snipet of union in C++11 for KMC Advent Calendar 2013

union4.cpp

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();
        break;
      case Tag::Vector:
        v.~vector();
        break;
      default:
        assert(0);  // Never reached
    }
  }
  void Assign(const std::string& src) { new (&s) std::string{src}; }
  void Assign(const std::vector<int>& src) { new (&v) std::vector<int>{src}; }
 public:
  U(const char* src) : tag{Tag::String}, s{src} {}
  U(std::initializer_list<int> src) : tag{Tag::Vector}, v{src} {}
  U(const U& src) : tag{src.tag} {
    switch (tag) {
      case Tag::String:
        Assign(src.s);
        break;
      case Tag::Vector:
        Assign(src.v);
        break;
      default:
        assert(0);  // Never reached
    }
  }
  U& operator=(const U& rhs) {
    if (tag == rhs.tag) {
      switch (tag) {
        case Tag::String:
          s = rhs.s;
          break;
        case Tag::Vector:
          v = rhs.v;
          break;
        default:
          assert(0);  // Never reached
      }
    } else {
      Clean();
      tag = rhs.tag;
      switch (tag) {
        case Tag::String:
          Assign(rhs.s);
          break;
        case Tag::Vector:
          Assign(rhs.v);
          break;
        default:
          assert(0);  // Never reached
      }
    }
    return *this;
  }
  ~U() { Clean(); }
};