20 General utilities library [utilities]

20.4 Pairs [pairs]

20.4.1 In general [pairs.general]

The library provides a template for heterogeneous pairs of values.
The library also provides a matching function template to simplify their construction and several templates that provide access to pair objects as if they were tuple objects (see [tuple.helper] and [tuple.elem]).

20.4.2 Class template pair [pairs.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 below) pair(); constexpr explicit(see below) pair(const T1& x, const T2& y); template<class U1 = T1, class U2 = T2> constexpr explicit(see below) pair(U1&& x, U2&& y); template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>& p); template<class U1, class U2> constexpr explicit(see below) 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 below); template<class U1, class U2> constexpr pair& operator=(pair<U1, U2>&& p); constexpr void swap(pair& p) noexcept(see below); }; template<class T1, class T2> pair(T1, T2) -> pair<T1, T2>; }
Constructors and member functions of pair do not throw exceptions unless one of the element-wise operations specified to be called for that operation throws an exception.
The defaulted move and copy constructor, respectively, of pair is a constexpr function if and only if all required element-wise initializations for move and copy, respectively, would satisfy the requirements for a constexpr function.
If (is_­trivially_­destructible_­v<T1> && is_­trivially_­destructible_­v<T2>) is true, then the destructor of pair is trivial.
pair<T, U> is a structural type ([temp.param]) if T and U are both structural types.
Two values p1 and p2 of type pair<T, U> are template-argument-equivalent ([temp.type]) if and only if p1.first and p2.first are template-argument-equivalent and p1.second and p2.second are template-argument-equivalent.
constexpr explicit(see below) pair();
Constraints:
  • is_­default_­constructible_­v<first_­type> is true and
  • is_­default_­constructible_­v<second_­type> is true.
Effects: Value-initializes first and second.
Remarks: The expression inside explicit evaluates to true if and only if either first_­type or second_­type is not implicitly default-constructible.
[Note 1:
This behavior can be implemented with a trait that checks whether a const first_­type& or a const second_­type& can be initialized with {}.
— end note]
constexpr explicit(see below) pair(const T1& x, const T2& y);
Constraints:
  • is_­copy_­constructible_­v<first_­type> is true and
  • is_­copy_­constructible_­v<second_­type> is true.
Effects: Initializes first with x and second with y.
Remarks: The expression inside explicit is equivalent to: !is_convertible_v<const first_type&, first_type> || !is_convertible_v<const second_type&, second_type>
template<class U1 = T1, class U2 = T2> constexpr explicit(see below) pair(U1&& x, U2&& y);
Constraints:
  • is_­constructible_­v<first_­type, U1> is true and
  • is_­constructible_­v<second_­type, U2> is true.
Effects: Initializes first with std​::​forward<U1>(x) and second with std​::​forward<U2>(y).
Remarks: The expression inside explicit is equivalent to: !is_convertible_v<U1, first_type> || !is_convertible_v<U2, second_type>
template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>& p);
Constraints:
  • is_­constructible_­v<first_­type, const U1&> is true and
  • is_­constructible_­v<second_­type, const U2&> is true.
Effects: Initializes members from the corresponding members of the argument.
Remarks: The expression inside explicit is equivalent to: !is_convertible_v<const U1&, first_type> || !is_convertible_v<const U2&, second_type>
template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>&& p);
Constraints:
  • is_­constructible_­v<first_­type, U1> is true and
  • is_­constructible_­v<second_­type, U2> is true.
Effects: Initializes first with std​::​forward<U1>(p.first) and second with std​::​forward<U2>(​p.second).
Remarks: The expression inside explicit is equivalent to: !is_convertible_v<U1, first_type> || !is_convertible_v<U2, second_type>
template<class... Args1, class... Args2> constexpr pair(piecewise_construct_t, tuple<Args1...> first_args, tuple<Args2...> second_args);
Mandates:
  • is_­constructible_­v<first_­type, Args1...> is true and
  • is_­constructible_­v<second_­type, Args2...> is true.
Effects: Initializes first with arguments of types Args1... obtained by forwarding the elements of first_­args and initializes second with arguments of types Args2... obtained by forwarding the elements of second_­args.
(Here, forwarding an element x of type U within a tuple object means calling std​::​forward<U>(x).)
This form of construction, whereby constructor arguments for first and second are each provided in a separate tuple object, is called piecewise construction.
constexpr pair& operator=(const pair& p);
Effects: Assigns p.first to first and p.second to second.
Returns: *this.
Remarks: This operator is defined as deleted unless is_­copy_­assignable_­v<first_­type> is true and is_­copy_­assignable_­v<second_­type> is true.
template<class U1, class U2> constexpr pair& operator=(const pair<U1, U2>& p);
Constraints:
  • is_­assignable_­v<first_­type&, const U1&> is true and
  • is_­assignable_­v<second_­type&, const U2&> is true.
Effects: Assigns p.first to first and p.second to second.
Returns: *this.
constexpr pair& operator=(pair&& p) noexcept(see below);
Constraints:
  • is_­move_­assignable_­v<first_­type> is true and
  • is_­move_­assignable_­v<second_­type> is true.
Effects: Assigns to first with std​::​forward<first_­type>(p.first) and to second with
std​::​forward<second_­type>(p.second).
Returns: *this.
Remarks: The exception specification is equivalent to: is_nothrow_move_assignable_v<T1> && is_nothrow_move_assignable_v<T2>
template<class U1, class U2> constexpr pair& operator=(pair<U1, U2>&& p);
Constraints:
  • is_­assignable_­v<first_­type&, U1> is true and
  • is_­assignable_­v<second_­type&, U2> is true.
Effects: Assigns to first with std​::​forward<U1>(p.first) and to second with
std​::​forward<U2>(p.second).
Returns: *this.
constexpr void swap(pair& p) noexcept(see below);
Preconditions: first is swappable with ([swappable.requirements]) p.first and second is swappable with p.second.
Effects: Swaps first with p.first and second with p.second.
Remarks: The exception specification is equivalent to: is_nothrow_swappable_v<first_type> && is_nothrow_swappable_v<second_type>

20.4.3 Specialized algorithms [pairs.spec]

template<class T1, class T2> constexpr bool operator==(const pair<T1, T2>& x, const pair<T1, T2>& y);
Returns: x.first == y.first && x.second == y.second.
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>& x, const pair<T1, T2>& y);
Effects: Equivalent to: if (auto c = synth-three-way(x.first, y.first); c != 0) return c; return synth-three-way(x.second, y.second);
template<class T1, class T2> constexpr void swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));
Constraints: is_­swappable_­v<T1> is true and is_­swappable_­v<T2> is true.
Effects: Equivalent to x.swap(y).
template<class T1, class T2> constexpr pair<unwrap_ref_decay_t<T1>, unwrap_ref_decay_t<T2>> make_pair(T1&& x, T2&& y);
Returns: pair<unwrap_ref_decay_t<T1>, unwrap_ref_decay_t<T2>>(std::forward<T1>(x), std::forward<T2>(y))
[Example 1:
In place of: return pair<int, double>(5, 3.1415926); // explicit types a C++ program may contain: return make_pair(5, 3.1415926); // types are deduced
— end example]

20.4.4 Tuple-like access to pair [pair.astuple]

template<class T1, class T2> struct tuple_size<pair<T1, T2>> : integral_constant<size_t, 2> { };
template<size_t I, class T1, class T2> struct tuple_element<I, pair<T1, T2>> { using type = see below ; };
Mandates: .
Type: The type T1 if I is 0, otherwise the type T2.
template<size_t I, class T1, class T2> constexpr tuple_element_t<I, pair<T1, T2>>& get(pair<T1, T2>& p) noexcept; template<size_t I, class T1, class T2> constexpr const tuple_element_t<I, pair<T1, T2>>& get(const pair<T1, T2>& p) noexcept; template<size_t I, class T1, class T2> constexpr tuple_element_t<I, pair<T1, T2>>&& get(pair<T1, T2>&& p) noexcept; template<size_t I, class T1, class T2> constexpr const tuple_element_t<I, pair<T1, T2>>&& get(const pair<T1, T2>&& p) noexcept;
Mandates: .
Returns:
  • If I is 0, returns a reference to p.first.
  • If I is 1, returns a reference to p.second.
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;
Mandates: T1 and T2 are distinct types.
Returns: A reference to p.first.
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;
Mandates: T1 and T2 are distinct types.
Returns: A reference to p.second.

20.4.5 Piecewise construction [pair.piecewise]

struct piecewise_construct_t { explicit piecewise_construct_t() = default; }; inline constexpr piecewise_construct_t piecewise_construct{};
The struct piecewise_­construct_­t is an empty class type used as a unique type to disambiguate constructor and function overloading.
Specifically, pair has a constructor with piecewise_­construct_­t as the first argument, immediately followed by two tuple arguments used for piecewise construction of the elements of the pair object.