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Showing content from https://en.cppreference.com/w/cpp/algorithm/../ranges/../../cpp/utility/pair/operator=.html below:

std::pair<T1,T2>::operator= - cppreference.com

(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.

3)

Assigns

other.first

first

and

other.second

second

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

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

6) Move assignment operator for const-qualified operand.

[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 of CopyAssignable from U1. (until C++11) -T2 must meet the requirements of CopyAssignable from U2. (until C++11) [edit] Return value

*this

[edit] Exceptions

1-4) May throw implementation-defined exceptions.

6-10) May throw implementation-defined exceptions.

[edit] Example

#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, {}}

[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 [edit] See also assigns the contents of one tuple to another
(public member function of std::tuple<Types...>) [edit]

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