# Standard library header <utility>

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This header is part of the general utility library.

### Includes

`<initializer_list>` (C++11)

### 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 (function template)
move (C++11) | obtains an rvalue reference (function template)
move_if_noexcept (C++11) | obtains an rvalue reference 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 its argument for use in unevaluated context (function template)
make_pair | creates a `pair` object of type, defined by the argument types (function template)
operator==operator!=operator<operator<=operator>operator>= | lexicographically compares the values in the pair (function template)
std::swap(std::pair) (C++11) | specializes the std::swap algorithm (function template)
std::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)
integer_sequence (C++14) | implements compile-time sequence of integers (class template)

### Helpers

piecewise_construct_t (C++11) | tag type used to select correct function overload for piecewise construction (class)
piecewise_construct (C++11) | an object of type `piecewise_construct_t` used to disambiguate functions for piecewise construction (constant)
in_place in_place_type in_place_index in_place_t in_place_type_t in_place_index_t (C++17) | in-place construction tag (class template)

##### Forward declarations

tuple (C++11) | implements fixed size container, which holds elements of possibly different types (class template)

### Synopsis

    #include <initializer_list>
    namespace std {
      // operators:
      namespace 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&);
      }
      // swap:
      template <class T>
      constexpr void swap(T& a, T& b) noexcept(/*see definition*/ );
      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>
      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

      // 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 definition*/ >;
      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)>;

      // pairs:
      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 bool operator< (const pair<T1, T2>&, const pair<T1, T2>&);
      template <class T1, class T2>
      constexpr bool operator!=(const pair<T1, T2>&, const pair<T1, T2>&);
      template <class T1, class T2>
      constexpr bool operator> (const pair<T1, T2>&, const pair<T1, T2>&);
      template <class T1, class T2>
      constexpr bool operator>=(const pair<T1, T2>&, const pair<T1, T2>&);
      template <class T1, class T2>
      constexpr bool 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 definition*/ make_pair(T1&&, T2&&);

      // tuple-like access to pair:
      template <class T> class tuple_size;
      template <size_t I, class T> class tuple_element;
      template <class T1, class T2> struct tuple_size<pair<T1, T2>>;
      template <class T1, class T2> struct tuple_element<0, pair<T1, T2>>;
      template <class T1, class T2> struct tuple_element<1, 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 T, class U>
      constexpr T& get(pair<T, U>& p) noexcept;
      template <class T, class U>
      constexpr const T& get(const pair<T, U>& p) noexcept;
      template <class T, class U>
      constexpr T&& get(pair<T, U>&& p) noexcept;
      template <class T, class U>
      constexpr const T&& get(const pair<T, U>&& p) noexcept;
      template <class T, class U>
      constexpr T& get(pair<U, T>& p) noexcept;
      template <class T, class U>
      constexpr const T& get(const pair<U, T>& p) noexcept;
      template <class T, class U>
      constexpr T&& get(pair<U, T>&& p) noexcept;
      template <class T, class U>
      constexpr const T&& get(const pair<U, T>&& p) noexcept;

      // pair piecewise construction
      struct piecewise_construct_t { };
      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{};
    }

#### 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 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;

        /*conditionally-explicit*/ constexpr pair();
        /*conditionally-explicit*/ constexpr pair(const T1& x, const T2& y);

        template<class U1, class U2>
        /*conditionally-explicit*/ constexpr pair(U1&& x, U2&& y);
        template<class U1, class U2>
        /*conditionally-explicit*/ constexpr pair(const pair<U1, U2>& p);
        template<class U1, class U2>
        /*conditionally-explicit*/ constexpr pair(pair<U1, U2>&& p);

        template<class... Args1, class... Args2>
        constexpr pair(piecewise_construct_t, tuple<Args1...> arg1, tuple<Args2...> arg2);

        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 definition*/);

        template<class U1, class U2> constexpr pair& operator=(pair<U1, U2>&& p);
        constexpr void swap(pair& p) noexcept(/*see definition*/);
      };

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

### See also

`<tuple>` | Defines std::tuple
Utility library

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