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

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
std::pair
(1)
pair & operator = ( const pair & other ) ;
(until C++20)
constexpr pair & operator = ( const pair & other ) ;
(since C++20)
constexpr const 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 >
constexpr const 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)
constexpr const 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 >
constexpr const 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 >
constexpr const 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 of other .

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 either std:: is_copy_assignable < T1 > :: value or std:: is_copy_assignable < T2 > :: value is false .

(since C++11)
2) Copy assignment operator for const-qualified operand.
This overload participates in overload resolution only if std:: is_copy_assignable_v < const T1 > and std:: is_copy_assignable_v < const T2 > are both true .
3) Assigns other. first to first and other. second to second .

This overload participates in overload resolution only if std:: is_assignable < T1 & , const U1 & > :: value and std:: is_assignable < T2 & , const U2 & > :: value are both true .

(since C++11)
4) Assigns other. first to first and other. second to second .
This overload participates in overload resolution only if std:: is_assignable_v < const T1 & , const U1 & > and std:: is_assignable_v < const T2 & , const U2 & > are both true .
5) Move assignment operator. Replaces the contents with those of other using move semantics.
This overload participates in overload resolution only if std:: is_move_assignable < T1 > :: value and std:: is_move_assignable < T2 > :: value are both true .
6) Move assignment operator for const-qualified operand.
This overload participates in overload resolution only if std:: is_assignable_v < const T1 & , T1 > and std:: is_assignable_v < const T2 & , T2 > are both true .
7) Assigns std:: forward < U1 > ( p. first ) to first and std:: forward < U2 > ( p. second ) to second .
This overload participates in overload resolution only if std:: is_assignable < T1 & , U1 > :: value and std:: is_assignable < T2 & , U2 > :: value are both true .
8) Assigns std:: forward < U1 > ( p. first ) to first and std:: forward < U2 > ( p. second ) to second .
This overload participates in overload resolution only if std:: is_assignable_v < const T1 & , U1 > and std:: is_assignable_v < const T2 & , U2 > are both true .
9) Assigns std :: get < 0 > ( std:: forward < P > ( u ) ) to first and std :: get < 1 > ( std:: forward < P > ( u ) ) to second .
This overload participates in overload resolution only if
10) Assigns std :: get < 0 > ( std:: forward < P > ( u ) ) to first and std :: get < 1 > ( std:: forward < P > ( u ) ) to second .
This overload participates in overload resolution only if

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 of CopyAssignable from U1 . (until C++11)
-
T2 must meet the requirements of CopyAssignable from U2 . (until C++11)

Return value

* this

Exceptions

1-4) May throw implementation-defined exceptions.
5)
noexcept specification:
noexcept (

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

)
6-10) May throw implementation-defined exceptions.

Example

Run this code 
#include <cstddef>
#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; // (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, {}}

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

See also

assigns the contents of one tuple to another
(public member function of std::tuple<Types...> )
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