W3cubDocs

/C++

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

	(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 );		(since C++11) (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.

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(u)) to first and std::get<1>(std::forward(u)) to second.

This overload participates in overload resolution only if

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

10) Assigns std::get<0>(std::forward(u)) to first and std::get<1>(std::forward(u)) to second.

This overload participates in overload resolution only if

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

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

Return value

*this.

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.

Example

#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.