Re: [boost] boost [msm] deferring event by base type.

Subject: Re: [boost] boost [msm] deferring event by base type.
From: Richard Szabo (sz.richard_at_[hidden])
Date: 2010-06-25 08:53:35

On 25 June 2010 12:59, Juraj Ivančić <juraj.ivancic_at_[hidden]> wrote:
> On 25.6.2010 10:40, Richard Szabo wrote:
>
>> I'm now using Row and Defer functor I have now an other problem whit the :
>>
>> Â  Â  void defer_event(Event const& Â e)
>> Â  Â  {
>> Â  Â  Â  Â  // to call this function, you need either a state with a
>> deferred_events typedef
>> Â  Â  Â  Â  // or that the fsm provides the activate_deferred_events typedef
>> Â  Â  Â  Â  BOOST_MPL_ASSERT(( has_fsm_deferred_events<library_sm> Â ));
>> Â  Â  Â  Â  execute_return (library_sm::*pf) (Event const& Â evt)=
>> &library_sm::process_event;
>> Â  Â  Â  Â  Event temp (e);
>> Â  Â  Â  Â  ::boost::function<execute_return ()> Â f= ::boost::bind(pf,
>> this,temp);
>> Â  Â  Â  Â  post_deferred_event(f);
>> Â  Â  }
>>
>> actually it tries to instantiate the base class ( Event temp (e); )
>> but in my case the base class is an abstract class -> Â compiler error;
>> and after deferring I want to dispatch the original derived event not
>> the copy constructed base class instance of the event.
>
> You might try the following:
>
> Instead of making your event class polymorphic, you add a shared pointer to
> polymorphic data. E.g.
>
> struct deferred_event
> {
> Â  Â boost::shared_ptr<deferred_event_data_base> event_data;
> };

I don't understand how this would help me with the differing based on
base type ?

Can you please provide a little example based on something like this ?

  struct BaseEvent
  {
    virtual ~BaseEvent() = 0;
  };

BaseEvent::~BaseEvent() {}

struct AEvent : public BaseEvent
{
};

struct BEvent : public BaseEvent
{
};

will not compile :
Row < State1, BaseEvent, none, Defer, none  >

you suggesting  :

struct deferred_event
{
 Â  Â boost::shared_ptr<BaseEvent> event_data;
};

now as soon as I assign AEvent to sharedPointer to BaseEvent I loose
the event type and I have to use static cast to actually get back the
AEvent.

and I don't see how deferred_event would fit together with other rows
which are using AEvent of BEvent ?

 I think he only think it would solve is avoiding the copy of
even_data during event queuing. And gives us problems with deletion if
the object is created by a factory and removed by a recycle. because
you can create  the shared pointer like this :
boost::shared_ptr<AEvent> mySPtrAEvent(myFactory::factory<AEvent>());
when there is a static factory method exist and returning a pointer to
the AEvent  for event creation. but how you call instead of delete
whit shared pointers something like myFactory::recycle(pAEevent) ?

thanks for your help !!!.
Br.
Richie

>
> This would solve both issues as you would have a single event to be
> deferred, and it would be copyable but would still retain polymorphic
> data needed to process event.
>
> HTH
>
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