Re: [boost] Proposal to add a Boost.FSM library

From: Andrey Semashev (andysem_at_[hidden])
Date: 2006-12-20 16:08:19

Hello Alexander,

Wednesday, December 20, 2006, 12:15:57 AM, you wrote:

> Andrey Semashev wrote:
>> A pointer to the state's data doesn't help if the state is actually
>> destroyed when being left (the pointer is freed and the data is lost).
>> And storing such pointer in a place somewhere out of the state (i.e.
>> in a base state, a state machine or its some common storage like
>> virtual bases in my implementation) means that data is exposed
>> and is no longer specific to the state. IMO, this breaks the natural
>> state encapsulation that is one of the main ideas of FSM concept.

> struct my_fsm : fsm<my_fsm>
> {
>     // Events
>     enum event { Start, Pause, Stop };

> private:
>     struct Data { /* ... */ };

>     // States
>     struct Operational { Data* data_ptr; };
>     struct Paused  : Operational {};
>     struct Running : Operational {};
>     struct Stopped {};
>     struct Initial {};

>     Data m_data; // Paused and Running will point to it.

>     Running on_process(id<1>, Initial,     integral_c<event,Start>) const;
>     Paused  on_process(id<2>, Running,     integral_c<event,Pause>) const;
>     Running on_process(id<3>, Paused,      integral_c<event,Pause>) const;
>     Stopped on_process(id<4>, Operational, integral_c<event,Stop> ) const;
>     // Last function is most interesting because it defines transitions
>     // for all states derived from Operational.

> public:
>     my_fsm();
> };

[snip on_process]

> In this example, all states and state data are private.

This is not quite what I meant.
In your example the Data structure holds variables specific to Paused
and Running states (let's assume the Operational state is only used as
a simple base class, not a final state). But instead of being hidden
inside of them it becomes the whole machine's member. Had I ten states
with their specific data, the my_fsm class would get too crowded.
Besides, nothing prevents you to access or modify this data from, say,
"Running on_process(id<1>, Initial, integral_c<event,Start>) const;"
handler which has nothing to do with neither Operational nor Running
or Paused states.

From my point of view a state should be a part of the state machine
since it may contain some specifiñ data. It may not be a lightweight
compound and it may want to keep its internal state through the whole
machine's lifetime, which may be long enough and even cyclic.

-- 
Best regards,
 Andrey                            mailto:andysem_at_[hidden]