# Standard library header <variant> (C++17)

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This header is part of the general utility library.

### Includes

<compare> (C++20) | Three-way comparison operator support

### 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) | hash support for `std::variant` (class template specialization)

### Constants

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 `variant`s (function template)
holds_alternative (C++17) | checks if a `variant` currently holds a given type (function template)
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>=operator<=> (C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20) | compares `variant` objects as their contained values (function template)
std::swap(std::variant) (C++17) | specializes the std::swap algorithm (function template)

### Synopsis

    #include <compare>

    namespace std {
      // class template 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>
        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>
        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...>&);
      template<class... Types> requires (three_way_comparable<Types> && ...)
        constexpr common_comparison_category_t<compare_three_way_result_t<Types>...>
          operator<=>(const variant<Types...>&, const variant<Types...>&);

      // visitation
      template<class Visitor, class... Variants>
        constexpr /* see description */ visit(Visitor&&, Variants&&...);
      template<class R, class Visitor, class... Variants>
        constexpr R visit(Visitor&&, Variants&&...);

      // class monostate
      struct monostate;

      // monostate relational operators
      constexpr bool operator==(monostate, monostate) noexcept;
      constexpr strong_ordering operator<=>(monostate, monostate) noexcept;

      // specialized algorithms
      template<class... Types>
        constexpr void swap(variant<Types...>&,
                            variant<Types...>&) noexcept(/* see description */);

      // 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>;
    }

    // deprecated
    namespace std {
      template<class T> struct variant_size<volatile T>;
      template<class T> struct variant_size<const volatile 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>;
    }

#### Class template std::variant

    namespace std {
      template<class... Types>
      class variant {
      public:
        // constructors
        constexpr variant() noexcept(/* see description */);
        constexpr variant(const variant&);
        constexpr variant(variant&&) noexcept(/* see description */);

        template<class T>
          constexpr variant(T&&) noexcept(/* see description */);

        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
        constexpr ~variant();

        // assignment
        constexpr variant& operator=(const variant&);
        constexpr variant& operator=(variant&&) noexcept(/* see description */);

        template<class T> constexpr variant& operator=(T&&) noexcept(/* see description */);

        // modifiers
        template<class T, class... Args>
          constexpr T& emplace(Args&&...);
        template<class T, class U, class... Args>
          constexpr T& emplace(initializer_list<U>, Args&&...);
        template<size_t I, class... Args>
          constexpr variant_alternative_t<I, variant<Types...>>& emplace(Args&&...);
        template<size_t I, class U, class... Args>
          constexpr variant_alternative_t<I, variant<Types...>>&
            emplace(initializer_list<U>, Args&&...);

        // value status
        constexpr bool valueless_by_exception() const noexcept;
        constexpr size_t index() const noexcept;

        // swap
        constexpr void swap(variant&) noexcept(/* see description */);
      };
    }

#### Class std::monostate

    struct monostate{};

#### Class std::bad_variant_access

    class bad_variant_access : public exception {
    public:
      // see [exception] for the specification of the special member functions
      const char* what() const noexcept override;
    };

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