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

 

C++

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Feature test macros (C++20)

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Ranges library (C++20)

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External libraries

 

Utilities library

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

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 first_type or second_type 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<first_type>::value or std::is_copy_assignable<second_type>::value is false.	(since C++11)

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

• This overload participates in overload resolution only if std::is_assignable<first_type&, const U1&>::value and std::is_assignable<second_type&, const U2&>::value are both true.

3) 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<first_type>::value and std::is_move_assignable<second_type>::value are both true.

4) 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<first_type&, U1>::value and std::is_assignable<second_type&, U2>::value are both true.

Parameters

other

-

pair of values to replace the contents of this pair

Return value

*this

Exceptions

1-2) (none)

3)

noexcept specification:  

noexcept(

    std::is_nothrow_move_assignable<T1>::value &&
    std::is_nothrow_move_assignable<T2>::value

)

4) (none)

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(19) << std::left
              << "p = q;" << "p" << p << "   q" << q << '\n';
 
    std::pair<short, std::vector<int>> r{ 4, {7,8,9} };
    p = r; // (2) operator=( const pair<U1,U2>& other );
    std::cout << std::setw(19)
              << "p = r;" << "p" << p << " r" << r << '\n';
 
    p = std::pair<int, std::vector<int>>{ 3, {4} };
    p = std::move(q); // (3) operator=( pair&& other );
    std::cout << std::setw(19)
              << "p = std::move(q);" << "p" << p << "   q" << q << '\n';
 
    p = std::pair<int, std::vector<int>>{ 5, {6} };
    p = std::move(r); // (4) operator=( pair<U1,U2>&& other );
    std::cout << std::setw(19)
              << "p = std::move(r);" << "p" << p << " r" << r << '\n';
}

Output:

(1) p = q;             p:{ 2, {5,6} }    q:{ 2, {5,6} } 
(2) p = r;             p:{ 4, {7,8,9} }  r:{ 4, {7,8,9} } 
(3) p = std::move(q);  p:{ 2, {5,6} }    q:{ 2, {} } 
(4) 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 result in unnecessary undefined behavior	constrained