std::pair<T1,T2>::operator=

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[edit]

Utilities library

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pair
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[edit]

std::pair

Member functions
pair::pair
pair::operator=
pair::swap (C++11)
Non-member functions
make_pair
operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
swap(std::pair) (C++11)
get(std::pair) (C++11)
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tuple_size<std::pair> (C++11)
tuple_element<std::pair> (C++11)
basic_common_reference<std::pair> (C++23)
common_type<std::pair> (C++23)
formatter<std::pair> (C++23)
piecewise_construct_t (C++11)
Deduction guides(C++17)

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	(1)
pair& operator=(const pair& other);			(until C++20)
constexpr pair& operator=(const pair& other);			(since C++20)
constexprconst pair& operator=(const pair& other)const;		(2)	(since C++23)
		(3)
template<class U1,class U2> pair& operator=(const pair<U1, U2>& other);				(until C++20)
template<class U1,class U2> constexpr pair& operator=(const pair<U1, U2>& other);				(since C++20)
template<class U1,class U2> constexprconst pair& operator=(const pair<U1, U2>& other)const;			(4)	(since C++23)
			(5)
pair& operator=( pair&& other)noexcept(/* see below */);					(since C++11) (until C++20)
constexpr pair& operator=( pair&& other)noexcept(/* see below */);					(since C++20)
constexprconst pair& operator=( pair&& other)const;				(6)	(since C++23)
				(7)
template<class U1,class U2> pair& operator=( pair<U1, U2>&& p);						(since C++11) (until C++20)
template<class U1,class U2> constexpr pair& operator=( pair<U1, U2>&& p);						(since C++20)
template<class U1,class U2> constexprconst pair& operator=( pair<U1, U2>&& p)const;					(8)	(since C++23)
template< pair-like P> constexpr pair& operator=( P&& u);					(9)	(since C++23)
template< pair-like P> constexprconst pair& operator=( P&& u)const;					(10)	(since C++23)

Replaces the contents of the pair.

1) Copy assignment operator. Replaces the contents with a copy of the contents ofother.

The assignment operator is implicitly declared. Using this assignment operator makes the program ill-formed if either`T1` or`T2` is a const-qualified type, or a reference type, or a class type with an inaccessible copy assignment operator, or an array type of such class.	(until C++11)
This overload is defined as deleted if eitherstd::is_copy_assignable<T1>::value orstd::is_copy_assignable<T2>::value isfalse.	(since C++11)

2) Copy assignment operator for const-qualified operand.

This overload participates in overload resolution only ifstd::is_copy_assignable_v<const T1> andstd::is_copy_assignable_v<const T2> are bothtrue.

3) Assignsother.first tofirst andother.second tosecond.

This overload participates in overload resolution only ifstd::is_assignable<T1&,const U1&>::value andstd::is_assignable<T2&,const U2&>::value are bothtrue.

(since C++11)

4) Assignsother.first tofirst andother.second tosecond.

This overload participates in overload resolution only ifstd::is_assignable_v<const T1&,const U1&> andstd::is_assignable_v<const T2&,const U2&> are bothtrue.

5) Move assignment operator. Replaces the contents with those ofother using move semantics.

This overload participates in overload resolution only ifstd::is_move_assignable<T1>::value andstd::is_move_assignable<T2>::value are bothtrue.

6) Move assignment operator for const-qualified operand.

This overload participates in overload resolution only ifstd::is_assignable_v<const T1&, T1> andstd::is_assignable_v<const T2&, T2> are bothtrue.

7) Assignsstd::forward<U1>(p.first) tofirst andstd::forward<U2>(p.second) tosecond.

This overload participates in overload resolution only ifstd::is_assignable<T1&, U1>::value andstd::is_assignable<T2&, U2>::value are bothtrue.

8) Assignsstd::forward<U1>(p.first) tofirst andstd::forward<U2>(p.second) tosecond.

This overload participates in overload resolution only ifstd::is_assignable_v<const T1&, U1> andstd::is_assignable_v<const T2&, U2> are bothtrue.

9) Assignsstd::get<0>(std::forward(u)) tofirst andstd::get<1>(std::forward(u)) tosecond.

This overload participates in overload resolution only if

std::same_as<std::remove_cvref_t,std::pair> isfalse,
std::remove_cvref_t is not a specialization ofstd::ranges::subrange,
std::is_assignable_v<T1&, decltype(std::get<0>(std::forward(p)))> istrue, and
std::is_assignable_v<T1&, decltype(std::get<1>(std::forward(p)))> istrue.

10) Assignsstd::get<0>(std::forward(u)) tofirst andstd::get<1>(std::forward(u)) tosecond.

This overload participates in overload resolution only if

std::same_as<std::remove_cvref_t,std::pair> isfalse,
std::remove_cvref_t is not a specialization ofstd::ranges::subrange,
std::is_assignable_v<const T1&, decltype(std::get<0>(std::forward(p)))> istrue, and
std::is_assignable_v<const T1&, decltype(std::get<1>(std::forward(p)))> istrue.

[edit]Parameters

other	-	pair of values to replace the contents of this pair
p	-	pair of values of possibly different types to replace the contents of this pair
u	-	`pair-like` object of values to replace the contents of this pair
Type requirements
- `T1` must meet the requirements ofCopyAssignable from`U1`.(until C++11)
- `T2` must meet the requirements ofCopyAssignable from`U2`.(until C++11)

[edit]Return value

*this

[edit]Exceptions

1-4) May throw implementation-defined exceptions.

noexcept specification:

noexcept(

std::is_nothrow_move_assignable<T1>::value&&
std::is_nothrow_move_assignable<T2>::value

)

6-10) May throw implementation-defined exceptions.

[edit]Example

Run this code

#include <cstddef>#include <iomanip>#include <iostream>#include <utility>#include <vector> template<class Os,class T>Os& operator<<(Os& os,conststd::vector<T>& v){    os<<'{';for(std::size_t t=0; t!= v.size();++t)        os<< v[t]<<(t+1< v.size()?", ":"");return os<<'}';} template<class Os,class U1,class U2>Os& operator<<(Os& os,conststd::pair<U1, U2>& pair){return os<<'{'<< pair.first<<", "<< pair.second<<'}';} int main(){std::pair<int,std::vector<int>> p{1,{2}}, q{2,{5,6}};     p= q;// (1) operator=(const pair& other);std::cout<<std::setw(23)<<std::left<<"(1) p = q;"<<"p: "<< p<<"     q: "<< q<<'\n'; std::pair<short,std::vector<int>> r{4,{7,8,9}};    p= r;// (3) operator=(const pair<U1, U2>& other);std::cout<<std::setw(23)<<"(3) p = r;"<<"p: "<< p<<"  r: "<< r<<'\n';     p=std::pair<int,std::vector<int>>{3,{4}};    p= std::move(q);// (5) operator=(pair&& other);std::cout<<std::setw(23)<<"(5) p = std::move(q);"<<"p: "<< p<<"     q: "<< q<<'\n';     p=std::pair<int,std::vector<int>>{5,{6}};    p= std::move(r);// (7) operator=(pair<U1, U2>&& other);std::cout<<std::setw(23)<<"(7) p = std::move(r);"<<"p: "<< p<<"  r: "<< r<<'\n';}

Output:

(1) p = q;             p: {2, {5, 6}}     q: {2, {5, 6}}(3) p = r;             p: {4, {7, 8, 9}}  r: {4, {7, 8, 9}}(5) p = std::move(q);  p: {2, {5, 6}}     q: {2, {}}(7) p = std::move(r);  p: {4, {7, 8, 9}}  r: {4, {}}

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 885	C++98	missing heterogeneous copy assignment	added (as overload(3))
LWG 2729	C++11	`pair::operator=` was unconstrained and might result in unnecessary undefined behavior	constrained