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[pairs.pair]
# 22 General utilities library [[utilities]](./#utilities)
## 22.3 Pairs [[pairs]](pairs#pair)
### 22.3.2 Class template 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](#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[.](#1.sentence-1)
[2](#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"))[.](#2.sentence-1)
[3](#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[.](#3.sentence-1)
[4](#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[.](#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[.](#4.sentence-2)
[🔗](#lib:pair,constructor)
`constexpr explicit(see below) pair();
`
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L835)
*Constraints*:
- [(5.1)](#5.1)
is_default_constructible_v<T1> is true and
- [(5.2)](#5.2)
is_default_constructible_v<T2> is true[.](#5.sentence-1)
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L842)
*Effects*: Value-initializes first and second[.](#6.sentence-1)
[7](#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[.](#7.sentence-1)
[*Note [1](#note-1)*:
This behavior can be implemented with a trait that checks
whether a const T1& or a const T2& can be initialized with {}[.](#7.sentence-2)
— *end note*]
[🔗](#lib:pair,constructor_)
`constexpr explicit(see below) pair(const T1& x, const T2& y);
`
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L864)
*Constraints*:
- [(8.1)](#8.1)
is_copy_constructible_v<T1> is true and
- [(8.2)](#8.2)
is_copy_constructible_v<T2> is true[.](#8.sentence-1)
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L871)
*Effects*: Initializes first with x and second with y[.](#9.sentence-1)
[10](#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](#11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L889)
*Constraints*:
- [(11.1)](#11.1)
is_constructible_v<T1, U1> is true and
- [(11.2)](#11.2)
is_constructible_v<T2, U2> is true[.](#11.sentence-1)
[12](#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)[.](#12.sentence-1)
[13](#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[.](#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](#14)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L925)
Let *FWD*(u) be static_cast<decltype(u)>(u)[.](#14.sentence-1)
[15](#15)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L928)
*Constraints*:
- [(15.1)](#15.1)
For the last overload,remove_cvref_t<P> is not a specialization of ranges​::​subrange,
- [(15.2)](#15.2)
is_constructible_v<T1, decltype(get<0>(*FWD*(p)))> is true, and
- [(15.3)](#15.3)
is_constructible_v<T2, decltype(get<1>(*FWD*(p)))> is true[.](#15.sentence-1)
[16](#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))[.](#16.sentence-1)
[17](#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[.](#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](#18)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L970)
*Mandates*:
- [(18.1)](#18.1)
is_constructible_v<T1, Args1...> is true and
- [(18.2)](#18.2)
is_constructible_v<T2, Args2...> is true[.](#18.sentence-1)
[19](#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[.](#19.sentence-1)
(Here, forwarding
an element x of type U within a tuple object means callingstd​::​forward<U>(x)[.](#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]")[.](#19.sentence-3)
[*Note [2](#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"))[.](#19.sentence-4)
— *end note*]
[🔗](#lib:operator=,pair)
`constexpr pair& operator=(const pair& p);
`
[20](#20)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1000)
*Effects*: Assigns p.first to first and p.second to second[.](#20.sentence-1)
[21](#21)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1004)
*Returns*: *this[.](#21.sentence-1)
[22](#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[.](#22.sentence-1)
[🔗](#lib:operator=,pair_)
`constexpr const pair& operator=(const pair& p) const;
`
[23](#23)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1021)
*Constraints*:
- [(23.1)](#23.1)
is_copy_assignable_v<const T1> is true and
- [(23.2)](#23.2)
is_copy_assignable_v<const T2> is true[.](#23.sentence-1)
[24](#24)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1030)
*Effects*: Assigns p.first to first and p.second to second[.](#24.sentence-1)
[25](#25)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1034)
*Returns*: *this[.](#25.sentence-1)
[🔗](#lib:operator=,pair__)
`template<class U1, class U2> constexpr pair& operator=(const pair<U1, U2>& p);
`
[26](#26)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1045)
*Constraints*:
- [(26.1)](#26.1)
is_assignable_v<T1&, const U1&> is true and
- [(26.2)](#26.2)
is_assignable_v<T2&, const U2&> is true[.](#26.sentence-1)
[27](#27)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1052)
*Effects*: Assigns p.first to first and p.second to second[.](#27.sentence-1)
[28](#28)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1056)
*Returns*: *this[.](#28.sentence-1)
[🔗](#lib:operator=,pair___)
`template<class U1, class U2> constexpr const pair& operator=(const pair<U1, U2>& p) const;
`
[29](#29)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1067)
*Constraints*:
- [(29.1)](#29.1)
is_assignable_v<const T1&, const U1&> is true, and
- [(29.2)](#29.2)
is_assignable_v<const T2&, const U2&> is true[.](#29.sentence-1)
[30](#30)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1076)
*Effects*: Assigns p.first to first and p.second to second[.](#30.sentence-1)
[31](#31)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1080)
*Returns*: *this[.](#31.sentence-1)
[🔗](#lib:operator=,pair____)
`constexpr pair& operator=(pair&& p) noexcept(see below);
`
[32](#32)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1091)
*Constraints*:
- [(32.1)](#32.1)
is_move_assignable_v<T1> is true and
- [(32.2)](#32.2)
is_move_assignable_v<T2> is true[.](#32.sentence-1)
[33](#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[.](#33.sentence-1)
[34](#34)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1103)
*Returns*: *this[.](#34.sentence-1)
[35](#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](#36)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1121)
*Constraints*:
- [(36.1)](#36.1)
is_assignable_v<const T1&, T1> is true and
- [(36.2)](#36.2)
is_assignable_v<const T2&, T2> is true[.](#36.sentence-1)
[37](#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[.](#37.sentence-1)
[38](#38)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1135)
*Returns*: *this[.](#38.sentence-1)
[🔗](#lib:operator=,pair______)
`template<class U1, class U2> constexpr pair& operator=(pair<U1, U2>&& p);
`
[39](#39)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1146)
*Constraints*:
- [(39.1)](#39.1)
is_assignable_v<T1&, U1> is true and
- [(39.2)](#39.2)
is_assignable_v<T2&, U2> is true[.](#39.sentence-1)
[40](#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[.](#40.sentence-1)
[41](#41)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1158)
*Returns*: *this[.](#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](#42)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1169)
*Constraints*:
- [(42.1)](#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)](#42.2)
remove_cvref_t<P> is not a specialization of ranges​::​subrange,
- [(42.3)](#42.3)
is_assignable_v<T1&, decltype(get<0>(std​::​forward<P>(p)))> is true, and
- [(42.4)](#42.4)
is_assignable_v<T2&, decltype(get<1>(std​::​forward<P>(p)))> is true[.](#42.sentence-1)
[43](#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[.](#43.sentence-1)
[44](#44)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1190)
*Returns*: *this[.](#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](#45)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1201)
*Constraints*:
- [(45.1)](#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)](#45.2)
remove_cvref_t<P> is not a specialization of ranges​::​subrange,
- [(45.3)](#45.3)
is_assignable_v<const T1&, decltype(get<0>(std​::​forward<P>(p)))> is true, and
- [(45.4)](#45.4)
is_assignable_v<const T2&, decltype(get<1>(std​::​forward<P>(p)))> is true[.](#45.sentence-1)
[46](#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[.](#46.sentence-1)
[47](#47)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1222)
*Returns*: *this[.](#47.sentence-1)
[🔗](#lib:operator=,pair_________)
`template<class U1, class U2> constexpr const pair& operator=(pair<U1, U2>&& p) const;
`
[48](#48)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1233)
*Constraints*:
- [(48.1)](#48.1)
is_assignable_v<const T1&, U1> is true, and
- [(48.2)](#48.2)
is_assignable_v<const T2&, U2> is true[.](#48.sentence-1)
[49](#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[.](#49.sentence-1)
[50](#50)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1247)
*Returns*: *this[.](#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](#51)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1259)
*Mandates*:
- [(51.1)](#51.1)
For the first overload,is_swappable_v<T1> is true andis_swappable_v<T2> is true[.](#51.1.sentence-1)
- [(51.2)](#51.2)
For the second overload,is_swappable_v<const T1> is true andis_swappable_v<const T2> is true[.](#51.2.sentence-1)
[52](#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[.](#52.sentence-1)
[53](#53)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1277)
*Effects*: Swapsfirst with p.first andsecond with p.second[.](#53.sentence-1)
[54](#54)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L1283)
*Remarks*: The exception specification is equivalent to:
- [(54.1)](#54.1)
is_nothrow_swappable_v<T1> && is_nothrow_swappable_v<T2> for the first overload, and
- [(54.2)](#54.2)
is_nothrow_swappable_v<const T1> && is_nothrow_swappable_v<const T2> for the second overload[.](#54.sentence-1)