[pairs]

# 22 General utilities library [[utilities]](./#utilities)

## 22.3 Pairs [pairs]

### [22.3.1](#general) General [[pairs.general]](pairs.general)

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L737)

The library provides a template for heterogeneous pairs of values[.](#general-1.sentence-1)

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]](tuple.helper "22.4.7 Tuple helper classes") and [[tuple.elem]](tuple.elem "22.4.8 Element access"))[.](#general-1.sentence-2)

### [22.3.2](#pair) Class template pair [[pairs.pair]](pairs.pair)

[🔗](#lib: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(pair<U1, U2>& p); 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 U1, class U2>constexpr explicit(*see below*) pair(const pair<U1, U2>&& p); template<[*pair-like*](tuple.syn#concept:pair-like "22.4.2 Header <tuple> synopsis [tuple.syn]") P>constexpr explicit(*see below*) pair(P&& 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); constexpr const pair& operator=(const pair& p) const; template<class U1, class U2>constexpr pair& operator=(const pair<U1, U2>& p); template<class U1, class U2>constexpr const pair& operator=(const pair<U1, U2>& p) const; constexpr pair& operator=(pair&& p) noexcept(*see below*); constexpr const pair& operator=(pair&& p) const; template<class U1, class U2>constexpr pair& operator=(pair<U1, U2>&& p); template<class U1, class U2>constexpr const pair& operator=(pair<U1, U2>&& p) const; template<[*pair-like*](tuple.syn#concept:pair-like "22.4.2 Header <tuple> synopsis [tuple.syn]") P>constexpr pair& operator=(P&& p); template<[*pair-like*](tuple.syn#concept:pair-like "22.4.2 Header <tuple> synopsis [tuple.syn]") P>constexpr const pair& operator=(P&& p) const; constexpr void swap(pair& p) noexcept(*see below*); constexpr void swap(const pair& p) const noexcept(*see below*); }; template<class T1, class T2> pair(T1, T2) -> pair<T1, T2>;}

[1](#pair-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L806)

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[.](#pair-1.sentence-1)

[2](#pair-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L811)

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 be constexpr-suitable ([[dcl.constexpr]](dcl.constexpr "9.2.6 The constexpr and consteval specifiers"))[.](#pair-2.sentence-1)

[3](#pair-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L817)

If (is_trivially_destructible_v<T1> && is_trivially_destructible_v<T2>) is true, then the destructor of pair is trivial[.](#pair-3.sentence-1)

[4](#pair-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L821)

pair<T, U> is a structural type ([[temp.param]](temp.param#term.structural.type "13.2 Template parameters"))
if T and U are both structural types[.](#pair-4.sentence-1)

Two values p1 and p2 of type pair<T, U> are template-argument-equivalent ([[temp.type]](temp.type "13.6 Type equivalence")) if and only ifp1.first and p2.first are template-argument-equivalent andp1.second and p2.second are template-argument-equivalent[.](#pair-4.sentence-2)

[🔗](#lib:pair,constructor)

`constexpr explicit(see below) pair();
`

[5](#pair-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L835)

*Constraints*: 

- [(5.1)](#pair-5.1)

is_default_constructible_v<T1> is true and

- [(5.2)](#pair-5.2)

is_default_constructible_v<T2> is true[.](#pair-5.sentence-1)

[6](#pair-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L842)

*Effects*: Value-initializes first and second[.](#pair-6.sentence-1)

[7](#pair-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L846)

*Remarks*: The expression inside explicit evaluates to true if and only if either T1 orT2 is not implicitly default-constructible[.](#pair-7.sentence-1)

[*Note [1](#pair-note-1)*:

This behavior can be implemented with a trait that checks
whether a const T1& or a const T2& can be initialized with {}[.](#pair-7.sentence-2)

— *end note*]

[🔗](#lib:pair,constructor_)

`constexpr explicit(see below) pair(const T1& x, const T2& y);
`

[8](#pair-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L864)

*Constraints*: 

- [(8.1)](#pair-8.1)

is_copy_constructible_v<T1> is true and

- [(8.2)](#pair-8.2)

is_copy_constructible_v<T2> is true[.](#pair-8.sentence-1)

[9](#pair-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L871)

*Effects*: Initializes first with x and second with y[.](#pair-9.sentence-1)

[10](#pair-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L875)

*Remarks*: The expression inside explicit is equivalent to:!is_convertible_v<const T1&, T1> || !is_convertible_v<const T2&, T2>

[🔗](#lib:pair,constructor__)

`template<class U1 = T1, class U2 = T2> constexpr explicit(see below) pair(U1&& x, U2&& y);
`

[11](#pair-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L889)

*Constraints*: 

- [(11.1)](#pair-11.1)

is_constructible_v<T1, U1> is true and

- [(11.2)](#pair-11.2)

is_constructible_v<T2, U2> is true[.](#pair-11.sentence-1)

[12](#pair-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L896)

*Effects*: Initializes first withstd​::​forward<U1>(x) and second with std​::​forward<U2>(y)[.](#pair-12.sentence-1)

[13](#pair-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L902)

*Remarks*: The expression inside explicit is equivalent to:!is_convertible_v<U1, T1> || !is_convertible_v<U2, T2>

This constructor is defined as deleted ifreference_constructs_from_temporary_v<first_type, U1&&> is true orreference_constructs_from_temporary_v<second_type, U2&&> is true[.](#pair-13.sentence-2)

[🔗](#lib:pair,constructor___)

`template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>& p);
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 U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>&& p);
template<[pair-like](tuple.syn#concept:pair-like "22.4.2 Header <tuple> synopsis [tuple.syn]") P> constexpr explicit(see below) pair(P&& p);
`

[14](#pair-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L925)

Let *FWD*(u) be static_cast<decltype(u)>(u)[.](#pair-14.sentence-1)

[15](#pair-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L928)

*Constraints*: 

- [(15.1)](#pair-15.1)

For the last overload,remove_cvref_t<P> is not a specialization of ranges​::​subrange,

- [(15.2)](#pair-15.2)

is_constructible_v<T1, decltype(get<0>(*FWD*(p)))> is true, and

- [(15.3)](#pair-15.3)

is_constructible_v<T2, decltype(get<1>(*FWD*(p)))> is true[.](#pair-15.sentence-1)

[16](#pair-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L942)

*Effects*: Initializes first with get<0>(*FWD*(p)) andsecond with get<1>(*FWD*(p))[.](#pair-16.sentence-1)

[17](#pair-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L947)

*Remarks*: The expression inside explicit is equivalent to:!is_convertible_v<decltype(get<0>(*FWD*(p))), T1> ||!is_convertible_v<decltype(get<1>(*FWD*(p))), T2>

The constructor is defined as deleted ifreference_constructs_from_temporary_v<first_type, decltype(get<0>(*FWD*(p)))> || reference_constructs_from_temporary_v<second_type, decltype(get<1>(*FWD*(p)))> is true[.](#pair-17.sentence-2)

[🔗](#lib:pair,constructor____)

`template<class... Args1, class... Args2>
  constexpr pair(piecewise_construct_t,
                 tuple<Args1...> first_args, tuple<Args2...> second_args);
`

[18](#pair-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L970)

*Mandates*: 

- [(18.1)](#pair-18.1)

is_constructible_v<T1, Args1...> is true and

- [(18.2)](#pair-18.2)

is_constructible_v<T2, Args2...> is true[.](#pair-18.sentence-1)

[19](#pair-19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L977)

*Effects*: Initializes first with arguments of typesArgs1... obtained by forwarding the elements of first_args and initializes second with arguments of types Args2... obtained by forwarding the elements of second_args[.](#pair-19.sentence-1)

(Here, forwarding
an element x of type U within a tuple object means callingstd​::​forward<U>(x)[.](#pair-19.sentence-2))

This form of construction, whereby constructor
arguments for first and second are each provided in a separatetuple object, is called [*piecewise construction*](#def:piecewise_construction "22.3.2 Class template pair [pairs.pair]")[.](#pair-19.sentence-3)

[*Note [2](#pair-note-2)*:

If a data member of pair is of reference type and
its initialization binds it to a temporary object,
the program is ill-formed ([[class.base.init]](class.base.init "11.9.3 Initializing bases and members"))[.](#pair-19.sentence-4)

— *end note*]

[🔗](#lib:operator=,pair)

`constexpr pair& operator=(const pair& p);
`

[20](#pair-20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1000)

*Effects*: Assigns p.first to first and p.second to second[.](#pair-20.sentence-1)

[21](#pair-21)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1004)

*Returns*: *this[.](#pair-21.sentence-1)

[22](#pair-22)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1008)

*Remarks*: This operator is defined as deleted unlessis_copy_assignable_v<T1> is true andis_copy_assignable_v<T2> is true[.](#pair-22.sentence-1)

[🔗](#lib:operator=,pair_)

`constexpr const pair& operator=(const pair& p) const;
`

[23](#pair-23)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1021)

*Constraints*: 

- [(23.1)](#pair-23.1)

is_copy_assignable_v<const T1> is true and

- [(23.2)](#pair-23.2)

is_copy_assignable_v<const T2> is true[.](#pair-23.sentence-1)

[24](#pair-24)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1030)

*Effects*: Assigns p.first to first and p.second to second[.](#pair-24.sentence-1)

[25](#pair-25)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1034)

*Returns*: *this[.](#pair-25.sentence-1)

[🔗](#lib:operator=,pair__)

`template<class U1, class U2> constexpr pair& operator=(const pair<U1, U2>& p);
`

[26](#pair-26)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1045)

*Constraints*: 

- [(26.1)](#pair-26.1)

is_assignable_v<T1&, const U1&> is true and

- [(26.2)](#pair-26.2)

is_assignable_v<T2&, const U2&> is true[.](#pair-26.sentence-1)

[27](#pair-27)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1052)

*Effects*: Assigns p.first to first and p.second to second[.](#pair-27.sentence-1)

[28](#pair-28)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1056)

*Returns*: *this[.](#pair-28.sentence-1)

[🔗](#lib:operator=,pair___)

`template<class U1, class U2> constexpr const pair& operator=(const pair<U1, U2>& p) const;
`

[29](#pair-29)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1067)

*Constraints*: 

- [(29.1)](#pair-29.1)

is_assignable_v<const T1&, const U1&> is true, and

- [(29.2)](#pair-29.2)

is_assignable_v<const T2&, const U2&> is true[.](#pair-29.sentence-1)

[30](#pair-30)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1076)

*Effects*: Assigns p.first to first and p.second to second[.](#pair-30.sentence-1)

[31](#pair-31)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1080)

*Returns*: *this[.](#pair-31.sentence-1)

[🔗](#lib:operator=,pair____)

`constexpr pair& operator=(pair&& p) noexcept(see below);
`

[32](#pair-32)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1091)

*Constraints*: 

- [(32.1)](#pair-32.1)

is_move_assignable_v<T1> is true and

- [(32.2)](#pair-32.2)

is_move_assignable_v<T2> is true[.](#pair-32.sentence-1)

[33](#pair-33)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1098)

*Effects*: Assigns std​::​forward<T1>(p.first) to first andstd​::​forward<T2>(p.second) to second[.](#pair-33.sentence-1)

[34](#pair-34)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1103)

*Returns*: *this[.](#pair-34.sentence-1)

[35](#pair-35)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1107)

*Remarks*: The exception specification is equivalent to:is_nothrow_move_assignable_v<T1> && is_nothrow_move_assignable_v<T2>

[🔗](#lib:operator=,pair_____)

`constexpr const pair& operator=(pair&& p) const;
`

[36](#pair-36)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1121)

*Constraints*: 

- [(36.1)](#pair-36.1)

is_assignable_v<const T1&, T1> is true and

- [(36.2)](#pair-36.2)

is_assignable_v<const T2&, T2> is true[.](#pair-36.sentence-1)

[37](#pair-37)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1130)

*Effects*: Assigns std​::​forward<T1>(p.first) to first andstd​::​forward<T2>(p.second) to second[.](#pair-37.sentence-1)

[38](#pair-38)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1135)

*Returns*: *this[.](#pair-38.sentence-1)

[🔗](#lib:operator=,pair______)

`template<class U1, class U2> constexpr pair& operator=(pair<U1, U2>&& p);
`

[39](#pair-39)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1146)

*Constraints*: 

- [(39.1)](#pair-39.1)

is_assignable_v<T1&, U1> is true and

- [(39.2)](#pair-39.2)

is_assignable_v<T2&, U2> is true[.](#pair-39.sentence-1)

[40](#pair-40)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1153)

*Effects*: Assigns std​::​forward<U1>(p.first) first andstd​::​forward<U2>(p.second) to second[.](#pair-40.sentence-1)

[41](#pair-41)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1158)

*Returns*: *this[.](#pair-41.sentence-1)

[🔗](#lib:operator=,pair_______)

`template<[pair-like](tuple.syn#concept:pair-like "22.4.2 Header <tuple> synopsis [tuple.syn]") P> constexpr pair& operator=(P&& p);
`

[42](#pair-42)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1169)

*Constraints*: 

- [(42.1)](#pair-42.1)

[*different-from*](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<P, pair> ([[range.utility.helpers]](range.utility.helpers "25.5.2 Helper concepts"))
is true,

- [(42.2)](#pair-42.2)

remove_cvref_t<P> is not a specialization of ranges​::​subrange,

- [(42.3)](#pair-42.3)

is_assignable_v<T1&, decltype(get<0>(std​::​forward<P>(p)))> is true, and

- [(42.4)](#pair-42.4)

is_assignable_v<T2&, decltype(get<1>(std​::​forward<P>(p)))> is true[.](#pair-42.sentence-1)

[43](#pair-43)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1185)

*Effects*: Assigns get<0>(std​::​forward<P>(p)) to first andget<1>(std​::​forward<P>(p)) to second[.](#pair-43.sentence-1)

[44](#pair-44)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1190)

*Returns*: *this[.](#pair-44.sentence-1)

[🔗](#lib:operator=,pair________)

`template<[pair-like](tuple.syn#concept:pair-like "22.4.2 Header <tuple> synopsis [tuple.syn]") P> constexpr const pair& operator=(P&& p) const;
`

[45](#pair-45)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1201)

*Constraints*: 

- [(45.1)](#pair-45.1)

[*different-from*](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<P, pair> ([[range.utility.helpers]](range.utility.helpers "25.5.2 Helper concepts"))
is true,

- [(45.2)](#pair-45.2)

remove_cvref_t<P> is not a specialization of ranges​::​subrange,

- [(45.3)](#pair-45.3)

is_assignable_v<const T1&, decltype(get<0>(std​::​forward<P>(p)))> is true, and

- [(45.4)](#pair-45.4)

is_assignable_v<const T2&, decltype(get<1>(std​::​forward<P>(p)))> is true[.](#pair-45.sentence-1)

[46](#pair-46)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1217)

*Effects*: Assigns get<0>(std​::​forward<P>(p)) to first andget<1>(std​::​forward<P>(p)) to second[.](#pair-46.sentence-1)

[47](#pair-47)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1222)

*Returns*: *this[.](#pair-47.sentence-1)

[🔗](#lib:operator=,pair_________)

`template<class U1, class U2> constexpr const pair& operator=(pair<U1, U2>&& p) const;
`

[48](#pair-48)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1233)

*Constraints*: 

- [(48.1)](#pair-48.1)

is_assignable_v<const T1&, U1> is true, and

- [(48.2)](#pair-48.2)

is_assignable_v<const T2&, U2> is true[.](#pair-48.sentence-1)

[49](#pair-49)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1242)

*Effects*: Assigns std​::​forward<U1>(p.first) to first andstd​::​forward<U2>(u.second) to second[.](#pair-49.sentence-1)

[50](#pair-50)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1247)

*Returns*: *this[.](#pair-50.sentence-1)

[🔗](#lib:swap,pair)

`constexpr void swap(pair& p) noexcept(see below);
constexpr void swap(const pair& p) const noexcept(see below);
`

[51](#pair-51)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1259)

*Mandates*: 

- [(51.1)](#pair-51.1)

  For the first overload,is_swappable_v<T1> is true andis_swappable_v<T2> is true[.](#pair-51.1.sentence-1)

- [(51.2)](#pair-51.2)

  For the second overload,is_swappable_v<const T1> is true andis_swappable_v<const T2> is true[.](#pair-51.2.sentence-1)

[52](#pair-52)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1272)

*Preconditions*: first is swappable with ([[swappable.requirements]](swappable.requirements "16.4.4.3 Swappable requirements")) p.first andsecond is swappable with p.second[.](#pair-52.sentence-1)

[53](#pair-53)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1277)

*Effects*: Swapsfirst with p.first andsecond with p.second[.](#pair-53.sentence-1)

[54](#pair-54)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1283)

*Remarks*: The exception specification is equivalent to:

- [(54.1)](#pair-54.1)

is_nothrow_swappable_v<T1> && is_nothrow_swappable_v<T2> for the first overload, and

- [(54.2)](#pair-54.2)

is_nothrow_swappable_v<const T1> && is_nothrow_swappable_v<const T2> for the second overload[.](#pair-54.sentence-1)

### [22.3.3](#spec) Specialized algorithms [[pairs.spec]](pairs.spec)

[🔗](#lib:operator==,pair)

`template<class T1, class T2, class U1, class U2>
  constexpr bool operator==(const pair<T1, T2>& x, const pair<U1, U2>& y);
`

[1](#spec-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1305)

*Constraints*: x.first == y.first and x.second == y.second are
valid expressions and
each of decltype(x.first == y.first) anddecltype(x.second == y.second) models *boolean-  
testable*[.](#spec-1.sentence-1)

[2](#spec-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1312)

*Returns*: x.first == y.first && x.second == y.second[.](#spec-2.sentence-1)

[🔗](#lib:operator%3c=%3e,pair)

`template<class T1, class T2, class U1, class U2>
  constexpr common_comparison_category_t<synth-three-way-result<T1, U1>,
                                         synth-three-way-result<T2, U2>>
    operator<=>(const pair<T1, T2>& x, const pair<U1, U2>& y);
`

[3](#spec-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1326)

*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);

[🔗](#lib:swap,pair_)

`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 void swap(const pair<T1, T2>& x, const pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));
`

[4](#spec-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1344)

*Constraints*: 

- [(4.1)](#spec-4.1)

  For the first overload,is_swappable_v<T1> is true andis_swappable_v<T2> is true[.](#spec-4.1.sentence-1)

- [(4.2)](#spec-4.2)

  For the second overload,is_swappable_v<const T1> is true andis_swappable_v<const T2> is true[.](#spec-4.2.sentence-1)

[5](#spec-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1357)

*Effects*: Equivalent to x.swap(y)[.](#spec-5.sentence-1)

[🔗](#lib:make_pair)

`template<class T1, class T2>
  constexpr pair<unwrap_ref_decay_t<T1>, unwrap_ref_decay_t<T2>> make_pair(T1&& x, T2&& y);
`

[6](#spec-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1369)

*Returns*: pair<unwrap_ref_decay_t<T1>,
 unwrap_ref_decay_t<T2>>(std::forward<T1>(x), std::forward<T2>(y))

[7](#spec-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1377)

[*Example [1](#spec-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*]

### [22.3.4](#pair.astuple) Tuple-like access to pair [[pair.astuple]](pair.astuple)

[🔗](#lib:tuple_size)

`template<class T1, class T2>
  struct tuple_size<pair<T1, T2>> : integral_constant<size_t, 2> { };
`

[🔗](#lib:tuple_element)

`template<size_t I, class T1, class T2>
  struct tuple_element<I, pair<T1, T2>> {
    using type = see below ;
  };
`

[1](#pair.astuple-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1405)

*Mandates*: I<2[.](#pair.astuple-1.sentence-1)

[2](#pair.astuple-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1409)

*Type*: The type T1 if I is 0, otherwise the type T2[.](#pair.astuple-2.sentence-1)

[🔗](#lib:get,pair)

`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;
`

[3](#pair.astuple-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1427)

*Mandates*: I<2[.](#pair.astuple-3.sentence-1)

[4](#pair.astuple-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1431)

*Returns*: 

- [(4.1)](#pair.astuple-4.1)

  If I is 0, returns a reference to p.first[.](#pair.astuple-4.1.sentence-1)

- [(4.2)](#pair.astuple-4.2)

  If I is 1, returns a reference to p.second[.](#pair.astuple-4.2.sentence-1)

[🔗](#lib:get,pair_)

`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;
`

[5](#pair.astuple-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1455)

*Mandates*: T1 and T2 are distinct types[.](#pair.astuple-5.sentence-1)

[6](#pair.astuple-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1459)

*Returns*: A reference to p.first[.](#pair.astuple-6.sentence-1)

[🔗](#lib:get,pair__)

`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;
`

[7](#pair.astuple-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1477)

*Mandates*: T1 and T2 are distinct types[.](#pair.astuple-7.sentence-1)

[8](#pair.astuple-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1481)

*Returns*: A reference to p.second[.](#pair.astuple-8.sentence-1)

### [22.3.5](#pair.piecewise) Piecewise construction [[pair.piecewise]](pair.piecewise)

[🔗](#lib:piecewise_construct_t)

`struct piecewise_construct_t {
  explicit piecewise_construct_t() = default;
};
inline constexpr piecewise_construct_t piecewise_construct{};
`

[1](#pair.piecewise-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1497)

The struct piecewise_construct_t is an empty class type
used as a unique type to disambiguate constructor and function overloading[.](#pair.piecewise-1.sentence-1)

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[.](#pair.piecewise-1.sentence-2)