queue::swap() in C++ STL

The swap() function in C++ STL exchanges the contents of two queues efficiently. It is a member function of the std::queue container adapter and is defined in the <queue> header file.

• Exchanges the contents of two queues without copying individual elements.
• Both queues must have the same element type and underlying container type.

#include <iostream>
#include <queue>
using namespace std;
int main()
{
    queue<int> q1, q2;
    q1.push(1);
    q1.push(2);
    q1.push(3);

    q2.push(10);
    q2.push(20);
    q2.push(30);
    q2.push(40);
    q1.swap(q2);

    cout << "q1: ";
    while (!q1.empty())
    {
        cout << q1.front() << " ";
        q1.pop();
    }
    cout << "\nq2: ";
    while (!q2.empty())
    {
        cout << q2.front() << " ";
        q2.pop();
    }
    return 0;
}

q1: 10 20 30 40 
q2: 1 2 3 

Explanation: After calling q1.swap(q2), the contents of the two queues are exchanged. The elements of q1 become the elements of q2 and vice versa.

Syntax

queue1.swap(queue2);
or
swap(queue1, queue2);

• Parameters: queue1 is the first queue and queue2 is the second queue.
• Return Value: The function does not return any value.

Using the Non-Member swap() Function

The STL also provides a non-member swap() function that performs the same operation.

#include <iostream>
#include <queue>
using namespace std;
int main()
{
    queue<char> q1, q2;
    q1.push('A');
    q1.push('B');
    q1.push('C');

    q2.push('X');
    q2.push('Y');

    swap(q1, q2);

    cout << "Front of q1: " << q1.front() << endl;
    cout << "Front of q2: " << q2.front() << endl;

    return 0;
}

Front of q1: X
Front of q2: A

Characteristics of queue::swap()

The swap() function provides an efficient way to exchange the contents of two queues.

• Both queues must have the same type.
• The sizes of the queues may differ.
• No elements are copied individually.
• The operation preserves the order of elements inside each queue.
• Both member swap() and non-member swap() perform the same task.

Errors and Exceptions

The queue::swap() function is safe and efficient, but there are a few conditions that should be kept in mind.

• A compilation error occurs if the two queues have different types.
• The function provides a no-throw guarantee when the underlying container supports a no-throw swap.
• The sizes of the two queues do not need to be the same for swapping.