as_cpp<Enum>

[romainfrancois]

It would be nice to have some mechanism to move enums back and forth between R and C++. I'm not sure what as_sexp() would do but as_cpp<Enum> could be:

namespace cpp11 {

template <typename E>
typename std::enable_if<std::is_enum<E>::value, E>::type as_cpp(SEXP from) {
  return E(cpp11::as_cpp<typename std::underlying_type<E>::type>(from));
}

}

If this is of interest, I'll polish and make it a proper pull request.

In Rcpp we had to use the RCPP_EXPOSED_ENUM_NODECL macro to declare each enum we wished to expose, manually, which sets some templates for Rcpp::as<> and Rcpp::wrap<>. IIRC, Rcpp cannot use the is_enum trait because it's a C++11 thing. https://github.com/RcppCore/Rcpp/blob/f3c5a34e06e774532227470b01c63a8f08ce4313/inst/include/Rcpp/macros/module.h#L76

[jimhester]

Sure, that seems reasonable to me.

In the past I have represented enums in R as named integers.

But I don't think it is possible to do that automatically in cpp11 until C++ has support for reflection over enums, so just doing this one way seem fine to me.

A PR with some basic tests in https://github.com/r-lib/cpp11/blob/master/cpp11test/src/test-as.cpp would be welcome!

[bkietz]

I'm not sure what a named integer is. However it is possible to extract string names of types, globals, and enum members:

enum class ValueKind { NULL_, BOOLEAN, NUMBER, STRING, ARRAY, OBJECT };
constexpr ValueKind kDocumentKind = ValueKind::OBJECT;
assert(nameof<ValueKind>() == "ValueKind");
assert(nameof<ValueKind, ValueKind::BOOLEAN>() == "BOOLEAN");
assert(nameof<ValueKind, kDocumentKind>() == "kDocumentKind");

Would that be enough reflection to accomplish named integers ergonomically?

EDIT: enum member names can't be extracted on all compilers; looks like clang can and gcc can't. Haven't tried msvc or others