NAME

std::pair::operator= - std::pair::operator=

Synopsis

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

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 (until C++11)
with an inaccessible copy assignment operator, or an array type of
such class.
* This overload is defined as deleted if either
std::is_copy_assignable<T1>::value or (since C++11)
std::is_copy_assignable<T2>::value is false.

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.

Parameters

other - pair of values to replace the contents of this pair

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

)
6-8) 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; // (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 2729 C++11 pair::operator= was unconstrained and might constrained
result in unnecessary undefined behavior