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

 

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Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

std::pair

Member functions

pair::pair

pair::operator=

pair::swap (C++11)

Non-member functions

make_pair

operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

swap(std::pair) (C++11)

get(std::pair) (C++11)

Helper classes

tuple_size<std::pair> (C++11)

tuple_element<std::pair> (C++11)

basic_common_reference<std::pair> (C++23)

common_type<std::pair> (C++23)

formatter<std::pair> (C++23)

piecewise_construct_t (C++11)

Deduction guides(C++17)

 

	(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

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

See also

operator=

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