C++ priority_queue::swap() Function

---

---

The C++ std::priority_queue::swap() function is used to swap the priority_queue efficiently, ensuring that their contents are exchanged without altering. It maitains the priority order of the elements within each queue.

The swap() function can be called in two ways: as a member function or as a non-member function. The time complexity of the swap() when used as member function is Constant i.e.O(1) orelse it is linear i.e.O(n) when used as non-member function. you can find the syntaxes of both the ways below.

Syntax

Following is the syntax for std::priority_queue::swap() function.

void swap (priority_queue& x) noexcept;
or
void swap (queue<T,Container,Compare>& q1,queue <T,Container,Compare>& q2) noexcept;

Parameters

• x − It indicates the another priority queue object of same type.
• q1 − It indicates the first priority_queue object.
• q2 − It indicates the second priority_queue object.

Return value

This function does not return anything.

Example

Let's look at the following example, where we are going to swap the two priority_queues.

#include <iostream>
#include <queue>
int main()
{
    std::priority_queue<int> a, b;
    a.push(1);
    a.push(2);
    b.push(11);
    b.push(22);
    a.swap(b);
    std::cout << "After swapping:";
    std::cout << "\nPriority_queue1: ";
    while (!a.empty()) {
        std::cout << a.top() << " ";
        a.pop();
    }
    std::cout << "\nPriority_queue2: ";
    while (!b.empty()) {
        std::cout << b.top() << " ";
        b.pop();
    }
    return 0;
}

Output

Output of the above code is as follows −

After swapping:
Priority_queue1: 22 11 
Priority_queue2: 2 1 

Example

Consider the following example, where we are going to swap an non empty queue with empty queue and observing the output.

#include <iostream>
#include <queue>
int main()
{
    std::priority_queue<int> a, b;
    a.push(1);
    a.push(2);
    a.swap(b);
    std::cout << "After swapping:\n";
    std::cout << "Priority_queue1 is empty";
    std::cout << "\nPriority_queue2: ";
    while (!b.empty()) {
        std::cout << b.top() << " ";
        b.pop();
    }
    return 0;
}

Output

Following is the output of the above code −

After swapping:
Priority_queue1 is empty
Priority_queue2: 2 1 

Example

In the following example, we are going to use the swap() function with move semantics to swap the priority_queue containing strings.

#include <iostream>
#include <queue>
#include <utility>
int main()
{
    std::priority_queue<std::string> a, b;
    a.push("Hi");
    a.push("Hello");
    b.push("Namaste");
    b.push("Vanakam");
    std::swap(a, b);
    std::cout << "\n\nAfter swapping:\n";
    std::cout << "Priority_queue1: ";
    while (!a.empty()) {
        std::cout << a.top() << " ";
        a.pop();
    }
    std::cout << "\nPriority_queue2: ";
    while (!b.empty()) {
        std::cout << b.top() << " ";
        b.pop();
    }
    return 0;
}

Output

If we run the above code it will generate the following output −

After swapping:
Priority_queue1: Vanakam Namaste 
Priority_queue2: Hi Hello