68 namespace optional {  namespace
69   template<typename T> struct optional {  struct
70     bool has;
71     union { T value; };
73     optional() : has(false) {}  in optional()  function
75     optional(U &&...u) : has(true), value(forward<U>(u)...) {}  in optional()  argument
77     optional(const optional &o) : has(o.has) {  in optional()  function
80     optional(optional &&o) : has(o.has) {  in optional()  function
84     optional &operator=(const optional &o) {  in operator =()
95     optional &operator=(optional &&o) {  in operator =()
107     ~optional() {  in ~optional()
112     explicit operator bool() const { return has; }  in operator bool()
113     T &operator*() { return value; }  in operator *()