# Standard library header <variant>

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### Classes

variant (since 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-contructible types (class)
bad_variant_access (C++17) | exception thrown on invalid accesses to the value of a variant (class)
variant_size variant_size_v (C++17) | obtains the size of the variant's list of alternatives at compile time (class template) (variable template)
variant_alternative variant_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)
std::uses_allocator<std::variant> (C++17) | specializes the std::uses_allocator type trait (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 given type appears exactly once in a variant (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>;
      // 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;
    };

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