amoeba commented on code in PR #45951:
URL: https://github.com/apache/arrow/pull/45951#discussion_r2020338733


##########
r/src/altrep.cpp:
##########
@@ -1267,24 +1304,27 @@ sexp test_arrow_altrep_copy_by_dataptr(sexp x) {
 
   if (TYPEOF(x) == INTSXP) {
     cpp11::writable::integers out(Rf_xlength(x));
-    int* ptr = reinterpret_cast<int*>(DATAPTR(x));
+    int* ptr = reinterpret_cast<int*>(INTEGER(x));
     for (R_xlen_t i = 0; i < n; i++) {
       out[i] = ptr[i];
     }
     return out;
   } else if (TYPEOF(x) == REALSXP) {
     cpp11::writable::doubles out(Rf_xlength(x));
-    double* ptr = reinterpret_cast<double*>(DATAPTR(x));
+    double* ptr = reinterpret_cast<double*>(REAL(x));
     for (R_xlen_t i = 0; i < n; i++) {
       out[i] = ptr[i];
     }
     return out;
   } else if (TYPEOF(x) == STRSXP) {
     cpp11::writable::strings out(Rf_xlength(x));
-    SEXP* ptr = reinterpret_cast<SEXP*>(DATAPTR(x));
-    for (R_xlen_t i = 0; i < n; i++) {
-      out[i] = ptr[i];
+    R_xlen_t len = Rf_xlength(x);
+
+    for (R_xlen_t i = 0; i < len; i++) {
+      SEXP str_elt = reinterpret_cast<SEXP>(STRING_ELT(x, i));
+      out[i] = str_elt;

Review Comment:
   `DATAPTR` and `STRING_ELT` work a bit different for altrep so it seems 
reasonable to me to assume we were implicitly materializing before and no 
longer are with `STRING_ELT`. It seems like this is an assumption of the test 
that no longer holds and the expectation could be removed.



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