snipet of union in C++03 for KMC Advent Calendar 2013

union2.cpp

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;
  } sp;
  void Clean() {
    if (IsAllocated()) { delete sp.p; }
  }
  void Assign(const T& src, bool alloc) {
    if (alloc) {
      sp.is_allocated = true;
      sp.p = new T(src);
    } else {
      si.is_allocated = false;
      si.i = src;
    }
  }
 public:
  U(const T& src, bool alloc) { Assign(src, alloc); }
  U(const U& src) { Assign(src.GetValue(), src.IsAllocated()); }
  U& operator=(const U& rhs) {
    if (IsAllocated() && rhs.IsAllocated()) {
      *sp.p = *rhs.sp.p;
    } else {
      Clean();
      Assign(rhs.GetValue(), rhs.IsAllocated());
    }
    return *this;
  }
  ~U() { Clean(); }
  T GetValue() const {
    if (IsAllocated()) {
      return *sp.p;
    } else {
      return si.i;
    }
  }
  bool IsAllocated() const { return si.is_allocated; }
};