// instead of #define N 5
const int N = 5;

int R;

class A {
    int a;

  public:
  	// int is already const: const int f(...)
    const int f(const int n) const {
      N = 1; // forbidden !!
      n = 3; // forbidden (2nd const)           
      a = 2; // forbidden (3rd const)

      R = 5; // allowed
    }
};