Return a zero length vector default constructed are converted to SEXP

When we default construct writable vectors we set them to R_Nilvalue, so
the default constructor cannot throw an exception. However this causes
problems if you then immediately return them. By checking for this
condition in the SEXP conversion function we can return a 0 length
vector in these cases, which is what the user likely intended.

Fixes #166

NEWS.md

@@ -1,5 +1,6 @@
 # cpp11 (development version)
 
+* `writable::r_vector` default constructors now return a 0 length vector when converted to `SEXP` (#166)
 * read-only `r_vector` constructors now disallow implicit construction with named arguments (#237)
 * read-only `r_vector.attr()` methods now return const objects, so it is a compile time error to try to assign to them (#237)
 * Allow `cpp11::matrix` to be accessed either row-wise or column-wise (the default) depending on the user's choice (@alyst, #229)

cpp11test/src/test-list.cpp

@@ -154,4 +154,12 @@ context("list-C++") {
     cpp11::strings nms(x.names());
     expect_true(x.size() == nms.size());
   }
+
+  test_that("We don't return NULL for default constructed vectors") {
+    cpp11::writable::list x;
+    SEXP y(x);
+
+    expect_true(Rf_xlength(y) == 0);
+    expect_true(y != R_NilValue);
+  }
 }

inst/include/cpp11/r_vector.hpp

@@ -900,8 +900,12 @@ inline SEXP truncate(SEXP x, R_xlen_t length, R_xlen_t capacity) {
 
 template <typename T>
 inline r_vector<T>::operator SEXP() const {
+  auto* p = const_cast<r_vector<T>*>(this);
+  if (data_ == R_NilValue) {
+    p->resize(0);
+    return data_;
+  }
   if (length_ < capacity_) {
-    auto* p = const_cast<r_vector<T>*>(this);
     p->data_ = truncate(p->data_, length_, capacity_);
     SEXP nms = names();
     auto nms_size = Rf_xlength(nms);