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Standard library header <variant>

Classes

variant (C++17)	a type-safe discriminated union (class template)
monostate (C++17)	placeholder type for use as the first alternative in a variant of non-default-constructible types (class)
bad_variant_access (C++17)	exception thrown on invalid accesses to the value of a variant (class)
variant_sizevariant_size_v (C++17)	obtains the size of the variant's list of alternatives at compile time (class template) (variable template)
variant_alternativevariant_alternative_t (C++17)	obtains the type of the alternative specified by its index, at compile time (class template) (alias template)
std::hash<std::variant> (C++17)	specializes the `std::hash` algorithm (class template specialization)

Helpers

variant_npos

(C++17)

index of the variant in the invalid state
(constant)

Functions

visit (C++17)	calls the provided functor with the arguments held by one or more variants (function template)
holds_alternative (C++17)	checks if a variant currently holds a given type (function template)
std::get(std::variant) (C++17)	reads the value of the variant given the index or the type (if the type is unique), throws on error (function template)
get_if (C++17)	obtains a pointer to the value of a pointed-to variant given the index or the type (if unique), returns null on error (function template)
operator==operator!=operator<operator<=operator>operator>= (C++17)	compares `variant` objects as their contained values (function template)
std::swap(std::variant) (C++17)	specializes the `std::swap` algorithm (function)

Synopsis

namespace std {
  // variant
  template <class... Types> class variant;
  // 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> inline 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...>>;
  inline constexpr size_t variant_npos = -1;
  // 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;
  // 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...>&);
  // visitation
  template <class Visitor, class... Variants>
  constexpr /*see definition*/ visit(Visitor&&, Variants&&...);
  // class monostate
  struct monostate;
  // 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;
  // specialized algorithms
  template <class... Types>
  void swap(variant<Types...>&, variant<Types...>&) noexcept(/*see definition*/);
  // class bad_variant_access
  class bad_variant_access;
  // hash support
  template <class T> struct hash;
  template <class... Types> struct hash<variant<Types...>>;
  template <> struct hash<monostate>;
}

Class template `std::variant`

template <class... Types>
class variant {
public:
  // constructors
  constexpr variant() noexcept(/*see definition*/);
  variant(const variant&);
  variant(variant&&) noexcept(/*see definition*/);
  template <class T> constexpr variant(T&&) noexcept(/*see definition*/);
  template <class T, class... Args>
  constexpr explicit variant(in_place_type_t<T>, Args&&...);
  template <class T, class U, class... Args>
  constexpr explicit variant(in_place_type_t<T>, initializer_list<U>, Args&&...);
  template <size_t I, class... Args>
  constexpr explicit variant(in_place_index_t<I>, Args&&...);
  template <size_t I, class U, class... Args>
  constexpr explicit variant(in_place_index_t<I>, initializer_list<U>, Args&&...);
  // destructor
  ~variant();
  // assignment
  variant& operator=(const variant&);
  variant& operator=(variant&&) noexcept(/*see definition*/);
  template <class T> variant& operator=(T&&) noexcept(/*see definition*/);
  // modifiers
  template <class T, class... Args> void emplace(Args&&...);
  template <class T, class U, class... Args>
  void emplace(initializer_list<U>, Args&&...);
  template <size_t I, class... Args> void emplace(Args&&...);
  template <size_t I, class U, class... Args>
  void emplace(initializer_list<U>, Args&&...);
  // value status
  constexpr bool valueless_by_exception() const noexcept;
  constexpr size_t index() const noexcept;
  // swap
  void swap(variant&) noexcept(/*see definition*/);
};

Class `std::monostate`

struct monostate{};

Class `std::bad_variant_access`

class bad_variant_access : public exception {
public:
  bad_variant_access() noexcept;
  virtual const char* what() const noexcept;
};

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