Nullable object (closes #363)

[eddelbuettel]

This (simple) PR has been tested in pending code that will may go into mvabund in a few days. (I may use a copy there to not have to rely on the very newest Rcpp there.) But as the feature is generally useful (and I also already spotted use cases for it in RcppDE).

The basic need was described in #363, and we had a lively discussion there. @dcdillon and went back and forth over some design with a templated class but this doesn't quite work: the is circular as we need a NULL protection layer around our proxy objects. As soon as we template with them, access when NULL leads to an instantiation which ... throws as before. Similarly, I got nowhere with the safe_bool_idiom approach.

So what is here works, but (as always) could be improved if someone is so inclined. I added a few tests to describe how it is used.

[dcdillon]

Given that isNull() throws if it has never been set, should you also add an isSet() function? This would give the user the ability to poll for the "third state" without resorting to exception handling.

[eddelbuettel]

You could you be "not set" but "not null" ?

Wait. I think I get it now. That is a good idea. Adding ...

[kevinushey]

Should this be checkIfSet()?

[eddelbuettel]

Absolutely. Now fixed.

I blame @dcdillon for enticing me to make changes, which I then changed midflight... ;-)

[kevinushey]

My only thought: the class name Nullable does not really evoke the type of the underlying object; e.g. I would much rather see Nullable<SEXP> to indicate that this is an object wrapping a SEXP that may be either unset or R_NilValue. It would be even better if this were a template class, e.g. Nullable<T> where T could be any Rcpp class type.

EDIT: Just saw your comment re: why you didn't go that route... it's a shame; I wonder if there's some other fix.

I'm still not quite sure on the utility -- can you provide some other examples where this would be used? (The example in #363 was a bit bare bones; a full example would be useful)

[kevinushey]

nit: probably don't need void in the parameter list

[eddelbuettel]

Nullable<SEXP> could work whereas Nullable<RcppANYTHING> does not seem to work given how our types are set up -- they bark at run-time when you inquire about their state and the payload was NULL.

One use case is in this PR where I carried a local copy over. We can now do this in a compact way whereas the current code is much uglier -- and exposes a SEXP in the nice Rcpp Attributes interface.

Suggestions for finer tuning welcome. What is there now is so far good enough for me.

[kevinushey]

Okay -- LGTM!

[eddelbuettel]

Thanks. @dcdillon and I went back and forth on this. We both really wanted Nullable<T> but you just instrument it as Nullable<NumericMatrix> and then pass NULL at run-time. It gets you back to NULL being used to instantiate NumericMatrix :-/

Maybe it can still be done -- suggestions welcome. We now have a few units describing what this should be good for, and one test use over in [RcppGSL] -- and those may help provide a framework for something better. But for now I'd say let's put this one too.

[eddelbuettel]

Ok, being stubborn helps sometimes.

We had another crack at this and @dcdillon spotted a really bad NULL that really wanted to be a R_NilValue plus a few more little things -- so we WILL have a templated version coming.