Why there is no output from `cout` but it is ok for `printf`

To clear illustrate the problem, here is a minimal code. Why there is no output for my line cout << z << endl;? I am running it on Mac OS, compiler is Apple clang version 12.0.0 (clang-1200.0.32.29) Target: x86_64-apple-darwin20.3.0

#include <iostream>

using namespace std;
int main() {
  int x = 15;
  int *y = &x;
  unsigned char *z = (unsigned char *)&x;
  cout << y << endl; //this line works OK
  cout << z << endl; //no output of this line
  printf("%x", z); // this line works OK
  return 0;
}