Standard library header <variant>

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Classes

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

Helpers

index of the variant in the invalid state
(constant)

Functions

(C++17)
calls the provided functor with the arguments held by one or more variants
(function template)
checks if a given type appears exactly once in a variant
(function template)
reads the value of the variant given the index or the type (if the type is unique), throws on error
(function template)
(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)
compares variant objects as their contained values
(function template)
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>;
  // allocator-related traits
  template <class T, class Alloc> struct uses_allocator;
  template <class... Types, class Alloc>
  struct uses_allocator<variant<Types...>, Alloc>;
}

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&&...);
  // allocator-extended constructors
  template <class Alloc>
  variant(allocator_arg_t, const Alloc&);
  template <class Alloc>
  variant(allocator_arg_t, const Alloc&, const variant&);
  template <class Alloc>
  variant(allocator_arg_t, const Alloc&, variant&&);
  template <class Alloc, class T>
  variant(allocator_arg_t, const Alloc&, T&&);
  template <class Alloc, class T, class... Args>
  variant(allocator_arg_t, const Alloc&, in_place_type_t<T>, Args&&...);
  template <class Alloc, class T, class U, class... Args>
  variant(allocator_arg_t, const Alloc&, in_place_type_t<T>,
          initializer_list<U>, Args&&...);
  template <class Alloc, size_t I, class... Args>
  variant(allocator_arg_t, const Alloc&, in_place_index_t<I>, Args&&...);
  template <class Alloc, size_t I, class U, class... Args>
  variant(allocator_arg_t, const Alloc&, 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;
};