# std::swap(std::unordered_map)

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< cpp‎ | container‎ | unordered map

Defined in header `<unordered_map>` |  |
template< class Key, class T, class Hash, class KeyEqual, class Alloc > void swap( std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& lhs, std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& rhs ); |  | (since C++11) (until C++17)
template< class Key, class T, class Hash, class KeyEqual, class Alloc > void swap( std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& lhs, std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& rhs ) noexcept(/* see below */); |  | (since C++17)

Specializes the std::swap algorithm for std::unordered_map. Swaps the contents of lhs and rhs. Calls lhs.swap(rhs).

### Parameters

lhs, rhs | - | containers whose contents to swap

### Return value

(none)

### Complexity

Constant.

### Exceptions

`noexcept` specification: noexcept(noexcept(lhs.swap(rhs))) | (since C++17)

### Notes

Although the overloads of std::swap for container adaptors are introduced in C++11, container adaptors can already be swapped by std::swap in C++98. Such calls to std::swap usually have linear time complexity, but better complexity may be provided.

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <unordered_map>

    int main()
    {
        std::unordered_map<int, char> alice{{1, 'a'}, {2, 'b'}, {3, 'c'}};
        std::unordered_map<int, char> bob{{7, 'Z'}, {8, 'Y'}, {9, 'X'}, {10, 'W'}};

        auto print = [](std::pair<const int, char>& n)
        {
            std::cout << ' ' << n.first << ':' << n.second;
        };

        // Print state before swap
        std::cout << "alice:";
        std::for_each(alice.begin(), alice.end(), print);
        std::cout << "\n" "bob  :";
        std::for_each(bob.begin(), bob.end(), print);
        std::cout << '\n';

        std::cout << "-- SWAP\n";
        std::swap(alice, bob);

        // Print state after swap
        std::cout << "alice:";
        std::for_each(alice.begin(), alice.end(), print);
        std::cout << "\n" "bob  :";
        std::for_each(bob.begin(), bob.end(), print);
        std::cout << '\n';
    }

Possible output:

    alice: 1:a 2:b 3:c
    bob  : 7:Z 8:Y 9:X 10:W
    -- SWAP
    alice: 7:Z 8:Y 9:X 10:W
    bob  : 1:a 2:b 3:c

### See also

swap (C++11) | swaps the contents (public member function)

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