Re: [Boost-users] [statechart] Working with state history - compilation errors

From: Mick Hollins (mick_at_[hidden])
Date: 2005-10-17 08:43:51

Andreas Huber wrote:
> Hi John
> 
> <john.wismar <at> autozone.com> writes:
> 
>>I'm playing with the statechart library,
>>in conjunction with Boost 1.33 and VC 7.1.  I'm having trouble using
>>the state history functionality; I can't get it to compile, even taking
>>into account the useful static assertions that ought to be helping me.
>> Can someone suggest what I might be missing?Here's a boiled down sample: 
> 
> there
> 
>>are 2 top-level states, StateA and StateB.  Event2 transitions between
>>them.  When in StateA, Event1 transitions between 2 sub-states, StateA1
>>and StateA2.  When transitioning from StateB to StateA, I want to
>>use StateA's history to determine which of StateA1 and Stae2 become active.
> 
> 
> I'm very sorry for the late answer, but I missed your post back in September 
> because it was sent in reply to an unrelated post (Boost Graph Library).
> 
> Mick's analysis of the problem is correct. When you make a transition to 
> history then you must specify the *default* *state* that is entered in case 
> the history has never been saved before. The *outer* *state* of the default 
> state must then pass the appropriate has_xxx_history.
> 
> Note that this is very much in line with the UML notation. A transition to 
> history always goes to a H disc first and from that H disc you have another 
> arrow to the default state.

Initially I thought it was not quite in line with UML since the boost::statechart implementation allows a different 
default arrow to be specified for each transition to a H disc, whereas UML only allows a single arrow out of a H disc.

But I just realised that afaict UML does not disallow having more than one H disc in a parent state. As such you can 
have 2 H discs each with a different default state, and boost::statechart supports that.

> The state containing the H disc is said to be the 
> one "having history".
> 
> 
>> From reading the tutorial, references, and sample code, it looks
>>like this should work:
> 
> [example snipped]
> 
> I checked the docs and I can't find any examples that display the wrong syntax 
> you used. A pointer would be appreciated.

I don't believe your docs are wrong. It is just that it is easy to misread them as it seems natural to expect that you 
would say:

     sc::shallow_history<StateA>

since you are requesting a transition to one of the sub-states of StateA. Saying:

     sc::shallow_history<StateA1>

just reads a little funny since you may or may not be transitioning to StateA1. The following, while redundant, might 
actually be easier for people to read:

      sc::shallow_history<StateA, StateA1>

where the first parameter specifies the parent state to which you are performing a history transition, and the second 
parameter specifies the default state.

cheers,
mick