boost statechart pass arguments with transition

Question

I'm trying to learn boost::statechart.

I want to make a little app which loads a file.

//  --------------------------------
// |                                |
// |           O     Project        |
// |           |                    |
// |           v                    |
// |  ----------------------------  |
// | |                            | |
// | |         Unloaded           | |
// |  ----------------------------  |
// |  |              ^              |
// |  | EvLoad       | EvUnload     |<-----O
// |  v              |              |
// |  ----------------------------  |
// | |                            | |
// | |         Loaded             | |
// |  ----------------------------  |
// |           |   ^                |
// |           |   | EvLoad         |
// |           -----                |
//  --------------------------------

But how do i transport arguments to the state, e.g. a filename? If i store the filename inside EvLoad i can access it easily for the in state reaction

struct Loaded : sc::simple_state< Loaded, Project>
{
    typedef sc::custom_reaction< EvLoad > reactions;
    sc::result react( const EvLoad & e )
    {
        //load file e.path()
        ...
        return discard_event();
    }
}

But when I'm in the Unloaded state then I'm invoking the constructor of Loaded and i can't pass arguments to it. The only workaround I came up with is reposting the event before doing the transition, but this looks a bit dirty to me.

struct Unloaded : sc::simple_state< Unloaded, Project >
{
    typedef sc::custom_reaction< EvLoad > reactions;
     sc::result react( const EvLoad & e )
     {
         post_event( e ); //workaround to pass the event to the loaded state
         return transit<Loaded>();
     }
};

Is there a better alternative?

Answer 1

We use the triggering_event method to pull the triggering event, and then just attach the data as member variables to triggering event. It saves a lot of coding effort and keeps us from having to generate custom reactions or attaching transition variables to the statechart (two common approaches I've seen).