[boost] Re: Storing variables in states [was: Re: RE: Re: overloads-20050207.tar.gz]

From: David Abrahams (dave_at_[hidden])
Date: 2005-02-14 10:13:34

"Andreas Huber" <ahd6974-spamgroupstrap_at_[hidden]> writes:

> David Abrahams wrote:
>> "Andreas Huber" <ahd6974-spamgroupstrap_at_[hidden]> writes:
>>
>>> You very often encounter the situation that you need a variable that
>>> is only used in a part of a state machine (i.e. only one state or a
>>> few states need access to it). IMO, it is good design to confine the
>>> lifetime of such a variable so that it will not even exist when the
>>> machine does not happen to reside in the states that need access to
>>> the variable. Even for a very tiny state machine like the StopWatch
>>> in my tutorial it made sense to limit the lifetime of one variable.
>>> See http://tinyurl.com/5q9hk (State-local storage) for details.
>>
>> But as I pointed out and -- you seem to agree -- that variable must
>> often persist through several states to which it applies.
>
> Yes, often.
>
>> In that case, if you want the lifetime to change based on those
>> states, you can't store the variable in the state object.
>
> Not in an innermost state, no. But you can simply store the variable in 
> an outer state that contains all the states that need to access the 
> variable. 

Can't we call that thing "the state's associated data object?"
Calling it "the state" gives me the willies.  It goes against the
grain of the English meaning of state (singular, not substance) and
also the one used in the FSM abstraction.

> More often than not the outer state already exists for other 
> reasons (e.g. reactions that all inner states have in common). E.g. in 
> the StopWatch example, the Active state not only exists so that it can 
> store the elapsedTime_ member but also because there is a 
> self-transition that can be triggered when the machine is in either the 
> Stopped or Running states.

Sorry, details please?  Where is this example?

>> boost::optional in the
>> state machine itself is the proper generalization of the idea.
>
> It makes things global that should rather be left local. Below, as a 
> counter-measure, you devise a system that does away with one aspect of 
> the globality (the lifetime). But, other negative aspects of the 
> globality remain, i.e. you still have to change the FSM class when you 
> want to introduce a new variable or change an existing one.

Fair enough.  It could also be done by non-intrusive specialization:

  template <>
  struct associated_data<
        player
      , vector_c<player::state, some-list-of-states>
  >
  {
       typedef some-data-type type;
  };
 

But I admit that's getting unweildy.

>>>> it doesn't match up with the domain abstraction of state
>>>> machines.
>>>
>>> How do you come to that conclusion?
>>
>> I have never seen a formal description of the FSM abstraction that
>> described states as carrying additional data inside them.  Have you?
>
> No I haven't. To me this just follows naturally and at least a few 
> boost::fsm users seem to agree. I think it is awkward to store variables 
> in the state machine object, where they can be accessed from states that 
> are not supposed to access them. Of course this can be managed by the 
> framework as you suggest below, but the FSM class still becomes a change 
> hotspot. To me, such hotspots are an indication that there is something 
> wrong with the design. Moreover, parameterized states (aka submachines, 
> which are reusable FSM building blocks) are hard to implement without 
> state-local storage.

Do you agree with Mr. Nasonov that outer states should be implemented
as base classes of inner states?  Because I don't see how the data
lifetime issues work out if you do that.  Transitioning from INNER1 to
INNER2 still causes all the data members of OUTER to be destroyed,
because the INNER1/2 instances have distinct OUTER sub-objects.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com