# Standard library header <utility>

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This header is part of the general utility library.

### Includes

<compare> (C++20) | Three-way comparison operator support
<initializer_list> (C++11) | std::initializer_list class template

### Namespaces

`rel_ops` | Provide automatic comparison operators
Defined in namespace `std::rel_ops`
operator!=operator>operator<=operator>= (deprecated in C++20) | automatically generates comparison operators based on user-defined operator== and operator< (function template)

### Functions

swap | swaps the values of two objects (function template)
exchange (C++14) | replaces the argument with a new value and returns its previous value (function template)
forward (C++11) | forwards a function argument and use the type template argument to preserve its value category (function template)
forward_like (C++23) | forwards a function argument as if casting it to the value category and constness of the expression of specified type template argument (function template)
move (C++11) | converts the argument to an xvalue (function template)
move_if_noexcept (C++11) | converts the argument to an xvalue if the move constructor does not throw (function template)
as_const (C++17) | obtains a reference to const to its argument (function template)
declval (C++11) | obtains a reference to an object of the template type argument for use in an unevaluated context (function template)
to_underlying (C++23) | converts an enumeration to its underlying type (function template)
cmp_equalcmp_not_equalcmp_lesscmp_greatercmp_less_equalcmp_greater_equal (C++20) | compares two integer values, ensuring that signed negative numbers are less than unsigned numbers (function template)
in_range (C++20) | checks if an integer value is in the range of a given integer type (function template)
unreachable (C++23) | marks unreachable point of execution (function)
make_pair | creates a `pair` object of type, determined by the argument types (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 `pair` (function template)
std::swap(std::pair) (C++11) | specializes the std::swap algorithm (function template)
get(std::pair) (C++11) | accesses an element of a `pair` (function template)

### Classes

pair | implements binary tuple, i.e. a pair of values (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::pair> (C++11) | obtains the size of a `pair` (class template specialization)
std::tuple_element<std::pair> (C++11) | obtains the type of the elements of `pair` (class template specialization)
integer_sequence (C++14) | implements compile-time sequence of integers (class template)

##### Forward declarations

Defined in header `<tuple>`
tuple (C++11) | implements fixed size container, which holds elements of possibly different types (class template)
Defined in header `<variant>`
monostate (C++17) | placeholder type for use as the first alternative in a `variant` of non-default-constructible types (class)

### Constants

ignore (C++11) | placeholder to skip an element when unpacking a `tuple` using `tie` (constant)

### Tags

piecewise_constructpiecewise_construct_t (C++11) | piecewise construction tag (tag)
in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t (C++17) | in-place construction tag (tag)
nontype nontype_t (C++26) | value construction tag (tag)

### Synopsis

    #include <compare>
    #include <initializer_list>

    namespace std {
      // swap
      template<class T>
        constexpr void swap(T& a, T& b) noexcept(/* see description */);
      template<class T, size_t N>
        constexpr void swap(T (&a)[N], T (&b)[N]) noexcept(is_nothrow_swappable_v<T>);

      // exchange
      template<class T, class U = T>
        constexpr T exchange(T& obj, U&& new_val);

      // forward/move
      template<class T>
        constexpr T&& forward(remove_reference_t<T>& t) noexcept;
      template<class T>
        constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
      template<class T, class U>
        constexpr /* see description */ forward_like(U&& x) noexcept;
      template<class T>
        constexpr remove_reference_t<T>&& move(T&&) noexcept;
      template<class T>
        constexpr conditional_t<
            !is_nothrow_move_constructible_v<T> && is_copy_constructible_v<T>, const T&, T&&>
          move_if_noexcept(T& x) noexcept;

      // as_const
      template<class T>
        constexpr add_const_t<T>& as_const(T& t) noexcept;
      template<class T>
        void as_const(const T&&) = delete;

      // declval
      template<class T>
        add_rvalue_reference_t<T> declval() noexcept;   // as unevaluated operand

      // integer comparison functions
      template<class T, class U>
        constexpr bool cmp_equal(T t, U u) noexcept;
      template<class T, class U>
        constexpr bool cmp_not_equal(T t, U u) noexcept;

      template<class T, class U>
        constexpr bool cmp_less(T t, U u) noexcept;
      template<class T, class U>
        constexpr bool cmp_greater(T t, U u) noexcept;
      template<class T, class U>
        constexpr bool cmp_less_equal(T t, U u) noexcept;
      template<class T, class U>
        constexpr bool cmp_greater_equal(T t, U u) noexcept;

      template<class R, class T>
        constexpr bool in_range(T t) noexcept;

      // to_underlying
      template<class T>
        constexpr underlying_type_t<T> to_underlying(T value) noexcept;

      // unreachable
      [[noreturn]] void unreachable();

      // compile-time integer sequences
      template<class T, T...>
        struct integer_sequence;
      template<size_t... I>
        using index_sequence = integer_sequence<size_t, I...>;

      template<class T, T N>
        using make_integer_sequence = integer_sequence<T, /* see description */>;
      template<size_t N>
        using make_index_sequence = make_integer_sequence<size_t, N>;

      template<class... T>
        using index_sequence_for = make_index_sequence<sizeof...(T)>;

      // class template pair
      template<class T1, class T2>
        struct pair;

      // pair specialized algorithms
      template<class T1, class T2>
        constexpr bool operator==(const pair<T1, T2>&, const pair<T1, T2>&);
      template<class T1, class T2>
        constexpr common_comparison_category_t</*synth-three-way-result*/<T1>,
                                               /*synth-three-way-result*/<T2>>
          operator<=>(const pair<T1, T2>&, const pair<T1, T2>&);

      template<class T1, class T2>
        constexpr void swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));

      template<class T1, class T2>
        constexpr /* see description */ make_pair(T1&&, T2&&);

      // tuple-like access to pair
      template<class T> struct tuple_size;
      template<size_t I, class T> struct tuple_element;

      template<class T1, class T2> struct tuple_size<pair<T1, T2>>;
      template<size_t I, class T1, class T2> struct tuple_element<I, pair<T1, T2>>;

      template<size_t I, class T1, class T2>
        constexpr tuple_element_t<I, pair<T1, T2>>& get(pair<T1, T2>&) noexcept;
      template<size_t I, class T1, class T2>
        constexpr tuple_element_t<I, pair<T1, T2>>&& get(pair<T1, T2>&&) noexcept;
      template<size_t I, class T1, class T2>
        constexpr const tuple_element_t<I, pair<T1, T2>>& get(const pair<T1, T2>&) noexcept;
      template<size_t I, class T1, class T2>
        constexpr const tuple_element_t<I, pair<T1, T2>>&& get(const pair<T1, T2>&&) noexcept;
      template<class T1, class T2>
        constexpr T1& get(pair<T1, T2>& p) noexcept;
      template<class T1, class T2>
        constexpr const T1& get(const pair<T1, T2>& p) noexcept;
      template<class T1, class T2>
        constexpr T1&& get(pair<T1, T2>&& p) noexcept;
      template<class T1, class T2>
        constexpr const T1&& get(const pair<T1, T2>&& p) noexcept;
      template<class T2, class T1>
        constexpr T2& get(pair<T1, T2>& p) noexcept;
      template<class T2, class T1>
        constexpr const T2& get(const pair<T1, T2>& p) noexcept;
      template<class T2, class T1>
        constexpr T2&& get(pair<T1, T2>&& p) noexcept;
      template<class T2, class T1>
        constexpr const T2&& get(const pair<T1, T2>&& p) noexcept;

      // pair piecewise construction
      struct piecewise_construct_t {
        explicit piecewise_construct_t() = default;
      };
      inline constexpr piecewise_construct_t piecewise_construct{};
      template<class... Types> class tuple;         // defined in <tuple>

      // in-place construction
      struct in_place_t {
        explicit in_place_t() = default;
      };
      inline constexpr in_place_t in_place{};

      template<class T>
        struct in_place_type_t {
          explicit in_place_type_t() = default;
        };
      template<class T> inline constexpr in_place_type_t<T> in_place_type{};

      template<size_t I>
        struct in_place_index_t {
          explicit in_place_index_t() = default;
        };
      template<size_t I> inline constexpr in_place_index_t<I> in_place_index{};

      // nontype argument tag
      template<auto V>
        struct nontype_t {
          explicit nontype_t() = default;
        };
      template<auto V> inline constexpr nontype_t<V> nontype{};
    }

    // deprecated
    namespace std::rel_ops {
      template<class T> bool operator!=(const T&, const T&);
      template<class T> bool operator> (const T&, const T&);
      template<class T> bool operator<=(const T&, const T&);
      template<class T> bool operator>=(const T&, const T&);
    }

#### Class template std::integer_sequence

    namespace std {
      template<class T, T... I> struct integer_sequence {
        using value_type = T;
        static constexpr size_t size() noexcept { return sizeof...(I); }
      };
    }

#### Class template std::pair

    namespace std {
      template<class T1, class T2>
      struct pair {
        using first_type  = T1;
        using second_type = T2;

        T1 first;
        T2 second;

        pair(const pair&) = default;
        pair(pair&&) = default;
        constexpr explicit(/* see description */) pair();
        constexpr explicit(/* see description */) pair(const T1& x, const T2& y);
        template<class U1 = T1, class U2 = T2>
          constexpr explicit(/* see description */) pair(U1&& x, U2&& y);
        template<class U1, class U2>
          constexpr explicit(/* see description */) pair(const pair<U1, U2>& p);
        template<class U1, class U2>
          constexpr explicit(/* see description */) pair(pair<U1, U2>&& p);
        template<class... Args1, class... Args2>
          constexpr pair(piecewise_construct_t,
                         tuple<Args1...> first_args, tuple<Args2...> second_args);

        constexpr pair& operator=(const pair& p);
        template<class U1, class U2>
          constexpr pair& operator=(const pair<U1, U2>& p);
        constexpr pair& operator=(pair&& p) noexcept(/* see description */);
        template<class U1, class U2>
          constexpr pair& operator=(pair<U1, U2>&& p);

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

      template<class T1, class T2>
        pair(T1, T2) -> pair<T1, T2>;
    }

### See also

<tuple> (C++11) | std::tuple class template

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