Re: [boost] [MSM] passing constructor arguments to frontend asreference.

Subject: Re: [boost] [MSM] passing constructor arguments to frontend asreference.
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2010-05-06 15:27:06

> Hi, All
>
> we might found a problem when passing constructor arguments to the
> front end in case constructor arguments are passed by reference.
> In that case you do not pass the argument as reference in the first
> place rather copy construct a new object and pass that object as
> reference to the front end.
> We looked into the preprocessor code which we believe verified our assumption:
>
> (code snippet only contains the first part with one constructor argument!)
>
> template < class ARG0>
> state_machine<Derived,HistoryPolicy,CompilePolicy >( ARG0 t0 )
> :Derived( t0) ,m_events_queue() ,m_deferred_events_queue()
> ,m_history() ,m_event_processing(false) ,m_is_included(false)
> ,m_visitors() ,m_substate_list() { ::boost::mpl::for_each<
> seq_initial_states, ::boost::msm::wrap<mpl::placeholders::_1> >
> (init_states(m_states)); m_history.set_initial_states(m_states);
> fill_states(this); }
>
>
> In our case, if we want to pass a reference, ARG0 has to be declared as follows:
> template < class ARG0>
> state_machine<Derived,HistoryPolicy,CompilePolicy >( ARG0& t0 )
> :Derived( t0) ....
>
> The "Derived" class looks like as follows:
> class "Derived"
> {
> public:
>  Derived(SomeClass& cl_) {}
> }
>
> Do you have any idea how to solve this?
>
> Thanks in advance.
>
> Richie & Michi

Hi,

It's a classical issue with template arguments and something I
definitely should have written in the doc.
To pass arguments by reference, you need to use boost::ref (or
boost::cref for const references), for example:

// The "Derived" class looks like as follows:
 class Derived : public msm::front::state_machine_def<Derived>
 {
 public:
  Derived(SomeClass& cl_) {}
...
 }

typedef msm::back::state_machine<Derived> MyFsm;

SomeClass stuff;
...
MyFsm fsm (boost::ref(stuff));

Thanks for pointing this out, I will update the doc.
Note: I fly tomorrow to the BoostCon, so I might be a bit less
responsive (jetlagged) the first few days.

HTH,
Christophe