From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2008-08-20 00:04:18
David Abrahams wrote:
> on Tue Aug 19 2008, Andrey Semashev <andrey.semashev-AT-gmail.com> wrote:
>
>> and, in fact, is more limiting, because it doesn't allow to specify
>> rules that apply to a subset of states and events. A small example
>> from the docs.
>>
>> // Let's define the fsm::transition's analogue that triggers
>> // not only when the event type is equal to the specified,
>> // but even when it is derived from it.
>> template<
>> typename CurrentStateT,
>> typename EventT,
>> typename TargetStateT
>> struct same_or_derived_event_transition :
>> public fsm::basic_transition< TargetStateT >
>> {
>> // We only need to specify the static predicate
>> // that selects the transition
>> template< typename StateT, typename EvtT >
>> struct is_applicable :
>> public mpl::and_<
>> // We should check that state type is correct
>> mpl::or_<
>> is_same< CurrentStateT, fsm::any_state >,
>> is_same< CurrentStateT, StateT >
>> >,
>
> Isn't the above "or" clause always true, and if not, why not?
No, it's not. The FSM instantiates is_applicable on every state and
every event type. The above condition yields true only for the selected
state (CurrentStateT), unless CurrentStateT is any_state. If it is, then
yes, the above "or" is true for all states.
>> // Now we shall check the event type
>> mpl::or_<
>> is_same< EventT, EvtT >,
>> is_base_and_derived< EventT, EvtT >
>> >
>> >
>> {
>> };
>> };
>>
>>>> In fact, I find your syntax more verbose for the unneeded fourth
>>>> column of the table.
>>> It's only unneeded if you think it doesn't convey information that's
>>> important to understanding the FSM. From my POV, it does. That's why
>>> arcs in an FSM diagram are often labeled with their associated
>>> transition actions. How does a user of your library associate actions
>>> with transitions if they don't go in the table?
>> The action is totally defined by the event and the current state.
>
> I don't see how that answers my question.
>
>> Handler names are always the same and thus need not to be present in the
>> table.
>
> Do you mean to say that every transition action is named the same and
> the one to execute is chosen by overload resolution?
Yes. Every transition rule (or a row in STT) has one or more "transit"
member functions that accept references to the current state and the
event being processed. So, when a particular transition rule is chosen,
the library simply calls the "transit" member in that particular rule.