the front-end*/ 3, "a",
              backend_args = (
                 init1=Paused("something"),
                 init2=Playing("more data"))
              );

or something like that.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com