# std::swap(std::queue)

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< cpp‎ | container‎ | queue

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

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

This overload participates in overload resolution only if std::is_swappable_v<Container> is true. | (since C++17)

### Parameters

lhs, rhs | - | containers whose contents to swap

### Return value

(none)

### Complexity

Same as swapping the underlying container.

### 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 <queue>

    int main()
    {
        std::queue<int> alice;
        std::queue<int> bob;

        auto print = [](const auto & title, const auto &cont)
        {
            std::cout << title << " size=" << cont.size();
            std::cout << " front=" << cont.front();
            std::cout << " back=" << cont.back() << '\n';
        };

        for (int i = 1; i < 4; ++i)
            alice.push(i);
        for (int i = 7; i < 11; ++i)
            bob.push(i);

        // Print state before swap
        print("alice:", alice);
        print("bob  :", bob);

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

        // Print state after swap
        print("alice:", alice);
        print("bob  :", bob);
    }

Output:

    alice: size=3 front=1 back=3
    bob  : size=4 front=7 back=10
    -- SWAP
    alice: size=4 front=7 back=10
    bob  : size=3 front=1 back=3

### See also

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

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