C++ reference member variable initialization

어렵다, 어려워.

struct Obj1 {
  int m;
  int& ref = m; // Ok
};

struct Obj2 {
  int m;
  int& ref{m}; // Ok
};

/*
struct Obj3 {
  int m;
  int& ref(m); // Error
};
*/

struct Obj4 {
  int m;
  int& ref;
  Obj4() : ref{m} {} // Ok
};

struct Obj5 {
  int m;
  int& ref;
  Obj5() : ref(m) {} // Ok
};