PROTECTing elements in VECSXP - r

Question

In the "Writing R Extensions" manual it says
Protecting an R object automatically protects all the R objects pointed to in the
corresponding SEXPREC, for example all elements of a protected list are automatically
protected.
Does this mean that I do not need to protect individual SEXP elements of a VECSXP. For example consider the following which contains two methods that I hope illustrate where I need clarification:
#include <Rinternals.h>
#include <R.h>
SEXP copy_int_vec(int *x, int size) {
SEXP ans = allocVector(INTSXP, size);
PROTECT(ans);
int *pans = INTEGER(ans);
for (int i = 0; i < size; i ++) {
pans[i] = x[i];
}
UNPROTECT(1);
return ans;
}
// method 1 - protect VECSXP when allocated BUT NOT individual array entries
SEXP method_one() {
// setup for rep example
int N = 3;
int *graph[N];
int tmp1[3] = {1, 2, 3};
int tmp2[4] = {1, 2, 3, 4};
int tmp3[2] = {3, 4};
graph[0] = tmp1;
graph[1] = tmp2;
graph[2] = tmp3;
int neighbours[3] = {3, 4, 2};
// method
SEXP ans = allocVector(VECSXP, N);
PROTECT(ans);
for (int i = 0; i < N; i++){
SET_VECTOR_ELT(ans, i, copy_int_vec(graph[i], neighbours[i]));
}
UNPROTECT(1);
return ans;
}
// method 2 - protect VECSXP when allocated AND ALSO individual array entries
SEXP method_two() {
// setup for rep example
int N = 3;
int *graph[N];
int tmp1[3] = {1, 2, 3};
int tmp2[4] = {1, 2, 3, 4};
int tmp3[2] = {3, 4};
graph[0] = tmp1;
graph[1] = tmp2;
graph[2] = tmp3;
int neighbours[3] = {3, 4, 2};
// method 2
SEXP ans = allocVector(VECSXP, N);
PROTECT(ans);
for (int i = 0; i < N; i++){
SEXP tmp = copy_int_vec(graph[i], neighbours[i]);
PROTECT(tmp);
SET_VECTOR_ELT(ans, i, tmp);
}
UNPROTECT(N + 1);
return ans;
}
I'm hoping the answer is the first method but would appreciate clarification.

Answer 1

I believe method_one is fine.
To do the analysis, you need to know every point in your code that does allocations, because an allocation might trigger garbage collection, and that will release any unprotected object.
So stepping through method_one:
SEXP method_one() {
// setup for rep example
int N = 3;
int *graph[N];
int tmp1[3] = {1, 2, 3};
int tmp2[4] = {1, 2, 3, 4};
int tmp3[2] = {3, 4};
graph[0] = tmp1;
graph[1] = tmp2;
graph[2] = tmp3;
int neighbours[3] = {3, 4, 2};
None of the code above uses R's allocator; those allocations are all on the C stack, so they are safe.
// method
SEXP ans = allocVector(VECSXP, N);
PROTECT(ans);
This does an R allocation, and then immediately protects it. A more common way to write this is
SEXP ans;
PROTECT(ans = allocVector(VECSXP, N));
which makes it harder to mess things up by inserting a statement in between the allocation and protection.
for (int i = 0; i < N; i++){
SET_VECTOR_ELT(ans, i, copy_int_vec(graph[i], neighbours[i]));
}
The copy_int_vec does allocations, so garbage collection could occur in any step of this loop. However, the allocated object is immediately assigned into ans, so each one is protected and safe.
UNPROTECT(1);
Nothing is protected now; remember not to insert any code before the return.
return ans;
}