cpp/variant/variant.h

#include <cstring>

template <typename T, typename ...Types>
struct get_index_by_type {
  static const size_t value = -1;
};

template <typename T, typename Head, typename ...Tail>
struct get_index_by_type<T, Head, Tail...> {
  static const size_t value = std::is_same_v<T, Head> ? 0 : get_index_by_type<T, Tail...>::value + 1;
};

template<typename T, typename ...Tail>
static const size_t get_index_by_type_v = get_index_by_type<T, Tail...>::value;

template<typename T, typename... Types>
struct t_is_on_of_types {
  static const bool value = false;
};

template<typename T, typename Head, typename... Tail>
struct t_is_on_of_types<T, Head, Tail...> {
  static const bool value = std::is_same_v<T, Head> || t_is_on_of_types<T, Tail...>::value;
};

template<typename T, typename... Types>
static const bool t_is_on_of_types_v = t_is_on_of_types<T, Types...>::value;

template<size_t N, typename... Types>
struct get_type_by_index {};

template<typename T, typename... Types>
struct get_type_by_index<0, T, Types...> {
  using value = T;
};

template<size_t N, typename T, typename... Types>
struct get_type_by_index<N, T, Types...> {
  using value = typename get_type_by_index<N - 1, Types...>::value;
};


template<typename T, typename... Types>
struct count_types_in_package {};

template<typename T, typename Head, typename... Tail>
struct count_types_in_package<T, Head, Tail...> {
  static const size_t value = std::is_same_v<T, Head> + count_types_in_package<T, Tail...>::value;
};

template<typename T>
struct count_types_in_package<T> {
  static const size_t value = 0;
};


template<size_t N, typename... Types>
using get_type_by_index_v = typename get_type_by_index<N, Types...>::value;

template<typename... Types>
struct get_max_size {
  static const size_t value = 0;
};

template<typename Head, typename... Tail>
struct get_max_size<Head, Tail...> {
  static const size_t value = std::max(sizeof(Head), get_max_size<Tail...>::value);
};

template<typename... Types>
static const size_t get_max_size_v = get_max_size<Types...>::value;

template <typename ...Types>
class Variant;

template<typename T, typename... Types>
struct VariantAlternative;

template<typename T, typename... Types>
struct VariantAlternative {
 public:
  VariantAlternative() {}
  VariantAlternative(const VariantAlternative&) {}

  VariantAlternative(const T& value) {
    auto* me = std::launder(static_cast<Variant<Types...>*>(this));
    me->index_ = get_index_by_type_v<T, Types...>;
    new(me->object_) T(value);
  }
  VariantAlternative(T&& value) {
    auto* me = std::launder(static_cast<Variant<Types...>*>(this));
    me->index_ = get_index_by_type_v<T, Types...>;
    new(me->object_) T(std::move(value));
  }
  VariantAlternative& operator=(const T& value) {
    auto* me = std::launder(static_cast<Variant<Types...>*>(this));
    me->destroy();
    me->index_ = get_index_by_type_v<T, Types...>;
    new(me->object_) T(value);

    return *this;
  }
  VariantAlternative& operator=(T&& value) {
    auto* me = static_cast<Variant<Types...>*>(this);
    me->destroy();
    me->index_ = get_index_by_type_v<T, Types...>;
    new(me->object_) T(std::move(value));
    return *this;
  }
  template<typename K, typename U = T, std::enable_if_t<!std::is_convertible_v<U, K> && std::is_constructible_v<U, K>, bool> = true>
  VariantAlternative(const K& value) {
    auto* me = std::launder(static_cast<Variant<Types...>*>(this));
    me->index_ = get_index_by_type_v<T, Types...>;

    me->index_ = get_index_by_type_v<T, Types...>;
    new(me->object_) T(value);
  }
  template<typename K, typename U = T, std::enable_if_t<!std::is_convertible_v<U, K> && std::is_constructible_v<U, K>, bool> = true>
  VariantAlternative(K&& value) {
    auto* me = std::launder(static_cast<Variant<Types...>*>(this));
    me->index_ = get_index_by_type_v<T, Types...>;

    me->index_ = get_index_by_type_v<T, Types...>;
    new(me->object_) T(std::forward<K>(value));
  }

  template<typename K, typename U = T, std::enable_if_t<!std::is_convertible_v<U, K> && std::is_assignable_v<U, K>, bool> = true>
  VariantAlternative& operator=(const K& value) {
    auto* me = static_cast<Variant<Types...>*>(this);
    me->destroy();
    me->index_ = get_index_by_type_v<T, Types...>;
    new(me->object_) T(value);
    return *this;
  }
  template<typename K, typename U = T, std::enable_if_t<!std::is_convertible_v<U, K> && std::is_assignable_v<U, K>, bool> = true>
  VariantAlternative& operator=(K&& value) {
    auto* me = static_cast<Variant<Types...>*>(this);
    me->destroy();
    me->index_ = get_index_by_type_v<T, Types...>;
    new(me->object_) T(std::forward<K>(value));
    return *this;
  }

  ~VariantAlternative() {
    auto* me = static_cast<Variant<Types...>*>(this);
    if (get_index_by_type_v<T, Types...> == me->index_) {
      me->destroy();
    }
  }
};

template<typename T, typename... TTypes>
bool holds_alternative(const Variant<TTypes...>& another) {
  return another.template holds_alternative<T>();
}

template<typename T, typename... TTypes>
const T& get(const Variant<TTypes...>& another) {
  static_assert(t_is_on_of_types_v<T, TTypes...>);
  static_assert(count_types_in_package<T, TTypes...>::value == 1);

  if (another.template holds_alternative<T>()) {
    return *reinterpret_cast<T*>(std::launder(const_cast<Variant<TTypes...>&>(another).object_));
  } else {
    throw std::bad_cast();
  }
}

template<typename T, typename... TTypes>
T& get(Variant<TTypes...>& another) {
  return const_cast<T&>(get<T>(const_cast<const Variant<TTypes...>&>(another)));
}

template<typename T, typename... TTypes>
T&& get(Variant<TTypes...>&& another) {
  return std::move(const_cast<T>(get<T>(const_cast<const Variant<TTypes...>&>(another))));
}

template<size_t N, typename... TTypes>
const get_type_by_index_v<N, TTypes...>& get(const Variant<TTypes...>& another) {
  return get<get_type_by_index_v<N, TTypes...>>(another);
}

template<size_t N, typename... TTypes>
get_type_by_index_v<N, TTypes...>& get(Variant<TTypes...>& another) {
  using R = get_type_by_index_v<N, TTypes...>;
  return const_cast<R&>(get<R>(const_cast<const Variant<TTypes...>&>(another)));
}

template<size_t N, typename... TTypes>
get_type_by_index_v<N, TTypes...>&& get(Variant<TTypes...>&& another) {
  using R = get_type_by_index_v<N, TTypes...>;
  return std::move(const_cast<R>(get<R>(const_cast<const Variant<TTypes...>&>(another))));
}

template <typename ...Types>
class Variant : public VariantAlternative<Types, Types...>... {
  template<typename TT, typename... TTypes>
  friend class VariantAlternative;


 private:
  static const size_t size_variant = get_max_size_v<Types...>;
  alignas(Types...) int8_t object_[size_variant];
  size_t index_;
  template<class T>
  bool holds_alternative() const {
    if (get_index_by_type_v<T, Types...> == -1)
      throw std::bad_cast();
    else
      return get_index_by_type_v<T, Types...> == index_;
  }
  template<size_t N = 0>
  void destroy() {
    if (index_ == N) {
      index_ = -1;
      using R = get_type_by_index_v<N, Types...>;
      reinterpret_cast<R*>(object_)->~R();
    } else {
      if constexpr (N + 1 < sizeof...(Types)) {
        destroy<N + 1>();
      }
    }
  }
  template<size_t N = 0>
  void copy_constructor(const Variant& another) {
    if (another.index_ == N) {
      using R = get_type_by_index_v<N, Types...>;
      new(object_) R(*reinterpret_cast<R const*>(std::launder(another.object_)));
      index_ = another.index_;
    } else {
      if constexpr (N + 1 < sizeof...(Types)) {
        copy_constructor<N + 1>(another);
      }
    }
  }
  template<size_t N = 0>
  void move_constructor(Variant&& another) {
    if (this == &another) return;
    if (another.index_ == N) {
      using R = get_type_by_index_v<N, Types...>;

      new(object_) R(std::move(*reinterpret_cast<R*>(std::launder(another.object_))));
      index_ = another.index_;
    } else {
      if constexpr (N + 1 < sizeof...(Types)) {
        move_constructor<N + 1>(std::move(another));
      }
    }
  }
  template<size_t N = 0, size_t M = 0>
  void swap(Variant& another) {
    if (index_ == N && another.index_ == M) {
      using R = get_type_by_index_v<N, Types...>;
      using S = get_type_by_index_v<M, Types...>;
      S tmp(std::move(*reinterpret_cast<S*>(another.object_)));
      new(another.object_) R(std::move(*reinterpret_cast<R*>(object_)));
      new(object_) S(std::move(tmp));
      std::swap(index_, another.index_);
    } else if (index_ != N) {
      if constexpr(N + 1 < sizeof...(Types))
        swap<N + 1, M>(another);
    } else {
      if constexpr(M + 1 < sizeof...(Types))
        swap<N, M + 1>(another);
    }
  }

 public:
  using VariantAlternative<Types, Types...>::operator=...;
  using VariantAlternative<Types, Types...>::VariantAlternative...;

  Variant() : index_(0) {
    new(object_) get_type_by_index_v<0, Types...>();
  }
  Variant(Variant&& another) {
    move_constructor(std::move(another));
  }
  Variant(const Variant& another) : Variant() {
    copy_constructor(another);
  }
  Variant& operator=(const Variant& another) {
    Variant tmp(another);
    swap(tmp);
    return *this;
  }
  Variant& operator=(Variant&& another) noexcept {
    if (&another == this) return *this;
    Variant tmp(std::move(another));
    swap(tmp);
    return *this;
  }
  template<typename T, typename... Args>
  T& emplace(Args&&... args) {
    destroy();
    index_ = get_index_by_type_v<T, Types...>;
    new(object_) T(std::forward<Args...>(args)...);
    return *reinterpret_cast<T*>(object_);
  }
  template<size_t I, typename... Args>
  get_type_by_index_v<I, Types...>& emplace(Args&&... args) {
    using T = get_type_by_index_v<I, Types...>;
    destroy();
    index_ = get_index_by_type_v<T, Types...>;
    new(object_) T(std::forward<Args...>(args)...);
    return *reinterpret_cast<T*>(object_);
  }
  template<typename T, typename U, typename... Args>
  T& emplace(std::initializer_list<U> il, Args&&... args) {
    destroy();
    index_ = get_index_by_type_v<T, Types...>;
    new(object_) T(il, std::forward<Args...>(args)...);
    return *reinterpret_cast<T*>(object_);
  }
  template<size_t I, typename U, typename... Args>
  get_type_by_index_v<I, Types...>& emplace(std::initializer_list<U> il, Args&&... args) {
    using T = get_type_by_index_v<I, Types...>;
    destroy();
    index_ = get_index_by_type_v<T, Types...>;
    new(object_) T(il, std::forward<Args...>(args)...);
    return *reinterpret_cast<T*>(object_);
  }
  size_t index() const {
    return index_;
  }
  bool valueless_by_exception() const {
    return index_ == -1;
  }
  template<typename T, typename... TTypes>
  friend bool holds_alternative(const Variant<TTypes...>& another);
  template<typename T, typename... TTypes>
  friend const T& get(const Variant<TTypes...>& another);
  template<typename T, typename... TTypes>
  friend T& get(Variant<TTypes...>& another);
  template<typename T, typename... TTypes>
  friend T&& get(Variant<TTypes...>&& another);
  template<size_t N, typename... TTypes>
  friend const get_type_by_index_v<N, TTypes...>& get(const Variant<TTypes...>& another);
  template<size_t N, typename... TTypes>
  friend get_type_by_index_v<N, TTypes...>& get(Variant<TTypes...>& another);
  template<size_t N, typename... TTypes>
  friend get_type_by_index_v<N, TTypes...>&& get(Variant<TTypes...>&& another);
};