fix ListOf compile by not relying on get__() in attribute accessors

AttributeProxyPolicy is a base class whose CLASS template parameter
need not provide get__() (that comes from the storage policy). ListOf
inherits AttributeProxyPolicy without a storage policy, so hasAttribute()
and attributeNames() failed to compile for it. Use the operator SEXP()
conversion every such class provides instead, matching the existing
R_getAttribNames() call. Fixes #1483.

Author: kevinushey

inst/NEWS.Rd

@@ -36,6 +36,9 @@
       has been fixed (Kevin in \ghpr{1475} fixing \ghit{1474})
       \item The \code{Nullable::operatorT()} has been added as a 'opt-out'
       (Dirk in \ghpr{1477} with coordination in \ghit{1472})
+      \item The attribute accessors in \code{AttributeProxyPolicy} no longer
+      rely on \code{get__()} so that classes such as \code{ListOf} compile
+      (Kevin in \ghpr{1484} fixing \ghit{1483})
     }
     \item Changes in Rcpp Documentation:
     \itemize{

inst/include/Rcpp/proxy/AttributeProxy.h

@@ -86,7 +86,7 @@ class AttributeProxyPolicy {
             v.push_back(std::string(CHAR(STRING_ELT(attrs, i))));
         }
 #else
-        SEXP attrs = ATTRIB( static_cast<const CLASS&>(*this).get__());
+        SEXP attrs = ATTRIB( static_cast<const CLASS&>(*this) );
         while( attrs != R_NilValue ){
             v.push_back( std::string(CHAR(PRINTNAME(TAG(attrs)))) ) ;
             attrs = CDR( attrs ) ;
@@ -97,7 +97,7 @@ class AttributeProxyPolicy {
 
     bool hasAttribute(const std::string& attr) const {
 #if R_VERSION >= R_Version(4, 6, 0)
-        return R_hasAttrib(static_cast<const CLASS&>(*this).get__(), Rf_install(attr.c_str()));
+        return R_hasAttrib(static_cast<const CLASS&>(*this), Rf_install(attr.c_str()));
 #else
         return static_cast<const CLASS&>(*this).attr(attr) != R_NilValue;
 #endif

inst/tinytest/cpp/ListOf.cpp

@@ -113,3 +113,13 @@ CharacterVector listof_names(ListOf<NumericVector> x) {
 SEXP listof_attr_foo(ListOf<NumericVector> x) {
     return x.attr("foo");
 }
+
+// [[Rcpp::export]]
+bool listof_has_attr(ListOf<NumericVector> x, std::string name) {
+    return x.hasAttribute(name);
+}
+
+// [[Rcpp::export]]
+std::vector<std::string> listof_attribute_names(ListOf<NumericVector> x) {
+    return x.attributeNames();
+}

inst/tinytest/test_listof.R

@@ -72,3 +72,15 @@ expect_equal(listof_names(l), c("a", "b", "c"))
 l <- list(a = 1L)
 attr(l, "foo") <- "bar"
 expect_equal(listof_attr_foo(l), "bar")
+
+#    test.ListOf.has.attribute <- function() {     # GH issue #1483
+l <- list(a = 1L)
+attr(l, "foo") <- "bar"
+expect_true(listof_has_attr(l, "foo"))
+expect_true(listof_has_attr(l, "names"))
+expect_false(listof_has_attr(l, "missing"))
+
+#    test.ListOf.attribute.names <- function() {   # GH issue #1483
+l <- list(a = 1L)
+attr(l, "foo") <- "bar"
+expect_true(all(c("names", "foo") %in% listof_attribute_names(l)))