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

From: Alexander Nasonov (alnsn_at_[hidden])
Date: 2006-12-19 16:15:57

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();
};

Running my_fsm::on_process( id<1>
                          , Initial
                          , integral_c<my_fsm::event,event::Start>
                          ) const
{
    Running result;
    result.data_ptr = &m_data;
    return result;
}

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

-- 
Alexander Nasonov
http://nasonov.blogspot.com
We need not think alike to love alike. -- Francis David --
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