Added as() and clone() to Nullable

[dcdillon]

Fixes #421

[eddelbuettel]

LGTM

[dcdillon]

So technically the call is just AWFUL it:

• Shallow copies in the return from Rcpp::as()
• Deep copies inside the `Rcpp::clone()' function
• Shallow copies in the return from Rcpp::clone()
• Shallow copies in the return from Nullable::clone()
• Probably shallow copies again when the end user receives it.

There should really be a better way. Best I can come up with is a SEXP Rcpp::clone(SEXP obj) function which would remove the need for some of this.

Any other thoughts?

[eddelbuettel]

Don't we usually call wrap() to convert to a SEXP which would make feeding in clone() more straightfoward?

[dcdillon]

OK. It's a lot better now. We keep it as a SEXP until return time. So now it's down to:

• Construct T
• Copy in return

Much more satisfying.

[dcdillon]

And same performance tweak to as(). Now I think it's good.

[eddelbuettel]

Well, nobody cared to second this but on the upside nobody howled either :)

Planning to fold this in later this morning.

[dcdillon]

No problem. It's just convenience stuff.

[dcdillon]

Added isUsable() to indicate whether or not the Nullable<T> can be used as a T. This is equivalent to checking isSet() followed by !isNull() but won't throw an exception.

[eddelbuettel]

No one shouted so will fold this one now.