Fixed missing Rcpp namespace in export interface generation

[coatless]

This PR adds the Rcpp:: namespace prefix to an as<>() call in the Rcpp Attributes generation procedure for a C++ interface. e.g.,

// [[Rcpp::interfaces(r, cpp)]]

In particular, appending Rcpp::, fixes the following errors:

error: expected '(' for function-style cast or type construction
            throw Rcpp::exception(as<std::string>(rcpp_result_gen).c_str());
                                     ~~~~~~~~~~~^
error: no member named 'c_str' in 'Rcpp::RObject_Impl<PreserveStorage>'
            throw Rcpp::exception(as<std::string>(rcpp_result_gen).c_str());
                                                 ~~~~~~~~~~~~~~~~~ ^

[eddelbuettel]

Can you remind me please in which issue ticket we discussed this?

[eddelbuettel]

OTOH seems harmless and minimal enough to not even warrant a ChangeLog or NEWS entry. I mean it add what, six characters in one place?

[coatless]

We didn't. I avoided filing one as it is a one-line fix isolated to cross-package exports with:

// [[Rcpp::interfaces(r, cpp)]

But, yes. It literally added six characters that were missing, which prevented the compilation from working correctly.

[eddelbuettel]

Out of curiousity, how did yout tickle this? That line has probably been three a while. I don't often set a cpp interface but as I recall I do in at least one package... Will just merge.

[coatless]

I was using cpp interface to share code between multiple packages. The generation under the first package went okay, but while trying to generate an interface with the second package an error was raised.

[eddelbuettel]

Nice. "Gallery post" ? I don't think we have usage examples of that.