index lapply operands through VectorBase so vectors yield their stored type

Holding the operand as the derived vector type made lapply() pass a
proxy (e.g. const_string_proxy for character vectors) to the function,
which no longer converted to a const CharacterVector& argument. Seen as
a compile failure in the touch package.

Author: kevinushey

inst/include/Rcpp/sugar/functions/lapply.h

@@ -40,7 +40,9 @@ class Lapply : public VectorBase<
 		vec(vec_.get_ref()), fun(fun_){}
 
         inline SEXP operator[]( R_xlen_t i ) const {
-		return Rcpp::wrap( fun( vec[i] ) );
+		// index through VectorBase so that vectors yield their stored
+		// type (e.g. SEXP for character vectors) rather than a proxy
+		return Rcpp::wrap( fun( static_cast<const VEC&>(vec)[i] ) );
 	}
         inline R_xlen_t size() const { return vec.size() ; }
 

inst/tinytest/cpp/sugar.cpp

@@ -245,6 +245,18 @@ List runit_lapply_seq( IntegerVector xx){
     return res ;
 }
 
+CharacterVector lapply_string_rev( const CharacterVector& x ){
+    std::string s = as<std::string>( x[0] ) ;
+    return CharacterVector::create( std::string( s.rbegin(), s.rend() ) ) ;
+}
+
+// the element passed to the function must be the stored SEXP, not a proxy
+// [[Rcpp::export]]
+List runit_lapply_string( CharacterVector xx ){
+    List res = lapply( xx, lapply_string_rev ) ;
+    return res ;
+}
+
 // [[Rcpp::export]]
 NumericVector runit_mapply2(NumericVector xx, NumericVector yy){
     NumericVector res = mapply(xx, yy, std::plus<double>());

inst/tinytest/test_sugar.R

@@ -341,6 +341,9 @@ expect_equal( fx( 1:10 ), as.list((1:10)^2) )
 fx <- runit_lapply_seq
 expect_equal( fx( 1:10 ), lapply( 1:10, seq_len ) )
 
+fx <- runit_lapply_string
+expect_equal( fx( c("abc", "de") ), list("cba", "ed") )
+
 
 #    test.sugar.mapply2 <- function( ){
 fx <- runit_mapply2