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Standard library header <utility>

This header is part of the general utility library.

Includes
(C++20)
 Three-way comparison operator support
(C++11)
 std::initializer_list class template

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

Functions
swaps the values of two objects
(function template)
(C++14)
 replaces the argument with a new value and returns its previous value
(function template)
(C++11)
 forwards a function argument
(function template)
(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)
(C++11)
 obtains an rvalue reference
(function template)
(C++11)
 obtains an rvalue reference if the move constructor does not throw
(function template)
(C++17)
 obtains a reference to const to its argument
(function template)
(C++11)
 obtains a reference to its argument for use in unevaluated context
(function template)
(C++23)
 converts an enumeration to its underlying type
(function template)
(C++20)
 compares two integer values without value change caused by conversion
(function template)
(C++20)
 checks if an integer value is in the range of a given integer type
(function template)
(C++23)
 marks unreachable point of execution
(function)
creates a pair object of type, defined by the argument types
(function template)
(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)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 accesses an element of a pair
(function template)

Classes
implements binary tuple, i.e. a pair of values
(class template)
(C++11)
 obtains the number of elements of a tuple-like type
(class template)
(C++11)
 obtains the element types of a tuple-like type
(class template)
(C++11)
 obtains the size of a pair
(class template specialization)
(C++11)
 obtains the type of the elements of pair
(class template specialization)
(C++14)
 implements compile-time sequence of integers
(class template)
Forward declarations
Defined in header <tuple>
(C++11)
 implements fixed size container, which holds elements of possibly different types
(class template)

Helpers
(C++11)
 tag type used to select correct function overload for piecewise construction
(class)
(C++11)
 an object of type piecewise_construct_t used to disambiguate functions for piecewise construction
(constant)
(C++17)
 in-place construction tag
(class template)

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

(C++11)
 std::tuple class template

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