C++ reference type

#include <cstdio>   
using namespace std;   
  
int zero = 0;   
class Foo {   
public:   
  int &a;   
  Foo(): a(zero) {}   
};   
  
int main() {   
  int &a = zero;   
  printf("%d\n", a);   
  
  Foo f;   
  printf("%d\n", f.a);   
  return 0;   
}