# Standard library header <tuple> (C++11)

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This header is part of the general utility library.

### Includes
<compare> (C++20) | Three-way comparison operator support
### Classes
tuple (C++11) | implements fixed size container, which holds elements of possibly different types (class template)
tuple_size (C++11) | obtains the number of elements of a tuple-like type (class template)
tuple_element (C++11) | obtains the element types of a tuple-like type (class template)
std::tuple_size<std::tuple> (C++11) | obtains the size of `tuple` at compile time (class template specialization)
std::tuple_element<std::tuple> (C++11) | obtains the type of the specified element (class template specialization)
std::uses_allocator<std::tuple> (C++11) | specializes the std::uses_allocator type trait (class template specialization)
### Constants
ignore (C++11) | placeholder to skip an element when unpacking a `tuple` using `tie` (constant)
### Functions
make_tuple (C++11) | creates a `tuple` object of the type defined by the argument types (function template)
tie (C++11) | creates a tuple of lvalue references or unpacks a tuple into individual objects (function template)
forward_as_tuple (C++11) | creates a `tuple` of forwarding references (function template)
tuple_cat (C++11) | creates a `tuple` by concatenating any number of tuples (function template)
std::get(std::tuple) (C++11) | tuple accesses specified element (function template)
operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20) | lexicographically compares the values in the tuple (function template)
std::swap(std::tuple) (C++11) | specializes the std::swap algorithm (function template)
apply (C++17) | calls a function with a tuple of arguments (function template)
make_from_tuple (C++17) | construct an object with a tuple of arguments (function template)

### Synopsis

    #include <compare>

    namespace std {
      // class template tuple
      template<class... Types>
        class tuple;

      // tuple creation functions
      inline constexpr /* unspecified */ ignore;

      template<class... TTypes>
        constexpr tuple<unwrap_ref_decay_t<TTypes>...> make_tuple(TTypes&&...);

      template<class... TTypes>
        constexpr tuple<TTypes&&...> forward_as_tuple(TTypes&&...) noexcept;

      template<class... TTypes>
        constexpr tuple<TTypes&...> tie(TTypes&...) noexcept;

      template<class... Tuples>
        constexpr tuple<CTypes...> tuple_cat(Tuples&&...);

      // calling a function with a tuple of arguments
      template<class F, class Tuple>
        constexpr decltype(auto) apply(F&& f, Tuple&& t);

      template<class T, class Tuple>
        constexpr T make_from_tuple(Tuple&& t);

      // tuple helper classes
      template<class T> struct tuple_size;                  // not defined
      template<class T> struct tuple_size<const T>;

      template<class... Types> struct tuple_size<tuple<Types...>>;

      template<size_t I, class T> struct tuple_element;     // not defined
      template<size_t I, class T> struct tuple_element<I, const T>;

      template<size_t I, class... Types>
        struct tuple_element<I, tuple<Types...>>;

      template<size_t I, class T>
        using tuple_element_t = typename tuple_element<I, T>::type;

      // element access
      template<size_t I, class... Types>
        constexpr tuple_element_t<I, tuple<Types...>>& get(tuple<Types...>&) noexcept;
      template<size_t I, class... Types>
        constexpr tuple_element_t<I, tuple<Types...>>&& get(tuple<Types...>&&) noexcept;
      template<size_t I, class... Types>
        constexpr const tuple_element_t<I, tuple<Types...>>&
          get(const tuple<Types...>&) noexcept;
      template<size_t I, class... Types>
        constexpr const tuple_element_t<I, tuple<Types...>>&&
          get(const tuple<Types...>&&) noexcept;
      template<class T, class... Types>
        constexpr T& get(tuple<Types...>& t) noexcept;
      template<class T, class... Types>
        constexpr T&& get(tuple<Types...>&& t) noexcept;
      template<class T, class... Types>
        constexpr const T& get(const tuple<Types...>& t) noexcept;
      template<class T, class... Types>
        constexpr const T&& get(const tuple<Types...>&& t) noexcept;

      // relational operators
      template<class... TTypes, class... UTypes>
        constexpr bool operator==(const tuple<TTypes...>&, const tuple<UTypes...>&);
      template<class... TTypes, class... UTypes>
        constexpr common_comparison_category_t</*synth-three-way-result*/<TTypes, UTypes>...>
          operator<=>(const tuple<TTypes...>&, const tuple<UTypes...>&);

      // allocator-related traits
      template<class... Types, class Alloc>
        struct uses_allocator<tuple<Types...>, Alloc>;

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

      // tuple helper classes
      template<class T>
        inline constexpr size_t tuple_size_v = tuple_size<T>::value;
    }

    // deprecated
    namespace std {
      template<class T> class tuple_size<volatile T>;
      template<class T> class tuple_size<const volatile T>;

      template<size_t I, class T> class tuple_element<I, volatile T>;
      template<size_t I, class T> class tuple_element<I, const volatile T>;
    }

#### Class template std::tuple

    namespace std {
      template<class... Types>
      class tuple {
      public:
        // tuple construction
        constexpr explicit(/* see description */) tuple();
        constexpr explicit(/* see description */)
          tuple(const Types&...);         // only if sizeof...(Types) >= 1
        template<class... UTypes>
          constexpr explicit(/* see description */)
            tuple(UTypes&&...);           // only if sizeof...(Types) >= 1

        tuple(const tuple&) = default;
        tuple(tuple&&) = default;

        template<class... UTypes>
          constexpr explicit(/* see description */) tuple(const tuple<UTypes...>&);
        template<class... UTypes>
          constexpr explicit(/* see description */) tuple(tuple<UTypes...>&&);

        template<class U1, class U2>
          constexpr explicit(/* see description */)
            tuple(const pair<U1, U2>&);   // only if sizeof...(Types) == 2
        template<class U1, class U2>
          constexpr explicit(/* see description */)
            tuple(pair<U1, U2>&&);        // only if sizeof...(Types) == 2

        // allocator-extended constructors
        template<class Alloc>
          constexpr explicit(/* see description */)
            tuple(allocator_arg_t, const Alloc& a);
        template<class Alloc>
          constexpr explicit(/* see description */)
            tuple(allocator_arg_t, const Alloc& a, const Types&...);
        template<class Alloc, class... UTypes>
          constexpr explicit(/* see description */)
            tuple(allocator_arg_t, const Alloc& a, UTypes&&...);
        template<class Alloc>
          constexpr tuple(allocator_arg_t, const Alloc& a, const tuple&);
        template<class Alloc>
          constexpr tuple(allocator_arg_t, const Alloc& a, tuple&&);
        template<class Alloc, class... UTypes>
          constexpr explicit(/* see description */)
            tuple(allocator_arg_t, const Alloc& a, const tuple<UTypes...>&);
        template<class Alloc, class... UTypes>
          constexpr explicit(/* see description */)
            tuple(allocator_arg_t, const Alloc& a, tuple<UTypes...>&&);
        template<class Alloc, class U1, class U2>
          constexpr explicit(/* see description */)
            tuple(allocator_arg_t, const Alloc& a, const pair<U1, U2>&);
        template<class Alloc, class U1, class U2>
          constexpr explicit(/* see description */)
            tuple(allocator_arg_t, const Alloc& a, pair<U1, U2>&&);

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

        template<class... UTypes>
          constexpr tuple& operator=(const tuple<UTypes...>&);
        template<class... UTypes>
          constexpr tuple& operator=(tuple<UTypes...>&&);

        template<class U1, class U2>
          constexpr tuple& operator=(const pair<U1, U2>&); // only if sizeof...(Types) == 2
        template<class U1, class U2>
          constexpr tuple& operator=(pair<U1, U2>&&);      // only if sizeof...(Types) == 2

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

      template<class... UTypes>
        tuple(UTypes...) -> tuple<UTypes...>;
      template<class T1, class T2>
        tuple(pair<T1, T2>) -> tuple<T1, T2>;
      template<class Alloc, class... UTypes>
        tuple(allocator_arg_t, Alloc, UTypes...) -> tuple<UTypes...>;
      template<class Alloc, class T1, class T2>
        tuple(allocator_arg_t, Alloc, pair<T1, T2>) -> tuple<T1, T2>;
      template<class Alloc, class... UTypes>
        tuple(allocator_arg_t, Alloc, tuple<UTypes...>) -> tuple<UTypes...>;
    }

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