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

	(1)
pair& operator=( const pair& other );			(until C++20)
constexpr pair& operator=( const pair& other );			(since C++20)
		(2)
template< class U1, class U2 > pair& operator=( const pair<U1,U2>& other );				(since C++11) (until C++20)
template< class U1, class U2 > constexpr pair& operator=( const pair<U1,U2>& other );				(since C++20)
			(3)
pair& operator=( pair&& other ) noexcept(/* see below */);					(since C++11) (until C++20)
constexpr pair& operator=( pair&& other ) noexcept(/* see below */);					(since C++20)
				(4)
template< class U1, class U2 > pair& operator=( pair<U1,U2>&& other );						(since C++11) (until C++20)
template< class U1, class U2 > constexpr pair& operator=( pair<U1,U2>&& other );						(since C++20)

Replaces the contents of the pair.

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

The assignment operator is implicitly declared. Using this assignment operator makes the program ill-formed if either `first_type` or `second_type` 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 either std::is_copy_assignable<first_type>::value or std::is_copy_assignable<second_type>::value is false.	(since C++11)

2) Assigns other.first to first and other.second to second.

This overload only participates in overload resolution if std::is_assignable<first_type&, const U1&>::value and std::is_assignable<second_type&, const U2&>::value are both true.

3) Move assignment operator. Replaces the contents with those of other using move semantics.

This overload only participates in overload resolution if std::is_move_assignable<first_type>::value and std::is_move_assignable<second_type>::value are both true.

4) Assigns std::forward<U1>(p.first) to first and std::forward<U2>(p.second) to second.

This overload only participates in overload resolution if std::is_assignable<first_type&, U1>::value and std::is_assignable<second_type&, U2>::value are both true.

Parameters

other

-

pair of values to replace the contents of this pair

Return value

*this

Exceptions

1-2) May throw implementation-defined exceptions.

3)

noexcept specification:

noexcept(

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

)

4) May throw implementation-defined exceptions.

Example

Run this code

#include <iomanip>
#include <iostream>
#include <utility>
#include <vector>
 
template <class Os, class T>
Os& operator<<(Os& os, const std::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, const std::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; // (2) operator=( const pair<U1,U2>& other );
    std::cout << std::setw(23)
              << "(2) p = r;" << "p" << p << " r" << r << '\n';
 
    p = std::pair<int, std::vector<int>>{ 3, {4} };
    p = std::move(q); // (3) operator=( pair&& other );
    std::cout << std::setw(23)
              << "(3) p = std::move(q);" << "p" << p << "   q" << q << '\n';
 
    p = std::pair<int, std::vector<int>>{ 5, {6} };
    p = std::move(r); // (4) operator=( pair<U1,U2>&& other );
    std::cout << std::setw(23)
              << "(4) p = std::move(r);" << "p" << p << " r" << r << '\n';
}

Output:

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

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 2729	C++11	`pair::operator=` was unconstrained and might result in unnecessary undefined behavior	constrained

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Technical Specifications

Member functions
pair::pair
pair::operator=
pair::swap
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)
std::swap
get (C++11)
Deduction guides(C++17)
Helper classes
tuple_size (C++11)
tuple_element (C++11)