more efficient DataFrame::nrows, i.e. one that does not have to pay the price for R special case handling of R_RowNamesSymbol

Author: romainfrancois

inst/include/Rcpp/DataFrame.h

@@ -61,11 +61,19 @@ namespace Rcpp{
         }
 
         inline int nrows() const {
-            SEXP rn = Rf_getAttrib( Parent::get__(), R_RowNamesSymbol );
-            if (TYPEOF(rn) == INTSXP && LENGTH(rn) == 2 && INTEGER(rn)[0] == NA_INTEGER)
-                return INTEGER(rn)[1];
+            SEXP rn = R_NilValue ;
+            SEXP att = ATTRIB( Parent::get__() )  ;
+            while( att != R_NilValue ){
+                if( TAG(att) == R_RowNamesSymbol ) {
+                    rn = CAR(att) ;
+                    break ;
+                }
+                att = CDR(att) ;    
+            }
             if (Rf_isNull(rn))
                 return 0;
+            if (TYPEOF(rn) == INTSXP && LENGTH(rn) == 2 && INTEGER(rn)[0] == NA_INTEGER)
+                return -INTEGER(rn)[1];
             return LENGTH(rn);
         }