std::pair<T1,T2>::operator=
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std::pair
pair::operator=
(C++11)
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
(C++11)
(C++11)
(C++11)
(C++11)
(C++23)
(C++23)
(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)
pair& operator=( const pair<U1, U2>& other );
template< class U1, class U2 >
constexpr pair& operator=( const pair<U1, U2>& other );
(since C++20)
constexpr pair& operator=( const pair<U1, U2>& other );
template< class U1, class U2 >
constexpr const pair& operator=( const pair<U1, U2>& other ) const;
(4)
(since C++23)
constexpr const pair& operator=( const pair<U1, U2>& other ) const;
(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) pair& operator=( pair<U1, U2>&& p );
(until C++20)
template< class U1, class U2 >
constexpr pair& operator=( pair<U1, U2>&& p );
(since C++20)
constexpr pair& operator=( pair<U1, U2>&& p );
template< class U1, class U2 >
constexpr const pair& operator=( pair<U1, U2>&& p ) const;
(8)
(since C++23)
constexpr const pair& operator=( pair<U1, U2>&& p ) const;
template< pair-like P >
constexpr pair& operator=( P&& u );
(9)
(since C++23)
constexpr pair& operator=( P&& u );
template< pair-like P >
constexpr const pair& operator=( P&& u ) const;
(10)
(since C++23)
constexpr const pair& operator=( P&& u ) 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 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 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.
This overload participates in overload resolution only if std::is_assignable <T1&, U1>::value and std::is_assignable <T2&, U2>::value are both true.
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.
[edit] 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
[edit] Return value
*this
[edit] 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.
[edit] 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, {}}
[edit] 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 mightresult in unnecessary undefined behavior |
constrained |