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

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< cpp‎ | utility‎ | pair

| (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 <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= (C++11) | assigns the contents of one `tuple` to another (public member function of `std::tuple<Types...>`)

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