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std::swap(std::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

#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

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

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