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

Subject: Re: [boost] [msm] Passing constructor arguments to submachine
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2010-06-03 13:37:47

>> 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.

I like this syntax too. Ok then I will give it a try and have a closer
look at Accumulator.
Thanks!