hatsusato

union Safe {
  Safe(const char* src) : s{src} {}
  ~Safe() { s.~basic_string(); }
  unsigned long long ull{0ULL};  // クラス内初期化は1つまで
  std::string s;
};
union Danger {
  Danger(const char* src) : s{src} {}
  ~Danger() { s.~basic_string(); }
  std::string s;

hatsusato

class U {
  enum class Tag { String, Vector } tag;
  union {
    std::string s;
    std::vector<int> v;
  };
  void Clean() {
    switch (tag) {
      case Tag::String:
        s.~basic_string();

hatsusato

template <typename T>
class BetterU {
  bool is_allocated;
  union {  // 余分な構造体が必要ない
    T i;
    T* p;
  };
  void Clean() {
    if (IsAllocated()) { delete p; }
  }

hatsusato

template <typename T>  // テンプレート共用体だって作れる
union U {
 private:
  struct {
    bool is_allocated;
    T i;
  } si;
  struct {
    bool is_allocated;
    T* p;

hatsusato

struct Si {
  char tag;
  int i;
};
struct Sp {
  char tag;
  int* p;
};
union U {
  int i;