Probably not "intentional". There is some encoding support in String.h ie some constructors have an second argument, Without it, maybe we are dropping infromation on Windows with is known to be difficult. I don't have good Windows access but maybe you try variants of test11() and test2() which call set_encoding() to test this?

I don't have access to a Windows machine either, but I see the encoding change from "UTF-8" to "unknown" on OS X, it just doesn't cause any downstream problems.

I looked at the Rcpp code and noticed two things:

1. In the copy constructor, I would have expected the encoding to just be copied over from other:
   

   Rcpp/inst/include/Rcpp/String.h

   Line 61 in 1789f09

   String(const String& other) : data(other.get_sexp()), valid(true), buffer_ready(false), enc(Rf_getCharCE(other.get_sexp())) {

2. In the assignment operator, encoding isn't set at all:
   

   Rcpp/inst/include/Rcpp/String.h

   Line 155 in 1789f09

   inline String& operator=(const String& other) { data = Rcpp_ReplaceObject(data, other.get_sexp()); valid = true; buffer_ready = false; return *this; }

   
   (In fact, none of the assignment operators set the encoding.)

I think this was a deliberate design choice for Rcpp::String circa 2015/6, c.f. original ticket #263, PR #310, and PR #529 that set defaults to CE_UTF8.

@clauswilke Re 1) Do you want to try that modification, maybe in the most careful that we'd enable it with an option? [Change management is hard with 1600+ client packages. Re 2): Unsure, but we had a number of discussions and attempts on that.

@coatless Thanks for digging up those tickets.

With greetings from Iceland...

Maybe we talk a little more about what the design exactly should be. I'm not sure I know enough about encoding and R internals to have a strong opinion. However, below I provide another reprex, now purely C++, that I'd argue is confusing to a downstream user. I create a string with defined encoding and then make a copy of that string, and the encoding changes.

#include <Rcpp.h>
using namespace Rcpp;

#include <string>

void output_encoding(const String &s, const std::string &name) {
  Rcout << "String " << name << " has encoding: ";

  cetype_t enc = s.get_encoding();

  switch(enc) {
  case CE_NATIVE:
    Rcout << "native" << std::endl;
    break;
  case CE_UTF8:
    Rcout << "utf8" << std::endl;
    break;
  default:
    Rcout << enc << std::endl;
  }
}

// [[Rcpp::export]]
void test_encoding(){
  String s("abcdabcd", CE_UTF8);
  output_encoding(s, "s");

  String s2(s);
  output_encoding(s2, "s2");

  String s3 = s;
  output_encoding(s3, "s3");
}

/*** R
test_encoding()
*/

Output:

Rcpp::sourceCpp("~/Desktop/test.cpp")
#> 
#> > test_encoding()
#> String s has encoding: utf8
#> String s2 has encoding: native
#> String s3 has encoding: native

^{Created on 2019-08-12 by the reprex package (v0.3.0)}

I am with you on things being surprising at times, but as far as I know (and there are areas of R I feel I know more about) the whole charactter is a mess particularly once Windows comes in so in that sense maybe the first recommendation (which we could make more public in the Rcpp FAQ) is to stick to CharacterVector instead. R added a lot of elbow grease to make this work for R, the best may just be to free ride as much as we can. String is fine, but may fall short on some of these corner cases. My $0.02.

Is it possible to take a specific element from a CharacterVector and stick it into a new CharacterVector without going through a String intermediate? It's not immediately obvious to me how, but I'm still learning Rcpp and may be missing something.

You can. As always, devil in the detail. Got my first attempt wrong, then cheated as usual and looked at the existing unit tests.

> cppFunction("CharacterVector getNth(CharacterVector v, int i) {   ## manual indent 
                              CharacterVector n(1); n[0] = v[i]; return(n); }")
> getNth(LETTERS, 2)
[1] "C"
> 

I can confirm this keeps the encoding intact.

Rcpp::cppFunction("CharacterVector getNth(CharacterVector v, int i) {
                              CharacterVector n(1); n[0] = v[i]; return(n); }")
x <- "special char: \u03bc"
getNth(x, 0)
#> [1] "special char: μ"
Encoding(x)
#> [1] "UTF-8"
Encoding(getNth(x, 0))
#> [1] "UTF-8"

^{Created on 2019-08-12 by the reprex package (v0.3.0)}

Great! It's a stop-gap.

In theory, there is no reason why String could not do the same.
In practice, it may be a beast to get there.