What is the correct way to exec bash with SUID SGID?

Question

C++ / C#

I want to set SUID to a binary that will exec in bash to have a shell under a different user, but the shell is launched with the rights of the launcher, not the owner. That is, there is no access, for example, to directories, files, etc. user 1000. The bash itself is executed, but when I try to read the file, it says

bash-4.3$ cat n
cat: n: Permission denied

The source of the binary is below.

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>

int main () {
  char *args [] = {"/bin/bash", (char *)NULL};
  setuid((uid_t)1000);
  setgid(1000);
  execvp ("/bin/bash", args);
  return 0;
}

What am I doing wrong? Thanks in advance.