20 General utilities library [utilities]

A variant object holds and manages the lifetime of a value. If the variant holds a value, that value's type has to be one of the template argument types given to variant. These template arguments are called alternatives.

Header <variant> synopsis

namespace std {
  // [variant.variant], variant
  template <class... Types> class variant;

  // [variant.helper], variant helper classes
  template <class T> struct variant_size; // not defined
  template <class T> struct variant_size<const T>;
  template <class T> struct variant_size<volatile T>;
  template <class T> struct variant_size<const volatile T>;
  template <class T> constexpr size_t variant_size_v
    = variant_size<T>::value;

  template <class... Types>
    struct variant_size<variant<Types...>>;

  template <size_t I, class T> struct variant_alternative; // not defined
  template <size_t I, class T> struct variant_alternative<I, const T>;
  template <size_t I, class T> struct variant_alternative<I, volatile T>;
  template <size_t I, class T> struct variant_alternative<I, const volatile T>;
  template <size_t I, class T>
    using variant_alternative_t = typename variant_alternative<I, T>::type;

  template <size_t I, class... Types>
    struct variant_alternative<I, variant<Types...>>;

  constexpr size_t variant_npos = -1;

  // [variant.get], value access
  template <class T, class... Types>
    constexpr bool holds_alternative(const variant<Types...>&) noexcept;

  template <size_t I, class... Types>
    constexpr variant_alternative_t<I, variant<Types...>>&
    get(variant<Types...>&);
  template <size_t I, class... Types>
    constexpr variant_alternative_t<I, variant<Types...>>&&
    get(variant<Types...>&&);
  template <size_t I, class... Types>
    constexpr const variant_alternative_t<I, variant<Types...>>&
    get(const variant<Types...>&);
  template <size_t I, class... Types>
    constexpr const variant_alternative_t<I, variant<Types...>>&&
    get(const variant<Types...>&&);

  template <class T, class... Types>
    constexpr T& get(variant<Types...>&);
  template <class T, class... Types>
    constexpr T&& get(variant<Types...>&&);
  template <class T, class... Types>
    constexpr const T& get(const variant<Types...>&);
  template <class T, class... Types>
    constexpr const T&& get(const variant<Types...>&&);

  template <size_t I, class... Types>
    constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>>
    get_if(variant<Types...>*) noexcept;
  template <size_t I, class... Types>
    constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
    get_if(const variant<Types...>*) noexcept;

  template <class T, class... Types>
    constexpr add_pointer_t<T> get_if(variant<Types...>*) noexcept;
  template <class T, class... Types>
    constexpr add_pointer_t<const T> get_if(const variant<Types...>*) noexcept;

  // [variant.relops], relational operators
  template <class... Types>
    constexpr bool operator==(const variant<Types...>&,
                              const variant<Types...>&);
  template <class... Types>
    constexpr bool operator!=(const variant<Types...>&,
                              const variant<Types...>&);
  template <class... Types>
    constexpr bool operator<(const variant<Types...>&,
                             const variant<Types...>&);
  template <class... Types>
    constexpr bool operator>(const variant<Types...>&,
                             const variant<Types...>&);
  template <class... Types>
    constexpr bool operator<=(const variant<Types...>&,
                              const variant<Types...>&);
  template <class... Types>
    constexpr bool operator>=(const variant<Types...>&,
                              const variant<Types...>&);

  // [variant.visit], visitation
  template <class Visitor, class... Variants>
  constexpr see below visit(Visitor&&, Variants&&...);

  // [variant.monostate], class monostate
  struct monostate;

  // [variant.monostate.relops], monostate relational operators
  constexpr bool operator<(monostate, monostate) noexcept;
  constexpr bool operator>(monostate, monostate) noexcept;
  constexpr bool operator<=(monostate, monostate) noexcept;
  constexpr bool operator>=(monostate, monostate) noexcept;
  constexpr bool operator==(monostate, monostate) noexcept;
  constexpr bool operator!=(monostate, monostate) noexcept;

  // [variant.specalg], specialized algorithms
  template <class... Types>
  void swap(variant<Types...>&, variant<Types...>&) noexcept(see below);

  // [variant.bad.access], class bad_variant_access
  class bad_variant_access;

  // [variant.hash], hash support
  template <class T> struct hash;
  template <class... Types> struct hash<variant<Types...>>;
  template <> struct hash<monostate>;

  // [variant.traits], allocator-related traits
  template <class T, class Alloc> struct uses_allocator;
  template <class... Types, class Alloc>
  struct uses_allocator<variant<Types...>, Alloc>;
}