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 > (until
pair& operator=( const pair<U1, U2>& other 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;
(since
pair& operator=( pair&& other ) noexcept(/* C++11)
see below */); (until
(1) C++20)
constexpr pair& operator=( pair&& other ) (since
noexcept(/* see below */); C++20)
constexpr const pair& operator=( pair&& (3) (6) (since
other ) const; C++23)
(since
template< class U1, class U2 > C++11)
pair& operator=( pair<U1, U2>&& p ); (until
(5) C++20)
template< class U1, class U2 > (since
constexpr pair& operator=( pair<U1, U2>&& p C++20)
);
template< class U1, class U2 > (7) (since
constexpr const pair& operator=( pair<U1, (8) C++23)
U2>&& p ) const;
template< pair-like P > (9) (since
constexpr pair& operator=( P&& u ); C++23)
template< pair-like P > (since
constexpr const pair& operator=( P&& u ) (10) C++23)
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 (until C++11)
const-qualified type, or a reference type, or a class type 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&, (since C++11)
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<P>(u)) to first and
std::get<1>(std::forward<P>(u)) to second.
This overload participates in overload resolution only if
* std::same_as<std::remove_cvref_t<P>, std::pair> is false,
* std::remove_cvref_t<P> is not a specialization of std::ranges::subrange,
* std::is_assignable_v<T1&, decltype(std::get<0>(std::forward<P>(p)))> is true,
and
* std::is_assignable_v<T1&, decltype(std::get<1>(std::forward<P>(p)))> is true.
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
* std::same_as<std::remove_cvref_t<P>, std::pair> is false,
* std::remove_cvref_t<P> is not a specialization of std::ranges::subrange,
* std::is_assignable_v<const T1&, decltype(std::get<0>(std::forward<P>(p)))> is
true, and
* std::is_assignable_v<const T1&, decltype(std::get<1>(std::forward<P>(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

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

See also¶

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

Category:¶

* conditionally noexcept