Add _event system variable if string events enabled

[sstiller]

Adding variable _event https://www.w3.org/TR/scxml/#SystemVariables
with the contents name and data. https://www.w3.org/TR/scxml/#InternalStructureofEvents
_event.name contains the event name as std::string
_event.data is a boost::any (new dependency)

The support for _event currently only works for string events. _event.data could be added to the other version of dispatch() later.

_event.data is really great to update the data model in scxml or for guard conditions for transitions.

[jp-embedded]

Great addition ☺️

I would prefer to use std:any instead boost:any to have a dependency on c++17 instead of boost

[sstiller]

In my targets I can only use C++14 (too old compilers).
Perhaps we can add a compiler switch.

[jp-embedded]

Yes, then both options are available. I think we should make the c++17 option the default because c++17 eventually will become more and more common. Also, we already have the c++17 dependency when using the --threadsafe option, where boost is a new one.

[sstiller]

Would you prefer

• a cmdline option for scxmlcc to specify the wanted C++ version or
• always adding #if __cplusplus >= 201703L to the generated code to use boost::any or std::any
  Then the type changes with the used c++ version automatically, what may not be wanted by the user.

Or do you have another idea?

[jp-embedded]

I think I prefer a 'use boost instead c++17' option rather than using the ifdef, because its a matter of using/not using an external library more than enabling/disabling a feature that requires c++17

Also, it could end up with a lot of ifdefs with the other method because the threadsafe option has the same issue which also requires c++17 (this could also be implemented with boost as an alternative)

[jp-embedded]

Thanks a lot :-)